This document summarizes the history and process of photosynthesis. It describes key discoveries such as Priestley observing that plants give off oxygen, and van Niel determining the reaction equation for photosynthesis in purple bacteria. It then explains the light and dark reactions of photosynthesis in detail, including Calvin's discovery of the Calvin cycle where carbon dioxide is incorporated into carbohydrates. The document also discusses different types of plants, including C3, C4, and CAM plants, and how they regulate photosynthesis through pathways like the Hatch-Slack cycle.
This PowerPoint presentation covers photosynthesis and was last revised in June 2008. It contains 27 slides discussing topics like the light and dark reactions of photosynthesis, chloroplast structure, and how plants use light energy from the sun to produce glucose and oxygen from water and carbon dioxide. The document provides an overview of photosynthesis and its key stages and equations.
Photosynthesis has two main stages:
1. The light reactions use light energy to convert water to oxygen and produce ATP and NADPH through the electron transport chain in the thylakoid membranes of chloroplasts.
2. The Calvin cycle uses the ATP and NADPH products from the light reactions to incorporate carbon from carbon dioxide into organic compounds to produce glucose in the stroma of the chloroplast.
3. The process is essential as it produces oxygen and carbohydrates for plants and food for animals and humans from carbon dioxide and water using energy from sunlight.
This document discusses the history and process of photosynthesis. It describes several key experiments that advanced understanding, including van Helmont's experiment showing plant mass comes from water, Priestley's showing plants release oxygen, and Ingenhousz's showing this only occurs with sunlight. The overall reaction of photosynthesis is described as using sunlight, water and carbon dioxide to produce sugars and oxygen. Chlorophyll is identified as the main light-absorbing pigment located in chloroplasts, with chlorophyll a directly participating in light reactions and chlorophyll b transferring energy.
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two stages: the light-dependent reactions and the light-independent reactions. The light-dependent reactions use energy from sunlight to produce ATP and NADPH through a process called photophosphorylation. The light-independent reactions, also called the Calvin cycle, use ATP and NADPH to fix carbon from carbon dioxide into organic molecules like glucose. The Calvin cycle is an enzyme-driven process where carbon is first fixed into the three-carbon molecule phosphoglycerate then reduced, regenerated, and used to produce glucose and other carbohydr
Photosynthesis is the process by which plants use sunlight, carbon dioxide, and water to produce oxygen and energy in the form of sugar. It occurs in two stages: the light-dependent reactions in the chloroplasts absorb sunlight and split water, producing oxygen and energized molecules. The Calvin cycle then uses this energy to fix carbon from carbon dioxide into organic compounds like glucose. Chlorophyll and other pigments capture sunlight and transfer the energy through electron transport chains to produce ATP and NADPH, which power the Calvin cycle to reduce carbon dioxide into sugars.
Photosynthesis involves the conversion of light energy from the sun into chemical energy. Chlorophyll, the main photosynthetic pigment, absorbs different wavelengths of light with red and blue absorbed more than green. Light energy is used to produce ATP and split water to form oxygen and hydrogen, while ATP and hydrogen are used to fix carbon dioxide and make glucose. The rate of photosynthesis is affected by temperature, light intensity, and carbon dioxide concentration.
Plants emit fluorescence during photosynthesis that is detectable by satellites in space. NASA scientists have developed a method to map this fluorescence globally using satellite data. The document then provides details on photosynthesis, including that it consists of two sets of reactions - the light reactions and Calvin cycle. It describes the light reactions in detail, including that they occur in the thylakoid membranes and produce ATP and NADPH using solar energy absorbed by chlorophyll. This energy is then used in the Calvin cycle to reduce carbon dioxide into carbohydrates.
This PowerPoint presentation covers photosynthesis and was last revised in June 2008. It contains 27 slides discussing topics like the light and dark reactions of photosynthesis, chloroplast structure, and how plants use light energy from the sun to produce glucose and oxygen from water and carbon dioxide. The document provides an overview of photosynthesis and its key stages and equations.
Photosynthesis has two main stages:
1. The light reactions use light energy to convert water to oxygen and produce ATP and NADPH through the electron transport chain in the thylakoid membranes of chloroplasts.
2. The Calvin cycle uses the ATP and NADPH products from the light reactions to incorporate carbon from carbon dioxide into organic compounds to produce glucose in the stroma of the chloroplast.
3. The process is essential as it produces oxygen and carbohydrates for plants and food for animals and humans from carbon dioxide and water using energy from sunlight.
This document discusses the history and process of photosynthesis. It describes several key experiments that advanced understanding, including van Helmont's experiment showing plant mass comes from water, Priestley's showing plants release oxygen, and Ingenhousz's showing this only occurs with sunlight. The overall reaction of photosynthesis is described as using sunlight, water and carbon dioxide to produce sugars and oxygen. Chlorophyll is identified as the main light-absorbing pigment located in chloroplasts, with chlorophyll a directly participating in light reactions and chlorophyll b transferring energy.
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two stages: the light-dependent reactions and the light-independent reactions. The light-dependent reactions use energy from sunlight to produce ATP and NADPH through a process called photophosphorylation. The light-independent reactions, also called the Calvin cycle, use ATP and NADPH to fix carbon from carbon dioxide into organic molecules like glucose. The Calvin cycle is an enzyme-driven process where carbon is first fixed into the three-carbon molecule phosphoglycerate then reduced, regenerated, and used to produce glucose and other carbohydr
Photosynthesis is the process by which plants use sunlight, carbon dioxide, and water to produce oxygen and energy in the form of sugar. It occurs in two stages: the light-dependent reactions in the chloroplasts absorb sunlight and split water, producing oxygen and energized molecules. The Calvin cycle then uses this energy to fix carbon from carbon dioxide into organic compounds like glucose. Chlorophyll and other pigments capture sunlight and transfer the energy through electron transport chains to produce ATP and NADPH, which power the Calvin cycle to reduce carbon dioxide into sugars.
Photosynthesis involves the conversion of light energy from the sun into chemical energy. Chlorophyll, the main photosynthetic pigment, absorbs different wavelengths of light with red and blue absorbed more than green. Light energy is used to produce ATP and split water to form oxygen and hydrogen, while ATP and hydrogen are used to fix carbon dioxide and make glucose. The rate of photosynthesis is affected by temperature, light intensity, and carbon dioxide concentration.
Plants emit fluorescence during photosynthesis that is detectable by satellites in space. NASA scientists have developed a method to map this fluorescence globally using satellite data. The document then provides details on photosynthesis, including that it consists of two sets of reactions - the light reactions and Calvin cycle. It describes the light reactions in detail, including that they occur in the thylakoid membranes and produce ATP and NADPH using solar energy absorbed by chlorophyll. This energy is then used in the Calvin cycle to reduce carbon dioxide into carbohydrates.
The document provides an overview of photosynthesis, including:
1) Photosynthesis uses light energy from the sun to convert carbon dioxide and water into sugars and oxygen through a two-stage process of light-dependent and light-independent reactions.
2) The light reactions convert solar energy to chemical energy stored in ATP and NADPH. The Calvin cycle then uses this chemical energy to fix carbon from carbon dioxide into sugars.
3) Two photosystems, Photosystem I and Photosystem II, work together to drive electron transport and generate a proton gradient used to produce ATP through chemiosmosis.
1. Photosynthesis occurs in leaves through two stages - the light dependent and light independent reactions.
2. In the light dependent reactions, light energy is captured by chloroplasts and used to convert carbon dioxide and water into oxygen and energy carriers (ATP and NADPH).
3. The light independent reactions, known as the Calvin cycle, use the energy from ATP and NADPH to fix carbon from carbon dioxide into sugars.
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It takes place in two stages - the light reactions where light energy is captured to make ATP and NADPH, and the dark reactions where ATP and NADPH are used to incorporate carbon from CO2 into organic compounds like glucose. The chloroplast is the organelle where photosynthesis takes place, containing chlorophyll pigments in the thylakoid membranes which absorb light energy to drive the light-dependent reactions.
Photosynthesis is the process by which plants and other organisms use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. Chlorophyll, located in chloroplasts, absorbs sunlight and uses the energy to convert carbon dioxide and water into oxygen and glucose through a two-step process - the light reactions and Calvin cycle. Plants appear green because chlorophyll, the main photosynthetic pigment, absorbs most wavelengths of visible light except green, which it reflects, giving leaves their green color.
Photosynthesis involves two main stages - the light reactions where light energy is converted to chemical energy stored in ATP and NADPH, and the Calvin cycle where carbon dioxide and this chemical energy are used to produce sugars like glucose. It is the most important chemical reaction on Earth because it provides the primary source of energy for nearly all living things through oxygen production and by forming organic compounds from carbon dioxide and water using sunlight.
This document summarizes key aspects of photosynthesis. It discusses that there are two types of organisms - autotrophs that get energy from sunlight and heterotrophs that get energy from food. It also describes ATP as a high-energy molecule used to store and transport energy in cells. The main ingredients for photosynthesis are outlined as water, carbon dioxide, sunlight, and chlorophyll, which is the green pigment found in chloroplasts. The overall equation for photosynthesis is provided. Light energy is absorbed by chlorophyll and converted to chemical energy, which breaks apart water molecules to release oxygen and provides energy to convert carbon dioxide into glucose.
This document provides an overview of photosynthesis presented by a group of 6 students. It describes the key processes and phases of photosynthesis including light reaction, dark reaction, photophosphorylation, photosystems, and alternative pathways such as C4 and CAM photosynthesis. The light reaction uses energy from sunlight to split water and produce ATP and NADPH. The dark reaction then uses these products to fix carbon from carbon dioxide into sugars.
Photosynthesis is a oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate level, the water and oxygen being by product.
1. Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two stages - the light reaction which converts solar energy to chemical energy in the form of ATP and NADPH, and the dark reaction which uses this energy to fix carbon dioxide and produce sugars.
2. The light reaction takes place in the thylakoid membranes of chloroplasts and involves two light-dependent reactions - photosystem I and photosystem II. Photosystem II uses light energy to split water, releasing electrons, protons and oxygen. Photosystem I uses these electrons to reduce NADP+ to NADPH.
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two stages: in the light-dependent reactions, sunlight is absorbed and used to convert carbon dioxide and water into glucose, producing oxygen as a byproduct. In the light-independent reactions, the glucose is then assembled from carbon dioxide using energy from the light reactions. The light reactions take place in chloroplasts within plant cells, while the dark reactions occur in the chloroplast stroma. Photosynthesis is essential for producing oxygen and food on Earth.
Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the sun, into chemical energy that can be used to fuel the organisms' activities. Carbohydrates, such as sugars, are synthesized from carbon dioxide and water.
WHAT IS PHOTOSYNTHESIS?, IMPORTANCE OF PHOTOSYNTHESIS, STRUCTURAL FEATURE OF LEAF ADVANTAGE FOR PHOTOSYNTHESIS,LEAVES AND LEAF STRUCTURE,CHLOROPHYLL, TYPES OF REACTIONS, LIGHT REACTION AND DARK REACTION, CYCLIC AND NON-CYCLIC PHOTOPHOSPORYLATION, MECAHANISM OF ATP SYNTHESIS, SCHEMATIC PRESENTATION OF LIGHT REACTION, CRASSULACEAN ACID METABOLISM (CAM), C3 AND C4 PLANTS, FACTORS AFFECTING RATE OF PHOTOSYNTHESIS, INTERNAL FACTORS AND EXTERNAL FACTORS,
This document provides an overview of photosynthesis, including essential questions, important vocabulary terms, the structure and function of leaves, chloroplasts, and the two main stages - the light reactions and light-independent reactions (also called the Calvin cycle). The light reactions use light energy to produce ATP and NADPH. The Calvin cycle uses these products to incorporate carbon dioxide into organic molecules to produce glucose. Factors that can affect the rate of photosynthesis such as light intensity, temperature, carbon dioxide and oxygen levels are also discussed.
Photosynthesis occurs in leaves through the processes of light-dependent and light-independent reactions. In the light-dependent reactions, chloroplasts use pigments like chlorophyll to absorb sunlight and drive photophosphorylation through non-cyclic or cyclic pathways. This generates ATP and NADPH that fuel the light-independent Calvin cycle where CO2 is fixed into glucose using enzymes like RuBisCO. The overall process requires chloroplast structures in mesophyll cells as well as transport of reactants and products through the leaf.
Photosynthesis is the primary source of food on Earth and releases oxygen into the atmosphere. Joseph Priestley and Jan Ingenhousz performed experiments in the 1700s that revealed plants require air and sunlight to grow. Ingenhousz showed oxygen bubbles form around green plant parts in sunlight. Later studies identified chloroplasts as the photosynthesis site in plant cells, with light and dark reactions taking place in the thylakoid membranes and stroma, respectively. The Calvin cycle was discovered to convert carbon dioxide into sugars using ATP and NADPH produced from the light reactions. Factors like light intensity, carbon dioxide levels, temperature and water availability can limit photosynthesis rates.
The document summarizes key aspects of photosynthesis. It describes:
1) The two main stages of photosynthesis - the light-dependent reactions where light energy is captured to form ATP and NADPH, and the light-independent Calvin cycle where carbon is fixed into sugars.
2) The role of chlorophyll, carotenoids, and other pigments in absorbing light and driving photosynthesis.
3) How the electron transport chain and chemiosmosis are used in the light reactions to generate ATP.
4) The steps of the Calvin cycle including carbon fixation, reduction, and regeneration of RuBP.
Photosynthesis is the process by which plants and other organisms convert sunlight, water and carbon dioxide into oxygen and energy in the form of sugars. It occurs in two main stages: the light reactions where sunlight is absorbed and used to produce ATP and NADPH, and the Calvin cycle where carbon dioxide is fixed using the ATP and NADPH to produce sugars. Overall, the process converts carbon dioxide and water into oxygen and energy-rich organic compounds like glucose.
1. The light reaction of photosynthesis occurs in the thylakoid membranes of chloroplasts and involves the absorption of light by photosynthetic pigments.
2. Energy from the absorbed light is used to transfer electrons along an electron transport chain, powering the synthesis of ATP through photophosphorylation and reducing NADP+ to NADPH.
3. The products of the light reaction, ATP and NADPH, are used in the Calvin cycle to fix carbon from CO2 into organic molecules like glucose.
Photosynthesis is the process by which plants convert energy from sunlight, water, and carbon dioxide to produce oxygen and energy-rich organic molecules like glucose. It occurs in two stages: the light-dependent reaction, which uses sunlight to produce ATP and NADPH, and the light-independent reaction (the Calvin cycle), which uses those products to incorporate carbon from carbon dioxide into organic molecules like glucose. Photosynthesis is essential as it provides energy and organic molecules for plants and forms the basis of the food chain supporting almost all life on Earth.
Photosynthesis is the process by which plants, algae, and cyanobacteria use sunlight, water, and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It occurs in two stages - the light-dependent reactions and the light-independent reactions. The light-dependent reactions use energy from sunlight to produce ATP and NADPH using a process called the Z-scheme that involves two photosystems. The light-independent reactions then use these products to incorporate carbon from carbon dioxide into organic compounds like glucose.
Photosynthesis is the process by which plants, algae, and cyanobacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It occurs in two phases: the light-dependent reactions and the light-independent reactions. The light reactions capture energy from sunlight and use it to make ATP and NADPH. The Calvin cycle uses these products to incorporate carbon from carbon dioxide into organic compounds to fuel the plant. Some plants use alternative pathways like C4 or CAM photosynthesis that help reduce photorespiration and increase water use efficiency.
1. Photosynthesis involves two main stages - the light dependent reaction where light energy is captured to make ATP and NADPH, and the light independent Calvin cycle where CO2 is fixed using ATP and NADPH to produce glucose.
2. The light reactions take place in the thylakoid membranes of chloroplasts and involve the photsytems which transfer electrons to make ATP and NADPH.
3. The Calvin cycle takes place in the stroma of the chloroplast and uses ATP and NADPH to convert CO2 into glucose through a series of reduction and phosphorylation reactions.
The document provides an overview of photosynthesis, including:
1) Photosynthesis uses light energy from the sun to convert carbon dioxide and water into sugars and oxygen through a two-stage process of light-dependent and light-independent reactions.
2) The light reactions convert solar energy to chemical energy stored in ATP and NADPH. The Calvin cycle then uses this chemical energy to fix carbon from carbon dioxide into sugars.
3) Two photosystems, Photosystem I and Photosystem II, work together to drive electron transport and generate a proton gradient used to produce ATP through chemiosmosis.
1. Photosynthesis occurs in leaves through two stages - the light dependent and light independent reactions.
2. In the light dependent reactions, light energy is captured by chloroplasts and used to convert carbon dioxide and water into oxygen and energy carriers (ATP and NADPH).
3. The light independent reactions, known as the Calvin cycle, use the energy from ATP and NADPH to fix carbon from carbon dioxide into sugars.
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It takes place in two stages - the light reactions where light energy is captured to make ATP and NADPH, and the dark reactions where ATP and NADPH are used to incorporate carbon from CO2 into organic compounds like glucose. The chloroplast is the organelle where photosynthesis takes place, containing chlorophyll pigments in the thylakoid membranes which absorb light energy to drive the light-dependent reactions.
Photosynthesis is the process by which plants and other organisms use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. Chlorophyll, located in chloroplasts, absorbs sunlight and uses the energy to convert carbon dioxide and water into oxygen and glucose through a two-step process - the light reactions and Calvin cycle. Plants appear green because chlorophyll, the main photosynthetic pigment, absorbs most wavelengths of visible light except green, which it reflects, giving leaves their green color.
Photosynthesis involves two main stages - the light reactions where light energy is converted to chemical energy stored in ATP and NADPH, and the Calvin cycle where carbon dioxide and this chemical energy are used to produce sugars like glucose. It is the most important chemical reaction on Earth because it provides the primary source of energy for nearly all living things through oxygen production and by forming organic compounds from carbon dioxide and water using sunlight.
This document summarizes key aspects of photosynthesis. It discusses that there are two types of organisms - autotrophs that get energy from sunlight and heterotrophs that get energy from food. It also describes ATP as a high-energy molecule used to store and transport energy in cells. The main ingredients for photosynthesis are outlined as water, carbon dioxide, sunlight, and chlorophyll, which is the green pigment found in chloroplasts. The overall equation for photosynthesis is provided. Light energy is absorbed by chlorophyll and converted to chemical energy, which breaks apart water molecules to release oxygen and provides energy to convert carbon dioxide into glucose.
This document provides an overview of photosynthesis presented by a group of 6 students. It describes the key processes and phases of photosynthesis including light reaction, dark reaction, photophosphorylation, photosystems, and alternative pathways such as C4 and CAM photosynthesis. The light reaction uses energy from sunlight to split water and produce ATP and NADPH. The dark reaction then uses these products to fix carbon from carbon dioxide into sugars.
Photosynthesis is a oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate level, the water and oxygen being by product.
1. Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two stages - the light reaction which converts solar energy to chemical energy in the form of ATP and NADPH, and the dark reaction which uses this energy to fix carbon dioxide and produce sugars.
2. The light reaction takes place in the thylakoid membranes of chloroplasts and involves two light-dependent reactions - photosystem I and photosystem II. Photosystem II uses light energy to split water, releasing electrons, protons and oxygen. Photosystem I uses these electrons to reduce NADP+ to NADPH.
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two stages: in the light-dependent reactions, sunlight is absorbed and used to convert carbon dioxide and water into glucose, producing oxygen as a byproduct. In the light-independent reactions, the glucose is then assembled from carbon dioxide using energy from the light reactions. The light reactions take place in chloroplasts within plant cells, while the dark reactions occur in the chloroplast stroma. Photosynthesis is essential for producing oxygen and food on Earth.
Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the sun, into chemical energy that can be used to fuel the organisms' activities. Carbohydrates, such as sugars, are synthesized from carbon dioxide and water.
WHAT IS PHOTOSYNTHESIS?, IMPORTANCE OF PHOTOSYNTHESIS, STRUCTURAL FEATURE OF LEAF ADVANTAGE FOR PHOTOSYNTHESIS,LEAVES AND LEAF STRUCTURE,CHLOROPHYLL, TYPES OF REACTIONS, LIGHT REACTION AND DARK REACTION, CYCLIC AND NON-CYCLIC PHOTOPHOSPORYLATION, MECAHANISM OF ATP SYNTHESIS, SCHEMATIC PRESENTATION OF LIGHT REACTION, CRASSULACEAN ACID METABOLISM (CAM), C3 AND C4 PLANTS, FACTORS AFFECTING RATE OF PHOTOSYNTHESIS, INTERNAL FACTORS AND EXTERNAL FACTORS,
This document provides an overview of photosynthesis, including essential questions, important vocabulary terms, the structure and function of leaves, chloroplasts, and the two main stages - the light reactions and light-independent reactions (also called the Calvin cycle). The light reactions use light energy to produce ATP and NADPH. The Calvin cycle uses these products to incorporate carbon dioxide into organic molecules to produce glucose. Factors that can affect the rate of photosynthesis such as light intensity, temperature, carbon dioxide and oxygen levels are also discussed.
Photosynthesis occurs in leaves through the processes of light-dependent and light-independent reactions. In the light-dependent reactions, chloroplasts use pigments like chlorophyll to absorb sunlight and drive photophosphorylation through non-cyclic or cyclic pathways. This generates ATP and NADPH that fuel the light-independent Calvin cycle where CO2 is fixed into glucose using enzymes like RuBisCO. The overall process requires chloroplast structures in mesophyll cells as well as transport of reactants and products through the leaf.
Photosynthesis is the primary source of food on Earth and releases oxygen into the atmosphere. Joseph Priestley and Jan Ingenhousz performed experiments in the 1700s that revealed plants require air and sunlight to grow. Ingenhousz showed oxygen bubbles form around green plant parts in sunlight. Later studies identified chloroplasts as the photosynthesis site in plant cells, with light and dark reactions taking place in the thylakoid membranes and stroma, respectively. The Calvin cycle was discovered to convert carbon dioxide into sugars using ATP and NADPH produced from the light reactions. Factors like light intensity, carbon dioxide levels, temperature and water availability can limit photosynthesis rates.
The document summarizes key aspects of photosynthesis. It describes:
1) The two main stages of photosynthesis - the light-dependent reactions where light energy is captured to form ATP and NADPH, and the light-independent Calvin cycle where carbon is fixed into sugars.
2) The role of chlorophyll, carotenoids, and other pigments in absorbing light and driving photosynthesis.
3) How the electron transport chain and chemiosmosis are used in the light reactions to generate ATP.
4) The steps of the Calvin cycle including carbon fixation, reduction, and regeneration of RuBP.
Photosynthesis is the process by which plants and other organisms convert sunlight, water and carbon dioxide into oxygen and energy in the form of sugars. It occurs in two main stages: the light reactions where sunlight is absorbed and used to produce ATP and NADPH, and the Calvin cycle where carbon dioxide is fixed using the ATP and NADPH to produce sugars. Overall, the process converts carbon dioxide and water into oxygen and energy-rich organic compounds like glucose.
1. The light reaction of photosynthesis occurs in the thylakoid membranes of chloroplasts and involves the absorption of light by photosynthetic pigments.
2. Energy from the absorbed light is used to transfer electrons along an electron transport chain, powering the synthesis of ATP through photophosphorylation and reducing NADP+ to NADPH.
3. The products of the light reaction, ATP and NADPH, are used in the Calvin cycle to fix carbon from CO2 into organic molecules like glucose.
Photosynthesis is the process by which plants convert energy from sunlight, water, and carbon dioxide to produce oxygen and energy-rich organic molecules like glucose. It occurs in two stages: the light-dependent reaction, which uses sunlight to produce ATP and NADPH, and the light-independent reaction (the Calvin cycle), which uses those products to incorporate carbon from carbon dioxide into organic molecules like glucose. Photosynthesis is essential as it provides energy and organic molecules for plants and forms the basis of the food chain supporting almost all life on Earth.
Photosynthesis is the process by which plants, algae, and cyanobacteria use sunlight, water, and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It occurs in two stages - the light-dependent reactions and the light-independent reactions. The light-dependent reactions use energy from sunlight to produce ATP and NADPH using a process called the Z-scheme that involves two photosystems. The light-independent reactions then use these products to incorporate carbon from carbon dioxide into organic compounds like glucose.
Photosynthesis is the process by which plants, algae, and cyanobacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of ATP and NADPH. It occurs in two phases: the light-dependent reactions and the light-independent reactions. The light reactions capture energy from sunlight and use it to make ATP and NADPH. The Calvin cycle uses these products to incorporate carbon from carbon dioxide into organic compounds to fuel the plant. Some plants use alternative pathways like C4 or CAM photosynthesis that help reduce photorespiration and increase water use efficiency.
1. Photosynthesis involves two main stages - the light dependent reaction where light energy is captured to make ATP and NADPH, and the light independent Calvin cycle where CO2 is fixed using ATP and NADPH to produce glucose.
2. The light reactions take place in the thylakoid membranes of chloroplasts and involve the photsytems which transfer electrons to make ATP and NADPH.
3. The Calvin cycle takes place in the stroma of the chloroplast and uses ATP and NADPH to convert CO2 into glucose through a series of reduction and phosphorylation reactions.
The document provides an overview of photosynthesis, including:
1) The importance of autotrophs in providing energy and carbon compounds for all organisms and oxygenating the atmosphere.
2) An overview of the light-dependent and light-independent reactions of photosynthesis, which use energy from sunlight to produce oxygen and carbohydrates from carbon dioxide and water.
3) The structures of leaves and chloroplasts that are important sites for photosynthesis.
This document discusses the roles of micro-organisms in ecosystems. It describes how viruses, bacteria, algae, fungi, and protists help control nutrient cycling and biodiversity. Bacteria are important for decomposition, recycling nitrogen, and producing nutrients for plants. Fungi decompose dead organic matter. The document also discusses symbiotic relationships between organisms, including mutualism, parasitism, and commensalism. An example of mutualism is nitrogen-fixing bacteria providing nitrates to plants in exchange for food.
Plants and algae perform photosynthesis, which converts carbon dioxide and water into glucose and oxygen using energy from sunlight. Photosynthesis occurs in chloroplasts and uses chlorophyll to absorb light, which drives the production of ATP and NADPH that fuel the Calvin cycle to produce glucose from carbon dioxide. It is an essential process that generates oxygen and forms the base of the food chain sustaining all life on Earth.
1. The document summarizes how photosynthesis works through light-dependent and light-independent reactions. It describes the process of capturing energy from sunlight to make ATP and NADPH, and then using these products to fix carbon dioxide and produce sugars.
2. Photosynthesis takes place in chloroplasts in plant cells, which contain thylakoid membranes where the light-dependent reactions occur. These reactions use light energy to convert water to oxygen and produce ATP and NADPH.
3. The light-independent reactions then use ATP and NADPH to fix carbon dioxide and produce glucose through the Calvin cycle. This provides the sugars and starches that plants need for growth and energy storage.
Cellular respiration involves the breakdown of glucose or other molecules to extract energy through a series of redox reactions. These reactions take place in the mitochondria, which contain an inner and outer membrane. The inner membrane contains cristae and enzymes that make up the electron transport chain (ETC), which pumps protons into the intermembrane space. This generates a proton gradient that is used by ATP synthase to produce ATP through oxidative phosphorylation. Glycolysis produces pyruvate and ATP in the cytosol. Pyruvate then enters the mitochondria and is converted to acetyl-CoA to feed into the Krebs cycle. The Krebs cycle produces NADH, FADH2, and GTP. These electron carriers are
The document outlines a lesson plan on aerobic respiration that includes three main steps: glycolysis, the Krebs cycle, and the electron transport chain. The teacher will motivate students with a puzzle activity and divide them into groups to research and present on each step. An evaluation asks students to identify the processes, steps, and important molecules in aerobic respiration using an illustration. The lesson aims to help students understand and appreciate the importance of aerobic respiration for animals and plants.
The document summarizes the sinking of the Titanic after it collided with an iceberg. It describes how the bow flooded, causing the stern to rise up at a 60 degree angle. The ship broke apart at an expansion joint between the third and fourth funnels. The bow sank completely, pulling the stern underwater vertically before it detached. The stern floated briefly before sinking completely. Decades later, Dr. Robert Ballard discovered the wreckage of the Titanic on the ocean floor in 1985.
The document discusses cellular respiration, which is the process by which cells break down food molecules to produce energy in the form of ATP. It describes the three main stages of cellular respiration: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis breaks down glucose and occurs in the cytoplasm, producing a small amount of ATP. The citric acid cycle further breaks down molecules in the mitochondria, producing more ATP and electrons. The electron transport chain uses these electrons and oxygen to produce the most ATP through oxidative phosphorylation. Aerobic respiration requires oxygen while anaerobic respiration occurs without oxygen through fermentation.
This document summarizes key aspects of photosynthesis. It begins by explaining that life is powered by the sun through photosynthesis and that organisms obtain energy and carbon either through autotrophy or heterotrophy. It then describes the two stages of photosynthesis - the light reactions which convert solar energy to ATP and NADPH and the Calvin cycle which uses this energy to fix carbon from CO2 into sugars. The document provides details on chloroplast structure, the electron transport chain, and mechanisms like cyclic and noncyclic electron flow. It also discusses C3, C4, and CAM carbon fixation pathways and the importance of photosynthesis for providing oxygen and energy to living things globally.
1. This document describes experiments to study the effect of pH on the rate of fermentation in yeast. Students will set up solutions with yeast in buffers of varying pH and measure the amount of carbon dioxide produced over time.
2. The results will be recorded and two graphs will be made: one showing gas production over time for each pH, the other showing gas produced at 40 minutes for each pH. This will help determine the effect of pH on the rate of fermentation.
3. The effect of pH and possible mechanisms will be discussed, drawing on cell biology and metabolic knowledge.
ATP, Photosynthesis, and Cellular Respirationlkocian
This document discusses energy flow through photosynthesis and cellular respiration. It begins by defining energy and the different forms it can take, including chemical energy stored in ATP. Photosynthesis uses light energy to convert carbon dioxide and water into glucose and oxygen through two reactions - the light-dependent reaction where ATP is produced, and the Calvin cycle where glucose is formed. Cellular respiration breaks down glucose and uses oxygen to release the energy stored in ATP through three stages - glycolysis, the Krebs cycle in the mitochondria, and the electron transport chain where a large amount of ATP is generated to store energy from food.
Cellular respiration involves three main stages to break down glucose and produce ATP as energy. [1] Glycolysis converts glucose to pyruvate in the cytoplasm, producing a small amount of ATP. [2] The pyruvate then enters the mitochondrion, where the citric acid cycle further oxidizes it and produces more ATP and electron carriers. [3] Finally, the electron transport chain uses these carriers to pump protons across a membrane and produce a large amount of ATP through chemiosmosis. Overall, the process fully oxidizes one glucose molecule into 6 carbon dioxide molecules and produces 38 ATP.
Photosynthesis is the process by which plants use sunlight, water and carbon dioxide to produce oxygen and energy in the form of sugar. It takes place in the chloroplasts of plant leaves using the green pigment chlorophyll. Chlorophyll absorbs sunlight which is used to convert water and carbon dioxide into oxygen and glucose through a pair of light-dependent and light-independent reactions. This process provides a crucial source of food for plants and oxygen for animals and is essential for life on Earth.
Plants use photosynthesis to convert carbon dioxide and water into glucose and oxygen using energy from sunlight. Photosynthesis is performed by plants and cyanobacteria and involves two photosystems, photosystem I and photosystem II. In photosynthesis, light energy is absorbed by chlorophyll, exciting electrons that are transferred through an electron transport chain between the two photosystems to generate ATP through chemiosmosis and reduce NADP+ to NADPH.
This document provides information about Module 5 of an alternative secondary education biology course on cellular respiration. The module contains 5 lessons that discuss how living organisms harvest energy stored in foods through cellular respiration. Key points covered include:
- Cellular respiration is the process where stored chemical energy in foods is converted to ATP in organisms.
- The mitochondria are where cellular respiration occurs in eukaryotic cells. They have inner and outer membranes with cristae infoldings containing protein complexes.
- Glycolysis is the first step where glucose is broken down, producing some ATP and NADH.
- An activity demonstrates how yeast cells respire and produce carbon dioxide from sugar, showing cellular respiration.
Photosynthesis converts sunlight, water and carbon dioxide into oxygen and energy in the form of ATP and NADPH. It takes place in chloroplasts and involves two stages: the light-dependent reactions capture energy from sunlight to produce ATP and NADPH, while the carbon fixation reactions use these products to incorporate CO2 into organic molecules like glucose. Many scientists contributed to discovering the process of photosynthesis, including how water is the source of oxygen produced.
Photosynthesis is the process by which plants, algae, and cyanobacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of food (sugars). It occurs in two stages: (1) the light reactions where sunlight is absorbed and converted to chemical energy in the form of ATP and NADPH, and (2) the dark reactions where carbon dioxide is fixed using the ATP and NADPH to produce sugars like glucose. The overall equation is: 6CO2 + 6H2O + sunlight → C6H12O6 (glucose) + 6O2. Photosynthesis provides a critical source of food for organisms and oxygen for respiration.
Cellular respiration is a catabolic process that uses oxygen to break down glucose and other organic molecules to extract energy in the form of ATP. It occurs in four main stages: 1) glycolysis in the cytosol, 2) transport of pyruvate into the mitochondria, 3) the Krebs cycle in the mitochondrial matrix, and 4) the electron transport chain and oxidative phosphorylation on the inner mitochondrial membrane. The overall process produces 38 ATP molecules from complete oxidation of one glucose molecule.
This document summarizes the key stages and discoveries in photosynthesis research:
1. Moll's half leaf experiment in the 1700s showed that CO2 is required for photosynthesis.
2. In the 1800s, Priestley, Ingenhousz, and van Niel discovered oxygen evolution and that sunlight and water are essential.
3. In the 1900s, Calvin used radioactive tracers to discover the Calvin cycle, where CO2 is fixed into 3-carbon compounds in chloroplasts, using enzymes like RuBisCO.
4. C4 and CAM pathways were discovered as alternatives to the C3 Calvin cycle that improve efficiency in hot/dry conditions through additional CO2
Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It occurs in two phases - the light-dependent reactions where sunlight is captured and its energy is used to produce ATP and NADPH, and the light-independent reactions where carbon dioxide is incorporated into organic compounds to produce carbohydrates like glucose. It is a critical process that supports life on Earth by producing food and oxygen and removing carbon dioxide from the atmosphere.
This document provides an overview of the early history and development of atomic theory in chemistry. It discusses how early Greek philosophers proposed ideas about atoms and the four classical elements. It then describes how alchemy dominated for 2000 years, during which time elements were discovered and mineral acids prepared. The foundations of modern chemistry were established in the 16th-17th centuries through quantitative experiments by Robert Boyle and Antoine Lavoisier's verification of the law of conservation of mass through careful weighing experiments. Lavoisier also discovered that combustion involved oxygen, not phlogiston as previously believed. After 1800, chemistry advanced through quantitative experiments determining chemical compositions and Proust's law of definite proportions.
The document summarizes the history of discoveries about photosynthesis from the 1600s to present. It describes Jan Baptista van Helmont's experiment in 1649 showing that a willow tree's mass gain came from water alone. Later experiments by John Woodward and Joseph Priestley helped establish that plants interact with air. Jan Ingenhousz's experiments in the late 1700s showed that plants produce oxygen through a process involving light. The document then provides details about chloroplasts, photosynthetic pigments, and explains the two-stage light-dependent and light-independent processes of photosynthesis.
The document discusses the element hydrogen. It notes that hydrogen is the lightest and most abundant element, and exists as a diatomic gas made of two hydrogen atoms bonded together. Hydrogen can exist in different phases such as compressed gas, liquid, solid, and even a predicted metallic liquid phase under extreme pressures. Hydrogen reacts with many elements to form compounds, taking on a positive charge due to being less electronegative than elements like oxygen or halogens. The Bohr model is discussed as a way to conceptualize hydrogen's electron energy levels.
Our understanding of photosynthesis has evolved over time. Early thinkers like Aristotle believed plants obtained nourishment from the soil and leaves provided shade. Jan van Helmont's willow tree experiment in 1643 showed that plant growth was due more to water than soil. Joseph Priestley's experiments in 1771 demonstrated that plants produce oxygen. Jan Ingenhousz discovered in 1779 that aquatic plants only produce oxygen in sunlight. Later, Melvin Calvin traced the chemical pathway of carbon in photosynthesis in 1948. Today we understand photosynthesis as the process by which plants use light, water and carbon dioxide to produce sugar and oxygen through a series of chemical reactions.
The origin and geological history of oxygenrita martin
Oxygen third most profusely found element in the universe Commercially, oxygen can be prepared by the process of liquefaction and fractional distillation of air and through electrolysis of water
This document provides an overview of photosynthesis in plants. It defines photosynthesis as the process by which plants convert light energy to chemical energy that is later used as fuel. Key points covered include: the importance of photosynthesis for producing oxygen and energy for life; early experiments in the 1700s and 1800s that advanced understanding; the two light-dependent stages of photosynthesis that produce ATP and NADPH; and the Calvin cycle and C4 pathway for carbon fixation. Factors affecting the rate of photosynthesis such as light intensity, carbon dioxide concentration, temperature, and water availability are also discussed.
Fullerene is a molecule composed entirely of carbon atoms arranged in a hollow sphere, ellipsoid or tube shape. Buckminsterfullerene or buckyball is the spherical form with 60 carbon atoms arranged in a geodesic dome structure. Fullerenes were discovered in 1985 and their discoverers were awarded the 1996 Nobel Prize in Chemistry. Common fullerenes include buckyballs (C60), nanotubes, and polymers. They are synthesized by evaporating graphite and trapping the carbon clusters, then purifying. Fullerenes are soluble in organic solvents and exhibit properties like conductivity, superconductivity, and use in applications such as photovoltaics, polymers, antioxidants, and catalysts.
The document discusses several key concepts in chemistry including Dalton's atomic theory, how compounds are formed, and laws of chemical combination. It specifically examines the law of conservation of matter proposed by Antoine Lavoisier, which states that the total mass of the reactants equals the total mass of the products in a chemical reaction. The document also explores Joseph Louis Proust's law of definite proportions, which specifies that the elements in a compound always combine in the same proportions by mass. An experiment is described demonstrating that the proportions of copper and oxygen in copper oxide samples were always in a 4:1 ratio, verifying the law.
Chemistry is the branch of science concerned with the substances that matter is composed of, their properties, and how they interact or combine. The document provides a brief history of chemistry, outlining key figures and discoveries from ancient civilizations through the 20th century that advanced the field. It describes the main branches of chemistry - organic, analytical, physical, inorganic, and biochemistry - and provides short definitions of each.
Andrew Fielding's doctoral research focused on understanding the mechanism of O2 activation and catalysis by two similar catechol dioxygenases: Fe(II)-homoprotocatechuate 2,3-dioxygenase (Fe-HPCD) and Mn(II)-MndD. He prepared and characterized the cobalt-substituted variant of HPCD, which showed higher activity than Mn- or Fe-HPCD despite Co(II) being a poorer reducing agent. Using electron-poor substrate analogs, he was able to trap and characterize three O2 intermediates by EPR, providing new insights into dioxygenase mechanisms. Comparing the properties of different metal-substituted enzymes allowed full characterization
This lab report examines the effect of light conditions on the rate of photosynthesis. The experiment measured the rate of photosynthesis by timing how long it took for photosynthesis to occur in leaf disks placed in a CO2 solution under different lighting conditions. As predicted, the results showed that the rate of photosynthesis was higher for leaf disks in direct sunlight compared to those in the shade. The conclusion was that light and carbon dioxide are necessary for photosynthesis, and that more light leads to a higher photosynthetic rate.
Lithium is a soft, silvery metal that is the lightest of all metals and the first element in the alkali metal family, having an atomic number of 3. As the least dense of all metals, lithium has a low melting point of 180 degrees Celsius and is very reactive, especially with water. Lithium and its compounds have various industrial and medical uses such as in lithium-ion batteries, psychiatric medications, and air treatment to remove carbon dioxide from the air.
Photosynthesis is the process by which plants produce their own food. It involves combining carbon dioxide, water, and sunlight to produce glucose and oxygen. There are four stages of photosynthesis: 1) Light absorption, where chlorophyll absorbs sunlight to power the process, 2) Electron transport, where electrons are transferred through membranes, 3) ATP generation, where ATP is synthesized, and 4) Carbon fixation, where carbon dioxide is converted into sugars using ATP and NADPH.
This document appears to be a student assignment analyzing the system development life cycle of Sharif Oxygen PVT LTD, a company that produces oxygen and nitrogen gases. The student discusses the scope, objectives, types of gases produced, history of oxygen and nitrogen, uses of the gases, SDLC phases including planning, analysis, design, and implementation. Key aspects of the company's gas collection method (air separation unit) and management structure are also summarized. The assignment includes tables of contents, acknowledgments and introduces the analysis of improving the company's management and sales systems.
The document discusses several theories of acids and bases that developed over time:
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- Liebig proposed in 1838 that acids contain replaceable hydrogen.
- Arrhenius' 1884 definition defined acids as producing hydrogen ions (H+) and bases as producing hydroxide ions (OH-) in aqueous solutions, which became the standard definition.
- Lewis in 1923 expanded the definition to electron pair transfers between any acids and bases, not just involving hydrogen.
- Other theories such as Lux-Flood's oxygen theory of 1939 and Pearson's hard/soft acid base principle
The document provides information about biology and bioenergetics for the 9th grade, including:
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2. Short definitions of terms like bioenergetics, ATP, photosynthesis, and the light and dark reactions of photosynthesis.
3. Longer explanations of the roles of chlorophyll and light in photosynthesis, and the limiting factors that can decrease the rate of photosynthesis, such as light intensity, temperature, and carbon dioxide and water availability.
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1. Photosynthesis
• The Dark Reactions of
Photosynthesis, Assimilation of
Carbon Dioxide And The
Calvin Cycle
• C3, C4 and CAM. Regulation of
The Activity of Photosynthesis
• The Light Reactions of
Photosynthesis
• The Photosynthetic Membrane
• Literature
The observation that a willow that has been cultivated in a container for five years with
enough watering gained more than half a centner weight although only two ounces of the
container's soil were lost goes back to J.B. van HELMONT (1577 - 1644). The British
natural scientist S. HALES (1677 - 1761) understood that air and light are necessary for
the nutrition of green plants. But it was not before the composition of air out of different
gases became known that their significance for plant nutrition was studies. In 1771
observed J. PRIESTLEY (1733 - 1804), one of the discoverers of oxygen, that green
plants give off oxygen and thus improve the air.
The priest J. SENEBIER (1742 - 1809) from Geneva discovered that the regeneration of
the air is based on the use of 'fixed air' (carbon dioxide). These observations were
confirmed and broadened by studies of the Dutch doctor J. INGENHOUSZ (1730 - 1799)
who recognized both the meaning of light and the fact that the whole carbon contained in
plants is of atmospheric origin. He, too, conceived that plants take up small amounts of
oxygen at night or in the shadow and give off carbon dioxide. In 1804 discovered Th. des
SAUSSURE (1767 - 1845) from Geneva that the plants' increase in weight cannot solely
be caused by the uptake of carbon and minerals, but is based on the binding of the water
components, too.
In 1894 constructed T. W. ENGELMANN (1843 - 1909) a gadget out of a modified
microscope condenser that allowed him to expose parts of photosynthetically active cells
(of the green alga Spirogyra) to a thin ray of light. His aim was to discover which
components of the cell functioned as light receptors. To measure the oxygen production,
he dispersed the thread-like Spirogyra in a bacteria-containing suspension. Whenever
parts of the chloroplast were illuminated, did the bacteria concentrate in this area (where
oxygen was available). The illumination of other parts of the cell resulted in no such
aggregations.
2. In an earlier study did he split white light into its spectral components using a prism. He
then illuminated a green alga, Chladophora, with this spectrum. In contrast to Spirogyra
are the Chladophora cells completely and evenly filled by the chloroplast. He observed
that the bacteria accumulated mainly in the blue and red light. A first action spectrum of
photosynthesis was thus yielded. It resembles roughly the absorption spectra of
chlorophyll a and b.
J. v. SACHS (1832 - 1897) could finally prove that chlorophyll is involved in
photosynthesis. In addition did he show that starch is produced in chloroplasts as a result
of the photosynthetic activities.
These results are in accord with the first law of thermodynamics, whose discoverer J. R.
MAYER postulated already in 1842 that plants take up energy in the form of light and
that they transform it into another, a chemical state of energy. Based on this assumption
was the reaction equation
6 carbon dioxide + 6 water > (chlorophyll) > glucose + 6 oxygen
formulated.
J.v. LIEBIG assumed that the oxygen stems from the breakdown of the carbon dioxide.
This idea was uncritically accepted by the plant physiologists of the late 19th
and the early
20th
century (SACHS, PFEFFER, JOST and others) although M. J. SCHLEIDEN had as
soon as 1842 realized that
1. glucose is produced as a result of photosynthesis (and he was closer to
reality than SACHS was later) and that
3. 2. it is very likely that it is water that is broken down. He wrote:
"It is well-known that CO2 is among the most stabile compounds and
that no chemical way of breaking it down is known while H2O is very
easily broken down.... and it does therefore seem likely that the 24 H2
of the 24 H2O are combined with the 12 CO2."
(from: Grundzüge der wissenschaftlichen Botanik). SCHLEIDEN's equations contain all
reaction compounds in double numbers. He gives C12H24O12 as the formula for glucose.
It was soon realized that the reaction equation above is a simplification and that
photosynthesis consists of a number of partial processes.
F. F. BLACKMAN and G. L. C. MATHGEL (1905, University of Cambridge, Great
Britain) were among the first to study this topic systematically. They cultivated plants
under different but controlled carbon dioxide concentrations, different light intensities
and different temperatures and they noted the effects of these parameters on the rate of
photosynthesis. Two decisive aspects were revealed. Under strong light and limited
amounts of carbon dioxide is the rate of photosynthesis dependent on the temperature.
This shows that the carbon dioxide fixation is based on normal, temperature-dependent
biochemical reactions. Under carbon dioxide excess and too little light was no
temperature-dependence found. This hints at the fact that the light-induced reactions are
independent of the temperature. This statement applies to all photochemical reactions.
In 1925 put O. WARBURG (Kaiser-Wilhelm-Institut [later Max-Planck-Institut] für
Zellphysiologie at Berlin-Dahlem) the results of BLACKMAN down to the existence of
two classes of photosynthetic reactions: the light and the dark reactions.
During the thirties analyzed C. B. van NIEL (Stanford University) the photosynthesis of a
number of purple bacteria. In addition to carbon dioxide do these bacteria need hydrogen
sulphide for photosynthesis. van NIEL was able to determine
4. 6 CO2 + 12 H2S > (light) > C6Hl2O6 + 12 S + 6 H2O
as the reaction's equation. Based on it did he extrapolate a general equation of
photosynthesis:
CO2 + 2 H2X > (light) > (CH2O) + H2O + 2 X
According to this equation is photosynthesis a redox reaction with H2X as the electron
donator (the oxydizable substance). In the case of green plants is it H2O and this means
that not the carbon dioxide but the water is broken down.
A first experimental prove that the oxygen developed during the photosynthesis of green
plants stems indeed from water was delivered by the British physiologist R. HILL. He
detected that isolated chloroplasts give off oxygen in the presence of unnatural reducing
agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The
reaction went down in literature as the HILL-reaction:
2 H2O + 2 A > (light, chloroplasts) > 2 AH2 + O2
where A is the electron acceptor. If A = FeIII
, then is
2 H2O + 4 FeIII
> (light, chloroplasts) > 4 FeII
+ O2 + 4 H+
The process is linked to a photolytic breakdown of water that precede the reductionn of
FeIII
.
4 H2O > (light, chloroplasts) > 4 H+
+ 4 OH-
This shows that
oxygen can also be set free in the absence of carbon dioxide,
the oxygen produced stems from the breakdown of water,
isolated chloroplasts are able to perform at least partial processes of photosynthesis.
The statement that the oxygen produced during photosynthesis stems only from the
breakdown of water was confirmed by S. M. RUBEN, M. RANDALL, M. KAMEN and
J. L. HYDE in 1941 after the isotope technique had found its way to biochemistry. They
could shown that a suspension of Chlorella grown in H2
18
O, gives off 18O2, after light
exposure. Shortly afterwards confirmed S. M. RUBEN and his collaborators the postulate
of O. WARBURG that the fixation of carbon dioxide is energy consuming but
independent of light. In addition could E. RACKER (Cornell University, Ithaca, N. Y.)
prove that light can be replaced by the addition of energy-rich compounds.
The Dark Reactions of Photosynthesis, Assimilation of Carbon
Dioxide And The CALVIN Cycle.
5. Due to the use of isotopes were M. CALVN and his collaborators at the University of
California, Berkeley able to reveal completely the reactions taking place during the
incorporation of carbon dioxide into carbohydrates in the relatively short period from
1946 - 1953. The quick success was based on the use of sensitive methods (two-
dimensional paper chromatography, autoradiography), a suitable specimen and the rapid
progresses of enzyme biochemistry. Cultures of the single-celled green alga Chlorella
pyrenoidosa (that was introduced to photosynthetic studies in 1919 by O. WARBURG)
were supplied with light and an even stream of air containing 12
CO2.
At a given time (t= 0) was 14
CO2 added to the stream of air for a short time. It was
assumed that the labelled carbon dioxide molecules were successively incorporated into
intermediates of the carbohydrate synthesis. After 3, 5 etc. seconds were the experiments
stopped by adding boiling alcohol and the newly produced 14
C-labelled intermediates
were separated and identified by paper chromatography.
1. The first stable compound that was labelled radioactively already after 3 seconds
was 3-phosphoglycerate (3-PG), a substance we got to know previously as an
intermediate of glycolysis. 14
C is found in the carboxyl group of 3-PG. At first
was it assumed that the molecule accepting the carbon dioxide would have to be a
C2 unit. But after a futile search was finally ribulose diphosphate (RuDP), a C5
unit identified as the acceptor
C5 + C1 = 2 C3
This reaction is catalyzed by ribulose bisphosphate carboxylase (also called
Rubisco or, formerly, fraction-1-protein), as far as quantity is concerned the most
common protein of the world. The protein complex of green plants consists of
eight times two subunits, eight large and eight small ones The picture to the right
shows part of the enzyme together with ribulose phosphate , CO2 , and a
magnesium ion (green ball) essential for the reaction. An interactive file
demonstrates the single, subsequent reactions.
After longer reaction periods (5 seconds, 10 seconds, etc.) were further labelled
compounds found. CALVIN and BENSON determined the sequence of the
incorporation and were able to unite the single steps to a pathway. Two results
were especially interesting:
o the resynthesis of ribulose diphosphate and
o the production of the assimilate (the net product of the carbon dioxide
assimilation).
The production of ribulose diphosphate is best described by a cycle (the CALVIN
cycle), while the assimilate production is a linear process. It is based on the fact
that an intermediate of the CALVIN cycle is deducted from it.
6. 2. 3-phosphoglycerate is reduced to glycerinaldehyde-3-phosphate (GAP), the
carboxyl group is transformed into an aldehyde group. The reaction consumes
ATP and NADPH2. The reverse reaction occurs, too, in glycolysis though in
photosynthesis NADP is needed instead of the NAD consumed during glycolysis.
It is known today that the two reactions (and all others, too) are catalyzed by
different enzymes and that the enzymes of photosynthesis use NADP (>
NADPH2) as a cofactor.
The CALVIN cycle has to be passed three times in order to produce one molecule
of glycerinaldehyde-3-phosphate (a C<SUB<3< sub> unit) via photosynthesis
since just one molecule of carbon dioxide is fixed in every round.
3. Just as in glycolysis is part of the glycerinaldehyde-3-phosphate converted into
dihydroxyacetonephosphate (DAP) by epimerization.
4. Fructose-1,6-diphosphate (F-1,6-P) is formed by addition of one molecule
glycerinaldehyde-3-phosphate and one molecule dihydroxyacetonephosphate.
5. Fructose-1,6-diphosphate is converted into fructose-6-phosphate (F-6-P) by
splitting off Pi. The F-6-P has two alternative fates:
o One of the F-6-P molecules is converted into glucose-6-phosphate (G-6-P)
that is sluiced away from the CALVIN cycle and is the net yield of
photosynthesis.
o The other F-6-P disintegrates into a C5 unit (xylulose-5-phosphate; X-5-P)
and a C1 unit that forms a C4 unit (erythrose-4-phosphate; E-4-P) together
with GAP.
6. The E-4-P is coupled to one molecule of dihydroxyacetonephosphate (DAP). The
result is a molecule of sedoheptulose-1,7-diphosphate (SDP), a C7 unit.
7. After splitting off one of the two phosphate residues reacts the sedoheptulose-7-
phosphate (S-7-P) with glycerinaldehydephosphate. Two C5 units are the result:
ribulose-5-phosphate (Ru-5-P) and xylulose-5-phosphate (X-5-P).
8. Ribulose-5-phosphate is phosphorylated to ribulose-1,5-diphosphate and starts a
new round of the CALVIN cycle.
In summary, one can describe the end result of the dark reactions as follows:
6 RuDP + 6 CO2 > 12 3-PG
12 3-PG + 12 NADPH2 + 12 ATP > 12 GAP + 12 ADP + 12 Pi + 12 NADP
12 GAP > 1 glucose (net synthesis product of carbon dioxide assimilation) + 10 GAP
10 GAP + 6 ATP > 6 RuDP
7. NADPH2 and ATP stem, as we will see, from the light reactions of photosynthesis in
which the light energy is converted into chemical energy.
After understanding the pathway in Chlorella pyrenoidosa arose the question whether it
occurs in all other green plants, too. It could be shown to be an important pathway of all
green plants. Even isolated chloroplasts (from spinach, for example) are still fully active
and all reactions of the CALVIN cycle take part within them.
C3, C4 and CAM. Regulation of The Activity of Photosynthesis
C4
A number of plants display an increased and more efficient net photosynthesis during
strong light intensities. A prime example are the Gramineae of warmer regions like maize
or sugar-cane.
At the beginning of the sixties observed H. KORTSCHAK (Hawaiian Sugar Planter's
Association) that the first product of photosynthesis in sugar-cane is not the C3 unit 3-
phosphoglycerate but a unit with four C-atoms. The Australian plant physiologist M. D.
HATCH and his English colleague C. R. SLACK confirmed this result and identified the
compound as oxaloacetate (OAA). It is produced by the addition of one molecule of
carbon dioxide to phosphoenolpyruvate (PEP). The cycle is also known as the HATCH-
SLACK-cycle or the C4 cycle. Plants with this cycle are called C4-plants (and CAM
plants, respectively) in contrast to C3 plants where the carbon dioxide is directly fed into
the CALVIN cycle. The oxaloacetate is usually converted into malate of which the
carbon dioxide is split off again with the help of an enzyme.
This carbon dioxide is now bound by ribulose-1,5-diphosphate and assimilated via the
CALVIN cycle. :
Some species use malate instead of aspartate
oxaloacetate + L-glutamate > aspartate + alpha-ketoglutarate.
The reversible binding of carbon dioxide has the function to accumulate and store CO2.
The process consumes energy, so that it could also be spoken of a carbon dioxide pump.
It should be mentioned that the HATCH-SLACK cycle requires two molecules of ATP
are per fixed carbon dioxide.
8. Photosynthesis of C4 plants. CO2 is bound to phosphoenolpyruvate (PEP) in mesophyll
cells. The product is oxaloacetate. The next step generates malate. In the cells of the
vascular bundle sheath, the 'Kranz' cells, is carbon dioxide split off the malate and fed
into the CALVIN cycle. The pyruvate is transported back into the mesophyll cells (active
transport) and is with the help of additional ATP phosphorylated to PEP.
The anatomy of C4 leaves with so-called 'Kranz' cells differs fundamentally from that of
C3 plants. The chloroplasts of C3 plants are of homogeneous structure, while two types of
chloroplasts occur in C4 plants. The mesophyll cells contain normal chloroplasts, that of
the vascular bundle sheath have chloroplasts without grana , i.e. they are partially
impaired in function. This peculiarity does not affect the CALVIN cycle, it concerns only
the light reactions of photosynthesis. The first binding of carbon dioxide (the HATCH-
SLACK reaction) occurs in the mesophyll cells, the incorporation into carbohydrates (the
9. CALVIN cycle) in the cells of the vascular bundle sheath. Both processes of
photosynthesis are spatially separated.
The Crassulacean Acid Metabolism (CAM)
CAM is the abbreviation of Crassulacean acid metabolism. The name points at the fact
that this pathway occurs mainly in Crassulacean species (and other succulent plants). The
chemical reaction of the carbon dioxide accumulation is similar to that of C4 plants but
here are carbon dioxide fixation and its assimilation not separated spatially but in time.
CAM plants occur mainly in arid regions. The opening of the stomata to take up carbon
dioxide is always connected with large losses of water. To inhibit this loss during intense
sun (the transpiration via the cuticle remains intact) has a mechanism developed that
allows the uptake of carbon dioxide during the night. The prefixed carbon dioxide is
stored in the vacuoles as malate (and isocitrate) and is used during the daytime for
photosynthesis.
Which Metabolism Goes With Which
Conditions?
The enzyme that catalyzes the primary carbon dioxide
fixation of C4 and CAM plants is phosphoenolpyruvate
carboxylase (PEPC). Its affinity for carbon dioxide is by far
higher than that of Rubisco, the first enzyme of the CALVIN
cycle. As a consequence are C4 plants able to use even trace
amounts of carbon dioxide. PEPC occurs in small amounts
(roughly 2 - 3 %) also in C3 plants, where it, too, has a key
position in the metabolic regulation.
Carbon dioxide yield of C4 and C3 plants of open
grasslands in different parts of the world. In temperate
regions is the rather low light intensity decisive for the
disadvantage of C4 plants. C3 plants have an advantage due
to their low rate of photorespiration and because they need
no energy for the previous fixation of CO2. (J. R.
EHRLICHER, 1978).
In growing roots (of maize seedlings, for example) does PEPC help to supply the lipid
synthesis with NADH + H+
. The following reactions take place:
1. phosphoenolpyruvate + HCO3- > oxaloacetate + Pi
2. oxaloacetate > malate. - During this step is NADH + H+
oxidized to NAD+
.
10. 3. malate > pyruvate + CO2.
During the last reaction is NAD reduced to NADH + H+
.
In the root nodules of leguminosae is nitrogen fixed. Enough carbon bodies have to be
supplied in order to incorporate the ammonia produced by the bacteria. the CO2-binding
via PEPC-reaction thus is an important supplement
Furthermore produces the PEPC intermediates of the citric acid cycle (oxaloacetate and /
or malate), a back-up in case of shortages. The activity of PEPC is controlled by extern
factors, the day length is decisive. In some cases have different isoenzymes be found in
different tissues. Their production is controlled by different triggers.
C3 plants can loose up to 20 percent of the carbon fixed in the CALVIN cycle at intense
radiation. Under strong light is the photorespiration 1.5 - 3.5 times as high as the usual
respiration in darkness. In C4 plants becomes the photorespiration drastically reduced, it
may even not be detectable any more. In other words:
The net rate of photosynthesis (and consequently also the net production of biomass) of
C4 plants is far larger at high light intensities than that of C3 plants. The optimal
temperature of photosynthesis is below that of the respiration in darkness. As a
consequence are losses caused by respiration larger at high than at low temperatures.
Where light is a limiting factor and temperatures are low (i.e. in temperate climatic
zones) have C3 plants the advantage, C4 plant do hardly occur (one of the exceptions is
Spartina townsendii). C4 plants, nearly always herbs or shrubs, are more successful in the
open country of warmer zones.
It has to be mentioned that two molecules of ATP are consumed in the HATCH-SLACK
cycle
C4 plants belong to numerous, phylogenetically not related monocotyledonous and
dicotyledonous families. Moreover have C4 activities also been detected in the blue-green
alga Anacystis nidulans as well as in some dinoflagellates.
Since the alternative C3 or C4 is accompanied by considerable changes of the leaf
anatomy has it to be assumed that the genetic potential for both pathways is quite
common in the plant kingdom and that, depending on the ecological needs, one way is
chosen by a species while a related species may choose the other one.
A well-studied example is the genus Atriplex, where both ways are realized. The C3
plants belong to one phylogenetic group, the C4 plants to another. In some cases can
hybrids of C3 and C4 species be generated.
11. Influence of different parameters on the efficiency of the carbon dioxide uptake
(ordinate) of a C3 plant (Atriplex patula, yellow line) and a C4 plant (Atriplex rosea, green
line). Measured parameters (from left to right): light intensity, leaf temperature and
concentration of carbon dioxide within the intercellular space (according to O.
BJÖRKMAN and J. BERRY, 1973).
In several plant species of the genera Zea, Mollugo, Moricandia, Flaveria, etc. occur both
types of CO2 fixation within one plant. In younger plants is usually the C3-, in older ones
the C4 pathway taken. The amount of C4 is controlled by environmental factors.
CAM: Advantages and Disadvantages. CAM has been detected in more than 1000
angiosperms of 17 different families. It is usually accompanied by succulence, though not
all Crassulaceae, for example, display CAM and succulence is no precondition of CAM.
Tillandsia usneoides of the bromelia family is not succulent, but uses CAM.
Mesemryanthemum crystallinum (a plant with succulent leaves) can use the C3 pathway
but switches to CAM when growing in saline soils. Under experimental conditions can
the shift be achieved by increasing the NaCl concentration of the nutrient medium (K.
WINTER and D. J. von WILLERT, 1972). While the advantage of C4 plants comes in
useful under high light intensities, is the degree of the CAM influence in CAM plants
regulated mainly by temperature, atmospheric humidity and salinity. Both strong and
weak CAM plants are known. In weak CAM plants becomes CAM only apparent at
certain differences between day and night temperature. CAM plants that store a lot of
malate and due to the thus high osmotic value also a lot of water, are usually less frost
resistant than C3 plants. Because of the high concentration of acid are they less heat
resistant, too. Species of arid regions are therefore forced to break their pool of malate
down during the daytime (R. LÖSCH and H. KAPPEN, Universität Kiel, 1985). Usually
do the C4 pathway and CAM exclude each other. An exception is the succulent C4
dicotyledon Portulaca oleracea that is able to choose the optimal pathway. under natural
conditions
12. The Light Reactions of Photosynthesis
It has been mentioned in the historical outline that photosynthesis is dependent on light.
The results of ENGELMANN and SACHS showed that it is absorbed by chlorophyll. We
also got to know that plants have two types of chlorophyll, chlorophyll a and b and that
both types display a characteristic absorption spectrum.
The action spectrum of photosynthesis resembles the absorption spectra of chlorophyll
though it is not identical. This means that further photoreceptors (so-called accessory
pigments) exist.
We already got to know something about the assimilation of carbon dioxide in the
previous section. Our question is now: which reactions are induced by light and how is
the light energy converted into chemical energy? Or, in other words, how are ATP and
NADPH2 produced?
CALVIN and his collaborators studied the dark reactions in intact, active cells. This
attempt proved to be insufficient for the light reactions. The results were contradictory.
Techniques to isolate active chloroplasts had to be developed.
After the use of fractions containing isolated chloroplasts became usual, were three
research groups at the same time (1951) and independent of each other able to show that
isolated chloroplasts reduce NADP to NADPH2 when exposed to light [W. VISHNIAC
and S. OCHOA (Rockefeller Institute, New York), L. J. TOLMACH (University of
Chicago) and D. I. ARNON (University of California, Berkeley)]. Shortly afterwards (in
1954) discovered ARNON and his collaborators that the production of ATP, too, is
dependent on light and that both ATP and NADPH2 can be produced simultaneously.
Both compounds are generated from precursors that are present in the chloroplast already
before photosynthesis starts, since no extern metabolites were supplied during the
experiments. Accordingly was light (photons) the only available source of energy. It
turned out that the production of ATP needs no oxygen, neither is oxygen produced
during the reaction. Consequently runs the equation as follows:
n ADP + n Pi > (photons) > n ATP
The process is termed photophosphorylation. It exists in bacteria and blue-green algae,
too, and is a general feature of photosynthetic processes.
After it was proven hat ATP is produced, was it asked how this is done. It seemed
unlikely that the light induces the production of ATP directly but it had turned out that
the production of ATP has to be preceded by an exposure to light. The concept of a light-
induced electron flow was developed. It assumes that one molecule of chlorophyll
absorbs one photon. As a consequence is an electrons of chlorophyll transferred to a
higher energy level.
13. This energy-rich electron is then transferred to a neighbouring electron acceptor with a
strong electronegative redox potential. The transfer of the electron from the activated
chlorophyll to the (first) acceptor is the first photochemical phase of photosynthesis. Its
decisive feature is the transformation of a photon flow (light) into a flow of electrons.
As soon as a strongly electronegative (reducing) substance has been produced can the
electron flow proceed with electron acceptors of less negative redox potentials. The
process releases chemical energy that is used for photophosphorylation. Already during
the fifties existed the first strong proves for the involvement of the chloroplasts'
cytochromes. It could be shown, too, that the electron is finally accepted by a
chlorophyll, so that its original state is restored again. The requirements of catalysis are
fulfilled. The process became known as cyclic phosphorylation (D. I. ARNON, 1959).
Such a cyclic flow of electrons that is powered by light and releases chemical energy
used for the production of ATP is unique. It is the outstanding property of photosynthetic
cells.
Concept of cyclic photophosphorylation (to the left). To the right: the original concept
of non-cyclic photophosphorylation (according to D. I. ARNON, 1971)
The only unexplained process remained was the photoreduction of NADP in chloroplasts.
It were again ARNON and his collaborators that were in 1957 able to discover a second
part of photophosphorylation. They could prove experimentally that the photoreduction
of NADP and the synthesis of ATP are coupled. In contrast to the cyclic
photophosphorylation is the production of ATP coupled stoichiometric to a light-induced
transfer of electrons from water to NADP and to the production of oxygen. The ATP
production of the whole system increases the reduction rate of NADP. This pointed at the
fact that the process is tightly coupled to cyclic photophosphorylation. Since electrons are
irreversibly transferred from chlorophyll to NADP, are substitutes needed and these
14. electrons stem from the breakdown of water. It is spoken of non-cyclic
photophosphorylation, since ATP is produced simultaneously. Ferredoxin (a heme-less
iron-sulphur protein) has a key position in this process. Its reduction potential is far more
negative than that of NADP so that an electron flow from ferredoxin to NADP was very
likely. The reduction of NADP is a three step reaction:
1. a photochemical reduction of ferredoxin that is followed by two 'dark' steps.
2. The re-oxidation of ferredoxin with the help of a ferredoxin-NADP-reductase (a
flavoprotein).
3. The re-oxidation of the ferredoxin-NADP-reductase by NADP.
Ferredoxin: Iron-sulphur-complex: - to the left: Fe2S2 protein - Planttype ferredoxins , to
the right: Fe4S4 proteins - Bacterialtype ferredoxins
from: PROMISE - The Prosthetic groups and Metal Ions in Protein Active Sites Database
What was at first regarded as a photoreduction of NADP proved to be an electron
transport chain that runs from ferredoxin via a flavin component to NADP.
The outstanding position of ferredoxin was strengthened even more after it was found out
that stoichiometric amounts of O2 and ATP are produced during the reaction. The non-
cyclic photophosphorylation can accordingly be described as follows:
4 ferredoxin (oxidized) + 2 ADP + 2 Pi + 2 H2O > (photons) > 4 ferredoxin (reduced) + 2
ATP + O2 + 4 H+
.
The results led to the question how this reaction is coupled to the cyclic
photophosphorylation discussed at the beginning. A series of experiments using specific
inhibitors showed that ferredoxin is a component of that pathway, too.
So much about the chemical data. More knowledge about the primary effect of the light
and about the significance of chlorophyll would have to exist to interpret them. These
problems, too, have a long past history.
15. Two Photosystems
In 1932 exposed R. EMERSON and U. ARNOLD of the University of Illinois at Urbana
Chlorella cells to a series of extremely short flashes of light. With this experiment did
they try to find out how many molecules of chlorophyll were necessary to use one photon
for the production of one molecule of oxygen. The result was that several hundred
chlorophyll molecules are necessary which means that not all of them are of the same
importance. Most act as light traps (or antennas) helping to transfer a photon to a reaction
centre where an especially exposed chlorophyll transforms light energy into chemical
energy. H. GAFFRON called this complex of several hundred chlorophyll molecules and
other pigments (carotenes, carotenoids, xanthophylls, etc.) a photosynthetic unit.
This aggregation of pigments seems to lead to an especially efficient use of the incoming
light. Still, a rather large part of the irradiated energy is lost. It does never reach the
reaction centre and is emitted as warmth or light (red autofluorescence of chlorophyll).
When regarding the absorption spectrum of photosynthesis does it stand out that the
efficiency of light of the wave length lambda > 680 nm decreases strongly although
chlorophyll a displays an absorption in that area. R. EMERSON (1957) discovered that
light of the wave length lambda > 700 (710) increases the rate of photosynthesis
drastically if light of the wave length lambda = 680 nm (or less) is present at the same
time.
When these two light qualities are used independent of each other or one after the other is
no increase measured. EMERSON concluded that two photochemical processes have to
exist that consist of different pigment systems (light receptors) but that do co-operate
(EMERSON-effect). According to a suggestion of L. N.M. DUYSENS are the two
systems called
photosystem I (PS I). It needs light of longer wave lengths (lambda > 700 nm)
and
photosystem II (PS II). It becomes active when exposed to shorter wave lengths
(lambda < 680 nm)
The ratio of chlorophyll a to chlorophyll b is higher in PS I than in PS II. The question
how the two systems co-operate and how they are coupled to the production of ATP and
NADPH2 remains to be settled.
ARNON and his collaborators could prove that the two systems are arranged in series and
that both systems are required to explain all effects that had been recognized as
photosynthetic ones. Only some bacteria that produce no oxygen during photosynthesis
lack photosystem II. This results hints at the suggestion that the splitting of water is
coupled to photosystem II and that photosystem I developed earlier in evolution.
16. The reaction centre of every photosystem is represented by one molecule of chlorophyll a
each (P 700 in PS I and P 680 in PS II, where P means pigment).
The absorption of a photon by P 680 (which has a positive redox potential of + 0,8 V in
its basic state) transfers P 680 into its excited state ( with a redox potential of 0,0 V) and
causes the formation of a strongly oxidizing component (Z+
) and a weakly reducing (Q-
)
one. Z+
withdraws electrons from water so that O2 and protons are set free.
4 Z+
+ 2 H2O > 4 Z + 4 H+
+ O2
The reducing component (a membrane-bound plastoquinone) feeds the electron into an
electron transport chain in the course of which it looses energy part of which is used for
the production of ATP. The electron does not return to its starting point (chlorophyll P
680) but is transferred to a chlorophyll molecule of photosystem I (P 700). The two
photosystems are thus coupled.
The absorption of a further photon excites the P 700 just mentioned (the redox potential
of which is + 0,4 - + 0,5 in its basic state). It transfers one electron to a membrane bound
ferredoxin (P430) which passes the electron on to a soluble ferredoxin. The following
steps are known.
In a stoichiometric sense is the outline above incomplete since ferredoxin transfers just
one electron at any given time while two electrons are needed for the production of one
NADPH2. The equation would consequently have to be:
2 ferredoxin (reduced) + 2 H+
+ NADP+
> 2 ferredoxin (oxidized) + NADPH2
In summary is the light energy used for the flow of electrons from water to NADPH2 and
for the simultaneous production of ATP (Z-scheme).
During our discussion did we neglect the cyclic photophosphorylation mentioned at the
beginning. It proved to be a parallel process that starts work as soon as enough NADPH2
but too small amounts of ATP are present. Only photosystem I participates in cyclic
photophosphorylation.
The Photosynthetic Membrane
In our discussion of photosynthesis have we thus far only regarded biochemical reactions.
In the section about glycolysis and other biosynthetic pathways was the significance of
single enzymes pointed out. Since quite some time know, for example, have all enzymes
involved in glycolysis been purified and isolated and each step of the pathway can be
analyzed under in vitro conditions. It has also been tried to isolate the complete set of
components necessary for photosynthesis in order to reconstitute the whole system. But
all attempts failed because the premises they were based on were wrong as we know
today.
17. A number of problems have not been taken into consideration until now. The terms
photosystem I and photosystem II, for example, have been introduced and all
participating pigments have been mentioned but the following subjects remain to be
discussed:
How are the photosystems organized?
How are the pigments arranged?
Why does one of the chlorophyll molecules react different than all the others?
Why are action and absorption spectra not quite congruent?
Why reacts P 680 (chlorophyll a) different than P 700 (chlorophyll a, too)?
How are electron transport chain and ATP production coupled?
How are photosystem I and II linked?
Which structural prerequisites have to exist in order for the two systems to co-operate?
It was always accepted that each of the biochemical reactions was catalyzed by a specific
enzyme and still, it took quite some time before it was realized that the chlorophyll and
the other pigments are protein-bound and that they are only active as protein-chlorophyll
(and protein-pigment, respectively) complexes. The isolated pigments themselves were
useless for photosynthesis. The pigment-protein complex, (most) proteins of the electron
transport chain as well as the catalyst of ATP synthesis (ATP synthase) are integral
compounds of the photosynthesis membrane(s) (= the thylacoid membranes of algae and
higher green plants, cytoplasmatic membranes of photosynthetically active bacteria and
blue-green algae). The location within the membrane (at the out- or the inside, for
example) and the relative arrangement of the proteins towards each other are important
prerequisites of energy transformation.
This is not only true for photosynthetic reactions but also for those of the respiratory
chain and for the enzymes located within the purple membrane of Halobacterium
halobium (an archaebacterium using light energy for the production of ATP without an
electron flow).
The requirements for energy transformation are even higher: completely intact
membranes that are impermeable for protons and that enclose compartments thus
maintaining a stable electrochemical gradient between inside and outside. The production
of ATP is based on a directed proton dislocation paralleled by a change of the
compartment's pH and of its membrane potential.
Proteins of The Photosynthetic Membrane
The research into the proteins essential for photosynthesis started very late. The reason is
that all of them are membrane-bound which rendered it nearly impossible to isolate and
characterize them with the classical methods of protein analysis.
Only after sensitive techniques like gel electrophoresis and the controlled use of
detergents like sodium dodecyl sulfate (SDS) had been developed, became it possible to
18. separate the proteins and to identify them as bands in a gel. A side product of this
technique is the determination of the molecular weights of the respective polypeptide
chain.
A second, independent attempt was and is the use of specific probes like fluorescence-
tagged antibodies that help to find out whether a certain protein (or part of a polypeptide
chain) is located at the inside or the outside of a membrane. The use of antibodies against
specific proteins allows, too, to precipitate these proteins selectively since only they are
able to form the extremely specific antigen - antibody complex.
Cross-linking agents render it possible to elucidate the surrounding of a molecule. And
the use of specific inhibitors helps localizing their site of effect. DCMU [3-(3', 4' -
dichlorphenyl) - 1,1 - dimethylurea] has since years been used to inhibit photosystem II.
It has no effect on photosystem I and was therefore used by ARNON and his
collaborators as an important help to study the electron transport chain that starts at
photosystem I independently of that induced by photosystem II.
We know today that DCMU does not effect chlorophyll itself but a certain protein, the
plastoquinone-binding protein.
A third possibility to characterize the photosynthetic membrane is the analysis of certain
mutants. The single-celled alga Chlamydomonas reinhardii proved to be a good test
object. Quite a range of mutants with photosynthetic defects are known. They can be
grouped in four classes:
1. mutants with a defect in photosystem I,
2. mutants with a defect in photosystem II,
3. mutants with a defect in photophosphorylation and
4. mutants with a defect in the antenna complex.
It is quite striking that almost all mutants are characterized not only by the loss or change
of a certain polypeptide chain but by the lack of a whole complex, for example that of PS
I. It seems therefore as if the mutations would lead to pleiotropic effects. Or, expressed
differently: when a polypeptide chain is changed or missing does the assembly of the
other polypeptide chains not work any more. This observation shows how tight the
interactions between the single polypeptide chains are and how important they are for
their mutual co-operation.
A further and not less important technique is electron microscopy usually used in
combination with freeze-etching.
The sequencing of membrane proteins remains difficult. And yet, the sequences of most
proteins involved in photosynthesis could be determined during the last years via the
sequencing of their respective genes. The most remarkable outcome of this work is that
these proteins contain (just like proteins of animal or bacterial membranes, too) a large
portion of alpha - helices. The lengths of the helices corresponds to the thickness of the
21. The Chemiosmotic Hypothesis of P. MITCHELL: a Model of The
Photosynthetic Membrane
Since quite some time has the ATP synthase been ascribed the function of a coupling factor. This means
that it is able to utilize the free energy released by electron transport. Such energy conservation is referred
to as energy coupling or energy transduction.
How does this work? It might have been assumed that the electron transport chain serves the production of
energy-rich intermediates and that these constitute an energy store for the production of ATP. Two
arguments against this idea exist:
1. No such substances have ever been isolated and
2. photophosphorylation (and the oxidative phosphorylation of the respiratory chain, respectively) is
only possible if the thylacoid membranes (the inner mitochondrial membrane, respectively) are
intact.
Another assumption had been that the ATP synthase changes its configuration and is thus itself transferred
into an activated state. In such a case would the energy be transiently stored in weak interactions. This
hypothesis, too, failed to withstand experimental scrutiny.
In 1961 proposed P. MITCHELL (Glynn Research Laboratories, Great Britain) that the energy set free
during the electron transport is conserved as a proton gradient across the membrane. The energy would then
not be stored as a chemical bond but as an electrochemical gradient. The electrochemical potential of this
gradient would be harnessed to synthesize ATP. The hypothesis explains several key observations:
1. It is consistent with the fact that oxidative phosphorylation requires an intact inner mitochondrial
membrane.
2. The inner membrane is impermeable to ions like H+
or OH-
whose free diffusion would discharge
the electrochemical gradient.
3. The electron transport results in the transport out of intact mitochondria (out of the thylacoid space
of chloroplasts) thereby creating a measurable electrochemical gradient across the inner
mitochondrial membrane (the thylacoid membrane of chloroplasts).
4. Substances that increase the permeability of the inner mitochondrial or the thylacoid membrane to
protons, thereby dissipating the electrochemical gradient, allow electron transport to continue but
inhibit ATP synthesis: they 'uncouple' electron transport from oxidative phosphorylation.
A very convincing experiment was performed by A. T. JAGENDORF (1966, Cornell University, Ithaca, N.
Y.): he took isolated thylacoids and incubated them in a pH buffer until the same pH (4) was measured at
both sides of the membrane. After the equilibrium had been achieved, did he quickly transfer the thylacoids
to a media of pH 8 containing both ADP and Pi. Immediately after the transfer was the pH between inside
and outside evened out while, at the same time, the system produced ATP. The proton flow across the
membrane was used for the production of ATP. The experiment worked only with intact membranes. In
addition have all biochemical processes to be directed (vectorial). All enzyme molecules have to have the
same direction so that the protons are transported in only one direction.
How can a flow of protons induce the production of ATP??
To understand the process is it useful to study the ATP production a little more. ADP and phosphate (Pi)
are its starting compounds. It is known that both are bound to neighbouring but separate binding sites of the
enzyme complex (the ATP synthase ). To produce ATP (ADP~P) has one H+
to be removed from ADP and
one OH-
from phosphate. In a formal sense is water split off. As soon as the ions have left the complex do
22. both combine with their counterions to water (not with each other). The end result is a directed flow of
protons.
This does not mean that one proton is transferred through the membrane via the ATP synthase complex.
Instead is a newly formed proton given off into solution at one side while another proton is captured and
neutralized (by a OH-
ion) at the other side of the membrane.
Independent of these observations could GRÄBER and WITT (Max-Vollmer-Institut, Technische
Universität Berlin) show that a direct coupling between proton gradient and the electron transport chains of
the photosystems I and II exists. It emerged that the already known 'Z'-scheme is no hypothetical product
but that the involved components are arranged like a Z within the photosynthetic membrane, i.e. the
structural basis for the process discussed in the previous sections became known.
D. von WETTSTEIN and R. P. OLIVER (Carlsberg Laboratorium, Copenhagen, 1985) summarized all
results and were thus able to develop a model that explains the topology of the single protein complexes.
The photosynthetic membrane contains four complexes altogether. Each has subunits encoded in the
nucleus and others encoded by plastids. Several proteins bind chlorophyll a, one binds chlorophyll a and b.
The PS II complex is mainly localized in stacked, PS I and the ATP synthase complex (CF1 - CF0) in non-
stacked thylacoid membranes.
A further remark: the reactions of the CALVIN cycle are catalyzed by soluble enzymes, localized within
the stroma.
Molecular Structure of The Reaction Centre
In 1985 was the structure of the photosynthetic reaction centre's protein subunits determined. J.
DEISENHOFER, H. MICHEL and R. HUBER (Max-Planck-Institut für Biochemie, Martinsried)
succeeded in crystallizing the protein complex of the photosynthetic membrane of the bacterium
Rhodopseudomonas viridis, they determined the folding of the polypeptide chain and the arrangement of
the chlorophyll molecules. In 1988 were they awarded the Nobel prize for this work. Together with the
tertiary and quaternary structure was also the amino acid sequence of the involved polypeptides
23. determined. The central part of the complex contains two subunits, L and M, each of which forms 5 helices
that span the photosynthetic membrane. Two further polypeptides, H and a cytochrome c - like protein are
associated. Furthermore belong 4 covalently linked heme groups, 4 bacteriochlorophyll b molecules, 2
molecules of bacteriopheophytin b, 2 quinones, 1 iron ion that is not linked to a heme group as well as
carotenoids as prosthetic groups to the complex. The structures found in bacteria are homologous to the
reaction centres of the photosystems I and II of green plants.