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  1. 1. 9/9/09 Chapter 6 Photosynthesis: Using Light to Make Food 1.  The Basics of Photosynthesis 2.  The Light Reactions 3.  Then comes the Calvin Cycle 4.  Water Saving Adaptations 5.  The Environmental Impact of Photosynthesis 1. The Basics of Photosynthesis 1
  2. 2. 9/9/09 1. The Basics of Photosynthesis Photosynthesis is the process by which plants, use the energy from sunlight to produce sugar. Almost all plants are photosynthetic autotrophs, as are some bacteria and protists, which means that they generate their own organic matter through photosynthesis. A. Chloroplasts: Sites of Photosynthesis – Are found in the interior cells of leaves. – Contain stroma, a thick fluid. – Contain thylakoids, membranous sacs. B. The Overall Equation for Photosynthesis The reactants and products of the reaction: Energized electrons are added to carbon dioxide to make sugar. Sunlight provides the energy. 2
  3. 3. 9/9/09 C. A Photosynthesis Road Map Photosynthesis is composed of 2 processes: 1. The light reactions convert 1 2 solar energy to chemical energy. 2. The Calvin cycle makes sugar from carbon dioxide. 2. The Light Reactions: Converting Solar Energy to Chemical Energy 2. The Light Reactions: Converting Solar Energy to Chemical Energy Chloroplasts are chemical factories powered by the sun; • That convert solar energy into chemical energy. 3
  4. 4. 9/9/09 A. The Nature of Sunlight So what’s INCREASING ENERGY sunlight made of? • It’s a type of energy called radiation or VISIBLE LIGHT electromagnetic energy. • The full range of radiation is called the electro-magnetic spectrum. B. The Process of Science: What Colors of Light Drive Photosynthesis? In 1883, German biologist Theodor Engelmann demonstrated that photosynthesis takes place inside the chloroplasts of plants and that the chloroplasts receive more energy from red and blue light than from other colours 1.  He illuminated an alga with light passed through a prism, exposing the alga to different wavelengths. 2.  Aerobic bacteria, which grow in the presence of oxygen, were used to determine where the alga was releasing the most oxygen and thus photosynthesizing most. 3.  They accumulated under red and blue light, showing that only some parts of the electromagnetic spectrum are used 4
  5. 5. 9/9/09 Chloroplasts absorb select wavelengths of light that drive photosynthesis. Reflected light is green, so you see the leaves Absorbed green! light is red and blue C. Chloroplast Pigments Chloroplasts contain several pigments: –  Chlorophyll a –  Chlorophyll b Green –  Carotenoids orange-yellow-brown Trees respond to the decreasing amount of sunlight by producing less and less chlorophyll. Eventually, a tree stops producing chlorophyll. When that happens, the carotenoid already in the leaves can finally show through. D. How Photosystems Harvest Light Energy A photosystem is an organized group of chlorophyll and other molecules. It is a light- gathering antenna. Light behaves as photons, discrete packets of energy. Chlorophyll molecules absorb photons. – Electrons in the pigment gain energy. – The energy is released and used. 5
  6. 6. 9/9/09 How the Light Reactions Generate ATP and NADPH The light reactions of photosynthesis: 2 types of photosystems cooperate in the light reactions. An electron transport chain • Connects the 2 photosystems. • Releases energy that the chloroplast uses to make ATP. 6
  7. 7. 9/9/09 And it all takes place in the thylakoid membrane of the chloroplast. 3. Then comes the Calvin Cycle: Making Sugar from Carbon Dioxide 3. Then comes the Calvin Cycle: Making Sugar from Carbon Dioxide It functions like a sugar factory within a chloroplast and regenerates the starting material with each turn. 7
  8. 8. 9/9/09 4. Water-Saving Adaptations 4. Water-Saving Adaptations All plants are classified as to their ability to convert sunlight into plant matter. Water enters leaf CO2 enters through stomata C3 plants – Use CO2 directly from the air. – Are very common and widely distributed. – Adaptive Value: efficient under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy). 8
  9. 9. 9/9/09 C4 plants – Close their stomata to save water during hot and dry weather. – Can still carry out photosynthesis. – Example: corn CAM plants – Open their stomata only at night to conserve water, and therefore carry photosynthesis during the night. – Example: cactus, orchids 5. The Environmental Impact of Photosynthesis 5. The Environmental Impact of Photosynthesis Photosynthesis has an enormous impact on the atmosphere. It swaps O2 for CO2. And the atmospheric oxygen we breathe is a by-product of photosynthesis…. 9
  10. 10. 9/9/09 How Photosynthesis Moderates Global Warming Greenhouses are used to grow plant indoors. They trap sunlight that warms the air inside. A similar process, the greenhouse effect: • Warms the atmosphere. • Is caused by atmospheric CO2. 10