Chapter 3-photosynthesis aa


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  • Some of the photons in sunlight carry a great deal of energy (xrays, uv light)
    Other carry very little energy (radio waves)
    High energy photons have shorter wavelength than low energy photons.
    The full range of these photons is called the electromagnetic spectrum.
  • Our eyes perceive photons carrying intermediate amounts of energy as visible light. Why?
    Because our eyes can only absorb photons of intermediate wavelengths.
    How do we, or for that matter plants, absorb these wavelengths? Through molecules called pigments.
  • The cells of plant leaves contain organelles called chloroplasts that carry out photosynthesis
    The internal membranes of chloroplasts are organized into flattened sacs called thylakoids.
    Often, numerous thylakoids are stacked on top of one another in columns called grana (singular granum).
    Surrounding the thylakoid membrane system is a semi liquid substance called stroma.
  • Plants absorb mainly blue and red light and reflect back what is left of the visible light, which is why they appear green.
    Chlorophyll a and b similar in structure, but have differences in absorption spectra
    An absorption spectra is a graph indicating how effectively a pigment absorbs different wavelengths of visible light.
  • The first two stages take place only in the presence of light and are commonly called light-dependent reactions.
    The third stage, the formation of organic molecules from atmospheric CO2 is called the Calvin Cycle. It is also referred to as light-independent or dark reactions because they do not require direct light.
    These reactions do depend indirectly on the light dependent reactions because they require the ATP and NADPH produced by the light-dependent reactions.
    We can sum up the overall process of photosynthesis by the following simple equation.
  • Electron Transport: the excited electron is then shuttled along a series of electron-carrier molecules embedded in the membrane.
    This is called the electron transport system.
    As the electron passes along the electron transport system, the energy from the electron is siphoned out in small amounts. This energy is used to pump hydrogen ions (protons) across the membrane, building up a high concentration of protons on one side of the membrane.
    Making ATP: the high concentration of protons can be used as an energy source to make ATP molecules (remember what an ATP molecule is?).
    Protons are only able to move back across the membrane via special channels. The kinetic energy that is released by the movement of protons is transferred to potential energy in the building of ATP molecules from ADP. This process is called chemiosmosis and makes the ATP that will be used in the Calvin Cycle to make carbohydrates.
    Making NADPH: the electron leaves the transport system and enters another photosystem where it is reenergized by the absorption of another photon of light. This energized electron enters another electron transport system where it is again shuttled along a series of electron-carrier molecules. The result of this electron transport system is not the synthesis of ATP, but the formation of NADPH (a coenzyme). The electron is transferred to a molecule (NADP) and a hydrogen ion that from NADPH. This molecule is important in the synthesis of carbohydrates in the Calvin Cycle.
  • If we break in down, photosynthesis is just a way of making organic molecules from carbon dioxide. To build organic molecules, cells use raw materials provided by the light-dependent reactions.
    Energy. ATP (provided by photosystem II) drives endergonic reactions
    Reducing Power. NADPH (provided by photosystem I) provides a source of hydrogens and energetic electrons needed to bind them to carbon atoms
    The actual assembly of new molecule employs a complex battery of enzymes in what is called the Calvin Cycle or C3 photsynthesis.
  • Many plants have trouble carrying out C3 photosynthesis when the weather is hot. (click)
    As temperature increases, plants partially close their leaf openings (called stomata) to conserve water. (Click)
    As a result, CO2 and O2 are not able to enetr and exit the leaves through these openings. (click)
    The concentration of CO2 falls, while the concentration of O2 in the leaves rises.
    Under these conditions rubisco, the enzyme that carries out the first step of the calvin cycle engages in photorespiration (click)
    Where the enzyme incorporates O2 not CO2 into the cycle and when this occurs, CO2 is ultimately released as a by-product.
    Photorespiration short circuits the successful performance of the calvin cycle.
  • Some plants are able to adapt to climate with higher temperatures by performing C4 photosysnthesis.
    In this process, plants such as sugar cane, corn, and many grasses are able to fix carbon using different types of cells and chemical reactions within their leaves and thereby avoiding a reduction in photosynthesis due to higher temperatures.
    C4 plants fix CO2 first as the four carbon molecule oxaloacetate (Hence name C4) rather than as the three carbon molecule phosphoglycerate of C3 photosynthesis.
    C4 plants carry out this process in the mesophyll cells of their leaves using a different enzyme.
    The oxaloacetate is then converted to malate which is transferred to the bundle-sheath cells of the leaf and there broken down to regenerate CO2 which then eneters the calvin cycle.
    The bundle sheath cells are impermeable to CO2 and therefore hold it within them
    The concentration of CO2 increases and thus decreases the occurrence of photorespiration.
  • A second strategy to decrease photorespiration is used by many succulent (water storing) plants such as cacti and pineapples.
    The mode of initial carbon fixation is called crassulacean acid metabolism (CAM) after the plant family Crassulaceae in which it was first discovered.
    In these plants, the stomata open during night when it’s cooler and close during the day.
    CAM plants initially fix CO2 into organic compounds at night, using the C4 pathway.
    These organic compounds accumulate at night and are broken down during the day, releasing CO2.
    These high levels of CO2 drive the Calvin Cycle and decrease photorespiration.
    CAM plants differ from C4 plants in that the C4 pathway anf the Calvin Cycle occur in the same cell, a mesophyll cell, but they occur at different times of the day, the C4 cycle at night and the Calvin cycle during the day.
  • Chapter 3-photosynthesis aa

    1. 1. Photosynthesis • An Overview of Photosynthesis • How Plants Capture Energy from Sunlight • Organizing Pigments into Photosystems • Light Reaction of Photosysnthesis Arba Minch University Dr. Chinthapalli Bhaskar Rao
    2. 2. An Overview of Photosynthesis • Photosynthesis is the process that captures light energy and transforms into the chemical energy of carbohydrates • It occurs in the – Plasma Membranes of Some Bacteria – Cells of Algae – Leaves of Plants
    3. 3. How Plants Capture Energy from Sunlight • Light has characteristic of both wave and particle • Wave: wavelength and frequency • Light is also a particle, which we call a photon. • Each photon contains an amount of energy that is called a quantum (plural quanta). • Its not continuous but rather is delivered in these discrete packets, the quanta. – High energy photons have shorter wavelengths than low energy photons • The full range of these photons is called the electromagnetic spectrum
    4. 4. Photons of different energy: the electromagnetic spectrum V I B G Y O R
    5. 5. Light absorption and emission by chlorophyll 1. Excited chlorophyll can re-emit a photon and thereby return to its ground state a process known as fluorescence. 2. The excited chlorophyll can return to its ground state by directly converting its excitation energy into heat, with no emission of a photon. 3.Chlorophyll may participate in energy transfer, from one molecule to another molecule. 4. A fourth process is photochemistry, in which the energy of the excited state causes chemical reactions to occur. The photochemical reactions of photosynthesis are among the fastest known chemical reactions. This extreme speed is necessary for photochemistry to compete with the three other possible reactions of the excited state just described.
    6. 6. Absorption spectra of Chlorophylls and Carotenoids
    7. 7. List of photosynthetic pigments Pigment Chlorophyll a Chlorophyll b Chlorophyll c Chlorophyll d Protochlorophyll Bacterio chlorophyll Bacterioviridin Phycocyanin Phyco erythrin Carotenoids Plant All green plants Green plants excluding red and blue green algae Brown algae, diatoms Red algae Etiolated plants Purple bacteria Green, sulphur bacteria Blue green algae Red, algae Most plants, bacteria Light absorbed Red and blue violet Red and blue violet Red and blue – violet Red and blue – violet Near red and blue violet Near red and blue violet Near red and blue violet Orange red Green Blue, blue green
    8. 8. What is a Chloroplast?
    9. 9. Organizing Pigments into Photosystems The protein components of thylakoid membrane are represented by 30 to 50 polypeptides disposed in different supramolecular complexes. This pigment-protein complex forms the photosystem
    10. 10. PS I complex: Pigments  Small and densely packed particles.  It consists of ~200 chlorophyll a, ~50 carotenoids.  The reaction centre is called P700, maximum absorption at 700 nm.  Energy funneling into P700 is responsible for the ejection of an election from the chlorophyll. PS II complex:  Its consists of ~200 molecules of chlorophyll a, ~200 molecules of carotenoids, chlorophyll b and chlorophyll c, depending upon the species.  Its reaction centre as P680 or shorter wavelength trap.  PS I and PS II are arranged near one another because they are functionally related.  Excitation energy originating from one system is shunted to another system.  Two photosystems are coupled chemically rather than through direct energy transfer.
    11. 11. Pigments contin… Cytochrome 559 and cytochrome 553:  This complex contains  one cytochrome f,  two cytochromes of b553,  one FeS center, and a polypeptide.  This system is uniformly distributed in the grana region. coupling factor I or CF I:  Synthesize ATP from ADP and Pi using the proton gradient. Light harvesting complex (LHC):  It contains two main polypeptides and both chlorophylls a and b.  The system remains mainly associated with PSII .  but may also be related to PSI.  This is mainly located in the stacked membranes.
    12. 12. Photosynthesis takes place in three stages Light-dependent reactions The Calvin Cycle or Light-independent reactions (Dark Reaction) – 1. Capturing energy from sunlight – 2. Using energy to make ATP and NADPH – 3. Using ATP and NADPH to power the synthesis of carbohydrates from CO2 6 CO2 + 12 H2O + Light energy C6H12O6 carbon dioxide glucose water + 6 H2 O + 6 O 2 water oxygen
    13. 13. Evidences in Support of Light Evidences in Support of Light and Dark Reaction and Dark Reaction  Evidences coefficient from temperature  Evidences from intermittent light  Evidences from carbon dioxide reduction in dark
    14. 14. Mechanism of Photosynthesis Mechanism of Photosynthesis   Until 1930s it was thought that photosynthetic reaction is reverse of respiration Though O2 evolved from CO2 Photosynthesis 6 CO2 + 12 H2O + Light energy carbon dioxide   water C6H12O6 Respiration glucose + 6 H2 O + 6 O 2 water oxygen In 1937 Robert Hill demonstrated that isolated chloroplasts evolved Oxygen, when illuminated with suitable electron acceptor Ferricyanide. This is called hill reaction. 4Fe3+ 2H2O Election acceptor 4eO2 + 4H+ 4Fe2+ Reduced Product
    15. 15. Mechanism of Photosynthesis continu… Mechanism of Photosynthesis continu…  Ruben, Randall and Kamen (1941) using heavy isotope of oxygen (O18) in their experiments provide direct proof.  Oxygen evolved in photosynthesis comes from water.
    16. 16. Oxygen-Evolving Organisms Have Two Oxygen-Evolving Organisms Have Two Photosystems That Operate in Series Photosystems That Operate in Series Red drop and Emerson Effect:  Photosynthesis is considered as a two quanta process  Two light quanta energy to drive one e Since 4e- are required, so eight quanta required to reduced and evolve one O2  Number of O2 molecules released is called Quantum yield. (1/8 or 12%) Emerson and Lewis worked on Photosynthesis in monochromatic light After 8 years Emerson and Chalmers measured the rate of photosynthesis separately with light of two different wavelengths and then used the two beams simultaneously
    17. 17. Light-Dependent Reactions • The light-dependent reactions take place in five stages – 1. – 2. – 3. – 4. – 5. Capturing light Exciting an electron Electron transport Making ATP Making NADPH
    18. 18. Production of Assimilatory Powers in Production of Assimilatory Powers in Photosynthesis Photosynthesis  Reduction of NADP or electron transport system.  Phosphorylation or Formation of ATP from ADP and Pi.
    19. 19. Photophosphorylation Photophosphorylation  Arnon and his associates (1954) first showed that isolated chloroplast can produce ATP when exposed to light.  This is phosphorylation or Photophosphorylation  The role of this ATP in two ways:  First, it suppliments the energy for the reduction of CO2 utilizing NADPH + H+ (end product of light reaction).  Secondly, this ATP is used in the phosphorylation of RUBP during its regeneration in Calvin cycle.  There are two different types of phosphorylation present.  Non-cyclic Photophosphorylation  Cyclic Photophosphorylation
    20. 20. How a Photosystem Works Lost electron is replaced by one from water breakdown Excitation energy is transferred between molecules
    21. 21. Non-cyclic Photophosphorylation Non-cyclic Photophosphorylation -0.6 -0.4 -0.2 Difference in redox potential of two cytochromes amounts to 0.33 eV, it is more than enough to accommodate phosphorylation of ADP FRS 2e- Fd NADP + H+ 2e- -0.0 PQ 2eCyt b6 2eCyt f +0.2 +0.4 ATP 2e+0.6 +0.8 2e- PC 2e- PS I (P700) ADP + Pi 2H2O ClMn++ PS II (P680) 2e- O2 + H2O 2OH + 2H+ NADP
    22. 22. Electron Transport System in Electron Transport System in Non-cyclic Photophosphorylation Non-cyclic Photophosphorylation Excited reaction center Energy of electrons Excited reaction center Ele e– Reaction center Photon P680 Photosystem II e- t ra ATP c t ro n tr Watersplitting enzyme ans p sys yst e m NADPH Reaction center Photon tem H+ Proton gradient formed for ATP synthesis Electron transport system ort s NADP+ + H+ e– o rt ns p P700 Photosystem I Electron transport system
    23. 23. Light Reactions and Non-Cyclic Photophosphorylation Hmmmm… Try to interpret this diagram in laymen’s terms. Non-cyclic photophosphorylation
    24. 24. The Photosynthetic Electron Transport System NADP+ picks up two electrons and a proton to become NADPH Calvin cycle ATP Photon Antenna complex Thylakoid membrane NADPH Photon H+ + NADP+ e- e- Stroma NADPH e- e- Light-dependent reactions Proton gradient H2O Thylakoid space Water-splitting enzyme 1/2 O2 H+ Thylakoid space 2H+ Photosystem II Electron transport system Photosystem I Electron transport system
    25. 25. Chemiosmosis in a Chloroplast H+ Photon Calvin cycle ATP H2 O H+ Thylakoid space ATP H+ e– NADPH Light-dependent reactions Membrane is impermeable to protons ADP H+ H+ + Thylakoid ½O2 2 H space Photosystem II H+ H+ Electron transport system ATP synthase
    26. 26. Cyclic Photophosphorylation -0.4 Difference in redox potential of two cytochromes amounts to 0.36 eV, and ferredoxin and cytochrome b6 is 0.32 eV Fd -0.2 e- ADP + Pi -0.0 +0.2 Cyt b6 ATP eCyt f e- PC ADP + Pi +0.4 e- ATP e- PS I (P700)
    27. 27. Differences between cyclic and noncyclic photophosphorylation 1. 2. 3. 4. 5. 6. Cyclic In this process PSI is involved Electron moves in closed circle Reduced NADPH2 is not formed and assimilation of CO2 is slow down. Oxygen is not evolved. The system is found dominantly in photosynthetic bacteria The process is not inhibited by DCMU 1. 2. 3. 4. 5. 6. Non-cyclic Both PSI and PSII are involved Not closed circle, water is the ultimate sources of electrons. NADPH2 is formed which is used in assimilation of carbon dioxide Oxygen as by produced is evolved The system is dominant is green plants The process is stopped by use of DCMU
    28. 28. In C3 plants: ATP Requirement 18 ATP molecules are required to synthesize one glucose molecule. 2 photons are required to drive 1e-. Four electrons removed from water. Eight quanta (photons) are required (4 at PSI and 4 at PSII) Only 18 ATP are generated in generation of 6O2. 18 ATP are required. Where additional 6 ATP come? Assumed that 2 additional quanta (photons) are required to generate 6 ATP molecules. i.e. 3 ATP +2NADPH for fixation of one molecule of CO2 6CO2 + 12NADPH + H+ + 18ATP C6H12O6 + 6H2O + 12NADP + 18ADP +18Pi C4 Plants: 30 ATP molecules are required to produce one molecule of glucose. Hatch and Slack (1970) proposed that C4 plants have higher capacity for photophosphorylation. They have higher chlorophyll ration of a/b ratio. But PSI component of chlorophyll a is also greater. Thus cyclic photophosphorylation supply abundant ATP molecules.
    29. 29. Part 2 Mechanism of Dark Reaction  Recent estimates indicate that about 200 billion tons of CO2 are converted to biomass each year.  About 40% of this mass originates from the activities of marine phytoplankton.  The bulk of the carbon is incorporated into organic compounds by the carbon reduction reactions associated with photosynthesis.  First time Blackman (1905) established that nonphotochemical process (dark reaction) is involved in photosynthesis.  In 1946 using radioactive materials and sophisticated techniques elucidate CO2 reduction .  Such techiques are done by Calvin and his coworkers.
    30. 30. THE LIGHT INDEPENDENT REACTION OR DARK REACTION • Enzyme controlled • Located in the stroma of the chloroplast • Occurs simultaneously with the light dependent reaction • It can continue in the dark provided the necessary raw materials are available (CO2, NADPH + H+ and ATP)
    31. 31. Enzyme controlled reaction pathways To find out the sequence of the reactions and the point at which X is added in, two approaches can be used: 1. Label and trace the products formed through time 2. Cut the supply of X and observe what happens to the intermediates in the pathway e.g. in studying photosynthesis, cut the CO2 supply or switch off the light so cutting the supply of ATP and NADPH+H+
    32. 32. Calvin and Benson 1946 to 1953 • Used 14C radioisotope for labelling • Unicellular algae: Chlorella and Scenedesmus • Simple plants which respond quickly to changes in the environment • So little time lag Image Credit Scenedesmus A flat-sided, round flask containing the culture of algae This shape: - provided even illumination of all the cells - permitted careful control of environmental conditions (e.g. pH, temperature) - permitted rapid mixing of contents - precise sampling time The “Lollipop” vessel
    33. 33. Labelling and tracing carbon using 14C A. Mixture placed at the origin D. 2nd run B.1st run C. Rotate the paper 90° E. Autoradiograph reveals the compound/s which are labelled with 14C • Add NaH14CO3 solution • At timed intervals the algae are sampled and killed by dropping in hot methanol • Two-way (2-dimensional) chromatography used to separate the compounds • Identify radioactively labelled compounds by autoradiography
    34. 34. C3 Cycle
    35. 35. Light Independent Pathway Ea A RUBISCO RBP CO2 Ec PGA Ee Ed GP 12 ATP 12 NADPH + H+ E Hexoses
    36. 36. Building New Molecules • In hot weather, plants have trouble with C3 photosynthesis – This leads to photorespiration – O2 is now consumed and CO2 is produced as a by-product – This decreases the photosynthetic yields
    37. 37. C4 Pathway – Some plants decrease photorespiration by performing C4 photosynthesis – CO2 is fixed initially into a four-carbon molecule – It is later broken down to regenerate CO2
    38. 38. Crassulacean acid metabolism (CAM) Pathway
    39. 39. • The C4 pathway is used by two types of plants – C4 plants • Examples: Sugarcane, corn • CO2 fixation and the Calvin cycle are separated in space, occurring in two different cells – CAM plants • Examples: Cacti, pineapples • Initial CO2 fixation is called crassulacean acid metabolism (CAM) • CO2 fixation and the Calvin cycle are separated in time, occurring in two different parts of the day
    40. 40. CAM plant pathways are separated temporally C4 plant pathways are separated spatially
    41. 41. Any Question?