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PHOTOSYNTHESIS

PHOTOSYNTHESIS. Plants, bacteria and various protista are autotrophs- they fix inorganic carbon from CO 2 into an organic molecule. We will focus on photosynthesis in plants. Chloroplasts are the site of PTN Basic formula:. 6CO 2 + 6 H 2 O+ light energy---> C 6 H 12 O 6 + 6O 2

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PHOTOSYNTHESIS

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  1. PHOTOSYNTHESIS

  2. Plants, bacteria and various protista are autotrophs- they fix inorganic carbon from CO2 into an organic molecule. We will focus on photosynthesis in plants. Chloroplasts are the site of PTN Basic formula: 6CO2 + 6 H2O+ light energy---> C6H12O6 + 6O2 Even simpler: CO2+H2O--->CH2O+O2

  3. Reactions in PTN, like cellular respiration are mostly redox reactions: reactions where electrons are transferred. Remember that Reduction is the GAIN of electrons and Oxidation is the LOSS of electrons: OIL RIG The loss and gain of electrons are hard to follow in organic rctns. It is also possible to see electron movement by looking at Hydrogen, since e- are transferred with H+, together making a hydrogen ATOM. So reduction can also be thought of as the gain of hydrogen. In the general formula: 6CO2 + 6 H2O+ ---> C6H12O6 + 6O2 What is being reduced and what is being oxidized? CO2 is reduced to glucose water is oxidized to oxygen

  4. Redox rctns are always coupled: they occur together

  5. CHLOROPLAST: site of PTN

  6. There are two main steps: Light dependant reactions- requires light. Light energy is absorbed and converted to chemical energy. This is stored in an intermediate E carrier called NADPH. Water is split, O2 is released. ATP is made by photophosphorylation. 2. Light independent reactions, Calvin cycle- light not needed. CO2 is fixed into an organic molecule, then reduced by NADPH to form a carbohydrate

  7. LIGHT REACTIONS Light energy is electromagnetic radiation. There are different wavelengths of light that correspond to the different colors

  8. The amount of energy in light is inversely proportional to its wavelength: shorter WL, more energy The energy in light can also be thought of as particles, called photons. A photon of light of a particular wavelength has a certain amount of energy. Plants absorb certain wavelengths better than others. The amount of light that a plant absorbs at different wavelengths is called the absorption spectrum. A graph showing how the different wavelengths actually power the PTN process is called the action spectrum

  9. Absorption and action spectra pigments Note differences in graphs (Y axis)

  10. Based on this information, which type of light do you think would cause the highest rate of photosynthesis? We are going to test this by putting light filters over a leaf to allow for selective transmittance of light. We can then indirectly test the rate of photosynthesis by looking at how much starch is produced.

  11. Pigments There are two main pigments that absorb light: chlorophyll a and chlorophyll b. They absorb different WL of light. When you look at the action spectrum, you see the result of light absorbed by both Chlorophylls and the carotenoids: accessory pigments The wavelengths that are most useful are blue and red. Green is not absorbed, it is reflected, which is why plants appear green

  12. Inside the chloroplast

  13. Absorption of light and excitation of pigments When a molecule of chlorophyll absorbs a photon of light, the energy is transferred to an electron, raising it to a higher energy state. 2. The molecule is now in an excited state, before it was in a ground state. 3. Photons are absorbed by a group of molecules in the thylakoid membrane called the antenna pigments 4. Molecules in this excited state are unstable, the electron would fall back to its ground state very quickly. 5. In a living chloroplast the electron will get passed along molecules in a photosystem rather than just fall back to its ground state.

  14. PHOTOSYSTEM Antenna pigments absorb light and become excited 2. Energy (but not e-) is then transmitted to other molecules until it reaches a particular chlorophyll that is located near a molecule known as the primary electron acceptor at the reaction center. 3. This PEA is reduced by the chlorophyll a, the chlorophyll is now oxidized and is “short” 2 electrons. 4. There are 2 photosystems in the thylakoid: I and II. PSI absorbs light best with a WL of 700nm, so its also called P700, PS II absorbs light of 680 best, so its called P680.

  15. The flow of electrons in the photosystems can happen in two ways: cyclic and non-cyclic. Noncyclic is the main one

  16. Steps in Noncyclic electron flow: The PEA in P680 (PSII) receives 2 e- from the chlorophyll 2. The chloropyll a that donated the e- has to have its e- replaced- it is a strong oxidizing agent 3. An enzyme splits water and this supplies the e- to P680. The chlorophyll a is reduced, the oxygen from the water combines with another to form O2, which is released. There are 4H+ left over. 4. Meanwhile the e- from PEA from p680 get passed down a series of molecules similar to ones in cellular resp. called cytochromes. 5. As the electrons fall down the ETC, this powers the production of ATP- called noncyclic photophosphorylation ( H+ ions are translocated across the membrane as the e- are transferred) 6. e- are then passed to P700

  17. 7. P700 functions like P680, it absorbs light energy and there will be an oxidized chlorophyll a looking to get its e’s replaced. These are supplied by the e- coming in from the ETC. 8. The PEA in P700 passes its e- to a second ETC which gives them to a protein called feredoxin (Fe). An enzyme, NADP+ reductase transfers e- from ferredoxin to NADP+ to form the reduced NADPH 9. Energy is transferred in the NADPH to the next step, the light independent reactions.

  18. Production of ATP by chemiosmosis ATP is synthesized by chemiosmosis in a similar mechanism to cellular respiration H+ ions that were produced by the splitting of water are are pumped from the stroma into the thylakoid space The energy for this is derived from the electron transport chain H+ ions then diffuse down their concentration gradient through the ATP synthase complex. ATP forms in the stroma

  19. http://www.sumanasinc.com/webcontent/animations/biology.html

  20. Cyclic electron flow Uses P700, not P680 No O2 is made, no NADPH is made Only ATP is made e- cycle back from ferredoxin to the cytochrome complex

  21. Why two different systems? Noncyclic PPh makes equal amounts NADPH and ATP, whereas cyclic only makes ATP The Calvin cycle will use more ATP than NADPH, so more ATP is needed so cyclic PPH makes up the difference. Some organisms with more primitive systems only have cyclic PPh

  22. PHOTOPHOSPHORYLATION up close 4. An electrochemical gradient is now set up, more H+ in thylakoid space 5. H+ moves down its concentration gradient thru ATP synthase complex. ADP+Pi--> ATP Water split in PSII e- transferred to cytochrome complex H+ removed from stroma when e- transferred to PQ

  23. Compare and contrast ATP production by chemiosmosis in photosynthesis and respiration Similarities: * ETC assembled in membrane of mitochondria or chloroplast pumps H+ ions across the membrane as e- are transported down the chain through a series of carriers, some are the same ie cytochrome * An electrochemical gradient is created * H+ ions flow down their concentration gradient through ATP synthase complex. ADP+Pi--> ATP

  24. Differences: * In resp. the high energy electrons that move down the chain come from oxidation of glucose, in PTN energy comes from light * Spatial organization is different: inner membrane of mitochondria pumps H+ from matrix to intermembrane space (inside to outside), in chloroplats H+ are pumped from stroma to thylakoid space (outside to inside).

  25. The products of the light dependant rctn: ATP and NADPH enter the next step: Calvin Cycle Carbon dioxide will be reduced using the energy in the ATP and NADPH to make sugar The product of one Calvin cycle is a three carbon sugar called G3P To synthesize one glucose the cycle must turn three times, fixing 3 molecules of CO2. This is because one of these carbon molecules must go back into the cycle to regenerate one of the intermediates.

  26. Steps in Calvin Cycle: Carbon fixation: a. CO2 molecule is attached to a 5 carbon molecule called ribulose bisphosphate (RuBP). The enzyme is RuBP carboxylase. Product is a 6 carbon molecule. b. This molecule is unstable and immediately splits to form two 3 carbon molecules: 3-phosphoglycerate

  27. 2. Reduction: a. Each 3 carbon molecule gets another P from ATP and becomes 1,3 bisphosphoglycerate. b. A pair of electrons is donated from NADPH to reduce the 1,3 bisphosphoglycerate and it releases one phosphate. Molecule is called glyceraldehyde-3-phosphate G3P or phosphoglyceraldehyde PGAL. (this is the same sugar that is made in glycolysis after 6C sugar is split) c. Because some of the G3P needs to regenerate RuBP, only one out of six of these will go one to make a 6 carbon sugar, the other 5 will stay in the cycle. d. So 6 molecules of CO2 are needed for a net gain of one G3P and one RuBP (a 5 carbon molecule)

  28. 3. Regeneration of RuBP a. five G3P molecules are rearranged to make 3 RuBP molecules. b. 5 X 3 carbons=15 and 3 X 5 carbons= 15 c. this uses 3 molecules of ATP d. RuBP is regenerated so cycle can continue e. So 3 molecules of CO2 combine with 3 molecules of RuBP to make 6 molecules of G3P. One will go on to make a 6 carbon sugar, the other 5 will regenerate RuBP. This requires 9 ATP and 6 NADPH

  29. http://www.science.smith.edu/departments/Biology/Bio231/calvin.htmlhttp://www.science.smith.edu/departments/Biology/Bio231/calvin.html http://www.sinauer.com/cooper/4e/animations0305.html

  30. Alternate mechanisms for PTN Some plants fix CO2 into a different molecule which is a four carbon molecule instead of a 3 carbon molecule- these are called C4 plants. Examples are: sugarcane, corn and grasses. CO2 combines with phosphoenolpyruvaye (PEP) to form oxaloacetae, using the enzyme PEP carboxylase. This enzyme has a high affinity for CO2 and fixes C02 more efficiently. Structure of leaves in C4 plants relates to differences in the mecahnisms of PTN. C4 plants have two photosynthetic cells: bundle sheaths and mesophylls.

  31. Bundle sheath and mesopyll cells are packed together around the veins. The Calvin cycle takes place in the bundle sheath cells, initial CO2 fixation takes place in the mesophyll cells. The mesopyll cells supply the bundle sheath cells with C02 in the form of PEP These plants are adapted to hot, high level conditions and can fix CO2 even when their stomata are partially closed. CAM plants (succulents, pineapples) use another adaptation to minimize water loss. They only open their stomata at night and fix C02 in crussulacean acid, which is stored until the daytime when light is available.

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