C3 Photosynthesis

Plants which use only the Calvin cycle for fixing the carbon dioxide from the air are known as C3 plants. In the first step of the cycle CO2 reacts with RuBP to produce two 3-carbon molecules of 3-phosphoglyceric acid (3-PGA). This is the origin of the designation C3 or C3 in the literature for the cycle and for the plants that use this cycle. The entire process, from light energy capture to sugar production occurs within the chloroplast. The light energy is captured by the non-cyclic electron transport process which uses the thylakoid membranes for the required electron transport. About 85% of plant species are C3 plants. They include the cereal grains: wheat, rice, barley, oats. Peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees are C3 plants. Most lawn grasses such as rye and fescue are C3 plants. C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration. When the CO2 concentration in the chloroplasts drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead. This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate. The problem of photorespiration is overcome in C4 plants by a two-stage strategy that keeps CO2 high and oxygen low in the chloroplast where the Calvin cycle operates. The class of plants called C3-C4 intermediates and the CAM plants also have better strategies than C3 plants for the avoidance of photorespiration. Energy cycle in living things Index Photosynthesis Concepts Reference Moore, et al. Ch 7 HyperPhysics***** Biology R Nave Go Back C4 Photosynthesis Sugarcane is a champion at photosynthesis under the right conditions and is a prime example of a C4 plant, one which uses C4 photosynthesis. Sugarcane has been recorded at 7% photosynthetic efficiency. C4 plants almost never saturate with light and under hot, dry conditions much outperform C3 plants. They use a two-stage process were CO2 is fixed in thin-walled mesophyll cells to form a 4-carbon intermediate, typically malate (malic acid). The reaction involves phosphoenol pyruvate (PEP) which fixes CO2 in a reaction catalyzed by PEP-carboxylate. It forms oxaloacetic acid (OAA) which is quickly converted to malic acid. The 4-carbon acid is actively pumped across the cell membrane into a thick-walled bundle sheath cell where it is split to CO2 and a 3-carbon compound. This CO2 then enters the Calvin cycle in a chloroplast of the bundle sheath cell and produces G3P and subsequently sucrose, starch and other carbohydrates that enter the cells energy transport system. The advantage that comes from this two-stage process is that the active pumping of carbon into the bundle sheath cell and the blocking of oxygen produce an environment with 10-120x as much CO2 available to the Calvin cycle and the rubisco tends to be optimally utilized. The high CO2 concentration and the absence of oxygen implies that the system never experiences the detractive effects of photorespiration. The drawback to C4 photosynthesis is the extra energy in the form of ATP that is used to pump the 4-carbon acids to the bundle sheath cell and the pumping of the 3-carbon compound back to the mesophyll cell for conversion to PEP. This loss to the system is why C3 plants will outperform C4 plants if there is a lot of water and sun. The C4 plants make some of that energy back in the fact that the rubisco is optimally used and the plant has to spend less energy synthesizing rubisco. Moore, et al. say that only about 0.4% of the 260,000 known species of plants are C4 plants. But that small percentage includes the important food crops corn, sorghum, sugarcane and millet. Also inluded are crabgrass and bermuda. Many tropical grasses and sedges are C4 plants. Energy cycle in living things Index Photosynthesis Concepts Reference Moore, et al. Ch 7 HyperPhysics***** Biology R Nave Go Back C3-C4 Intermediate Photosynthesis Moore, et al. point to Flaveria (Asteraceae), Panicum (Poaceae) and Alternanthera (Amarantheceae) as genera that contain species that are intermediates between C3 and C4 photosynthesis. These plants have intermediate leaf anatomies that contain bundle sheath cells that are less distinct and developed than the C4 plants. These intermediates are characterized by their resistance to photorespiration so that they can operate in higher temperatures and dryer environments than C3 plants. At right, the ranges of CO2 compensation points for the three types of plants are shown. These compensation points are the values at which the plants cease to provide net photosynthesis. The connection to hot and dry conditions comes from the fact that all the plants will close their stomata in hot and dry weather to conserve moisture, and the continuing fixation of carbon from the air drops the CO2 dramatically from the atmospheric concentration of nominally 38,000 ppm. If the CO2 compensation point is lower on the above scale, the plant can operate in hotter and dryer conditions. The limits are placed by the fact that rubisco begins to fix oxygen rather than CO2, undoing the work of photosynthesis. C4 plants shield their rubisco from the oxygen, so can operate all the way down to essentially zero CO2 without the onset of photorespiration. Energy cycle in living things Index Photosynthesis Concepts Reference Moore, et al. Ch 7 HyperPhysics***** Biology R Nave Go Back Crassulacean Acid Metabolism (CAM) The CAM plants represent a metabolic strategy adapted to extremely hot and dry environments. They represent about 10% of the plant species and include cacti, orchids, maternity plant, wax plant, pineapple, Spanish moss, and some ferns. The only agriculturally significant CAM plants are the pineapple and an Agave species used to make tequila and as a source of fiber. The sketch below of the day-night cycle of the CAM plants is patterned after Moore, et al. The name Crassulacean Acid Metabolism came from the fact that this strategy was discovered in a member of the Crassulaceae which was observed to become very acidic at night and progressively more basic during the day. The acidity was found to arise from the opening of their stomata at night to take in CO2 and fix it into malic acid for storage in the large vacuoles of their photosynthetic cells. It could drop the pH to 4 with a malic acid concentration up to 0.3M . Then in the heat of the day, the stomata close tightly to conserve water and the malic acid is decarboxylated to release the CO2 for fixing by the Calvin cycle. PEP is used for the initial short-term carbon fixation as in the C4 plants, but the entire chain of reactions occurs in the same cell rather than handing off to a separate cell as with the C4 plants. In the CAM strategy, the processes are separated temporally, the initial CO2 fixation at night, and the malic acid to Calvin cycle part taking place during the day. With stomata open only at night when the temperature is lower and the relative humidity higher, the CAM plants use much less water than either C3 plants or C4 plants. Some varieties convert to C3 plants at the end of the day when their acid stores are depleted if they have adequate water, and even at other times when water is abundant. http://www.xtec.net/~jcoll/activitats/els_protozous.htm ons A. Two Pathways 1. Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway. 2. Both pathways produce ATP but only the noncyclic pathway also produces NADPH. 3. ATP production during photosynthesis is sometimes called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation. B. Noncyclic Electron Pathway (*SPLITS WATER, PRODUCES NADPH & ATP) 1. This pathway occurs in the thylakoid membranes and requires participation of two light-gathering units: photosystem I (PS I) and photosystem II (PS II). 2. A photosystem is a photosynthetic unit comprised of a pigment complex and electron acceptor; solar energy is absorbed and high-energy electrons are generated. 3. Each photosystem has a pigment complex composed of green chlorophyll a and chlorophyll b molecules and orange and yellow accessory pigments (e.g., carotenoid pigments). 4. Absorbed energy is passed from one pigment molecule to another until concentrated in reaction-center chlorophyll a. 5. Electrons in reaction-center chlorophyll a become excited; they escape to electron-acceptor molecule. 6. The noncyclic pathway begins with PSII; electrons move from H2O through PS II to PS I and then on to NADP+. 7. The PS II pigment complex absorbs solar energy; high-energy electrons (e-) leave the reaction-center chlorophyll a molecule. 8. PS II takes replacement electrons from H2O, which splits, releasing O2 and H+ ions: 9. Oxygen is released as oxygen gas (O2). 10. The H+ ions temporarily stay within the thylakoid space and contribute to a H+ ion gradient. 11. As H+ flow down electrochemical gradient through ATP synthase complexes, chemiosmosis occurs. 12. Low-energy electrons leaving the electron transport system enter PS I. 13. When the PS I pigment complex absorbs solar energy, high-energy electrons leave reaction-center chlorophyll a and are captured by an electron acceptor. 14. The electron acceptor passes them on to NADP+. 15. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e- + H+ NADPH. 16. NADPH and ATP produced by noncyclic flow electrons in thylakoid membrane are used by enzymes in stroma during light-independent reactions. Cyclic Electron Pathway 1. The cyclic electron pathway begins when the PS I antenna complex absorbs solar energy. 2. High-energy electrons leave PS I reaction-center chlorophyll a molecule. 3. Before they return, the electrons enter and travel down an electron transport system. a. Electrons pass from a higher to a lower energy level. b. Energy released is stored in form of a hydrogen (H+) gradient. c. When hydrogen ions flow down their electrochemical gradient through ATP synthase complexes, ATP production occurs. d. Because the electrons return to PSI rather than move on to NADP+, this is why it is called cyclic and also why no NADPH is produced. D. ATP Production (chemiosmosis) 1. The thylakoid space acts as a reservoir for H+ ions; each time H2O is split, two H+ remain. 2. Electrons move carrier-to-carrier, giving up energy used to pump H+ from the stroma into the thylakoid space. 3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to produce ATP from ADP + P by using an ATP synthase enzyme Phase 1: Carbon Fixation CO2 comes into the stroma of the chloroplast via the stomata of the leaves. Rubisco catalyzes the bonding of CO2 to RuBP to create an unstable 6-carbon molecule that instantly splits into two 3-carbon molecules of 3-PG. Phase 2: Reduction ATP phosphorylates each 3-PG molecule and creates 1,3-bisphosphoglycerate. This in turn results in the loss of the terminal phosphate group from ATP (adenosine triphosphate) thus making ADP (adenosine diphosphate). NADPH reduces 1,3-bisphosphoglycerate which causes the phosphate group to break off once again. The molecule then picks up a proton (H+) from the medium to become glyceraldehyde-3-phosphate. The broken off phosphate group also gains a proton to become H3PO4. NADPH is oxidized by this process and becomes NADP+.Below are the three intermediates of the reduction phase of the Calvin cycle. Note the only difference between each molecule is the group attached to the primary carbon (the lowest carbon -- with the carbonyl.) Phase 3: Regeneration For every six molecules of G3P created five molecules continue on to phase 3 while one leaves to be used for organic compounds ATP is once again needed. However, this time it phosphorylates G3P to regenerate RuBP after some rearrangement.