Photosynthesis: Carbon Reactions - ATP and NADPH are generated from the oxidation of water to O2 through photochemical reactions in the thylakoid membranes. - The reduction of CO2 to carbohydrates is coupled to consumption of ATP and NADPH by enzymes found in the stroma (stroma reactions, dark reactions, light-independent reactions more properly referred to as the carbon reactions of photosynthesis).

The Calvin Cycle - The Calvin cycle is the most important pathway of autotrophic CO2 fixation. - This metabolic pathway reduces CO2 to carbohydrates. - Energy from ATP, electrons from NADPH, and carbon from CO2 are combined to produce organic molecules which are converted to sugar molecules. - The cycle starts by incorporating CO2 into organic compounds: carbon fixation.

-Three stages: Fixation, reduction, and regeneration. 1- CO2 and water from the environment are enzymatically combined with a five-carbon acceptor molecule (ribulose-1,5-biphosphate) to generate two molecules of a three-carbon intermediate (3-phosphoglycerate). Phosphoglycerate is first phosphorylated to 1,3-bisphosphoglycerate through use of the ATP generated in the light reactions. Then it is reduced to glyceraldehyde-3-P through use of the NADPH generated by the light reactions. 2- The intermediate (3-phosphoglycerate) is reduced to carbohydrate by enzymatic reactions driven by the ATP and NADPH. 3- The cycle is completed by regeneration of the ribulose-1,5-biphosphate (RuBP), catalyzed by enzyme, ribulose bisphosphate carboxylase/oxygenase, Rubisco.

- The continued uptake of CO2 requires that the CO2 acceptor, RuBP, be constantly regenerated. To prevent depletion of Calvin cycle intermediates, three molecules of RuBP (15 carbons total) are formed by reactions that reshuffle the carbons from the five molecules of triose phosphate (5 × 3 = 15 carbons). - The carboxylation of three molecules of ribulose-1,5- biphosphate leads to the net synthesis of one molecule of G-3-P and the regeneration of the three molecules of starting material. - An important property of rubisco is its ability to catalyze both the carboxylation and oxygenation of RuBP.

- In most plants, initial fixation of carbon occurs via rubisco, the Calvin cycle enzyme that adds CO2 to ribulose bisphosphate. - Such plants are called C3 plants because the first organic product of carbon fixation is a three-carbon compound, 3-phosphoglycerate. - When their stomata partially close on hot, dry days, C3 plants produce less sugar because the declining level of CO2 in the leaf starves the Calvin cycle.

Ribulose-1,5-bisphosphate carboxylase/oxygenase wasteful oxygenase activity and slow turnover of Rubisco, the enzyme is among the most important targets for improving the photosynthetic efficiency of vascular plants

- Rubisco adds O2 to the Calvin cycle instead of CO2 - Rubisco adds O2 to the Calvin cycle instead of CO2. The product splits, and a two-carbon compound leaves the chloroplast. Peroxisomes and mitochondria rearrange and split this compound, releasing CO2. - The process is called photorespiration because it occurs in the light (photo) and consumes O2 while producing CO2 (respiration). However, photorespiration generates no ATP and produces no sugar. Photorespiration plays a protective role in plants. It acts to neutralize the damaging products of the light reactions (excess ATP and NADPH), which build up when a low CO2 concentration limits the progress of the Calvin cycle.

Calvin cycle Overall reaction for production of one hexose sugar: 6 CO2 + 11 H2O + 12 NADPH + 18 ATP Fructose-6-phosphate + 12 NADP+ + 6 H+ + 18 ADP + 17 Pi Efficiency? Need 8 photons to fix 1 CO2 1 quantum mole of photons = 175 kJ (for red light) To fix 6 (moles) CO2 you need 8400 kJ Oxidizing one mole of F-6-P yields 2804 kJ Efficiency ~ 33% Most NRG lost from light during synthesis of ATP and NADPH

Examples of C3 plants cotton (Gossypium spp.), rice (Oryza sativa), sugar beets (Beta vulgaris), tobacco (Nicotiana tabacum), spinach (Spinacea oleracea), potato (Solanum tuberosum); most trees and lawn grasses rice (Oryza sativa), wheat (Triticumspp.),  barley (Hordeum vulgare), rye (Secale cereale), and oat (Avena sativa);  soybean (Gycine max), peanut (Arachis hypogaea),

C3 and C4 plants

C4 Photosynthesis Low [CO2] Frequent plasmodesmata facilitate transfer of C3/C4 acids between mesophyll and BSC High [CO2] CO2 Pump

The C4 photosynthetic pathway

The Basic C4 Cycle

1. Fixation of CO2 by the carboxylation of phosphoenolpyruvate (PEP) in the mesophyll to form a C4 acid (malate and/or aspartate). 2. Transport of the C4 acids to the bundle sheath. 3. Decarboxylation of the C4 acids and generation of CO2, which is then reduced to carbohydrate via the Calvin cycle. 4. Transport of the C3 acid (pyruvate or alanine) that is formed by the decarboxylation step back to the mesophyll, and regeneration of PEP.

The C4 photosynthetic pathway

- Shuttling of metabolites between mesophyll and bundle sheath cells is driven by diffusion gradients along numerous plasmodesmata. - Transport within the cells is regulated by concentration gradients. - The cycle effectively shuttles CO2 from the atmosphere into the bundle sheath cells, which generates a much higher concentration of CO2, high enough for Rubisco to bind CO2 rather than O2.

- The cyclic series of reactions involving PEP carboxylase and regeneration of PEP can be thought of as a CO2- concentrating pump that is powered by ATP. - C4 photosynthesis minimizes photo-respiration and enhances sugar production. - This adaptation is especially advantageous in hot regions with intense sunlight, where stomata partially close during the day, and it is in such environments that C4 plants evolved and thrive today.

Examples maize (Zea mays), sugarcane (Saccharum officinarum), sorghum (Sorghum bicolor), and millets switchgrass (Panicum virganum) 

Crassulacean Acid Metabolism (CAM) - A second photosynthetic adaptation to arid conditions has evolved in many succulent (water-storing) plants, numerous cacti, pineapples, and representatives of several other plant families. - CAM plants are typical of desert environments. - These plants open their stomata during the night and close them during the day, just the reverse of how other plants behave.

- Closing stomata during the day helps desert plants conserve water, but it also prevents CO2 from entering the leaves. - During the night, when their stomata are open, these plants take up CO2 and incorporate it into a variety of organic acids. - The mesophyll cells store the organic acids they make during the night in their vacuoles until morning, when the stomata close.

Examples Ferns Moringa Aloe Cactus Epiphytes

C4 CAM C3 Pathway of CO2 fixation via C3 cycle only via C3 and C4 cycles, spatially (C4 in the mesophyll cells followed by C3 in the bundle sheath cells via C3 and C4 cycles, both spatially (in different parts of same cells) and temporally (C4 at night, C3 at daytime) Normal diurnal occurence Daytime Day and night Initial CO2 acceptor Ribulose-1,5-bisphosphate (RuBP) Phosphoenolpyruvate (PEP) PEP at night, RuBP in the day CO2-fixing enzyme Ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBisCo) Phosphoenolpyruvate carboxylase (PEPcase) then Rubisco PEPcase at night, Rubisco at daytime First stable product of CO2 fixation 3-phosphoglycerate (3-PGA) Oxaloacetate (OAA) in C4 cycle OAA at night, 3-PGA at daytime Cells Involved Mesophyll cells C4- mesophyll cells, C3- bundle sheath cells Both C3 and C4 in same mesophyll cells

Energy needed for complete reduction of one molecule of CO2 3 ATP, 2 NADPH 5 ATP, 2 NADPH Environmental conditions favoring most efficient photosynthesis moderate conditions; temperature 15 °C-25°C hot, dry conditions; temperature 30°C-47°C extremely dry or xeric conditions; temperature ~35°C Amount of water needed to produce 1 g dry matter 450-950 g 250-350 g 50-55 g CO2 compensation point 30-70 ppm 0-10 ppm 0-5 ppm Photorespiration Yes Absent or suppressed Annual dry matter production per hectare ~20-25 tons ~35-40 tons Usually low and variable Sources: Mathews and Van Holde (1990); Hopkins (1999); Moore et al. (2003); Simpson (2010)