Photosynthesis: Carbon Reactions ATP and NADPH are generated from the oxidation of water to O2 through photochemical reactions in the thylakoid membranes (Light Reaction). 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 Basic C4 Cycle

The C4 photosynthetic pathway

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: carboxylation, reduction, and regeneration. Carboxylation: 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). Reduction: The intermediate (3-phosphoglycerate) is reduced to carbohydrate by enzymatic reactions driven by the ATP and NADPH. Regeneration: The cycle is completed by regeneration of the ribulose-1,5-biphosphate (RuBP).

To prevent depletion of Calvin cycle intermediates, The carboxylation of RuBP is catalyzed by the 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).

An important property of rubisco is its ability to catalyze both the The carboxylation of three molecules of ribulose-1,5- bis phosphate 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.

Photorespiration 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. In some plant species, alternate modes of carbon fixation have evolved. These plants have normal rubiscos, and their lack of photorespiration is a consequence of mechanisms that concentrate CO2 in the rubisco environment and thereby suppress the oxygenation reaction.

C4 photosynthetic carbon fixation (C4) Also called CO2 - concentrating mechanism. The first carbon compound will be C4 acid (oxaloacetate) Plants with C4 metabolism are often found in hot environments (C4 plants). A unique leaf anatomy is correlated with C4 plants. There are two distinct chloroplast-containing cells: bundle sheath and mesophyll cells. The Calvin cycle is confined to the chloroplasts of the bundle-sheath cells. However, the cycle is preceded by incorporation of CO2 into organic compounds in the mesophyll cells.

C4 leaf anatomy

The Basic C4 Cycle

Fixation of CO2 by the carboxylation of phosphoenolpyruvate (PEP) in the mesophyll to form a C4 acid (malate and/or aspartate). Transport of the C4 acids to the bundle sheath. Decarboxylation of the C4 acids and generation of CO2, which is then reduced to carbohydrate via the Calvin cycle. 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.

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.

Succulent Plants They have sunken stomata (stomata are deeply inside the leaves) They have succulent leaves and stems Stomata open in night-time and close in daytime.