Photosynthesis Light-independent Reactions Fixing Carbon Dioxide

Calvin Cycle Chloroplasts contain enzymes in the stroma that convert carbon dioxide to carbohydrates The energy needed is supplied by ATP and NADPH that was formed in the light- dependent reactions The key initial step is to convert CO 2 into organic compounds (CO 2 assimilation)

Calvin cycle The assimilation of CO 2 is carried out by a cyclical pathway that continually regenerates its intermediates It is called the Calvin cycle in honour of Melvin Calvin who identified the pathway in the 1950s Also called the dark reactions and the Calvin- Benson cycle

Phase 1: Carbon Dioxide Fixation The key is the chemical bonding of carbon atom in CO 2 to a pre-existing molecule called ribulose-1,5-biphosphate (RuBP) RuBP is a five-carbon compound The resulting six-carbon compound is very unstable and immediately breaks down into two identical three-carbon compounds called 3-phosphogycerate (PGA)

C 3 Photosynthesis Because PGA is the first stable product of the process, plants that use this method for photosynthesis are called C 3 plants The process is called C 3 photosynthesis ed/media/ch10/c3-plants.html rice wheat soybeans

Phase 1 CO 2 + RuBP unstable C 6 2PGA The reaction is catalysed by the enzyme ribulose biphosphate carboxylase (rubisco) Rubisco is probably the most abundant protein on earth

Phase 2: Reduction The newly formed PGA are in a low-energy state To convert them to a higher-energy state they are activated by ATP and reduced by NADPH This results in 2 molecules of gyceraldyhyde-3- phosphate (G3P)

Phase 2 Some G3P leave the cycle and may be used to make glucose and other carbohydrates The remaining G3P molecules move on to the third phase to replenish the RuBP

Phase 3: Regenerating RuBP Most of the reduced G3P are used to make more RuBP ATP is required to break and reform the bonds to make the 5-carbon RuBP from the 3-carbon G3P The Calvin cycle must be completed 6 times to form one glucose molecule

Net equation: Calvin Cycle 6CO ATP + 12 NADPH + water 2 G3P + 16P i + 18 ADP + 12 NADP + Much of the G3P produced is transported out of the chloroplasts into the cytoplasm

G3P: A Crucial Molecule In Plant Metabolism In the cytoplasm, G3P is used to make: – sucrose, a key sugar in plants – starch when photosynthesis is intense – cellulose – plant oils such as corn, olive and safflower oil – amino acids (with a source of nitrogen)

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Adaptations to Photosynthesis Photorespiration is the reaction of oxygen with RuBP in a process that reverses carbon fixation and reduces the efficiency of photosynthesis CO 2 and O 2 compete for the same active site on the rubisco enzyme The energy used to regenerate RuBP is wasted

Adaptations Under normal conditions (temperatures near 25°) C3 plants lose 20% of the energy used to fix one CO 2 molecule Under hot dry conditions, leaves begin to lose water through the stomata so the stomata close With stomata closed, O 2 accumulates inside the leaves and CO 2 cannot enter Photorespiration increases

Adaptations Plants that are native to hot dry regions have evolved mechanisms to reduce the amount of photorespiration These plants fit into two categories: C4 and CAM plants

C4 plants Some examples: crabgrass corn (maize) sugarcane sorghum Although only ~3% of the angiosperms are C4 plants, they are responsible for ~25% of all the photosynthesis on land.

Alternate mechanisms: C4 C4 plants separate the uptake of CO 2 from the Calvin cycle into different types of cells: – Uptake is in the mesophyll cells – Calvin cycle occurs in the bundle sheath cells

C4 Plants

C4 adaptations Mesophyll cells: – are close to the surface – are exposed to high levels of O 2 – but have no rubisco so cannot start photorespiration (nor the reactions of the Calvin cycle)

C4 adaptations Bundle sheath cells: – are deep in the leaf so atmospheric oxygen cannot diffuse easily to them – often have thylakoids with reduced photosystem II complexes (the one that produces O 2 ). – Both of these features keep oxygen levels low.

The Details of the C4 cycle After entering through stomata, CO 2 diffuses into a mesophyll cell. the CO 2 is inserted into a 3-carbon compound called phosphoenolpyruvate (PEP) forming the 4-carbon compound oxaloacetate

The Details of the C4 cycle Oxaloacetate is converted into malic acid or aspartic acid (both have 4 carbons), which is transported into a bundle sheath cell. In the bundle sheath cells the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle and pyruvic acid which is transported back to a mesophyll cell where it is converted back into PEP.

CAM Plants Use a pathway identical to C4 plants The reactions take place in the same cell The carbon fixation is separated from the Calvin cycle by time of day CAM stands for crassulacean acid metabolism because it was first studied in members of the plant family Crassulaceae.)

Some examples of CAM plants: cacti Bryophyllum the pineapple and all bromeliads sedums

Details of the CAM cycle At night: CAM plants take in CO 2 through their open stomata (they tend to have reduced numbers of them) The CO 2 joins with phosphoenolpyruvate (PEP) to form the 4-carbon oxaloacetate. This is converted to 4-carbon malic acid that accumulates during the night in the central vacuole of the cells.

Details of the CAM cycle In the morning: the stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen). The accumulated malic acid leaves the vacuole and is broken down to release CO 2 The CO 2 is taken up into the Calvin cycle

That’s All