The Calvin Cycle Anabolic reaction (builds sugar) Requires ATP and NADPH (reducing power) Requires 9 ATP and 6 NADPH (which are regenerated by light reactions) Carbon enters in the form of CO2 and exits as a glucose molecule For every three molecules of CO2, one G3P(sugar) is produced (net gain)

Phases of The Calvin Cycle Phase I: Carbon Fixation CO2 attaches to a five-carbon sugar ribulose biophosphate (RuBP) Enzyme (RuBP carboxylase aka rubisco most abundant protein) catalyzes this reaction Produces a six carbon molecule but is unstable and split to form a three carbon molecule 3-phosphoglycerate **For each CO2 molecule

Phase II: Reduction Each 3-phosphoglycerate receives an additional phosphate group Enzyme transfers phosphate from ATP to the molecule A pair of electrons donated from NADPH reduces the molecule to G3P (potential energy)

Phase III: Regeneration of CO2 acceptor RuBP five molecules of G3P are arranged into three molecules of RuBP uses 3 ATP molecules to do this RuBP is now prepared again to receive CO2 and the cycle continues

What would happen if you closed the stomata on a leaf? Build up of oxygen No water would be released No carbon dioxide would enter Why would a plant close its stomata? Hot dry environments (prevent dehydration) But this limits photosynthesis production

Alternate Methods of Carbon Fixation Photorespiration: C3 plants (most plants) Produce G3P by fixating carbon using rubisco. Examples:Rice, wheat, soybean

Stomata close therefore CO2 decreases and O2 increases Rubisco adds O2 to the Calvin Cycle which forms two carbon molecule broken down by mitochondria/peroxisomes into CO2 Process is called photorespiration No ATP is generated No food is generated Seems wasteful

Modern Plants have Evolved C4 Plants Produces a four carbon molecule Examples: sugarcane, corn, grass

Contain two types of photosynthetic cells Bundle sheath cells Mesophyll cells

CO2 is added to PEP (phosphoenopyruvate) Enzyme PEP carboxylase combines the two due to enzymes high affinity for CO2 After C4 plants fixes CO2 then the mesophyll cells exports the four carbon into bundle sheath cells Enhances sugar production (b/c CO2 is available)

CAM Plants (Crassulacean Acid Metabolism) Cacti, pineapples

CAM Plants (Crassulacean Acid Metabolism) Cacti, pineapples open stomata at night CO2 gained at night forms organic acid Mesophyll cells of CAM plants store the organic acid During daylight CO2 (from organic acid) is released into the bundle sheath and made into sugar

** CAM, C3 and C4 plants all eventually use the Calvin Cycle to make sugar from CO2