Download The Calvin Cycle

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