Photosynthesis – The Calvin Cycle
Calvin Cycle
• Incorporates atmospheric CO2 and uses ATP/NADPH from light reaction
• Named for Dr. Melvin Calvin• He & other scientists worked out many of
the steps in the 1940s• Sometimes called “dark” reaction
Overview
• Occurs in the stroma• CO2 enters the cycle and leaves as sugar• Spends the energy of ATP and NADPH • Glucose not produced - yield is:
glyceraldehyde-3-phosphate (G3P)
• WHERE HAVE WE SEEN G3P BEFORE?
One turn of the Calvin…
• Each turn of the Calvin cycle fixes 1C• For net synthesis of one G3P molecule,
cycle must occur 3X, fixing 3CO2
• To make one glucose molecule:6 cycles and the fixation of 6CO2
Calvin Cycle has 3 Phases
1. Carbon Fixation Phase (Carboxylation)
2. Reduction
3. Regeneration of CO2 acceptor (RuBP)
1. Carbon Fixation
• 1CO2 attaches to a 5C sugar• ribulose 1,5 bisphosphate (RuBP)
• Catalyzed by (RuBisCO)• ribulose-1,5-bisphosphate
carboxylase/oxygenase
1. Carbon Fixation
• 6C intermediate is unstable• Immediately splits in half:
• forms 2 molecules of 3-phosphoglycerate
2. Reduction
• 2 ATP needed for this step (per 1CO2)
• Each 3-phosphoglycerate is phosphorylated• forms 1,3-bisphosphoglycerate
• Pair of e- from NADPH reduces each 1,3-bisphosphoglycerate to:• G3P• Reduction of a carboxyl group to a carbonyl
Crunch the Numbers…• To produce one G3P net:
• start with 3CO2 (3C) and 3RuBP (15C)
• After fixation/reduction:• 6 molec of G3P (18C)• One of these 6 G3P (3C) is a net gain of a
carbohydrate• Molec. can exit cycle to be used by plant cell
• Other 5 G3P (15C) must remain in the cycle to regenerate 3RuBP
3. Regeneration of CO2
• The 5 G3P molecules are rearranged to form 3 RuBP molecules
• 3 molecules of ATP spent (one per RuBP) to complete the cycle and prepare for the next
Crunch the Numbers…again
• Net synthesis of 1 G3P molecule, Calvin cycle consumes 9ATP and 6NAPDH
• “Costs” three ATP and two NADPH per CO2
Dehydration
• Land plants can easily dehydrate• Stomata open to allow O2/CO2 exchange
• Allows for evaporative loss of H2O
• Hot dry days – plants close stomata to conserve H2O
PROBLEM!
C3 Plants
• C3 plants (most plants – rice, wheat, soy are examples) use RuBisCO and end product is G3P
• Stomata closed• CO2 levels drop (consumed by Calvin)
• O2 levels rise (produced by light rxn)
• When O2 / CO2 ratio increases, RuBisCO can add O2 to RuBP
Photorespiration• O2 + RuBP yields 3C and 2C pieces
(photorespiration)• 2C piece exported from chloroplast,
peroxisomes & mitochondria degrade to CO2
• Produces no ATP, no organic molecules
• Photorespiration decreases photosynthetic output
WHY?• EVOLUTION!
• Early Earth had little O2, lots of CO2
• Alternative pathway negligible
• TODAY…• Photorespiration can drain up to 50% of fixed
carbon on a hot day
• Might evolution have come into play again?
C4 Plants
• Very common pathway – sugarcane, corn• Mesophyll cells incorporate CO2 into
organic molec• Phosphoenolpyruvate carboxylase adds CO2
to phosphoenolpyruvate (PEP) to form OXALOACETATE.
• PEP Carboxylase has a high affinity for CO2 – can fix C when RuBisCo can’t (i.e. when stomata are closed)
• Mesophyll cells pump 4C cmpds to bundle sheath cells• BS cells strip a C (as CO2) and return the 3C
to mesophyll• BS cells then use RuBisCO to start Calvin
Cycle
• So…• Mesophyll cells pump CO2 into BS cells, so
RuBisCO doesn’t need to utilize O2.
• C4 plants minimize photorespiration & promote sugar production
• Thrive in hot regions with intense sun
CAM Plants• Other plants have evolved another
strategy to minimize photorespiration• Succulents:
• Cacti, pineapples, several others
• CAM – Crassulacean Acid Metabolism• Stomata open at night ONLY!
• Night: • Fix CO2 into a variety of organic acids in
mesophyll
• Day:• Light rxns supply ATP & NADPH to Calvin;
CO2 released from acids
CAM Mechanism
CAM & C4
• Add CO2 to organic intermediates before entering Calvin• In C4, carbon fixation and Calvin cycle
PHYSICALLY (space) separated• In CAM, carbon fixation and Calvin cycle are
TEMPORALLY (time) separated
