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