Photosynthesis – The Calvin Cycle

Calvin Cycle Incorporates atmospheric CO 2 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 CO 2 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 3CO 2 To make one glucose molecule: 6 cycles and the fixation of 6CO 2

Calvin Cycle has 3 Phases 1. Carbon Fixation Phase (Carboxylation) 2. Reduction 3. Regeneration of CO 2 acceptor (RuBP)

1. Carbon Fixation 1CO 2 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 1CO 2 ) 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 3CO 2 (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 CO 2 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 CO 2

Dehydration Land plants can easily dehydrate Stomata open to allow O 2 /CO 2 exchange Allows for evaporative loss of H 2 O Hot dry days – plants close stomata to conserve H 2 O PROBLEM!

C 3 Plants C3 plants (most plants – rice, wheat, soy are examples) use RuBisCO and end product is G3P Stomata closed CO 2 levels drop (consumed by Calvin) O 2 levels rise (produced by light rxn) When O 2 / CO 2 ratio increases, RuBisCO can add O 2 to RuBP

Photorespiration O 2 + RuBP yields 3C and 2C pieces (photorespiration) 2C piece exported from chloroplast, peroxisomes & mitochondria degrade to CO 2 Produces no ATP, no organic molecules Photorespiration decreases photosynthetic output

WHY? EVOLUTION! Early Earth had little O 2, lots of CO 2 Alternative pathway negligible TODAY… Photorespiration can drain up to 50% of fixed carbon on a hot day Might evolution have come into play again?

C 4 Plants Very common pathway – sugarcane, corn Mesophyll cells incorporate CO 2 into organic molec Phosphoenolpyruvate carboxylase adds CO 2 to phosphoenolpyruvate (PEP) to form OXALOACETATE. PEP Carboxylase has a high affinity for CO 2 – 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 CO 2 ) and return the 3C to mesophyll BS cells then use RuBisCO to start Calvin Cycle

So… Mesophyll cells pump CO 2 into BS cells, so RuBisCO doesn’t need to utilize O 2. C 4 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 CO 2 into a variety of organic acids in mesophyll Day: Light rxns supply ATP & NADPH to Calvin; CO 2 released from acids CAM Mechanism

CAM & C 4 Add CO 2 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