Photosynthesis The Calvin Cycle Calvin Cycle Incorporates atmospheric

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 1940 s 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 (G 3 P) WHERE HAVE WE SEEN G 3 P BEFORE?

One turn of the Calvin… • • • Each turn of the Calvin cycle fixes 1 C For net synthesis of one G 3 P molecule, cycle must occur 3 X, fixing 3 CO 2 To make one glucose molecule: 6 cycles and the fixation of 6 CO 2

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

1. Carbon Fixation • 1 CO 2 attaches to a 5 C sugar • • ribulose 1, 5 bisphosphate (Ru. BP) Catalyzed by (Ru. Bis. CO) • ribulose-1, 5 -bisphosphate carboxylase/oxygenase

1. Carbon Fixation • • 6 C intermediate is unstable Immediately splits in half: • forms 2 molecules of 3 -phosphoglycerate

2. Reduction • • 2 ATP needed for this step (per 1 CO 2) Each 3 -phosphoglycerate is phosphorylated • • forms 1, 3 -bisphoglycerate Pair of e- from NADPH reduces each 1, 3 bisphoglycerate to: G 3 P • Reduction of a carboxyl group to a carbonyl •

Crunch the Numbers… • To produce one G 3 P net: • • start with 3 CO 2 (3 C) and 3 Ru. BP (15 C) After fixation/reduction: 6 molec of G 3 P (18 C) • One of these 6 G 3 P (3 C) is a net gain of a carbohydrate • Molec. can exit cycle to be used by plant cell • • Other 5 G 3 P (15 C) must remain in the cycle to regenerate 3 Ru. BP

3. Regeneration of CO 2 • • The 5 G 3 P molecules are rearranged to form 3 Ru. BP molecules 3 molecules of ATP spent (one per Ru. BP) to complete the cycle and prepare for the next

Crunch the Numbers…again • • Net synthesis of 1 G 3 P molecule, Calvin cycle consumes 9 ATP and 6 NAPDH “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 • • C 3 plants (most plants – rice, wheat, soy are examples) use Ru. Bis. CO and end product is G 3 P Stomata closed CO 2 levels drop (consumed by Calvin) • O 2 levels rise (produced by light rxn) • • When O 2 / CO 2 ratio increases, Ru. Bis. CO can add O 2 to Ru. BP

Photorespiration • O 2 + Ru. BP yields 3 C and 2 C pieces (photorespiration) 2 C 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 Ru. Bis. Co can’t (i. e. when stomata are closed)

• Mesophyll cells pump 4 C cmpds to bundle sheath cells BS cells strip a C (as CO 2) and return the 3 C to mesophyll • BS cells then use Ru. Bis. CO to start Calvin Cycle •

• So… Mesophyll cells pump CO 2 into BS cells, so Ru. Bis. CO 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!

CAM Mechanism • 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 & C 4 • Add CO 2 to organic intermediates before entering Calvin In C 4, carbon fixation and Calvin cycle PHYSICALLY (space) separated • In CAM, carbon fixation and Calvin cycle are TEMPORALLY (time) separated •