Dark Reactions

Cyclic photosphorylation generates ATP PQ PSIIPSI Stromal side Thylakoid space side Fd P700

Non-Cyclic photosphorylation generates ATP and NADPH PQ PSII PSI Stromal side Thylakoid space side Fd NADP+ NADPH Q P700P680 H 2 0 2H+ & 2e- & 1/2 O 2

Light-independent reactions (Dark Reactions)

The Calvin/Benson/C3 cycle Has 3 phases: Has 3 phases: 1. carbon fixation phase carbon fixation phase carbon fixation phase 2. reduction of CO2 phase reduction of CO2 phase reduction of CO2 phase 3. regeneration of RuBP phase regeneration of RuBP phase regeneration of RuBP phase

carbon fixation phase

each CO 2 molecule is attached to a five- carbon sugar, ribulose bisphosphate (RuBP) each CO 2 molecule is attached to a five- carbon sugar, ribulose bisphosphate (RuBP) catalyzed by RuBP carboxylase or rubisco catalyzed by RuBP carboxylase or rubisco six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate

reduction of CO2 phase

each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3 bisphosphoglycerate. each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3 bisphosphoglycerate. pair of electrons from NADPH reduces,Gain, each 1,3 bisphosphoglycerate to G3P. pair of electrons from NADPH reduces,Gain, each 1,3 bisphosphoglycerate to G3P. – Electrons from NADPH change a carboxyl group to a carbonyl group.

regeneration of RuBP phase

regeneration of the CO 2 acceptor (RuBP), these five G3P molecules are rearranged to form 3 RuBP molecules. regeneration of the CO 2 acceptor (RuBP), these five G3P molecules are rearranged to form 3 RuBP molecules. cycle must spend 3 more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next. cycle must spend 3 more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next.

Hatch & Slack/C4 cycle Inefficient as RUBISCO can act as both a carboxylase and as an oxygenase. Inefficient as RUBISCO can act as both a carboxylase and as an oxygenase. Oxygenase activity leads to loss of carbon that has already been fixed. Oxygenase activity leads to loss of carbon that has already been fixed. Some plants have an alternative pathway where CO 2 is first fixed into C4 organic acids (C4 pathway) and then liberated later to undergo the Calvin/Benson cycle (C3 metabolism). Some plants have an alternative pathway where CO 2 is first fixed into C4 organic acids (C4 pathway) and then liberated later to undergo the Calvin/Benson cycle (C3 metabolism).

2 types of C4 cycle: 2 types of C4 cycle: 1. C3 and C4 metabolism are separated in space (different cells) e.g. sugar cane Characteristic aggregation of cells around the vascular bundles - Krantz morphology (C4 cycle) C4 cycleC4 cycle 2. C3 and C4 metabolism are separated in time (same cell=mesophyl) e.g. pineapple (CAM cycle). CAM cycleCAM cycle

Comparison of C3 & C4 leaves C3 - note the lack of chloroplasts in the bundle sheath C3 - note the lack of chloroplasts in the bundle sheath C4 - note the extensive chloroplasts in the bundle sheath C4 - note the extensive chloroplasts in the bundle sheath

Separation in space (C4 cycle) Vascular tissue Malat Bundle sheath cell Mesophyll cell surrounding bundle sheath CO 2 PEP (3C) C3 CO 2 OAA (4C) Pyruvate (C3) ATP ADP sugar

Separation in time (CAM cycle) Mesophyl Malat In the light In the dark CO 2 PEP (3C) C3 CO 2 OAA (4C) Pyruvate (C3) ATP ADP sugar

Comparison of C3 & C4 leaves

Details of C4 cycle

Details of CAM cycle