Fig Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2 Chloroplast in Mesophyll cells

Mechanisms of Carbon Fixation KEY DIFFERENCE- HOW CO 2 IS CAPTURED

Fig. 10-UN2 Regeneration of CO 2 acceptor 1 G3P (3C) Reduction Carbon fixation 3 CO 2 Calvin Cycle 6  3C 5  3C 3  5C Overview of Calvin Cycle ALL PLANTS UTILIZE CALVIN CYCLE; C3 PLANTS TRANSFER CO 2 DIRECTLY INTO PATHWAY; C4 AND CAM USE CAPTURE CO 2 IN 4-CARBON ACIDS

How Calvin worked out C-3 cycle using C-14 Link to animation Link to animation

Link to McGraw Calvin cycle

Figure The Calvin cycle (Layer 1)

The addition of CO 2 to RuBP to form 3-PGA is catalyzed by the enzyme Rubisco -Very inefficient enzyme -Slow rate means that many copies of enzyme are required in each chloroplast

Figure The Calvin cycle (Layer 1)

Figure The Calvin cycle (Layer 2)

Figure The Calvin cycle (Layer 3)

Link to Smith Calvin cycle animation

Rubisco can also catalyze the addition of O 2 to PGA with disasterous consequences for plants -Rubisco evolved at time in earth’s history in which atmospheric oxygen concentration was very low -No evolutionary pressure to exclude O 2 - Rubisco binds CO 2 more tightly than O 2, but if [O 2 ] is much higher than [CO 2 ], O 2 will out compete CO 2 for active site; also higher temperatures increase relative affinity for O 2. -Transfer of O 2 ultimately causes loss of carbon, instead of gain of carbon in Calvin cycle

Impact of Photorespiration on Calvin cycle

H 2 O escapes from stomata on hot, dry days Hot, dry conditions → Stomata close to reduce H 2 O loss. Stomata closed →[CO 2 ] ↓, [O 2 ]↑ O 2 outcompetes CO 2 to bind to Rubisco O 2 is added to RuBP, siphons C out of Calvin cycle H2OH2O

Link to Boyer photorespiration animation

CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO C4 AND CAM PATHWAYS: ADAPTIONS BY SOME PLANTS TO HOT, DRY CONDITIONS

CAM plants fix carbon in mesophyll cells but only at night ENZYME PEP CATALYZES CAPTURE OF CO 2 AS A FOUR CARBON ORGANIC ACID

CAM plants capture CO 2 as 4 - carbon organic acids, oxaloacetate at night and release CO 2 to Calvin cycle during the day

Fig C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue C4 plants have a different anatomy than C3 plants and capture CO 2 initially not in 3-carbon PGA but rather in 4 –carbon oxaloacetate Note that Calvin cycle takes place in a completely separate cell, the Bundle –sheath cell, than CO 2 capture.

NOTE HOW CO 2 IS CAPTURED AS AN ORGANIC ACID, OXALOACETATE IN REACTION CATALYZED BY ENZYME PEP

Figure C 4 and CAM photosynthesis compared Table Link

Figure A review of photosynthesis

C3 vs C4 plants as a function of [CO 2 ]; C4 plants capture CO 2 with greater efficiency than C3 at low [CO 2 ]; C3 plants are more efficient at higher [CO 2 ] due to C4 plants reaching saturation at lower [CO 2 ]