Carbon Fixation & Plant Diversity

Carbon Fixation Reactions Forming organic molecules from CO 2

Calvin Cycle Used by most plants to fix C Occurs in the stroma 13 total reactions broken down into 3 key phases: o CO 2 uptake o C reduction (redox) o RuBP regeneration

CO 2 Uptake CO 2 fixed to 5-C RuBP (ribulose biphosphate) o X 6 Enzyme that catalyzed this is rubisco - most abundant protein on planet o RuBP carboxylase oxygenase o What does this mean?? 6-C product immediately breaks down into 2 3-C phosphoglygerate molecules

Carbon Reduction Energy from ATP and NADPH generated in light reactions needed PGA is converted into G 3 P o For every 6 CO 2 that enter Calvin cycle, 12 G 3 P are created o 2 molecules of G 3 P leave Calvin & react to create glucose or fructose

RuBP Regeneration 10 molecules of G3P are left in Calvin cycle o = 30 total C atoms o 10 reactions rearrange these C into 6 RuBP y/Bio231/calvin.html y/Bio231/calvin.html

In Summary… Inputs into Calvin Cycle: o 6 CO 2 o ATP and NADPH from light reactions Key output: carbohydrate Plants that go through Calvin cycle are called C 3 plants (1 st product is 3-C) o 95% of plants on earth

On hot dry days… Plants close stomata to prevent water loss This means less CO 2 available for photosynthesis O 2 builds up in mesophyll and competes with CO 2 for active site of rubisco o Remember – carboxylase oxygenase

Some intermediates of Calvin degraded into CO 2 and H 2 0 – photorespiration o Occurs in presence of light o Requires O 2 o Results in CO 2 and H 2 0 (same as cellular respiration), but no ATP produced Photorespiration reduces photosynthetic efficiency

C 4 and CAM plants have adaptations to reduce photorespiration

Review of Leaf Cross- Sections: C 3 Leaf C 4 Leaf

C 4 Plants Examples: Sugarcane, corn, crabgrass, sorghum First fix CO 2 into 4-C oxaloacetate o Uses enzyme PEP carboxylase – high affinity for CO 2 o Mesophyll cells o Prior to Calvin Cycle Oxaloacetate, through intermediates, delivers CO 2 to Calvin Cycle o In bundle sheath cells o Requires input of energy to recycle PEP

Pros & Cons In high light areas, photorespiration is negliglible in C 4 plants o Photosynthetic yield is worth expenditure of 30 ATPs In lower light/temperature, C 3 plants are favored

CAM Plants Found in plants in xeric conditions: ex. cacti Only open stomata at night Use PEP carboxylase to fix CO 2 During day, CO 2 is removed from malate o CO 2 then enters Calvin

C 4 vs. CAM plants Both use PEP carboxylase and form 4-C oxaloacetate In C 4 plants – C 4 and C 3 pathways occur in different locations In CAM plants, CAM and C 3 pathways occur at different times