Chapter 10: Photosynthesis Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates.

Dehydration & Tradeoffs with Photosynthesis On hot, dry days, plants conserve water by closing or partially closing their stomata. Unfortunately, this limits CO2 uptake for making sugars through photosynthesis. The closing of stomata also causes O2 released from the light reactions to build up in leaves. These conditions favor a seemingly wasteful process called “photorespiration.”

Photorespiration In most plants, initial fixation of CO2 occurs via rubisco, forming a 3-carbon compound that is used to make sugar. (For this reason, these plants are called “C3” plants.) In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2, forming a two-carbon compound that must exit the chloroplast to be rearranged in peroxisomes and mitochondria.

Photorespiration: Using oxygen and fuel and producing carbon dioxide in the presence of light.

Photorespiration Is Wasteful! Photorespiration consumes O2 and RuBP (fuel) while releasing CO2. However, unlike cellular respiration, it does not produce ATP. Photorespiration inputs into the Calvin cycle only three PGA (3-phosphoglycerate) molecules for each O2 instead of 6 PGA for each CO2.

Photorespiration Is Wasteful! On a hot, dry day photorespiration can drain as much as 50% of the carbon fixed by the Calvin cycle (three PGA instead of six PGA). From our heterotrophic perspective, this is needlessly wasteful. Crop yields would be much higher if photorespiration did not occur. Scientists are trying to bioengineer C3 crops that can minimize photorespiration.

Photorespiration: Evolutionary Baggage? Photorespiration occurs because rubisco has an affinity for both CO2 and O2. Rubisco first evolved at a time when the atmosphere had far less O2 and more CO2. Under these conditions, the inability of rubisco’s active site to exclude O2 made little difference. Modern rubisco retains some of its affinity for O2, making photorespiration inevitable.

C4 and CAM plants have adaptations that minimize photorespiration. Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid C4 Plants Spatial Separation CAM Plants Temporal Separation Bundle- sheath cell CO2 CO2 Day Organic acids release CO2 to Calvin cycle CALVIN CYCLE CALVIN CYCLE Sugar Sugar Spatial separation of steps Temporal separation of steps

C4 Plants Spatially Separate Carbon Fixation and the Calvin Cycle C4 plants minimize photorespiration by incorporating CO2 into 4-carbon compounds in their mesophyll cells. These 4-carbon compounds are then transferred to bundle-sheath cells, where they release CO2 to be fixed by rubisco in the Calvin cycle.

Photosynthetic cells of C4 plant leaf Mesophyll cell Bundle- sheath Vein (vascular tissue) C4 leaf anatomy Stoma Bundle- sheath Pyruvate (3 C) CO2 Sugar Vascular tissue CALVIN CYCLE PEP (3 C) ATP ADP Malate (4 C) Oxaloacetate (4 C) The C4 pathway PEP carboxylase Mesophyll

How do C4 plants minimize photorespiration? In C4 mesophyll cells, PEP carboxylase fixes CO2. PEP carboxylase has no affinity for O2. C4 plants effectively “pump” CO2 into bundle sheath cells, keeping the CO2 level high enough for rubisco to bind CO2 rather than O2. This cyclic series of reactions is essentially a CO2-concentrating pump powered by ATP. Even with stomata partially closed, C4 plants can still produce sugar and avoid protorespiration.

CAM Plants Temporally Separate Carbon Fixation and the Calvin Cycle CAM plants close their stomata during the day and open them at night. At night, CAM plants fix CO2 into organic acids which are stored in their vacuoles. CO2 is then released during the day to be fixed by rubisco in the Calvin cycle.

How do CAM plants minimize photorespiration? In CAM plants, water loss is minimized by having the stomata closed throughout the day. The release of CO2 from organic acids during the day keeps the CO2 levels in the mesophyll cells high, minimizing the fixation of O2 by rubisco. Even with stomata closed during the day, CAM plants can still produce sugar and avoid photorespiration.

Comparison of Alternate Methods of CO2 Fixation