11.4: Innovations in Photosynthesis

photorespiration

rubisco fixes oxygen instead of CO2 to RuBP to form 2-phosphoglycolate = decreases photosynthesis

Rubisco’s affinity for CO2 and O2

low O2 and high CO2 within leaf = carboxylase favoured

high O2 and low CO2 within leaf = oxygenase favoured

when is oxygenase activity typically preferred

On hot dry days where stomata are closed to prevent water loss = prevents gases from entering and leaving = CO2 decreases as used in photosynthesis and increases O2 = oxygenase activity is favoured

C3 plants

uses Calvin cycle directly with carbon fixation done by rubisco in mesophyll cells. more efficient in cooler, moist environments as prone to photorespiration in warmer conditions

C3 leaf structure

high density of mesophyll cells with rubisco where CO2 fixation occurs and has bundle sheath cells with few chloroplasts and little rubisco so don’t fix CO2

C4

Have a pre-Calvin cycle initial carbon fixation by PEP carboxylase in mesophylls. Calvin cycle with Rubisco then occurs in bundle sheath cells = minimises photorespiration by concentrating CO2 around rubisco = suited for hot dry conditions

C4 process

PEP carboxylase in mesophyll cells fixes carbon to form oxaloacetate then converted into malate. Diffuses into bundle sheath cells which have modified chloroplasts that concentrate CO2 around rubisco. Malate decarboxylated into pyruvate and CO2 - pyruvate returns to mesophyll to regenerate PEP and CO2 used in Calvin = 2 cells involved

PEP carboxylase advantages over rubisco

no oxygenase activity and fixes CO2 even at very low CO2 levels = enhances photosynthetic efficiency in C4 plants.

Crassulacean acid metabolism (CAM)

carbon fixation at night with CO2 stored as organic acid then used during day in Calvin cycle. Similar to C4 but carbon fixation occurs in same cell but at different times

which plants better in cold conditions

C3

which plants are better in warm climates

C4