11.4: Innovations in Photosynthesis

photorespiration

a process occurring in plants where the enzyme RuBisCO fixes oxygen instead of carbon dioxide, leading to a decrease in photosynthetic efficiency.

how is phosphoglycerate eliminated

from the cycle during photorespiration, it is converted into glycolate which is subsequently processed and released.

Rubisco’s affinity for CO2 and O2

higher affinity for CO2 = CO2 fixation preferred even if CO2 concentration in air is lower than 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

those that use the C3 carbon fixation pathway as the sole mechanism for photosynthesis which utilizes RuBisCO to convert CO2 into three-carbon compounds, primarily phosphoglycerate. C3 plants are typically more efficient in cooler, moist environments.

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

initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells which allows C4 plants to efficiently fix CO2 even in hot, dry conditions, reducing photorespiration.

C4 leaf structure

mesophyll cells contain PEP carboxylase which catalyses reduction of CO2 and PEP to form oxaloacetate which is converted into malate then diffused to bundle sheath cells which have modified chloroplasts that concentrate CO2 around rubisco. Malate is decarboxylated to pyruvate and CO2 and pyruvate moves back into mesophyll to regenerate PEP = 2 cells are involved in reactions

PEP carboxylase advantages over rubisco

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

Crassulacean acid metabolism (CAM)

A photosynthetic adaptation where carbon fixation occurs at night, incorporating CO2 into organic acids, which are then used during the day in the Calvin cycle. Similar to C4 but  in C4 initial steps of carbon fixation are separated structurally but in CAM the 2 steps occur within the same cell but separate times

which plants better in cold conditions

C3 plants have an advantage as they don't expect energy concentrating CO2 around rubisco

which plants are better in warm climates

C4 have an advantage as they cause photorespiration doesn't occur and photosynthesis rates don't fall