Govt Exams
Apparent (net) photosynthesis = Gross photosynthesis - Respiration. True (gross) photosynthesis is always greater since some fixed CO2 is released during concurrent respiration.
The correct sequence after PSII photolysis is: Plastoquinone accepts electrons → Cytochrome b6f complex → Plastocyanin → PSI, following the Z-scheme.
During midday, increased light stimulates photosynthesis in guard cells, producing sugars and activating K+ uptake pumps, while ABA levels decrease, both promoting stomatal opening.
The C4 pathway's spatial separation of initial CO2 fixation (PEP carboxylase in mesophyll) and Calvin cycle (in bundle sheath) concentrates CO2 around Rubisco, reducing photorespiration and increasing CO2 fixation efficiency.
At high temperatures, Rubisco and other enzymes denature reducing their activity. Additionally, stomata close to prevent excessive water loss, reducing CO2 availability and thus net photosynthesis.
Some C3 plants show facultative CAM characteristics under severe drought stress, temporarily shifting to CAM metabolism to conserve water.
The cytochrome b6f complex transfers electrons between PSII and PSI while simultaneously pumping H+ ions into thylakoid lumen for chemiosmosis.
In the Z-scheme, electrons from PSI reduce NADP+ to NADPH with the help of ferredoxin and NADP reductase enzyme.
Km (Michaelis constant) is the substrate concentration at half Vmax, and it inversely represents enzyme affinity for substrate. Lower Km means higher affinity.
Photorespiration occurs when RuBisCO catalyzes oxygenation of RuBP instead of carboxylation, which happens under high O2 and low CO2 conditions.