Carotenoids are accessory pigments that absorb light and protect against photoinhibition but do NOT participate directly in CO2 fixation.
In darkness, photosynthesis stops, but aerobic respiration continues, consuming O2 and releasing CO2, resulting in measurable gas exchange.
The proton gradient (pH gradient) is established by water photolysis at PSII and the Q-cycle in cytochrome b6f complex, which pumps H+ into the thylakoid lumen.
CAM plants open stomata at night to fix CO2 as malic acid, stored in vacuoles. During the day, stomata close to reduce water loss while malic acid is decarboxylated for the Calvin cycle.
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.
Photoinhibition occurs when light intensity exceeds the photosynthetic capacity, causing photodamage to PSII reaction center and photodissociation of D1 protein.
Light intensity directly affects photosynthesis but has no effect on respiration, which occurs in both light and dark conditions.