The proton gradient across thylakoid membrane in photosynthesis is maintained primarily by:
AActive transport of H+ ions by ATP
BWater splitting at PSII and electron transport through cytochrome b6f
CCO2 fixation in the stroma
DGlucose breakdown in mitochondria
Correct Answer:
B. Water splitting at PSII and electron transport through cytochrome b6f
EXPLANATION
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.
Which statement about CAM (Crassulacean Acid Metabolism) plants is CORRECT?
AThey open stomata during the day for CO2 uptake
BThey store CO2 as malic acid in vacuoles during the night
CThey are less drought-resistant than C3 plants
DThey have higher transpiration rates than C4 plants
Correct Answer:
B. They store CO2 as malic acid in vacuoles during the night
EXPLANATION
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.
The apparent photosynthesis is ALWAYS greater than true photosynthesis because:
AStomata open wider in light
BApparent photosynthesis includes gross photosynthesis minus respiration
CTrue photosynthesis occurs only in dark
DApparent photosynthesis includes respiration that also consumes CO2
Correct Answer:
B. Apparent photosynthesis includes gross photosynthesis minus respiration
EXPLANATION
Apparent (net) photosynthesis = Gross photosynthesis - Respiration. True (gross) photosynthesis is always greater since some fixed CO2 is released during concurrent respiration.
A plant exhibits high transpiration rate during midday despite adequate soil moisture. Which mechanism BEST explains stomatal opening under these conditions?
ADecreased turgor pressure in guard cells
BAccumulation of potassium ions and organic solutes in guard cells
CReduced ABA concentration in guard cells
DBoth B and C
Correct Answer:
D. Both B and C
EXPLANATION
During midday, increased light stimulates photosynthesis in guard cells, producing sugars and activating K+ uptake pumps, while ABA levels decrease, both promoting stomatal opening.
In the Hatch and Slack pathway, malic acid is synthesized in the mesophyll cells and transported to bundle sheath cells. This adaptation is particularly advantageous because it:
AReduces the photorespiration rate
BIncreases the rate of CO2 uptake
CDecreases the need for light
DBoth A and B
Correct Answer:
D. Both A and B
EXPLANATION
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.
The phenomenon of photoinhibition in plants occurs due to:
AExcess water in soil
BExtremely high light intensity causing damage to PSII
CLow CO2 concentration
DInsufficient chlorophyll production
Correct Answer:
B. Extremely high light intensity causing damage to PSII
EXPLANATION
Photoinhibition occurs when light intensity exceeds the photosynthetic capacity, causing photodamage to PSII reaction center and photodissociation of D1 protein.
