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.
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.
A researcher observes that a plant's net photosynthesis decreases with increase in temperature above 35°C. This is primarily because:
AChlorophyll is denatured
BRubisco enzyme activity decreases due to denaturation and stomatal closure to prevent water loss
CThylakoids are destroyed
DPhotolysis of water stops
Correct Answer:
B. Rubisco enzyme activity decreases due to denaturation and stomatal closure to prevent water loss
EXPLANATION
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.
