In C4 photosynthesis, the primary CO2 acceptor molecule is:
Answer: B
In C4 plants, CO2 first combines with PEP (3-carbon molecule) to form oxaloacetate (4-carbon), which is why it's called C4 photosynthesis.
Q.242Easy
Which hormone is primarily responsible for closing stomata under water stress conditions?
Answer: B
ABA accumulates in guard cells during water stress and causes stomatal closure by reducing turgor pressure in guard cells.
Q.243Medium
The compensation point in photosynthesis is defined as the light intensity at which:
Answer: B
At compensation point, the rate of CO2 fixation in photosynthesis equals the rate of CO2 release in respiration, resulting in net zero gas exchange.
Q.244Medium
Which of the following statements about the light reaction of photosynthesis is INCORRECT?
Answer: C
Light reactions do not require CO2. CO2 is used in the dark reaction (Calvin cycle). Light reactions produce ATP, NADPH, and O2.
Q.245Medium
In the process of guttation, water is expelled through:
Answer: B
Guttation occurs through specialized pores called hydathodes located at the leaf margins, allowing liquid water to be exuded when root pressure exceeds transpiration.
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Q.246Medium
The quantum yield of photosynthesis refers to:
Answer: B
Quantum yield represents the photosynthetic efficiency, defined as the number of CO2 molecules fixed per photon absorbed (typically 8-10 photons per CO2).
Q.247Medium
Which of the following affects the rate of photosynthesis but NOT the rate of respiration?
Answer: A
Light intensity directly affects photosynthesis but has no effect on respiration, which occurs in both light and dark conditions.
Q.248Medium
The phenomenon of photoinhibition in plants occurs due to:
Answer: B
Photoinhibition occurs when light intensity exceeds the photosynthetic capacity, causing photodamage to PSII reaction center and photodissociation of D1 protein.
Q.249Hard
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:
Answer: D
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.
Q.250Hard
A plant exhibits high transpiration rate during midday despite adequate soil moisture. Which mechanism BEST explains stomatal opening under these conditions?
Answer: D
During midday, increased light stimulates photosynthesis in guard cells, producing sugars and activating K+ uptake pumps, while ABA levels decrease, both promoting stomatal opening.
Q.251Hard
Which of the following correctly represents the sequence of electron carriers in the electron transport chain of photosynthesis (from PSII)?
Answer: A
The correct sequence after PSII photolysis is: Plastoquinone accepts electrons → Cytochrome b6f complex → Plastocyanin → PSI, following the Z-scheme.
Q.252Hard
The apparent photosynthesis is ALWAYS greater than true photosynthesis because:
Answer: B
Apparent (net) photosynthesis = Gross photosynthesis - Respiration. True (gross) photosynthesis is always greater since some fixed CO2 is released during concurrent respiration.
Q.253Medium
Which statement about CAM (Crassulacean Acid Metabolism) plants is CORRECT?
Answer: B
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.
Q.254Hard
The proton gradient across thylakoid membrane in photosynthesis is maintained primarily by:
Answer: B
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.
Q.255Easy
A student observes that a plant kept in a completely dark room still shows some gas exchange. This can be attributed to:
Answer: B
In darkness, photosynthesis stops, but aerobic respiration continues, consuming O2 and releasing CO2, resulting in measurable gas exchange.
Q.256Medium
The role of carotenoids in photosynthesis includes all EXCEPT:
Answer: C
Carotenoids are accessory pigments that absorb light and protect against photoinhibition but do NOT participate directly in CO2 fixation.
Q.257Medium
In plants experiencing drought stress, stomatal closure occurs primarily due to a decrease in:
Answer: B
During drought, water loss causes cells to lose turgor pressure. ABA accumulation further reduces guard cell turgor by opening K+ efflux channels, closing stomata.
Q.258Easy
During photosynthesis, the splitting of water molecules occurs in which compartment of the chloroplast?
Answer: B
Water photolysis (photosystem II) occurs in the thylakoid lumen where oxygen is released as a byproduct.
Q.259Medium
Which of the following is the correct sequence of electron carriers in the light-dependent reactions?
Answer: A
The correct electron flow is: PSII (P680) → Plastoquinone → Cytochrome b6f complex → Plastocyanin → PSI (P700) → NADP+ reductase.
Q.260Medium
At what light intensity does compensation point occur in a typical C3 plant?
Answer: A
The light compensation point for C3 plants typically occurs at 50-100 μmol/m²/s where photosynthesis equals respiration rate.
