The anatomical adaptation of the leaf of Nerium (oleander) for xerophytic conditions includes all of the following EXCEPT:
Answer: C
Xerophytes like Nerium have reduced intercellular spaces (not large) to minimize water loss. They have sunken stomata, thick cuticle, and multiple palisade layers.
Q.42Hard
Which statement best describes the relationship between vessel elements and tracheids in angiosperm xylem?
Answer: A
Vessel elements in angiosperms have perforations (lacking end walls) allowing efficient water transport, while tracheids with bordered pits provide both transport and mechanical support.
Q.43Hard
The anatomical basis for the high tensile strength of wood in dicots is primarily due to:
Answer: B
Xylem fibers, which constitute a significant portion of secondary xylem, have thick lignified walls that provide tensile strength to the wood.
Q.44Hard
In the phloem, sieve plate perforations serve the function of:
Answer: A
Sieve plate perforations with reduced cell wall material allow efficient translocation of assimilates between sieve tube elements with minimal resistance.
Q.45Hard
During the development of a dicot leaf, the dorsiventral structure is established with palisade parenchyma on the adaxial surface and spongy mesophyll on the abaxial surface. Which combination of factors is primarily responsible for maintaining this distinct anatomical differentiation?
Answer: A
The dorsiventral structure of dicot leaves is primarily established by differential light exposure (adaxial surface receives more direct light, promoting compact palisade development) combined with auxin gradients that influence cell differentiation patterns. The abaxial surface, receiving diffuse light and lower auxin concentration, develops the loosely packed spongy mesophyll with intercellular spaces for gas exchange.
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Q.46Hard
The ascent of sap in tall trees is best explained by:
Answer: C
The cohesion-tension theory explains water transport in tall trees through cohesive forces between water molecules and tension created by transpiration.
Q.47Hard
Which of the following is a characteristic of phloem transport?
Answer: C
Phloem transport is bidirectional and can move sugars upwards or downwards depending on source-sink relationships and requires metabolic energy.
Q.48Hard
The ratio of photosynthesis to respiration in a plant is represented by:
Answer: D
Apparent photosynthesis (net photosynthesis) represents the actual photosynthesis minus respiration measured as the gaseous exchange in the plant.
Q.49Hard
Which enzyme catalyzes the fixation of CO2 in the Calvin cycle?
Answer: B
RuBisCO catalyzes the carboxylation of RuBP with CO2 to form 3-phosphoglycerate, the first stable product of dark reactions in C3 plants.
Q.50Hard
The process of photorespiration in C3 plants is initiated when:
Answer: C
Photorespiration occurs when RuBisCO catalyzes oxygenation of RuBP instead of carboxylation, which happens under high O2 and low CO2 conditions.
Q.51Hard
The Km value of an enzyme indicates:
Answer: B
Km (Michaelis constant) is the substrate concentration at half Vmax, and it inversely represents enzyme affinity for substrate. Lower Km means higher affinity.
Q.52Hard
During the Z-scheme of photosynthesis, electrons move from PSII to PSI. What happens to electrons after they leave PSI?
Answer: B
In the Z-scheme, electrons from PSI reduce NADP+ to NADPH with the help of ferredoxin and NADP reductase enzyme.
Q.53Hard
In the electron transport chain of photosynthesis, the role of cytochrome b6f complex is to:
Answer: B
The cytochrome b6f complex transfers electrons between PSII and PSI while simultaneously pumping H+ ions into thylakoid lumen for chemiosmosis.
Q.54Hard
Which of the following C3 plants can show characteristics similar to CAM plants under water stress?
Answer: C
Some C3 plants show facultative CAM characteristics under severe drought stress, temporarily shifting to CAM metabolism to conserve water.
Q.55Hard
A researcher observes that a plant's net photosynthesis decreases with increase in temperature above 35°C. This is primarily because:
Answer: B
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.
Q.56Hard
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.57Hard
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.58Hard
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.59Hard
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.60Hard
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.
