Regarding primary carbon fixation in C₄ mesophyll cells:
I. The primary CO₂ acceptor in mesophyll cells is Phosphoenolpyruvate (PEP), a 3-carbon compound. II. Fixation of CO₂ in mesophyll cells is catalysed by PEP carboxylase (PEPcase). III. Mesophyll cells of C₄ plants contain abundant RuBisCO enzyme. IV. Initial CO₂ fixation in mesophyll cells yields Oxaloacetic acid (OAA).
Which of the statement(s) given above is/are correct?
Statements I, II and IV are correct because PEP accepts CO₂ under the action of PEP carboxylase to produce OAA in mesophyll cells. Statement III is incorrect because mesophyll cells of C₄ plants lack RuBisCO.
Question 92 of 100
📘 CLASS XI
Consider the following statements regarding enzyme distribution in C₄ plants:
I. Mesophyll cells possess PEPcase but lack RuBisCO. II. Bundle sheath cells possess RuBisCO but lack PEPcase. III. The Calvin cycle in C₄ plants takes place exclusively in bundle sheath cells. IV. Primary CO₂ fixation and the Calvin cycle take place simultaneously in mesophyll cells.
Which of the statement(s) given above is/are correct?
Statements I, II and III are correct because C₄ plants exhibit spatial separation of photosynthesis: mesophyll cells contain PEPcase (without RuBisCO) for primary CO₂ fixation, whereas bundle sheath cells contain RuBisCO (without PEPcase) where the Calvin cycle operates. Statement IV is incorrect because the Calvin cycle does not occur in mesophyll cells.
Question 93 of 100
📘 CLASS XI
Regarding transport and decarboxylation steps in the Hatch and Slack pathway:
I. Oxaloacetic acid (OAA) formed in mesophyll cells is converted to malic acid or aspartic acid. II. Malic acid or aspartic acid is transported into bundle sheath cells through plasmodesmata. III. In bundle sheath cells, 4-carbon acids are decarboxylated to release CO₂ and a 3-carbon acid. IV. The 3-carbon acid returns to mesophyll cells where it is converted back to PEP.
Which of the statement(s) given above is/are correct?
All four statements are correct. OAA is converted to malate or aspartate in mesophyll cells, transported to bundle sheath cells, decarboxylated to release CO₂ and a 3-carbon acid (pyruvate), which returns to mesophyll cells for regeneration of PEP.
Question 94 of 100
📘 CLASS XI
Consider the following statements regarding RuBisCO dual activity:
I. RuBisCO can bind to both CO₂ and O₂ at its active site. II. RuBisCO has a significantly higher affinity for CO₂ than for O₂. III. Carboxylation versus oxygenation depends on the relative concentrations of O₂ and CO₂ at the enzyme site. IV. In C₃ plants, when O₂ binds to RuBisCO, RuBP is converted into two molecules of phosphoglycolate.
Which of the statement(s) given above is/are correct?
Statements I, II and III are correct because RuBisCO binds both CO₂ and O₂ competitively and has a higher affinity for CO₂. Statement IV is incorrect because oxygenation of RuBP yields one molecule of 3-PGA and one molecule of phosphoglycolate.
Question 95 of 100
📘 CLASS XI
Regarding the photorespiratory pathway in C₃ plants:
I. Photorespiration involves the binding of oxygen to RuBisCO. II. Photorespiration leads to the net synthesis of ATP and NADPH. III. Photorespiration results in the release of CO₂ with the utilization of ATP. IV. Photorespiration produces neither sugar nor ATP.
Which of the statement(s) given above is/are correct?
Statements I, III and IV are correct because photorespiration begins with oxygenation by RuBisCO, releases CO₂, consumes ATP, and produces neither ATP nor sugars. Statement II is incorrect because ATP and NADPH are not synthesized during photorespiration.
Question 96 of 100
📘 CLASS XI
Consider the following statements regarding the absence of photorespiration in C₄ plants:
I. Photorespiration is absent in C₄ plants because they possess a mechanism that increases CO₂ concentration at the enzyme site. II. Decarboxylation of C₄ acid in bundle sheath cells elevates intracellular CO₂ level around RuBisCO. III. High concentration of CO₂ ensures that RuBisCO functions as a carboxylase, suppressing oxygenase activity. IV. The absence of photorespiration contributes to higher photosynthetic productivity and yields in C₄ plants.
Which of the statement(s) given above is/are correct?
All four statements are correct. C₄ plants concentrate CO₂ around RuBisCO by decarboxylating C₄ acids in bundle sheath cells, thereby suppressing oxygenase activity, preventing photorespiration, and increasing productivity.
Question 97 of 100
📘 CLASS XI
Regarding Blackman’s Law of Limiting Factors (1905):
I. Blackman proposed that when a biological process is controlled by multiple factors, its rate is governed by the factor closest to its minimal value. II. A limiting factor is one that directly affects the process if its quantity is changed. III. If a leaf is exposed to optimal light and CO₂ levels, photosynthesis will proceed at maximum rate regardless of temperature. IV. Light, CO₂ concentration, temperature, and water can all act as limiting factors for photosynthesis.
Which of the statement(s) given above is/are correct?
Statements I, II and IV are correct because Blackman's Law states that the factor nearest its minimum limits the rate of a process. Statement III is incorrect because temperature can become limiting even when light and CO₂ are optimal.
Question 98 of 100
📘 CLASS XI
Consider the following statements concerning light intensity as a factor affecting photosynthesis:
I. At low light intensities, there is a linear relationship between incident light and CO₂ fixation rates. II. Light saturation occurs at approximately 10% of full sunlight. III. Beyond light saturation point, further increases in incident light cause chlorophyll breakdown (photo-oxidation). IV. Except for plants in shade or dense forests, light is rarely a limiting factor in nature.
Which of the statement(s) given above is/are correct?
All four statements are correct. Photosynthesis increases linearly with low light, saturates at about 10% of full sunlight, excessive light causes photo-oxidation, and light is seldom limiting in nature except under shaded conditions.
Question 99 of 100
📘 CLASS XI
Regarding carbon dioxide concentration as a limiting factor for C₃ and C₄ plants:
I. Carbon dioxide is the major limiting factor for photosynthesis in nature. II. Atmospheric CO₂ concentration is very low, between 0.03% and 0.04%. III. C₄ plants show saturation at lower CO₂ levels (approximately 360 µL L⁻¹) than C₃ plants (greater than 450 µL L⁻¹). IV. C₃ plants respond to higher CO₂ concentration by showing increased photosynthetic rates, a fact utilized in greenhouse crop production.
Which of the statement(s) given above is/are correct?
All four statements are correct. Atmospheric CO₂ is naturally low, C₄ plants saturate at lower CO₂ concentrations than C₃ plants, and CO₂ enrichment is used in greenhouses to improve yields of C₃ crops.
Question 100 of 100
📘 CLASS XI
Consider the following statements regarding the effects of temperature and water stress on photosynthesis:
I. The dark reactions, being enzymatic, are temperature-controlled to a greater extent than light reactions. II. C₄ plants respond to higher temperatures and have a higher temperature optimum than C₃ plants. III. Water stress affects photosynthesis directly by altering the primary structure of RuBisCO. IV. Water stress causes stomatal closure (limiting CO₂ availability) and leaf wilting (reducing surface area and metabolic activity).
Which of the statement(s) given above is/are correct?
Statements I, II and IV are correct because dark reactions are enzyme-mediated and temperature-sensitive, C₄ plants have higher temperature optima, and water stress reduces photosynthesis indirectly through stomatal closure and wilting. Statement III is incorrect because water stress does not directly alter the primary structure of RuBisCO.
