Questions & explanations
1. According to Blackman's law of limiting factors, which of the following statements is correct?
- A. The rate of photosynthesis is limited by the factor present in the maximum amount.
- B. The rate of photosynthesis is limited by the factor present in the minimum amount relative to the requirement.
- C. All factors affect photosynthesis equally at any given time.
- D. The limiting factor is always carbon dioxide.
Answer: B. The rate of photosynthesis is limited by the factor present in the minimum amount relative to the requirement.
Blackman's law of limiting factors states that when a process is affected by multiple factors, its rate is limited by the factor that is in the shortest supply (minimum) relative to the requirement. Option A is incorrect because the factor in maximum amount is not limiting. Option C is incorrect because factors do not affect equally; the most deficient one limits the rate. Option D is incorrect because the limiting factor can vary (e.g., light, CO2, temperature) depending on conditions.
2. In many natural conditions, which of the following is often the major limiting factor for photosynthesis?
- A. Light intensity
- B. Carbon dioxide concentration
- C. Oxygen concentration
- D. Soil pH
Answer: B. Carbon dioxide concentration
Carbon dioxide is a key raw material for photosynthesis, and its atmospheric concentration (0.03-0.04%) is often the limiting factor, especially in bright light and optimal temperatures. Light intensity (A) can be limiting in shade but not generally the major limiting factor in many conditions. Oxygen concentration (C) affects photorespiration but is not a major limiting factor. Soil pH (D) affects mineral availability but not directly photosynthesis rate.
3. When light intensity is high and temperature is optimal, but the rate of photosynthesis is still low, the most likely limiting factor is:
- A. Carbon dioxide concentration.
- B. Oxygen concentration.
- C. Water availability.
- D. Chlorophyll content.
Answer: A. Carbon dioxide concentration.
Under high light and optimal temperature, CO2 is often the limiting factor because its atmospheric concentration is low. Oxygen concentration (B) affects photorespiration but is not typically limiting. Water availability (C) could be limiting if there is drought, but the question implies optimal temperature, so water is likely adequate. Chlorophyll content (D) is rarely limiting in healthy plants.
4. Water stress reduces photosynthesis primarily because:
- A. Water is a direct reactant in the light reactions.
- B. Stomata close to conserve water, reducing CO2 uptake.
- C. Chlorophyll degrades in dry conditions.
- D. RuBisCO is inactivated by low water potential.
Answer: B. Stomata close to conserve water, reducing CO2 uptake.
Under water stress, plants close stomata to reduce transpiration, which also limits CO2 entry into leaves, thus reducing photosynthesis. While water is a reactant (A), the amount used in photosynthesis is negligible compared to transpiration, so the primary effect is via stomatal closure. Chlorophyll degradation (C) is not immediate. RuBisCO inactivation (D) is not the primary effect.
5. In a graph of photosynthesis rate versus light intensity at two different CO2 concentrations (low and high), which of the following is observed?
- A. The initial slope is steeper at low CO2 concentration.
- B. The light saturation point is higher at high CO2 concentration.
- C. The maximum photosynthesis rate is the same regardless of CO2 concentration.
- D. At very low light, CO2 concentration has a large effect on the rate.
Answer: B. The light saturation point is higher at high CO2 concentration.
At high CO2 concentration, photosynthesis can continue to increase with light intensity to a higher level before saturation, so the light saturation point is higher. The initial slope (A) is determined by light reactions and is similar at different CO2 levels. Maximum rate (C) is higher at high CO2. At very low light (D), light is limiting, so CO2 concentration has little effect.
6. As light intensity increases, the rate of photosynthesis initially increases linearly, but at high light intensities it plateaus. This plateau is called the:
- A. Light compensation point.
- B. Light saturation point.
- C. CO2 compensation point.
- D. Optimum temperature point.
Answer: B. Light saturation point.
The light saturation point is the light intensity beyond which further increase in light does not increase the rate of photosynthesis, due to other factors becoming limiting. The light compensation point (A) is where photosynthesis equals respiration. CO2 compensation point (C) is related to CO2 concentration. Optimum temperature point (D) is for temperature.
7. Which of the following statements about the effect of temperature on photosynthesis is correct?
- A. C3 plants have a higher optimum temperature for photosynthesis than C4 plants.
- B. C4 plants have a higher optimum temperature for photosynthesis than C3 plants.
- C. Both C3 and C4 plants have the same optimum temperature around 25°C.
- D. Temperature has no effect on photosynthesis as long as water is available.
Answer: B. C4 plants have a higher optimum temperature for photosynthesis than C3 plants.
C4 plants are adapted to hot climates and have an optimum temperature around 30-45°C, whereas C3 plants have a lower optimum around 20-30°C. Option A is opposite. Option C is incorrect because they differ. Option D is incorrect because temperature affects enzyme activity and thus photosynthesis.
8. In the C4 pathway (Hatch and Slack cycle), the first stable product of carbon fixation is:
- A) 3-phosphoglycerate (PGA)
- B) Oxaloacetate (OAA)
- C) Ribulose-1,5-bisphosphate (RuBP)
- D) Malate
Answer: B) Oxaloacetate (OAA)
In C4 plants, CO2 is initially fixed by PEP carboxylase in mesophyll cells to form oxaloacetate (OAA), a 4-carbon compound. PGA is the first stable product in C3 plants, RuBP is the CO2 acceptor in C3, and malate is a reduced form of OAA but not the first product.
9. Which of the following correctly compares C3, C4, and CAM plants?
- A) C4 plants have the highest photorespiration rate
- B) CAM plants fix CO2 only during the day
- C) C3 plants have a lower CO2 compensation point than C4 plants
- D) C4 plants have Kranz anatomy, while CAM plants have temporal separation of carbon fixation
Answer: D) C4 plants have Kranz anatomy, while CAM plants have temporal separation of carbon fixation
C4 plants use spatial separation (Kranz anatomy) to concentrate CO2, while CAM plants use temporal separation (night fixation, day decarboxylation). C4 plants have lower photorespiration and lower CO2 compensation point than C3 plants. CAM plants fix CO2 at night.
10. In CAM plants, stomata open at night to fix CO2 into:
- A) Oxaloacetate, which is then converted to malic acid and stored in vacuoles
- B) 3-phosphoglycerate, which enters the Calvin cycle immediately
- C) RuBP, which is stored until daytime
- D) Glucose, which is used for respiration
Answer: A) Oxaloacetate, which is then converted to malic acid and stored in vacuoles
CAM plants fix CO2 at night via PEP carboxylase into oxaloacetate, which is reduced to malate and stored in vacuoles. During the day, malate is decarboxylated to release CO2 for the Calvin cycle, allowing stomata to remain closed to conserve water.
11. Photorespiration in C3 plants is initiated by the oxygenase activity of RuBisCO, leading to the formation of:
- A) 3-phosphoglycerate and glycolate
- B) 2-phosphoglycolate and 3-phosphoglycerate
- C) Oxaloacetate and malate
- D) Glycolate and serine
Answer: B) 2-phosphoglycolate and 3-phosphoglycerate
When RuBisCO uses O2 instead of CO2, it produces one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate. The glycolate pathway then converts 2-phosphoglycolate, leading to loss of fixed carbon and no ATP/NADPH gain.
12. Which of the following is an advantage of C4 plants over C3 plants?
- A) Higher rate of photorespiration
- B) Higher CO2 compensation point
- C) Efficient photosynthesis at high temperature and light intensity
- D) Better adaptation to cold climates
Answer: C) Efficient photosynthesis at high temperature and light intensity
C4 plants have a mechanism to concentrate CO2, which suppresses photorespiration and allows efficient photosynthesis under high temperature and light. They have a lower CO2 compensation point and are adapted to hot, dry environments.