AP Biology: Unit 3, Energy, ATP & Free Energy (Drill 8)

Question 1. At 0 lux light intensity, the net oxygen production is -3.2 micromol/g/min. Which of the following best explains why the value is negative under these conditions?

• A) The plant is releasing carbon dioxide at a higher rate than it is consuming oxygen in the light reactions.
• B) In the absence of light, the plant switches entirely to an oxygen-consuming fermentation pathway running in the chloroplast stroma under these conditions.
• C) Photorespiration occurs at low light intensity, consuming oxygen without producing usable energy.
• D) In the absence of light, photosynthesis cannot occur, so only cellular respiration proceeds, consuming oxygen and releasing carbon dioxide. ✓

Explanation: Correct: (D) In the absence of light, the light-dependent reactions of photosynthesis cannot proceed. The plant still requires energy for cellular maintenance, so cellular respiration continues, consuming oxygen and releasing carbon dioxide. The net oxygen production value of -3.2 micromol/g/min represents the rate of cellular respiration alone, because gross photosynthesis is zero when no light is available. This value is important because it establishes the baseline respiration rate used to calculate gross photosynthesis at other light intensities. Incorrect: (A) This statement conflates two separate processes. At 0 lux the plant is consuming oxygen through respiration, not releasing it. Carbon dioxide release is a product of respiration, but the probe measures oxygen, not CO2. Incorrect: (B) Fermentation is an anaerobic pathway that does not consume oxygen; it occurs when oxygen is absent. The plant is in an oxygenated aquatic environment and is performing aerobic cellular respiration, not fermentation. Incorrect: (C) Photorespiration involves the oxygenase activity of RuBisCO and occurs when light is present and O2 concentration is high relative to CO2. At 0 lux there is no light-driven photosynthesis, so photorespiration cannot take place.

Question 2. The light compensation point is the light intensity at which the rate of gross photosynthesis exactly equals the rate of cellular respiration, resulting in no net change in oxygen. Based on the data, which of the following correctly identifies the light compensation point and explains its significance?

• A) The light compensation point is at 1,000 lux, because this is the first intensity at which net oxygen production is positive, indicating photosynthesis exceeds respiration.
• B) The light compensation point is at approximately 500 lux, because net oxygen production equals zero at this intensity, meaning gross photosynthesis rate equals the cellular respiration rate. ✓
• C) The light compensation point cannot be determined from this data because the net oxygen production values are not symmetric around zero.
• D) The light compensation point is at 0 lux, because this is the baseline condition from which all oxygen production begins and net O2 first becomes measurable as light increases in this context.

Explanation: Correct: (B) The light compensation point is the specific intensity at which net O2 production = 0, meaning gross photosynthesis produces oxygen at the same rate that cellular respiration consumes it. The data show net O2 = 0.0 micromol/g/min at exactly 500 lux, making 500 lux the light compensation point. At any intensity below 500 lux, respiration exceeds photosynthesis and net O2 is negative. Above 500 lux, photosynthesis exceeds respiration and net O2 is positive. Incorrect: (A) At 1,000 lux, net O2 is +4.1, meaning photosynthesis already substantially exceeds respiration. The compensation point is the crossover from negative to zero, which the data show at 500 lux, not 1,000 lux. Incorrect: (C) The compensation point is identified directly from where net O2 = 0. Symmetry of values around zero is irrelevant. The data provide this crossover value directly at 500 lux. Incorrect: (D) At 0 lux, net O2 = -3.2, meaning oxygen is being consumed, not produced. This is the maximum respiration rate with zero photosynthesis, the opposite of the compensation point.

Question 3. The data show that net oxygen production increases substantially between 500 and 2,000 lux, but increases only slightly between 2,000 and 4,000 lux (from 7.8 to 8.1 micromol/g/min). Which of the following best explains why increasing light intensity beyond 2,000 lux produces diminishing gains in oxygen production?

• A) At high light intensities, the plant increases its rate of cellular respiration to process the additional energy, consuming most of the newly produced oxygen.
• B) At high light intensities, photoinhibition damages the photosystems faster than they can be repaired, causing the absolute rate of oxygen production to decrease above 2,000 lux.
• C) At high light intensities, a factor other than light, most likely the availability of CO2 or the capacity of the Calvin cycle, becomes the limiting factor for the overall rate of photosynthesis. ✓
• D) At high light intensities, chlorophyll molecules become saturated with photons and begin reflecting light rather than absorbing it, preventing further increases in the reaction rate at the light intensities described.

Explanation: Correct: (C) The rate of photosynthesis is limited by whichever factor is in shortest supply relative to demand. At low light intensities, light is the limiting factor. As light intensity increases, light is no longer limiting and the bottleneck shifts to another step, most likely the availability of CO2 at the fixed concentration used in this experiment, or the capacity of Calvin cycle enzymes (particularly RuBisCO) to process carbon. At high light intensities, adding more light cannot increase the overall rate further because a downstream factor has become limiting. Incorrect: (A) Cellular respiration does not substantially increase in response to high light intensity in this scenario. The respiration rate is established as 3.2 micromol/g/min at 0 lux. Increased light does not drive proportional increases in respiration that would offset photosynthetic gains. Incorrect: (B) While photoinhibition is a real phenomenon at very high light intensities, the data do not show a decrease in net O2 at 4,000 lux; they show a near-plateau. Photoinhibition would cause a measurable decline, not simply a leveling off. The plateau is more parsimoniously explained by a downstream limiting factor. Incorrect: (D) Chlorophyll does not begin reflecting light in response to saturation. Chlorophyll molecules can become saturated in the sense that they absorb photons faster than the reaction center can process them, but this is a reaction-center processing limit, not a reflection phenomenon.

Question 4. Using the 0 lux data point, a student claims that the gross rate of photosynthesis at 500 lux is 3.2 micromol O2/g/min. Which of the following best evaluates this claim?

• A) The claim is correct, because at 500 lux net O2 = 0, meaning gross photosynthesis exactly offsets the respiration rate established at 0 lux (3.2 micromol/g/min), so gross photosynthesis = 3.2 micromol/g/min. ✓
• B) The claim is incorrect, because gross photosynthesis at 500 lux equals net O2 production (0.0) minus the respiration rate (3.2), which yields a value of exactly zero rather than 3.2.
• C) The claim cannot be evaluated without knowing the exact CO2 concentration used in the experiment, because gross photosynthesis depends directly on CO2 availability.
• D) The claim is incorrect, because gross photosynthesis at 500 lux must be greater than 3.2 micromol/g/min to account for the extra oxygen needed to replace cellular losses in plant tissues.

Explanation: Correct: (A) The relationship is: Net O2 = Gross photosynthesis minus Cellular respiration. At 0 lux, gross photosynthesis = 0, so net O2 = -3.2, establishing that the respiration rate = 3.2 micromol/g/min. At 500 lux, net O2 = 0, so: 0 = Gross photosynthesis minus 3.2, therefore Gross photosynthesis = 3.2 micromol/g/min. The claim correctly applies this identity. The key assumption, that cellular respiration rate remains constant across light intensities, is standard for this type of analysis and is supported by the controlled experimental design. Incorrect: (B) This applies the formula incorrectly. Gross photosynthesis = Net O2 plus Respiration rate = 0.0 plus 3.2 = 3.2 micromol/g/min (positive). The correct operation uses the signed respiration rate as a positive number to add back, not a negative number to subtract. Incorrect: (C) CO2 concentration affects the maximum gross photosynthesis rate, but the data at 500 lux already tell us the net rate. The student's claim is evaluable from the data as provided, without additional information about CO2. Incorrect: (D) This introduces a correction factor with no basis in the data or in the standard framework for analyzing net versus gross photosynthesis. The standard calculation is Net O2 = Gross photosynthesis minus Respiration, with no additional adjustment.

Question 5. The experiment was conducted at a constant CO2 concentration. If the same experiment were repeated with CO2 concentration doubled while all other conditions remained identical, which of the following best predicts the effect on net oxygen production at 4,000 lux?

• A) Net oxygen production at 4,000 lux would decrease because higher CO2 concentrations inhibit the light reactions by competing with water for binding sites in Photosystem II.
• B) Net oxygen production at 4,000 lux would remain unchanged because light intensity, not CO2 concentration, is always the primary limiting factor for photosynthesis at high light levels.
• C) Net oxygen production at 4,000 lux would decrease because higher CO2 promotes photorespiration, which consumes oxygen in the peroxisome without producing any useful carbon fixation products.
• D) Net oxygen production at 4,000 lux would increase because the additional CO2 would relieve the Calvin cycle limitation, allowing the light reactions to drive more O2 production before a new plateau is reached. ✓

Explanation: Correct: (D) The near-plateau at high light intensities (7.8 to 8.1 between 2,000 and 4,000 lux) suggests that a downstream process, most likely CO2 availability at the fixed concentration used or Calvin cycle enzyme capacity, has become limiting rather than light itself. Doubling CO2 concentration would increase substrate availability for RuBisCO, allowing the Calvin cycle to process ATP and NADPH from the light reactions at a higher rate. This would relieve the downstream bottleneck, and net O2 production would increase before reaching a new plateau at the higher CO2 level. Incorrect: (A) CO2 does not compete with water for binding sites in Photosystem II. Water is oxidized at the oxygen-evolving complex of Photosystem II to release electrons, protons, and O2. CO2 is used by RuBisCO in the stroma as the carbon fixation substrate, a completely separate location and reaction. Incorrect: (B) This answer correctly identifies that a downstream process is limiting at 4,000 lux, but draws the wrong conclusion. If CO2 availability or Calvin cycle capacity is the bottleneck, increasing CO2 would relieve that limitation and allow greater O2 production. Light intensity is not always the primary limiting factor, the identity shifts depending on which resource is in shortest supply. Incorrect: (C) This reverses the effect of CO2 on photorespiration. Higher CO2 concentration suppresses photorespiration by outcompeting O2 for the active site of RuBisCO. It is lower CO2 relative to O2 that promotes photorespiration. Doubling CO2 would decrease photorespiration, not increase it.