- A student designs an experiment to measure the effect of light wavelength on the photosynthetic rate of the aquatic plant Elodea by counting the number of oxygen bubbles released per minute. Which of the following represents the most significant source of error specifically related to this method of data collection?
Answer: The volume of oxygen in each bubble can vary, so bubble count is an imprecise measure of the actual volume of gas produced.
While controlling temperature (D) and accounting for respiration (A) are important for experimental design, the most significant flaw in the measurement technique of counting bubbles is its imprecision. The size of bubbles is not uniform, meaning that counting 10 small bubbles represents a different volume of oxygen than counting 10 large bubbles. A more accurate method would be to collect the gas produced and measure its volume directly.
- Water absorbs red and blue wavelengths of light more strongly than green wavelengths. What is the most likely consequence of this physical property for photosynthetic life in a deep, clear ocean?
Answer: Organisms at greater depths possess accessory pigments that are efficient at absorbing green light.
Because red and blue light are filtered out in the upper layers of water, the light that penetrates to deeper zones is enriched in green wavelengths. Organisms that can survive at these depths have evolved accessory pigments, such as phycoerythrin in red algae, which absorb green light efficiently and pass the energy to chlorophyll for photosynthesis. This allows them to exploit the available light spectrum.
- A scientist predicts that rising atmospheric CO₂ levels will cause a sustained, proportional increase in global photosynthesis rates. Which statement identifies the most significant biological limitation to this prediction based on the principle of limiting factors?
Answer: The rate of photosynthesis is often limited by other factors, such as the availability of nitrogen or phosphorus in the soil.
The principle of limiting factors states that a process's rate is limited by the scarcest resource. While increasing CO₂ can boost photosynthesis (the CO₂ fertilization effect), this increase can only be sustained if other resources are not limiting. In many ecosystems, the growth of plants and thus the overall rate of photosynthesis is limited by the availability of essential nutrients like nitrogen and phosphorus, or by water availability, not just CO₂.
- In a classic experiment, a plant is supplied with water containing a heavy isotope of oxygen, H₂¹⁸O, and carbon dioxide containing normal oxygen, C¹⁶O₂. After a period of photosynthesis, where would the heavy oxygen isotope (¹⁸O) be primarily detected?
Answer: In the atmospheric oxygen gas (O₂) released by the plant.
This type of isotope tracing experiment was crucial in determining that the oxygen released during photosynthesis originates from the splitting of water molecules (photolysis) in the light-dependent reactions. The oxygen atoms from the carbon dioxide molecules are incorporated into the synthesized glucose and other organic molecules.
- A plant that is actively photosynthesizing is suddenly placed in an environment with no carbon dioxide but with continued bright illumination. What will be the immediate consequence for the levels of RuBP and glycerate-3-phosphate (GP) in the Calvin cycle? [HL]
Answer: The concentration of RuBP will accumulate, and the concentration of GP will fall.
In the absence of CO₂, the enzyme RuBisCO cannot perform its carboxylase function, so the conversion of RuBP into GP ceases. This causes the level of GP to fall as the existing pool is converted to other intermediates. Meanwhile, the light-dependent reactions are still producing ATP and NADPH, which continue to power the latter part of the Calvin cycle, regenerating RuBP from triose phosphates. As RuBP is being produced but not consumed, its concentration will accumulate.
- Chlorophylls absorb light most strongly in the blue-violet and red regions of the spectrum, while carotenoids absorb most strongly in the blue-violet region. Neither absorbs well in the green-yellow region. If an experiment measures the rate of oxygen production versus wavelength of light (an action spectrum), what would be observed?
Answer: Two major peaks of oxygen production, in the blue-violet and red regions, with a trough in the green region.
An action spectrum shows the rate of a physiological activity (like photosynthesis) at different wavelengths. Since photosynthesis is driven by light absorbed by pigments, the action spectrum should closely match the combined absorption spectra of the pigments involved. Therefore, the highest rates of photosynthesis (and thus O₂ production) will occur at the wavelengths where chlorophylls and carotenoids absorb most strongly (blue-violet and red), and the lowest rate will be where they absorb poorly (green).
- A chloroplast is isolated and kept in complete darkness. It is then supplied with an abundance of ATP, NADPH, and CO₂. Under these conditions, which statement accurately predicts the activity within the chloroplast?
Answer: The production of organic molecules such as triose phosphate will occur via the Calvin cycle.
The Calvin cycle is light-independent, not light-requiring. Its essential inputs are ATP and NADPH (from the light-dependent reactions) and CO₂. If these substrates are provided artificially in the dark, the enzymes of the Calvin cycle in the stroma will proceed to fix carbon and synthesize sugars. This demonstrates the chemical, rather than photochemical, nature of the cycle.
- A leaf disc is placed in a solution of hydrogencarbonate indicator. The solution is initially red. After being exposed to dim light for one hour, the solution remains red. What can be deduced about the rates of photosynthesis and cellular respiration in the leaf disc under these conditions?
Answer: The rate of CO₂ consumption by photosynthesis is equal to the rate of CO₂ release by respiration.
Hydrogencarbonate indicator is sensitive to CO₂ concentration, which affects pH. Red indicates a neutral pH. If the solution remains red, there is no net change in CO₂ concentration. This means the amount of CO₂ being removed from the solution by photosynthesis is exactly balanced by the amount of CO₂ being added to the solution by cellular respiration. This point is known as the light compensation point.
- In a hot, dry environment, a plant's stomata close to conserve water. This leads to an increase in O₂ concentration and a decrease in CO₂ concentration within the leaf's air spaces. Which statement correctly applies the consequences of this change to the Calvin cycle? [HL]
Answer: RuBisCO will function as an oxygenase, fixing O₂ to RuBP, which leads to a net loss of fixed carbon from the cycle.
RuBisCO can act as both a carboxylase (fixing CO₂) and an oxygenase (fixing O₂). When the CO₂/O₂ ratio is low, its oxygenase activity increases. This process, called photorespiration, consumes RuBP and produces compounds that result in a net loss of previously fixed carbon, reducing the efficiency of photosynthesis. A is incorrect because regeneration depends on the cycle progressing. C is incorrect as GP is the product of carboxylation, not oxygenation. D is incorrect as this active pumping mechanism (C4 pathway) is a separate adaptation not present in all plants.
- Which sequence correctly describes the primary energy conversions that occur during the entire process of photosynthesis?
Answer: Light energy → chemical energy in ATP and NADPH → chemical energy in glucose
The overall process of photosynthesis involves two main stages of energy conversion. First, in the light-dependent reactions, light energy is captured and transformed into temporary chemical energy in the form of ATP and NADPH. Second, in the light-independent reactions (Calvin cycle), the energy stored in ATP and NADPH is used to convert CO₂ into stable, long-term chemical energy in the form of glucose and other organic compounds.
- Under which conditions would the rate of cyclic photophosphorylation be expected to increase relative to non-cyclic photophosphorylation in a chloroplast? [HL]
Answer: When the Calvin cycle slows down, leading to an accumulation of NADPH and a shortage of NADP⁺ to act as an electron acceptor.
Non-cyclic photophosphorylation requires a final electron acceptor, NADP⁺. If the Calvin cycle slows (e.g., due to low CO₂), NADPH is not oxidized back to NADP⁺. The resulting low concentration of NADP⁺ prevents electrons from flowing through the complete non-cyclic pathway. To continue producing some ATP, the chloroplast will switch to cyclic photophosphorylation, where electrons are cycled from Photosystem I back into the electron transport chain, generating ATP without making NADPH.
- A plant is grown in a sealed container at 25°C with optimal light intensity and a CO₂ concentration of 400 ppm. The rate of photosynthesis is measured. If the temperature is then increased to 35°C, which is closer to the enzyme optimum, and the CO₂ concentration is simultaneously reduced to 200 ppm, what is the most likely outcome for the rate of photosynthesis?
Answer: The rate will decrease because the significant reduction in CO₂ concentration will become the primary limiting factor.
The rate of photosynthesis is determined by the limiting factor, which is the resource in shortest supply. While increasing the temperature from 25°C to 35°C might move it closer to the optimum for enzymes like RuBisCO, halving the concentration of a key substrate (CO₂) will have a more pronounced effect. At optimal light and a favorable temperature, CO₂ concentration is very likely to become the new limiting factor, causing the overall rate to decrease.
- A chemical is introduced into a chloroplast that makes the thylakoid membrane permeable to protons (H⁺), allowing them to dissipate from the lumen into the stroma. Assuming constant illumination, what is the most direct consequence?
Answer: The synthesis of ATP by ATP synthase will be severely inhibited.
ATP synthesis in the chloroplast is driven by chemiosmosis, which relies on a high concentration of protons (a proton motive force) being built up in the thylakoid lumen. If the membrane becomes leaky to protons, this gradient cannot be maintained, and the flow of protons through ATP synthase, which powers ATP production, will stop. The electron transport chain that splits water and reduces NADP⁺ is not directly dependent on the gradient itself and will continue to function, at least initially.
- The Z-scheme model indicates that for every molecule of O₂ evolved, four electrons must pass through both Photosystem II and Photosystem I. To excite one electron at each photosystem requires one photon. Based on this model, what is the theoretical minimum number of photons required to produce the ATP and NADPH needed to fix one molecule of CO₂?
Answer: 8 photons
This requires multi-step reasoning. The overall balanced equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This shows a 1:1 ratio between CO₂ fixed and O₂ evolved. The problem states that evolving 1 O₂ requires 4 electrons to pass through the Z-scheme. Each electron requires 2 photons (one at PSII, one at PSI). Therefore, the total number of photons required to evolve 1 O₂ is 4 electrons × 2 photons/electron = 8 photons. Since evolving 1 O₂ corresponds to fixing 1 CO₂, the theoretical minimum is 8 photons per molecule of CO₂.
- A photosynthetically active palisade mesophyll cell is in bright sunlight. How does its photosynthetic activity directly support its own mitochondrial activity?
Answer: By producing oxygen and organic molecules, which are the essential substrates for aerobic respiration.
Photosynthesis and cellular respiration are complementary processes within a plant cell. Photosynthesis produces organic molecules (like glucose) and oxygen. These are the very molecules that the cell's mitochondria use as the primary fuel and final electron acceptor, respectively, for aerobic respiration to produce ATP for all cellular functions.
- To produce the amount of ATP and NADPH needed to fix one molecule of CO₂, non-cyclic photophosphorylation is the primary pathway. However, the Calvin cycle requires more ATP than NADPH (a 3:2 ratio). How does the chloroplast produce the additional ATP required to sustain carbon fixation? [HL]
Answer: Through cyclic photophosphorylation, which generates ATP without producing NADPH.
The Calvin cycle consumes ATP and NADPH in a 3:2 ratio per molecule of CO₂ fixed. Non-cyclic photophosphorylation produces ATP and NADPH in roughly a 1:1 ratio. To balance this stoichiometry, chloroplasts use cyclic photophosphorylation. In this pathway, electrons from Photosystem I are cycled back to the electron transport chain, pumping more protons and generating ATP via ATP synthase without splitting water or reducing NADP⁺.
- The herbicide paraquat accepts electrons from Photosystem I and prevents them from reducing NADP⁺. It then reacts with oxygen to form destructive reactive oxygen species. What is an immediate effect of paraquat on photosynthesis?
Answer: The production of NADPH will be inhibited, but ATP synthesis may continue for a short time.
By intercepting electrons after Photosystem I, paraquat directly prevents the reduction of NADP⁺ to NADPH. However, the electron flow from Photosystem II to Photosystem I, which includes the splitting of water and the pumping of protons into the thylakoid lumen, can continue. This proton gradient can still power ATP synthase to produce ATP, at least until the system is damaged by the reactive oxygen species. This process is essentially a form of cyclic photophosphorylation, driven by an external agent.
- A scientist measures the gas exchange of a plant in a closed system. In the light, the net rate of oxygen production is +15 µmol min⁻¹. In complete darkness, the rate of oxygen consumption is -5 µmol min⁻¹. Assuming the rate of cellular respiration is constant in both light and dark conditions, what is the gross rate of photosynthesis?
Answer: 20 µmol min⁻¹
The net rate of photosynthesis is the total (gross) amount of photosynthesis minus what is consumed by respiration. The measurement in the dark gives the rate of respiration (-5 µmol min⁻¹, or a consumption of 5). The gross rate of photosynthesis is therefore the net rate measured in the light plus the rate of respiration. Gross Photosynthesis = Net Photosynthesis + Respiration Rate = 15 µmol min⁻¹ + 5 µmol min⁻¹ = 20 µmol min⁻¹.
- Some desert plants employ a photosynthetic adaptation where they open their stomata only at night to take in CO₂, which is temporarily stored as malic acid. During the day, the stomata close and the malic acid releases CO₂ for the Calvin cycle. What is the primary selective advantage of this temporal separation?
Answer: It maximizes water retention by preventing transpiration during the hottest and driest part of the day.
This adaptation, known as Crassulacean Acid Metabolism (CAM), is primarily a mechanism for water conservation. By opening stomata for gas exchange only during the cooler, more humid night, the plant drastically reduces water loss through transpiration. The stored CO₂ is then available for photosynthesis during the day when light is available for the light-dependent reactions, even though the stomata are closed.
- A toxin is introduced that specifically inhibits the enzyme that regenerates RuBP from triose phosphate in the Calvin cycle. Assuming the light-dependent reactions continue to function, what would be the immediate effect on the concentrations of RuBP and glycerate-3-phosphate (GP)? [HL]
Answer: The concentrations of both RuBP and GP will decrease as the cycle halts.
If the regeneration of RuBP is blocked, the existing pool of RuBP will continue to be consumed by RuBisCO to produce GP. This causes the concentration of RuBP to decrease. As the RuBP concentration drops, the rate of CO₂ fixation and thus the rate of GP production will also decrease. Meanwhile, existing GP is still being converted to triose phosphate. The net result is that the levels of both the initial reactant (RuBP) and the initial product (GP) will fall, and the entire cycle will grind to a halt.
