- If the gene encoding the enzyme NADP⁺ reductase were non-functional, how would this most directly impact the Calvin cycle?
Answer: Glycerate-3-phosphate would accumulate because its reduction would be inhibited.
NADP⁺ reductase catalyzes the final step of the light-dependent reactions, producing NADPH. NADPH is the reducing agent required to convert glycerate-3-phosphate (GP) into triose phosphate in the Calvin cycle. Without NADPH, this reduction step cannot occur, leading to a build-up of the substrate, GP, and a depletion of the products, including triose phosphate and regenerated RuBP.
- Which statement accurately distinguishes between the absorption spectrum of chlorophyll and the action spectrum of photosynthesis?
Answer: The action spectrum has peaks that are broader than the absorption spectrum because accessory pigments transfer absorbed energy to chlorophyll.
The action spectrum shows the overall rate of photosynthesis at different wavelengths. The absorption spectrum shows which wavelengths are absorbed by a specific pigment like chlorophyll. The action spectrum is broader because accessory pigments (like carotenoids) absorb light at wavelengths where chlorophyll absorbs poorly and then transfer that energy to chlorophyll, thus contributing to photosynthesis. This makes the overall action spectrum cover a wider range than the absorption spectrum of chlorophyll alone.
- A student designs an experiment to measure the effect of temperature on the rate of photosynthesis in spinach leaves. Which of the following is a crucial variable that must be kept constant to ensure a valid conclusion?
Answer: The concentration of carbon dioxide available to the leaves.
In this experiment, temperature is the independent variable and the rate of photosynthesis is the dependent variable. To ensure that only temperature is affecting the rate, all other potential limiting factors must be controlled (kept constant). These include light intensity and CO₂ concentration. The rate of O₂ production and change in biomass are measures of the dependent variable, not controlled variables.
- An aquatic plant is photosynthesizing at a constant, maximum rate under conditions of high light intensity and optimal temperature. If the concentration of CO₂ is suddenly doubled, what is the most likely immediate effect on the concentrations of ATP and NADPH in the stroma?
Answer: Both ATP and NADPH will decrease because the Calvin cycle will accelerate.
The plant is initially limited by CO₂. Doubling the CO₂ concentration will increase the rate of the light-independent reactions (Calvin cycle). This cycle consumes ATP and NADPH. As the cycle speeds up, the consumption rate of ATP and NADPH will exceed their production rate from the light-dependent reactions, leading to an immediate decrease in their stromal concentrations.
- The light-independent reactions of photosynthesis are collectively known as the Calvin cycle. Why is this series of reactions described as a 'cycle'?
Answer: Because the starting compound, RuBP, is regenerated at the end of the reactions.
A metabolic cycle is a series of reactions where the starting molecule is regenerated at the end, allowing the process to continue. In the Calvin cycle, CO₂ is fixed to a 5-carbon compound, ribulose bisphosphate (RuBP). After a series of reactions that produce carbohydrate (triose phosphate), the majority of the molecules are used to regenerate the initial RuBP, thus completing the cycle.
- In an experiment, a plant is supplied with water containing a heavy isotope of oxygen (H₂¹⁸O) and carbon dioxide containing a common isotope of oxygen (C¹⁶O₂). After several hours of photosynthesis, where would the ¹⁸O isotope primarily be found?
Answer: In the oxygen gas released as a waste product from the leaves.
The oxygen gas released during photosynthesis is derived exclusively from the photolysis (splitting) of water molecules in the light-dependent reactions. The oxygen atoms in the carbon dioxide reactant become incorporated into the glucose and water molecules produced by the overall process. Therefore, the heavy ¹⁸O isotope from the water will be found in the released O₂ gas.
- What are the net inputs for the production of one molecule of triose phosphate (G3P) that can be used to synthesize glucose?
Answer: 3 CO₂, 9 ATP, and 6 NADPH
To produce one net triose phosphate (G3P), the Calvin cycle must 'fix' 3 molecules of CO₂. Each CO₂ molecule fixed requires 3 ATP and 2 NADPH to proceed through the reduction and regeneration phases. Therefore, for 3 CO₂ molecules, the total required input is 3 x 3 = 9 ATP and 3 x 2 = 6 NADPH.
- What feature of chloroplast structure is key to establishing the proton gradient during the light-dependent reactions?
Answer: The impermeability of the thylakoid membrane to protons, allowing for their accumulation.
Chemiosmosis depends on creating a high concentration of protons in one area (the thylakoid space) relative to another (the stroma). This is only possible if the membrane separating these two compartments—the thylakoid membrane—is largely impermeable to protons. This impermeability prevents the protons from leaking back into the stroma and forces them to pass through ATP synthase, thereby driving ATP production.
- Which of the following comparisons between photophosphorylation and oxidative phosphorylation is correct?
Answer: Both processes use energy from an electron transport chain to create a proton gradient.
Both photophosphorylation (in chloroplasts) and oxidative phosphorylation (in mitochondria) utilize an electron transport chain embedded in a membrane to pump protons, creating an electrochemical gradient. This gradient then powers ATP synthase (chemiosmosis). The final electron acceptor in photophosphorylation is NADP⁺, while in oxidative phosphorylation it is O₂. The source of electrons in photophosphorylation is water, while in oxidative phosphorylation it is NADH and FADH₂ derived from catabolism.
- The leaves of most plants appear green. What is the most accurate explanation for this phenomenon in the context of photosynthesis?
Answer: Chlorophyll primarily absorbs light in the blue-violet and red regions and reflects or transmits green light.
Pigments have their characteristic colour because they reflect the wavelengths of light that they do not absorb. The absorption spectrum for chlorophyll shows major peaks in the blue-violet and red parts of the spectrum. Green light is absorbed very poorly; instead, it is mostly reflected or transmitted through the leaf, which is why we perceive leaves as green.
- If a plant undergoing photosynthesis is moved from a light environment to complete darkness, what are the immediate changes in the stromal concentrations of ribulose bisphosphate (RuBP) and glycerate-3-phosphate (GP)?
Answer: RuBP decreases and GP increases.
In darkness, the light-dependent reactions stop, ceasing production of ATP and NADPH. Carbon fixation (RuBP + CO₂ → 2 GP) continues for a short time, consuming RuBP. The reduction of GP to triose phosphate, which requires ATP and NADPH, stops. Consequently, RuBP is consumed but not regenerated, so its concentration falls, while GP is produced but not converted, so its concentration rises.
- The final product of the Calvin cycle is triose phosphate (G3P). Which of the following is NOT a direct fate of G3P within the chloroplast?
Answer: Being immediately oxidized in the Krebs cycle to produce ATP.
Triose phosphate has several fates: most is used within the Calvin cycle to regenerate RuBP; some is converted to starch and stored in the chloroplast; and some is exported to the cytoplasm to be used for sucrose synthesis or cellular respiration. However, the Krebs cycle occurs in the mitochondria, not the chloroplast. G3P would first need to be exported and then enter the glycolysis pathway before its products could enter the Krebs cycle.
- What is the primary function of cyclic photophosphorylation in chloroplasts?
Answer: To generate ATP independently of NADPH production to balance the energy budget of the cell.
The Calvin cycle requires more ATP than NADPH (in a 3:2 ratio for producing one G3P). When non-cyclic photophosphorylation produces ATP and NADPH in roughly equal amounts, a deficit of ATP can occur. Cyclic photophosphorylation involves only photosystem I and generates ATP via chemiosmosis without producing NADPH or O₂. This allows the chloroplast to supplement the ATP supply to match the demands of the Calvin cycle.
- The enzyme ATP synthase plays a similar role in both photosynthesis and aerobic respiration. Which statement describes a key difference in its function between these two processes?
Answer: In photosynthesis the proton gradient is generated by light energy, while in respiration it is generated by the oxidation of chemical compounds.
The fundamental energy source for creating the proton gradient differs. In photosynthesis (photophosphorylation), light energy excites electrons, which power the proton pumps. In aerobic respiration (oxidative phosphorylation), chemical energy from the oxidation of NADH and FADH₂ powers the proton pumps. In both cases, the flow of protons through ATP synthase drives ATP production. Note that ATP synthase is on the inner mitochondrial membrane, not the outer.
- During chemiosmosis in chloroplasts, a proton gradient is established. How does this gradient directly lead to the synthesis of ATP?
Answer: The flow of protons down their concentration gradient through ATP synthase provides the energy to form ATP.
The electron transport chain pumps protons (H⁺) from the stroma into the thylakoid space, creating a high concentration of protons inside. This electrochemical gradient represents potential energy. The protons then flow passively back into the stroma through a channel in the ATP synthase enzyme. This flow of protons drives the rotation of a part of the enzyme, providing the energy to synthesize ATP from ADP and inorganic phosphate.
- The enzyme RuBisCO is crucial for the Calvin cycle. Which property of RuBisCO can lead to a reduction in photosynthetic efficiency, particularly at high temperatures?
Answer: It can bind to O₂ as a substrate, initiating the process of photorespiration.
RuBisCO has an active site that can bind to both CO₂ (carboxylase activity) and O₂ (oxygenase activity). At high temperatures and low CO₂/O₂ ratios, the oxygenase activity increases. When RuBisCO binds O₂, it initiates photorespiration, a wasteful pathway that consumes ATP and releases previously fixed CO₂, thereby reducing the net efficiency of photosynthesis.
- A herbicide, DCMU, blocks the electron transport chain between photosystem II and photosystem I. Which of the following would be the most immediate consequence of applying this herbicide to a plant?
Answer: A cessation of O₂ production and a decrease in ATP synthesis.
Blocking the electron transport chain after photosystem II stops the flow of electrons. This prevents the photolysis of water, which is the source of O₂. It also stops the pumping of protons into the thylakoid space, which dissipates the proton gradient needed for ATP synthesis by ATP synthase. Therefore, both O₂ production and ATP synthesis will cease.
- A scientist is measuring the rate of photosynthesis by monitoring the change in pH of the stroma. If the light intensity is increased, leading to a higher rate of photosynthesis, what change in the stroma's pH would be expected and why?
Answer: pH will increase because protons are pumped from the stroma into the thylakoid space.
The light-dependent reactions use energy from light to power an electron transport chain that actively pumps protons (H⁺) from the stroma into the thylakoid space. This removal of H⁺ from the stroma makes it more alkaline, causing its pH to increase. The protons released from water photolysis accumulate inside the thylakoid space, not the stroma.
