Questions: Photosynthesis: Light and Dark Reactions

Question 1

On a hot, dry day, a plant closes its stomata to prevent water loss. What is the most likely consequence for the two stages of photosynthesis?

• A. Only the dark reactions slow, because CO₂ cannot enter; the light reactions continue producing ATP and NADPH normally
• B. Only the light reactions slow, because less water is available to be split at Photosystem II
• C. Both stages slow: CO₂ depletion stalls the Calvin cycle, which causes NADPH and ATP to accumulate and NADP⁺ and ADP to become depleted, eventually stalling the light reactions too
• D. Both stages accelerate temporarily because the plant redirects more energy to photosynthesis when water stress is detected

Answer: (C)

The tight coupling between the two stages is the key insight. When stomata close, CO₂ supply drops, slowing the Calvin cycle. The Calvin cycle consumes ATP and NADPH and regenerates ADP and NADP⁺. When it slows, NADPH and ATP accumulate while ADP and NADP⁺ become scarce. The light reactions need NADP⁺ as the terminal electron acceptor and ADP + Pi to make ATP — without these substrates, the electron transport chain backs up and the light reactions stall. Option A is the common mistake: assuming the two stages are independent when they are mutually dependent.

Question 2

Why is the term 'dark reactions' considered misleading for the Calvin cycle?

• A. Because the Calvin cycle is actually light-dependent — RuBisCO requires photons directly to catalyze CO₂ fixation
• B. Because the Calvin cycle occurs in the thylakoid membrane alongside the light reactions, not separately
• C. Because the Calvin cycle operates during daylight hours and is light-independent (not darkness-requiring) — it can run whenever ATP and NADPH are available
• D. Because darkness actually inhibits the Calvin cycle by reducing chloroplast pH

Answer: (C)

The Calvin cycle is 'light-independent' in the sense that it does not directly use photons — but it is not 'dark-requiring.' During the day, both stages run simultaneously: light reactions in the thylakoid membranes generate ATP and NADPH, which the Calvin cycle in the stroma immediately consumes. The name 'dark reactions' implies the cycle runs in darkness, but in practice it runs when ATP and NADPH are supplied, which is during light exposure. At night, the Calvin cycle slows because its substrates are depleted.

Question 3

Dark reactions in photosynthesis occur primarily at night, when the light reactions have ceased and their products (ATP and NADPH) have accumulated to sufficient levels.

• T. True
• F. False

Answer: False

This is the misconception targeted by the term 'light-independent reactions.' The Calvin cycle does not require darkness — it requires ATP and NADPH, which are produced by the light reactions during daylight. In practice, both stages run simultaneously in the light. At night, the Calvin cycle actually slows or stops because the light reactions cease and the supply of ATP and NADPH is depleted. The 'dark' label refers to the absence of a direct photon requirement, not to temporal activity during night.

Question 4

If the Calvin cycle were completely inhibited (e.g., by an enzyme inhibitor targeting RuBisCO), the light reactions would eventually slow down even though light is still available and photons continue to excite chlorophyll.

• T. True
• F. False

Answer: True

The Calvin cycle regenerates ADP, Pi, and NADP⁺ — the substrates the light reactions need to produce ATP and NADPH. Without Calvin cycle activity, NADPH accumulates and NADP⁺ becomes depleted, and the electron transport chain has no terminal electron acceptor. ATP also accumulates while ADP becomes scarce. The light reactions cannot continue without these substrates, so they stall despite the continued availability of light. This demonstrates the mutual dependence: the light reactions depend on the Calvin cycle just as the Calvin cycle depends on the light reactions.

Question 5

Explain the metabolic coupling between the light reactions and the Calvin cycle. What would happen if you could block the Calvin cycle from regenerating ADP and NADP⁺?

Model answer: The light reactions produce ATP and NADPH and consume ADP, Pi, and NADP⁺. The Calvin cycle consumes ATP and NADPH and regenerates ADP, Pi, and NADP⁺. This creates a closed loop: each stage's products are the other stage's substrates. If the Calvin cycle stopped regenerating ADP and NADP⁺, these molecules would be depleted from the system. The light reactions require NADP⁺ as the terminal electron acceptor (at Photosystem I) and ADP + Pi as the substrate for ATP synthase. Without them, electrons would back up in the electron transport chain and proton pumping would cease, halting ATP synthesis. The entire photosynthetic machine would stop, even with abundant light.

This coupling also explains why photosynthetic rate depends on environmental factors that affect only one stage: drought (limiting CO₂ for the Calvin cycle) ultimately limits both stages; cold (slowing Calvin cycle enzymes) also limits the light reactions by depleting their substrates. The two stages are not independent modules that can be regulated separately — they are a single coupled system.