Prediction plus mechanism

Trace the block: no electron transfer to O₂, no pumping, the gradient falls, ATP synthase slows; NADH builds up, so fermentation rises to regenerate NAD⁺. Structures such as mitochondria do not change in minutes.

• O₂ used by the cells per minute: decreases. Electrons can no longer be passed to O₂, so O₂ stops being reduced to water.
• H⁺ gradient across the inner mitochondrial membrane: decreases. With electrons stuck, the chain stops pumping H⁺, while H⁺ keeps flowing back through ATP synthase, so the gradient runs down.
• ATP made by ATP synthase: decreases. ATP synthase is driven by H⁺ flowing down its gradient; a shrinking gradient drives less synthesis.
• Lactate released by the cells: increases. NADH cannot unload electrons to the chain, so NAD⁺ runs short; fermentation regenerates NAD⁺ by reducing pyruvate to lactate, letting glycolysis continue.
• Number of mitochondria per cell: no change. Cyanide blocks a protein's function; in a few minutes it does not change how many mitochondria the cell has.

The Krebs cycle needs NAD⁺ (and FAD) to accept electrons. With the chain blocked, NADH cannot be oxidized back to NAD⁺, so the cycle stalls even though it uses no O₂ itself.

• Correct: It slows, because NADH cannot pass its electrons to the blocked chain, so NAD⁺ runs short and the cycle's oxidation steps cannot continue.: Correct: a mechanism that links the block to the cycle through NAD⁺.
• It speeds up, because the cells need more ATP, so they send more pyruvate into the mitochondria to make up the lost ATP.: Need does not drive a pathway; without NAD⁺ the cycle cannot run faster.
• It stays the same, because the Krebs cycle does not use oxygen directly, so a block at the end of the chain has no route to reach it in the mitochondrion.: It does not use O₂ directly, but it depends on NAD⁺ that only the chain regenerates in mitochondria.
• It stops at once, because cyanide binds the enzymes of the Krebs cycle as well as the last protein of the chain.: The description says cyanide binds the last protein of the chain; the cycle slows indirectly.

Cyanide does not keep O₂ out; it blocks its use. A full prediction names a measurable change (ATP falls, lactate rises) and the chain of causes from the block.

• Correct: Oxygen still reaches the cells; the problem is that electrons cannot be passed to it, and no steps link the block to the outcome.: Correct: wrong mechanism, and no causal steps.
• It is too detailed, because a prediction should name just the variable that changes and leave out the cause and any other explanation.: Exam predictions earn more credit with a mechanism, not less.
• It is wrong, because muscle cells use fermentation for their ATP and are not affected by cyanide in any way.: Fermentation yields far less ATP; cyanide does affect these cells.
• It uses the word "because", which belongs in reasoning rather than in a prediction about an experiment.: A prediction with mechanism is expected to use "because".

The block stops the flow of electrons from water through to NADP⁺, so O₂ release, NADPH and (through lost ATP and NADPH) carbon fixation all fall. The light supply is unchanged.

• O₂ released by water splitting: decreases. Electrons cannot leave photosystem II, so it cannot accept new electrons from water, and water splitting stops.
• NADPH produced: decreases. No electrons reach photosystem I and NADP⁺, so NADPH production stops.
• CO₂ fixed by the Calvin cycle: decreases. The Calvin cycle needs ATP and NADPH from the light reactions; with less of both, it slows.
• Light reaching the chloroplasts: no change. DCMU blocks electron flow; it does not change the lamp or how much light reaches the leaf.

Photosystem II gives up electrons to the chain and refills them by splitting water, which releases O₂. If its electrons cannot move on, water splitting stops and O₂ release flattens.

• Correct: With electrons unable to leave photosystem II, it cannot replace them from water, so water splitting and O₂ release stop.: Correct.
• DCMU destroys the chlorophyll in the disks, so they can no longer absorb any of the light energy that reaches them from the lamp.: The description says DCMU binds a protein of the chain, not chlorophyll.
• DCMU blocks the Calvin cycle directly, so O₂ is used up as fast as it is made.: The Calvin cycle does not consume O₂, and DCMU acts on the chain.
• DCMU closes the leaf's stomata, so the O₂ that is made is trapped inside the leaf disk.: The curve shows O₂ release stopping; nothing in the setup involves stomata.

Trace the model: the compound gives electrons a new exit before the block, so photosystem II keeps working and water splitting, with O₂ release, resumes.

• Correct: O₂ release resumes, because photosystem II can again pass on electrons and refill them by splitting water.: Correct: a model-based prediction.
• O₂ release stays at zero, because DCMU still blocks the chain and so no electrons can leave photosystem II.: The compound takes electrons before the block, so they no longer need to pass DCMU.
• NADPH production resumes, because the compound carries the electrons on to photosystem I and NADP⁺.: Nothing says the compound passes electrons to photosystem I; NADPH stays low.
• O₂ release rises above the control, because the compound adds extra electrons to photosystem II.: The compound accepts electrons; it does not supply them, and O₂ comes from water.