6 questions about cellular respiration
Cellular respiration converts the chemical energy of glucose into ATP through linked pathways: glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation (electron transport chain plus chemiosmosis). The six questions below target the most tested concepts and calculations in introductory biology.
Question 1: Write the overall balanced equation and explain the redox roles
Prompt. Write the overall equation for aerobic cellular respiration of glucose. Identify what is oxidized, what is reduced, and why oxygen is required.
Answer (with reasoning). The overall equation is:
\[ \mathrm{C_6H_{12}O_6 + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O} \]
- Glucose is oxidized: its carbon atoms lose electrons (and hydrogen equivalents) as glucose is converted to \( \mathrm{CO_2} \).
- Oxygen is reduced: \( \mathrm{O_2} \) gains electrons and protons to form water, \( \mathrm{H_2O} \).
- Why oxygen is required: oxygen serves as the terminal electron acceptor in the electron transport chain; without it, electrons cannot flow through the chain, NADH and FADH\(_2\) cannot be oxidized back to NAD\(^+\) and FAD, and ATP production by oxidative phosphorylation collapses.
Question 2: Where do the stages occur in a eukaryotic cell?
Prompt. State the cellular location of glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation in a eukaryotic cell.
Answer (with reasoning).
- Glycolysis: cytosol.
- Pyruvate oxidation (pyruvate \(\rightarrow\) acetyl-CoA): mitochondrial matrix.
- Citric acid cycle: mitochondrial matrix.
- Oxidative phosphorylation: inner mitochondrial membrane (ETC proteins embedded in the membrane; proton gradient across the membrane; ATP synthase uses the gradient).
Question 3: Track carbon atoms from glucose to carbon dioxide
Prompt. One glucose molecule has 6 carbons. How many \( \mathrm{CO_2} \) molecules are released during (a) glycolysis, (b) pyruvate oxidation, and (c) the citric acid cycle?
Answer (with reasoning).
- Glycolysis: produces 2 pyruvate (3 carbons each) and releases 0 \( \mathrm{CO_2} \).
- Pyruvate oxidation: each 3-carbon pyruvate becomes 2-carbon acetyl-CoA, releasing 1 \( \mathrm{CO_2} \). With 2 pyruvate: 2 \( \mathrm{CO_2} \).
- Citric acid cycle: each acetyl-CoA yields 2 \( \mathrm{CO_2} \) per turn. With 2 acetyl-CoA per glucose: 4 \( \mathrm{CO_2} \).
Total \( \mathrm{CO_2} \) per glucose: \[ 0 + 2 + 4 = 6\ \mathrm{CO_2} \]
Question 4: Calculate ATP yield per glucose (state assumptions)
Prompt. Using modern P/O ratios (approximately \(2.5\) ATP per NADH and \(1.5\) ATP per FADH\(_2\)), estimate total ATP produced per glucose in aerobic respiration. Show the accounting.
Answer (with reasoning). Substrate-level phosphorylation yields ATP directly, while oxidative phosphorylation yields ATP from NADH and FADH\(_2\).
| Stage | ATP (substrate-level) | NADH produced | FADH2 produced |
|---|---|---|---|
| Glycolysis | \(+2\) (net) | \(2\) | \(0\) |
| Pyruvate oxidation | \(0\) | \(2\) | \(0\) |
| Citric acid cycle (2 turns) | \(+2\) (as GTP/ATP) | \(6\) | \(2\) |
Totals: \(10\) NADH and \(2\) FADH\(_2\), plus \(4\) ATP by substrate-level phosphorylation. Oxidative phosphorylation estimate:
\[ \text{ATP from NADH} = 10 \times 2.5 = 25 \] \[ \text{ATP from FADH}_2 = 2 \times 1.5 = 3 \] \[ \text{Total ATP} \approx 4 + 25 + 3 = 32 \]
Many textbooks also report a range (often \(30\)–\(32\) ATP) because the effective ATP yield depends on how cytosolic NADH from glycolysis is shuttled into mitochondria and on proton leak and transport costs.
Question 5: Predict the effects of blocking the electron transport chain
Prompt. A toxin inhibits the final electron transfer to oxygen in the electron transport chain. Predict what happens to (a) oxygen consumption, (b) NADH levels, (c) the proton gradient, and (d) ATP production.
Answer (with reasoning).
- Oxygen consumption: decreases sharply because oxygen can no longer accept electrons at the end of the chain.
- NADH levels: increase (and NAD\(^+\) decreases) because NADH cannot be oxidized efficiently back to NAD\(^+\).
- Proton gradient: dissipates because electron flow is needed to pump protons; with the chain stalled, pumping stops and the gradient cannot be maintained.
- ATP production: oxidative phosphorylation drops dramatically; only substrate-level ATP (glycolysis and the GTP/ATP from the citric acid cycle, if the cycle can continue) remains, but the cycle typically slows because it also depends on NAD\(^+\) and FAD regeneration.
Question 6: Compare aerobic respiration and fermentation
Prompt. When oxygen is absent, how does fermentation allow glycolysis to continue, and what is the ATP yield compared with aerobic respiration?
Answer (with reasoning).
- Problem without oxygen: NADH accumulates and NAD\(^+\) becomes limiting; glycolysis requires NAD\(^+\) to oxidize glyceraldehyde-3-phosphate.
- Fermentation solution: fermentation transfers electrons from NADH back to an organic molecule, regenerating NAD\(^+\).
- ATP yield: fermentation itself does not add ATP; the cell gets the net glycolysis yield only: \[ \text{ATP per glucose (fermentation)} = 2 \] Compared with aerobic respiration (\(\approx 30\)–\(32\) ATP per glucose), fermentation yields far less ATP but maintains short-term ATP production under anaerobic conditions.