Unit 2: Metabolic Processes — Practice Quiz (AS Learning)

Topic 2.1 — Glycolysis & Krebs Cycle

Question 1

Glycolysis occurs in the ___ and produces a NET of ___ ATP per glucose.

Mitochondrial matrix; 2
Cytoplasm; 2
Cytoplasm; 4
Inner mitochondrial membrane; 36

Solution: Glycolysis: cytoplasm. 4 ATP produced (substrate-level phosphorylation) − 2 ATP invested = NET 2 ATP. Also yields 2 NADH and 2 pyruvate.

Question 2

Per glucose molecule, the Krebs (citric acid) cycle directly produces:

2 ATP, 6 NADH, 2 FADH₂, 4 CO₂
4 ATP, 10 NADH, 2 FADH₂
32 ATP
2 NADH, 2 ATP, 6 O₂

Solution: Krebs runs twice per glucose (one per acetyl-CoA). Per turn: 1 ATP, 3 NADH, 1 FADH₂, 2 CO₂. ×2 = 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂. (The 2 CO₂ from pyruvate oxidation give 6 CO₂ total released.)

Question 3

Calculate total CO₂ released per glucose in aerobic respiration.

Solution: Pyruvate oxidation: 2 CO₂ (one per pyruvate × 2). Krebs: 4 CO₂ (2 per turn × 2). Total = 6 CO₂, matching the equation C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O.

Topic 2.2 — Electron Transport Chain & Chemiosmosis

Question 4

In chemiosmosis, ATP is generated when:

H+ ions flow down their gradient through ATP synthase
Electrons leave NADH directly
O₂ is split
Glucose is broken in half

Solution: ETC pumps H+ from matrix to intermembrane space, building a proton-motive force. H+ flows back through ATP synthase, rotating its F₀ rotor and driving ADP + P_i → ATP.

Question 5

The final electron acceptor in aerobic ETC is:

NAD+
Pyruvate
O₂ → forms H₂O
CO₂

Solution: O₂ accepts electrons at Complex IV and combines with H+ to form H₂O. Without O₂ the ETC stalls; NAD+ can't be regenerated and respiration halts (cells switch to fermentation).

Question 6

Approximately how many ATP per glucose are produced by ETC + chemiosmosis (oxidative phosphorylation)?

Solution: Modern stoichiometry: 10 NADH × 2.5 ATP + 2 FADH₂ × 1.5 ATP = 25 + 3 = ~28 ATP, minus shuttle costs for cytosolic NADH ≈ 26. (Older texts use 3 and 2 → ~34 from oxphos for total ~38.) Either 26 or 28 accepted.

Topic 2.3 — Photosynthesis

Question 7

The light-dependent reactions of photosynthesis occur in the ___ and produce ___.

Stroma; glucose
Thylakoid membrane; ATP, NADPH, O₂
Outer membrane; CO₂
Cytoplasm; pyruvate

Solution: Photosystems II and I in the thylakoid membrane absorb light, splitting H₂O (releasing O₂) and pumping H+ into the thylakoid lumen. ATP synthase in the thylakoid produces ATP; NADP+ is reduced to NADPH. ATP and NADPH then power the Calvin cycle in the stroma.

Question 8

In the Calvin cycle, the enzyme RuBisCO catalyzes:

Splitting of water
Fixation of CO₂ onto RuBP, forming a 6-C intermediate that splits into two 3-PGA
Production of ATP from sunlight
Regeneration of chlorophyll

Solution: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) attaches CO₂ to RuBP (5-C); the unstable 6-C immediately splits into two 3-PGA. ATP and NADPH then reduce 3-PGA → G3P; 5 of 6 G3P regenerate RuBP, 1 leaves as net product.

Question 9

How many CO₂ molecules and how many turns of the Calvin cycle are needed to produce one glucose (C₆)?

Solution: One turn fixes 1 CO₂ and produces ⅓ G3P (net). Six turns fix 6 CO₂ and yield 2 G3P (3-C each), which combine to form 1 glucose (C₆). Energy cost: 18 ATP and 12 NADPH per glucose.

Topic 2.4 — Fermentation

Question 10

Lactic-acid fermentation in human muscle cells occurs because:

NAD+ must be regenerated to keep glycolysis running when O₂ is limited
Lactic acid is needed for muscle structure
Mitochondria are absent
CO₂ is not allowed in muscle

Solution: Without O₂, ETC halts and NADH cannot be re-oxidized. Lactate dehydrogenase reduces pyruvate to lactate, regenerating NAD+ so glycolysis (and its 2 ATP/glucose) can continue.

Question 11

Compared to aerobic respiration, alcoholic fermentation in yeast yields:

The same ATP per glucose
Far less ATP (only 2 net) and produces ethanol + CO₂
More ATP because no O₂ is wasted
Lactic acid only

Solution: Aerobic ≈ 30–32 ATP per glucose. Fermentation = 2 ATP per glucose (only glycolysis). Yeast converts pyruvate → acetaldehyde → ethanol, releasing CO₂ (used in baking, brewing).

Question 12

If oxygen is suddenly removed from a respiring cell, the immediate consequence is:

ETC halts, NADH accumulates, NAD+ depletes, Krebs slows, ATP drops
Photosynthesis starts
All ATP is destroyed
The mitochondria multiply

Solution: No O₂ → no terminal e- acceptor → ETC stops → H+ gradient collapses → ATP synthase stops → ATP drops. NADH accumulates and NAD+ runs out, halting Krebs and pyruvate oxidation. Cell switches to fermentation to keep glycolysis (and minimal ATP) going.