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How do cellular processes work?
the inputs, outputs and locations of glycolysis, the Krebs cycle and the electron transport chain in aerobic cellular respiration, and anaerobic fermentation in animal and plant cells
A focused answer to the VCE Biology Unit 3 dot point on cellular respiration. Covers glycolysis in the cytosol, the Krebs cycle in the mitochondrial matrix, oxidative phosphorylation at the inner mitochondrial membrane, and anaerobic respiration to lactate or ethanol.
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What this dot point is asking
VCAA wants the three stages of aerobic cellular respiration, their locations, their inputs and outputs, the approximate ATP yield, and the difference between anaerobic respiration in animal and plant or yeast cells.
The answer
Cellular respiration is the controlled breakdown of glucose to release energy as ATP. The overall summary equation for aerobic respiration is:
C6H12O6 + 6O2 -> 6CO2 + 6H2O + ATP (around 30 to 32 ATP per glucose)
Respiration takes place in three linked stages, plus an anaerobic alternative when oxygen is absent.
Glycolysis (cytosol)
Location. The cytosol of every living cell.
Inputs. One glucose, 2 ATP (investment phase), 4 ADP + Pi, 2 NAD+.
Outputs. 2 pyruvate (3 carbons each), 2 NADH, net 2 ATP (4 made, 2 invested).
Process. Glucose (6 carbons) is phosphorylated twice using 2 ATP, split into two 3-carbon intermediates, and oxidised. NAD+ accepts electrons and H+ to form NADH. ADP is phosphorylated to ATP by substrate-level phosphorylation.
Glycolysis does not require oxygen and is the starting point for both aerobic and anaerobic pathways.
Link reaction (mitochondrial matrix)
If oxygen is present, each pyruvate enters the mitochondrial matrix and is converted to acetyl-CoA (2 carbons), releasing one CO2 and producing one NADH. Two pyruvates per glucose therefore yield 2 acetyl-CoA, 2 CO2 and 2 NADH.
Krebs cycle (mitochondrial matrix)
Location. Mitochondrial matrix.
Inputs (per turn). Acetyl-CoA, oxaloacetate (regenerated), 3 NAD+, 1 FAD, ADP + Pi.
Outputs (per turn). 2 CO2, 3 NADH, 1 FADH2, 1 ATP. The cycle regenerates oxaloacetate.
Per glucose (two turns). 4 CO2, 6 NADH, 2 FADH2, 2 ATP.
Process. Acetyl-CoA combines with oxaloacetate (4C) to form citrate (6C). A series of oxidations and decarboxylations releases CO2 and transfers electrons and H+ to NAD+ and FAD, regenerating oxaloacetate to start again.
The Krebs cycle does not make much ATP directly. Its main job is to fill up NADH and FADH2 to feed the electron transport chain.
Electron transport chain and oxidative phosphorylation (inner mitochondrial membrane)
Location. Inner mitochondrial membrane (folded into cristae).
Inputs. NADH and FADH2 (from glycolysis, link reaction and Krebs), O2, ADP + Pi.
Outputs. Water, NAD+ and FAD (recycled), around 26 to 28 ATP (giving the overall ~30 to 32 ATP per glucose when combined with glycolysis and Krebs).
Process.
- NADH and FADH2 deliver electrons to membrane protein complexes.
- Electrons pass along the chain through redox reactions. Energy released is used to pump H+ from the matrix into the intermembrane space, creating a proton gradient.
- O2 is the final electron acceptor. It combines with electrons and H+ to form H2O.
- H+ flows back through ATP synthase down its gradient, driving the phosphorylation of ADP to ATP. This is chemiosmosis, and the overall process is oxidative phosphorylation.
Anaerobic respiration (cytosol)
If oxygen is absent, the electron transport chain stalls. NADH cannot be reoxidised to NAD+, so glycolysis would also stall. Fermentation regenerates NAD+ in the cytosol so glycolysis can continue producing 2 ATP per glucose.
Animal cells (and many bacteria). Pyruvate is reduced to lactate (lactic acid) by lactate dehydrogenase. NADH is reoxidised to NAD+. No CO2 is released. Lactate builds up in working muscle and is later cleared by the liver.
Plant and yeast cells. Pyruvate is decarboxylated to acetaldehyde (releasing CO2), then reduced to ethanol by alcohol dehydrogenase. NADH is reoxidised to NAD+. This is the basis of brewing and bread-making.
Both pathways yield only 2 ATP per glucose, compared with around 30 to 32 ATP for full aerobic respiration.
ATP yield at a glance
| Stage | Location | Net ATP | Other products |
|---|---|---|---|
| Glycolysis | Cytosol | 2 | 2 NADH, 2 pyruvate |
| Link reaction | Matrix | 0 | 2 NADH, 2 CO2 |
| Krebs cycle | Matrix | 2 | 6 NADH, 2 FADH2, 4 CO2 |
| Electron transport chain | Inner membrane | ~26 to 28 | H2O |
| Total aerobic | ~30 to 32 | 6 CO2, 6 H2O | |
| Anaerobic (animal) | Cytosol | 2 | 2 lactate |
| Anaerobic (yeast/plant) | Cytosol | 2 | 2 ethanol, 2 CO2 |
Worked example
A sprinter runs at maximum effort for 200 metres. Oxygen delivery cannot keep up with ATP demand in the leg muscles. Glycolysis continues at high rate; pyruvate is reduced to lactate; NAD+ is regenerated so glycolysis can continue. Lactate accumulates, contributing to muscle fatigue. After the sprint, the runner pants to repay the oxygen debt: lactate is transported to the liver and converted back to pyruvate or glucose, and the muscle resumes aerobic respiration.
Common traps
Saying glycolysis occurs in the mitochondrion. It happens in the cytosol.
Forgetting the link reaction. Pyruvate must be converted to acetyl-CoA before it enters the Krebs cycle. This step releases CO2 and produces NADH.
Calling oxidative phosphorylation "the Krebs cycle". Oxidative phosphorylation is the ATP synthesis on the inner mitochondrial membrane, driven by the chemiosmotic gradient. The Krebs cycle is a separate matrix process whose main job is generating NADH and FADH2.
Confusing fermentation products. Animal cells make lactate (no CO2). Yeast and plant cells make ethanol and CO2.
Saying anaerobic respiration produces "no ATP". It still produces 2 ATP per glucose from glycolysis; it just produces far less than aerobic respiration.
In one sentence
Aerobic cellular respiration breaks glucose down through glycolysis (cytosol, 2 ATP and 2 NADH), the Krebs cycle (matrix, NADH, FADH2 and CO2) and the electron transport chain (inner mitochondrial membrane, around 32 ATP via oxidative phosphorylation with oxygen as the final electron acceptor), while anaerobic fermentation produces only 2 ATP per glucose, with lactate in animal cells and ethanol plus CO2 in yeast and plant cells.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2022 VCE4 marksDescribe the location, inputs and outputs of each stage of aerobic cellular respiration.Show worked answer →
A 4-mark answer needs all three stages with location, inputs and outputs.
- Glycolysis. Location: cytosol. Inputs: glucose, 2 ATP (investment), 2 NAD+, 4 ADP + Pi. Outputs: 2 pyruvate, 2 NADH, net 2 ATP. Does not require oxygen.
- Krebs cycle (after pyruvate enters the mitochondrial matrix and is converted to acetyl-CoA, releasing CO2 and NADH). Location: mitochondrial matrix. Inputs per acetyl-CoA: NAD+, FAD, ADP + Pi. Outputs per glucose (2 turns): 4 CO2, 6 NADH, 2 FADH2, 2 ATP.
- Electron transport chain and oxidative phosphorylation. Location: inner mitochondrial membrane (cristae). Inputs: NADH, FADH2, O2, ADP + Pi. Outputs: H2O, NAD+, FAD, around 32 ATP (depending on the cell). Electrons pass along carriers, H+ is pumped into the intermembrane space, and ATP synthase makes ATP as H+ flows back.
Markers reward correct location for each stage and consistent input-output bookkeeping.
2024 VCE3 marksCompare anaerobic fermentation in animal muscle cells and in yeast cells.Show worked answer →
A 3-mark answer needs glycolysis as the common step, the regeneration of NAD+, and the different products.
Both animal muscle and yeast use glycolysis to make a net 2 ATP per glucose in the cytosol, producing 2 pyruvate and 2 NADH. Because there is no oxygen, the electron transport chain cannot run, so NADH must be reoxidised to NAD+ to keep glycolysis going.
In animal muscle cells, pyruvate is reduced to lactate (lactic acid) by lactate dehydrogenase, regenerating NAD+. No CO2 is released.
In yeast cells, pyruvate is first decarboxylated to acetaldehyde, releasing CO2, then reduced to ethanol by alcohol dehydrogenase, regenerating NAD+.
Both yield only 2 ATP per glucose (the glycolytic net), far less than aerobic respiration (around 30 to 32 ATP).
2026 VCE2 marksExplain why oxygen is described as the final electron acceptor in aerobic cellular respiration.Show worked answer →
A 2-mark answer needs the role at the end of the chain and the consequence if oxygen is missing.
Oxygen accepts electrons at the end of the electron transport chain on the inner mitochondrial membrane. It combines with these electrons and H+ ions to form water. By removing electrons, oxygen keeps the chain flowing, so H+ continues to be pumped and ATP synthase continues to produce ATP.
Without oxygen, electrons back up, NADH and FADH2 cannot be reoxidised, the chain stops, and ATP production by oxidative phosphorylation halts. The cell must rely on glycolysis and fermentation.
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