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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.

Generated by Claude Opus 4.813 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. ATP yield at a glance
  4. Examples in context
  5. Try this

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 diagram traces aerobic respiration through its three stages, the cell location of each, and the net ATP yield.

Aerobic cellular respiration overview Three sequential boxes: glycolysis in the cytosol, Krebs cycle in the mitochondrial matrix, oxidative phosphorylation on the inner mitochondrial membrane. Arrows show glucose moving through pyruvate and acetyl coenzyme A. Net ATP yields are two from glycolysis, two from Krebs, and about twenty six from oxidative phosphorylation, total around thirty to thirty two ATP per glucose. Aerobic cellular respiration 1. Glycolysis Cytosol Glucose (6C) → 2 pyruvate (3C) +2 ATP, +2 NADH 2. Krebs cycle Mitochondrial matrix Pyruvate → acetyl-CoA → CO₂ + electron carriers +2 ATP, +6 NADH, +2 FADH₂ 3. Oxidative phosphorylation Inner mitochondrial membrane NADH/FADH₂ donate e⁻ → H⁺ gradient + O₂ +~26 ATP, H₂O released Total: ~30 to 32 ATP per glucose C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP Anaerobic respiration stops after step 1 and yields only 2 ATP.

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
  1. NADH and FADH2 deliver electrons to membrane protein complexes.
  2. 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.
  3. O2 is the final electron acceptor. It combines with electrons and H+ to form H2O.
  4. 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

Examples in context

Example 1. Anaerobic fermentation in Yarra Valley wineries. Yarra Valley winemakers rely on Saccharomyces cerevisiae yeasts fermenting grape sugars anaerobically. Glycolysis runs in the yeast cytoplasm, splitting one glucose into two pyruvate, two ATP and two NADH. Without oxygen, the Krebs cycle and ETC do not operate; instead, pyruvate is converted to ethanol and carbon dioxide, regenerating NAD+ so glycolysis can continue. Net yield: 2 ATP per glucose - one twentieth of aerobic respiration. Winemakers monitor sugar drop (Baume scale) and temperature to control fermentation, since the yeast struggle above 30 degrees C and fail above 12 percent alcohol when ethanol denatures their enzymes. The wine is the metabolic waste of anaerobic yeast.

Example 2. Muscle physiology in Cadel Evans hill stages. Sports scientists at the Victorian Institute of Sport explain why cyclists fatigue on Cadel Evans Great Ocean Road Race climbs. At low effort, leg muscles use aerobic respiration: glycolysis followed by Krebs cycle in mitochondrial matrix and ETC on the inner membrane, yielding around 36 ATP per glucose. On a steep climb, oxygen demand exceeds supply. Glycolysis continues but pyruvate is reduced to lactate in the cytoplasm, regenerating NAD+ and yielding 2 ATP per glucose. Lactate accumulation lowers pH, inhibits enzymes and is felt as the "burn". Recovery requires the Cori cycle in the liver, which uses ATP to convert lactate back to glucose.

Try this

Q1. State the location, net ATP yield and main output of glycolysis, the Krebs cycle and the electron transport chain. [4 marks]

  • Cue. Glycolysis: cytoplasm, 2 ATP net, 2 pyruvate plus 2 NADH. Krebs cycle: mitochondrial matrix, 2 ATP, NADH, FADH2, CO2. ETC: inner membrane, around 32 to 34 ATP, water.

Q2. One glucose yields about 36 ATP under aerobic conditions but only 2 ATP under anaerobic. (a) Calculate the efficiency loss percentage. (b) Explain why yeasts can still grow vigorously despite the lower ATP. [3 marks]

  • Cue. (a) Around 94 percent less ATP. (b) Substrates are abundant in grape juice; yeasts compensate by consuming much more glucose per unit time.

Q3. Refer to a Yarra Valley fermentation. (a) Write the word equation for ethanolic fermentation. (b) Predict what happens to the yeast at 14 percent alcohol. (c) Explain why CO2 is bubbled out of an open fermenter but not from a sealed wine bottle. [2+2+2 marks]

  • Cue. (a) Glucose to ethanol + CO2 + ATP. (b) Ethanol denatures membrane proteins and enzymes; yeast cease activity. (c) CO2 dissolves under pressure in sealed bottle (sparkling wine if held); escapes in open vessel.

Exam-style practice questions

Practice questions written in the style of VCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2022 VCE4 marksDescribe the location, inputs and outputs of each stage of aerobic cellular respiration.
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A 4-mark answer needs all three stages with location, inputs and outputs.

  1. 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.
  2. 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.
  3. 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.
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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).

2025 VCAA-style2 marksExplain why oxygen is described as the final electron acceptor in aerobic cellular respiration.
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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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