Difficulty: Introductory | Prerequisites: Photosynthesis notes (Units 5–6), basic cell organelle knowledge.
Cellular respiration is how cells extract usable energy (ATP) from glucose. It is, in effect, the reverse of photosynthesis: where photosynthesis builds glucose from CO₂ and H₂O using light energy, cellular respiration breaks glucose back down, releasing that stored energy. In a biology course, these two processes are taught together because the products of one are the reactants of the other. You should already understand photosynthesis, ATP structure and the general idea that energy is transferred through electron carriers.
Cells break down glucose through glycolysis, the Krebs Cycle and the electron transport chain, producing up to 36 ATP per glucose molecule. Oxygen is the final electron acceptor, which is why the process requires O₂ (aerobic). Without oxygen, cells fall back on fermentation, which yields far less energy.
Cellular Respiration
The metabolic process that breaks down glucose in the presence of oxygen to produce ATP, CO₂ and H₂O. In simple terms, this is how your cells "burn" food for energy.
Glycolysis
The first stage of cellular respiration, occurring in the cytoplasm, in which one glucose molecule is split into two pyruvate molecules, yielding a net gain of 2 ATP and 2 NADH. Think of it as the opening act: it does not need oxygen and happens in every living cell.
Krebs Cycle (Citric Acid Cycle)
A series of chemical reactions in the mitochondrial matrix that completes the breakdown of pyruvate, producing 2 ATP, electron carriers (NADH, FADH₂) and CO₂ per glucose molecule. In simple terms, this is the stage that strips the remaining energy from what is left of glucose and loads it onto carrier molecules.
Electron Transport Chain (ETC)
A series of protein complexes on the inner mitochondrial membrane that use electrons from NADH and FADH₂ to generate the bulk of ATP (about 32 per glucose). Think of it as the main power generator of the cell.
Fermentation
An anaerobic process that regenerates NAD⁺ from NADH so glycolysis can continue when oxygen is unavailable. Produces either ethanol (alcohol fermentation) or lactic acid (lactic acid fermentation). In simple terms, fermentation is the backup plan when there is no oxygen.
Aerobic
A process that requires oxygen. Think: "aero" = air.
Anaerobic
A process that does not require oxygen. Think: "an-" = without, so "without air."
Oxidation
The loss of electrons from a molecule. In simple terms, when a molecule is oxidised it gives away electrons (and often energy).
Reduction
The gain of electrons by a molecule. In simple terms, when a molecule is reduced it picks up electrons. Remember "OIL RIG": Oxidation Is Loss, Reduction Is Gain.
NAD⁺ / NADH
NAD⁺ is the oxidised form of the electron carrier; NADH is its reduced (electron-loaded) form. NADH delivers high-energy electrons to the electron transport chain. Think of NAD⁺ as the empty taxi and NADH as the taxi carrying a passenger (electrons) to the ETC.
FADH₂
A second electron carrier produced during the Krebs Cycle that also delivers electrons to the ETC, though it yields slightly less ATP than NADH.
Kilocalorie (kcal)
The quantity of heat needed to raise the temperature of 1 kg of water by 1 °C. This is the "Calorie" (capital C) you see on food labels.
Occurs in the cytoplasm of all cells, prokaryotic and eukaryotic.
One 6-carbon glucose molecule is split into two 3-carbon pyruvate molecules.
Uses 2 ATP to start, produces 4 ATP, for a net gain of 2 ATP.
Also produces 2 NADH (which carry electrons to the ETC if oxygen is available).
Does not require oxygen, so it is technically anaerobic.
Takes place in the matrix of the mitochondria.
Each glucose produces two pyruvate molecules, so the Krebs Cycle turns twice per glucose.
Per glucose: produces 2 ATP, 8 NADH (including those from the link reaction) and 2 FADH₂.
Releases 6 CO₂ molecules total.
The main purpose is to load up electron carriers (NADH and FADH₂) for the next stage.
NADH and FADH₂ donate their electrons to a chain of carrier proteins.
As electrons move down the chain, energy is released and used to pump H⁺ ions, creating a gradient that drives ATP synthase.
Produces approximately 32 ATP per glucose.
Oxygen is the final electron acceptor: it combines with electrons and H⁺ to form water.
Without oxygen, the chain stalls and aerobic respiration stops.
Occurs when oxygen is unavailable.
NADH is oxidised back to NAD⁺ so that glycolysis can continue.
Two types:
Lactic acid fermentation: occurs in animal muscle cells during intense exercise. Pyruvate is converted to lactic acid.
Alcohol (ethanol) fermentation: occurs in yeast and some bacteria. Pyruvate is converted to ethanol and CO₂. Animal cells cannot perform this.
Yields only the 2 ATP from glycolysis (no ETC, no Krebs).
Glycolysis: 2 ATP (net)
Krebs Cycle: 2 ATP
Electron Transport Chain: 32 ATP
Total: approximately 36 ATP per glucose molecule
Also produced per glucose: 8 NADH, 2 FADH₂, 6 CO₂
Overall Cellular Respiration Equation
6\,\text{O}_2 + \text{C}_6\text{H}_{12}\text{O}_6 \longrightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{Energy (ATP)}Oxidation and Reduction (OIL RIG)
Oxidation: loss of electrons
Reduction: gain of electrons
Fermentation
NADH → NAD⁺ (NADH is oxidised to regenerate NAD⁺ for glycolysis)
Cellular respiration explains why you breathe harder during exercise: your muscles demand more ATP, so the process speeds up and consumes more oxygen. It is also why food labels list kilocalories (Calories), which measure the energy your body can extract from glucose through respiration. Fermentation is the basis of bread-making (yeast produces CO₂, which makes dough rise) and brewing (yeast produces ethanol).
Students often think glycolysis is aerobic because it is part of cellular respiration. Glycolysis itself does not use oxygen; it occurs whether oxygen is present or not.
Students sometimes believe fermentation produces a large amount of ATP. It does not. The only ATP comes from glycolysis (2 net ATP). Fermentation's purpose is to regenerate NAD⁺.
Students frequently confuse where each stage happens. Glycolysis is in the cytoplasm; the Krebs Cycle is in the mitochondrial matrix; the ETC is on the inner mitochondrial membrane.
Students often mix up oxidation and reduction. Use the mnemonic OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain.
⚠️ Know the overall cellular respiration equation and be able to write it from memory.
⚠️ Be able to state the ATP yield from each stage: 2 (glycolysis) + 2 (Krebs) + 32 (ETC) = 36 total.
⚠️ Know the location of each stage: cytoplasm, mitochondrial matrix, inner mitochondrial membrane.
⚠️ Understand that oxygen is the final electron acceptor in the ETC and that water is produced there.
⚠️ Expect questions comparing aerobic and anaerobic processes, and know that animal cells cannot perform alcohol fermentation.
True or False: Glycolysis requires oxygen. (False. Glycolysis is anaerobic.)
Fill in the blank: The final electron acceptor in the electron transport chain is ________. (oxygen)
True or False: One molecule of glucose produces a total of 36 ATP through cellular respiration. (True.)
Fill in the blank: In fermentation, NADH is oxidised to regenerate ________. (NAD⁺)
True or False: Animal cells can carry out alcohol fermentation. (False. Only yeast and some bacteria can.)
Q: What is the overall equation for cellular respiration?
A: 6 O₂ + C₆H₁₂O₆ → 6 CO₂ + 6 H₂O + Energy (ATP)
Q: Where does glycolysis take place, and what is its net ATP yield?
A: Glycolysis takes place in the cytoplasm and yields a net gain of 2 ATP per glucose molecule.
Q: How many times does the Krebs Cycle turn for each molecule of glucose, and why?
A: Twice, because glycolysis splits one glucose into two pyruvate molecules, each of which enters the cycle separately.
Q: What is the role of oxygen in the electron transport chain?
A: Oxygen serves as the final electron acceptor. It combines with electrons and hydrogen ions to form water.
Q: Why does fermentation produce so much less ATP than aerobic respiration?
A: Fermentation only runs glycolysis (2 ATP). Without oxygen, the Krebs Cycle and ETC cannot operate, so the 34 additional ATP from those stages are never produced.
Q: What is the difference between oxidation and reduction?
A: Oxidation is the loss of electrons; reduction is the gain of electrons (OIL RIG).
Cellular respiration is the direct complement to photosynthesis: the glucose and oxygen photosynthesis produces are the exact inputs respiration requires, and the CO₂ and H₂O respiration releases feed back into photosynthesis. This relationship underpins the carbon and oxygen cycles in ecology. The concept of electron transport chains also reappears in more advanced biochemistry and in topics such as chemiosmosis and oxidative phosphorylation.
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