Cells break down glucose to produce ATP, the molecule that powers nearly every cellular process. The main route, aerobic respiration, runs through four linked stages (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation) and yields up to 36–38 ATP per glucose molecule. When oxygen is unavailable, cells fall back on anaerobic respiration or fermentation, which produce far less ATP but keep the process running.
Difficulty: Introductory to Intermediate
Prerequisites: Basic understanding of cell structure (what a mitochondrion is, what the cytoplasm is). Familiarity with ATP as an energy currency helps.
Cellular respiration is the process cells use to convert the chemical energy stored in food (mainly glucose) into ATP, which drives everything from muscle contraction to nerve signalling. It sits at the centre of metabolism: the macronutrients you eat (carbohydrates, proteins, fats) are all funnelled through these pathways. You should already know what a cell looks like and where its major organelles sit, because where each stage happens (cytoplasm vs. mitochondrial matrix vs. inner mitochondrial membrane) matters for understanding the logic of the pathway. This topic connects directly to photosynthesis (which builds the glucose that respiration breaks down) and to enzyme kinetics (since every step is enzyme-catalysed).
ATP (Adenosine Triphosphate)
The cell's main energy currency. Energy is released when the terminal phosphate bond is broken, converting ATP to ADP. Think of it as a rechargeable battery that every cellular process draws on.
Glycolysis
The splitting of one 6-carbon glucose molecule into two 3-carbon pyruvate molecules, producing a net gain of 2 ATP and 2 NADH. In simple terms, this is the first stage of breaking down sugar, and it happens in the cytoplasm without needing oxygen.
Pyruvate
The 3-carbon end product of glycolysis. Its fate depends on whether oxygen is available: with oxygen it enters the mitochondria; without, it is converted via fermentation.
Acetyl CoA (Acetyl Coenzyme A)
The 2-carbon molecule formed when pyruvate is oxidised and joined to coenzyme A during the link reaction. Think of it as the "entry ticket" to the Krebs cycle.
Link Reaction (Pyruvate Oxidation)
The bridge between glycolysis and the Krebs cycle. Each pyruvate loses one carbon as CO₂, is oxidised (producing NADH), and the remaining 2-carbon fragment bonds to coenzyme A.
Krebs Cycle (Citric Acid Cycle / TCA Cycle)
A cyclic series of reactions in the mitochondrial matrix that fully oxidises the acetyl group from acetyl CoA, releasing CO₂ and generating NADH, FADH₂ and a small amount of ATP (as GTP). In simple terms, this is where the carbon skeleton of glucose is completely dismantled.
NADH and FADH₂
Reduced coenzymes that carry high-energy electrons to the electron transport chain. Think of them as electron shuttle buses: NADH carries a slightly higher energy load than FADH₂.
Electron Transport Chain (ETC)
A series of protein complexes embedded in the inner mitochondrial membrane that pass electrons from NADH and FADH₂ down an energy gradient, pumping H⁺ ions across the membrane as they go.
Oxidative Phosphorylation
The process by which the H⁺ gradient generated by the ETC drives ATP synthase to produce large quantities of ATP. This is the stage that requires oxygen as the final electron acceptor, and it is where the bulk of ATP is made.
Anaerobic Respiration
A form of cellular respiration that uses an electron transport chain but substitutes a molecule other than oxygen (such as sulphate, nitrate or carbonate) as the final electron acceptor. Yields less ATP than aerobic respiration.
Fermentation
A pathway that regenerates NAD⁺ from NADH so glycolysis can continue when no electron transport chain is available. It uses an organic molecule as the electron acceptor and produces either ethanol (alcoholic fermentation) or lactate (lactic acid fermentation). Net yield: 2 ATP per glucose.
Substrate-Level Phosphorylation
Direct transfer of a phosphate group from a substrate molecule to ADP, forming ATP, without involving the electron transport chain. This is how ATP is made in glycolysis and the Krebs cycle.
Chemiosmosis
The movement of H⁺ ions down their concentration gradient through ATP synthase, driving ATP production. In simple terms, the proton gradient acts like water behind a dam, and ATP synthase is the turbine.
Tags: glycolysis, sugar splitting, glucose breakdown, cytoplasm, 10-step pathway
Location: Cytoplasm (does not require a mitochondrion, so it occurs in virtually all living cells).
Input: 1 glucose (6C), 2 ATP, 2 NAD⁺
Output: 2 pyruvate (3C), 4 ATP (net gain of 2 ATP), 2 NADH
Glycolysis has two phases:
Energy investment phase (steps 1–5): 2 ATP are spent to phosphorylate glucose and split it into two 3-carbon molecules (glyceraldehyde-3-phosphate).
Energy payoff phase (steps 6–10): Each G3P is oxidised, generating 2 NADH total and 4 ATP total by substrate-level phosphorylation.
The net ATP gain is 2 per glucose because 2 were invested at the start.
Glycolysis does not require oxygen. It is the one stage shared by aerobic respiration, anaerobic respiration and fermentation.
The 2 NADH produced here will later deliver their electrons to the electron transport chain (if oxygen is available), contributing further ATP.
Tags: link reaction, pyruvate oxidation, transition reaction, pyruvate decarboxylation, acetyl CoA formation
Location: Mitochondrial matrix
Input (per pyruvate): 1 pyruvate (3C), 1 NAD⁺, 1 coenzyme A
Output (per pyruvate): 1 acetyl CoA (2C), 1 CO₂, 1 NADH
Because glycolysis produces 2 pyruvates per glucose, the link reaction runs twice per glucose molecule. That gives 2 acetyl CoA, 2 CO₂ and 2 NADH per glucose.
This step is irreversible. The carbon lost as CO₂ is gone for good, which is why fatty acids (which enter as acetyl CoA) cannot be converted back to glucose in animals.
The link reaction is sometimes called the "transition reaction" or "pyruvate decarboxylation." All three names refer to the same process.
No ATP is produced directly in this stage. Its purpose is to prepare the 2-carbon acetyl group for the Krebs cycle and to harvest one pair of electrons (as NADH) per pyruvate.
Tags: Krebs cycle, citric acid cycle, TCA cycle, tricarboxylic acid cycle, mitochondrial matrix
Location: Mitochondrial matrix
Input (per turn): 1 acetyl CoA (2C), 3 NAD⁺, 1 FAD, 1 ADP (or GDP)
Output (per turn): 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP (or GTP)
The cycle runs twice per glucose (once for each acetyl CoA).
Per glucose, the Krebs cycle therefore produces: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP (or 2 GTP).
The 2-carbon acetyl group enters by combining with the 4-carbon oxaloacetate to form 6-carbon citrate. Through a series of oxidations and decarboxylations, two carbons are released as CO₂ and oxaloacetate is regenerated, closing the cycle.
The main role of the Krebs cycle is not direct ATP production. It is the generation of NADH and FADH₂, which carry their electrons to the electron transport chain where the large ATP payoff occurs.
All six carbons of the original glucose molecule have now been released as CO₂ (2 in the link reaction, 4 in the Krebs cycle). From this point on, the energy is carried entirely by the reduced coenzymes.
Tags: oxidative phosphorylation, electron transport chain, ETC, chemiosmosis, ATP synthase, inner mitochondrial membrane, oxygen, final electron acceptor
Location: Inner mitochondrial membrane
Input: NADH, FADH₂, O₂, ADP + Pᵢ
Output: NAD⁺, FAD, ATP (approximately 26 ATP per glucose), H₂O
Electrons from NADH and FADH₂ are passed through a series of membrane protein complexes (Complexes I–IV). As they move, energy is released and used to pump H⁺ ions from the matrix into the intermembrane space.
This creates a steep electrochemical gradient (the proton-motive force). H⁺ ions flow back into the matrix through ATP synthase, which harnesses that flow to phosphorylate ADP into ATP. This coupling of electron transport to ATP synthesis via a proton gradient is chemiosmosis.
Oxygen is the final electron acceptor at the end of the chain. It picks up the spent electrons along with H⁺ ions to form water. Without oxygen, the chain stalls, NADH and FADH₂ cannot be re-oxidised, and the Krebs cycle stops as well.
NADH donates electrons at Complex I and yields roughly 2.5 ATP per molecule. FADH₂ enters at Complex II and yields roughly 1.5 ATP per molecule. This is why NADH is worth more in terms of ATP.
The total from oxidative phosphorylation (approximately 26 ATP) accounts for the vast majority of the 36–38 ATP produced per glucose in aerobic respiration.
The 36–38 range (rather than a single number) arises because the NADH produced in glycolysis is in the cytoplasm and must be shuttled into the mitochondria. Depending on which shuttle system is used, each cytoplasmic NADH yields either 1.5 or 2.5 ATP.
Tags: anaerobic respiration, fermentation, lactic acid fermentation, alcoholic fermentation, ethanol, lactate, NAD⁺ regeneration
These are the fallback pathways when oxygen is limited or absent.
Anaerobic respiration
Still uses an electron transport chain, but the final electron acceptor is an inorganic molecule other than O₂, such as sulphate (SO₄²⁻), nitrate (NO₃⁻) or carbonate (CO₃²⁻).
Still runs glycolysis, the link reaction and the Krebs cycle.
ATP yield varies (2–36 ATP depending on the electron acceptor), but is always less than aerobic respiration because alternative acceptors release less energy than oxygen.
Found mainly in certain bacteria and archaea, not in human cells.
Fermentation
Does not use an electron transport chain at all. Uses organic compounds as the electron acceptor.
Only glycolysis produces ATP (net 2 ATP per glucose). The purpose of the fermentation reactions that follow is to regenerate NAD⁺ so glycolysis can keep running.
Two main types:
Alcoholic (ethanol) fermentation: Pyruvate is converted to ethanol and CO₂. Used by yeast and some bacteria. This is the basis of brewing and bread-making.
Lactic acid fermentation: Pyruvate is converted to lactate. Occurs in human muscle cells during intense exercise when oxygen delivery cannot keep up with demand, and in certain bacteria (used in yogurt and cheese production).
Fermentation is far less efficient than aerobic respiration (2 ATP vs. 36–38 ATP), but it is fast and requires no oxygen.
Overall equation for aerobic respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (as ATP + heat)
ATP yield summary per glucose molecule (aerobic respiration):
Stage | ATP produced | NADH produced | FADH₂ produced | CO₂ released |
|---|---|---|---|---|
Glycolysis | 2 (net) | 2 | 0 | 0 |
Link reaction (×2) | 0 | 2 | 0 | 2 |
Krebs cycle (×2) | 2 | 6 | 2 | 4 |
Oxidative phosphorylation | ~26 | 0 (consumes 10 NADH) | 0 (consumes 2 FADH₂) | 0 |
Total | ~30–32 directly + 4 substrate-level = 36–38 | 6 |
Maximum net ATP yield by pathway type:
Pathway | Maximum net ATP per glucose |
|---|---|
Aerobic respiration | 36–38 |
Anaerobic respiration | 2–36 (varies by electron acceptor) |
Fermentation | 2 |
Brewing and baking: Yeast performs alcoholic fermentation, producing the ethanol in beer and wine and the CO₂ that makes bread rise.
Exercise physiology: When you sprint, your muscles outstrip their oxygen supply and switch to lactic acid fermentation. The lactate build-up contributes to the burning sensation and fatigue. Recovery involves re-oxidising the lactate once oxygen is available again.
Cyanide poisoning: Cyanide blocks Complex IV of the electron transport chain. Without a functioning ETC, cells cannot produce ATP via oxidative phosphorylation and rapidly run out of energy, which is why cyanide is so lethal.
Food preservation: Lactic acid fermentation by bacteria is the basis of yogurt, sauerkraut and kimchi. The acidic environment inhibits the growth of spoilage organisms.
Students often think glycolysis requires oxygen. It does not. Glycolysis is anaerobic and occurs in the cytoplasm regardless of whether oxygen is present.
Students often confuse anaerobic respiration with fermentation. They are different processes. Anaerobic respiration still uses an electron transport chain (just with a non-oxygen acceptor); fermentation does not use an ETC at all.
Students often believe oxygen is "used up" in the Krebs cycle. Oxygen is not directly involved until oxidative phosphorylation, where it serves as the final electron acceptor. The Krebs cycle itself does not consume O₂.
Students often think lactic acid causes muscle soreness the day after exercise. The lactate is cleared within an hour or so of stopping. Delayed-onset muscle soreness is caused by microscopic muscle fibre damage, not lactate accumulation.
⚠️ Know the ATP yield at each stage and be able to account for the total 36–38. This is one of the most commonly tested calculations in intro biology.
⚠️ Be able to explain why the yield is a range (36–38) rather than a fixed number. The answer involves the NADH shuttle systems (malate-aspartate vs. glycerol-3-phosphate).
⚠️ Understand the role of oxygen as the final electron acceptor. A common exam question: "What happens to cellular respiration if oxygen is removed?" Answer: oxidative phosphorylation stops, NADH/FADH₂ accumulate, NAD⁺/FAD are not regenerated, the Krebs cycle stalls, and the cell falls back on fermentation.
⚠️ Be clear on where each stage occurs. Glycolysis in the cytoplasm; link reaction and Krebs cycle in the mitochondrial matrix; oxidative phosphorylation on the inner mitochondrial membrane.
⚠️ Know the difference between substrate-level phosphorylation and oxidative phosphorylation. Both make ATP, but by completely different mechanisms.
True or False: Glycolysis requires oxygen. (False. It occurs in the cytoplasm and is entirely anaerobic.)
Fill in the blank: The final electron acceptor in aerobic respiration is ________. (Oxygen / O₂)
True or False: The Krebs cycle produces more ATP directly than oxidative phosphorylation. (False. The Krebs cycle produces only 2 ATP per glucose; oxidative phosphorylation produces roughly 26.)
Fill in the blank: Fermentation regenerates ________ so that glycolysis can continue. (NAD⁺)
True or False: Anaerobic respiration and fermentation are the same thing. (False. Anaerobic respiration uses an ETC with a non-oxygen acceptor; fermentation does not use an ETC.)
Q: Name the four stages of aerobic respiration in order and state where each occurs.
A: Glycolysis (cytoplasm), link reaction (mitochondrial matrix), Krebs cycle (mitochondrial matrix), oxidative phosphorylation (inner mitochondrial membrane).
Q: Explain why cells produce only 2 ATP per glucose during fermentation, compared with 36–38 during aerobic respiration.
A: In fermentation, only glycolysis generates ATP (2 net). The electron transport chain does not operate, so the large ATP yield from oxidative phosphorylation is lost. The fermentation step itself produces no ATP; its sole purpose is to regenerate NAD⁺.
Q: What is the role of NAD⁺ in cellular respiration?
A: NAD⁺ is an electron carrier. It is reduced to NADH during glycolysis, the link reaction and the Krebs cycle by accepting electrons from substrate molecules. These electrons are later donated to the electron transport chain, where their energy is used to drive ATP synthesis.
Q: A student claims that the Krebs cycle cannot run without oxygen, yet oxygen does not appear in any Krebs cycle reaction. Resolve this apparent contradiction.
A: The Krebs cycle itself does not use oxygen directly. However, it depends on a supply of NAD⁺ and FAD, which are regenerated when NADH and FADH₂ donate their electrons to the oxygen-dependent electron transport chain. Without oxygen, the ETC stalls, the coenzymes stay reduced, and the Krebs cycle has no oxidised coenzymes to accept electrons, so it stops.
Q: Compare and contrast anaerobic respiration and fermentation.
A: Both operate when oxygen is unavailable. Anaerobic respiration still uses an electron transport chain, with an inorganic molecule other than oxygen (e.g. sulphate, nitrate) as the final electron acceptor; it can therefore still run the Krebs cycle and produce a variable amount of ATP. Fermentation does not use an ETC at all; it relies solely on glycolysis for ATP (2 net) and uses an organic molecule to regenerate NAD⁺, producing either ethanol or lactate as a by-product.
This material connects directly to photosynthesis, which is essentially the reverse process: photosynthesis uses light energy to build glucose from CO₂ and H₂O, while cellular respiration breaks glucose back down. The two cycles are complementary, and understanding the inputs/outputs of one helps you learn the other.
It also connects to enzyme kinetics and regulation. Every step of glycolysis and the Krebs cycle is enzyme-catalysed, and key regulatory enzymes (phosphofructokinase in glycolysis, isocitrate dehydrogenase in the Krebs cycle) are subject to feedback inhibition by ATP and other products. Understanding allosteric regulation of these enzymes explains how cells match ATP production to demand.
Finally, the metabolic overview diagram in the source material shows how proteins and fats also feed into these pathways. Amino acids can enter at various points (some as pyruvate, some as acetyl CoA, some as Krebs cycle intermediates), and fatty acids are broken down via beta-oxidation into acetyl CoA. This is the foundation for understanding metabolism as a whole, not just carbohydrate breakdown.
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