Difficulty: Intermediate | Prerequisites: Metabolism and Enzymes notes, basic understanding of redox reactions
Cellular respiration is the catabolic pathway that converts the chemical energy in glucose into ATP, the energy currency every cell uses to do work. It is the practical application of the metabolism and thermodynamics principles from the previous set of notes. This topic is one of the most heavily tested in introductory biology, and questions regularly ask you to trace the flow of energy through each stage and account for every ATP produced.
Cellular respiration breaks one glucose molecule into CO2 and water, yielding a total of 38 ATP (in prokaryotes; 36 in most eukaryotes, though this exam uses 38). It proceeds through four stages: glycolysis (cytoplasm, 2 net ATP), the transition reaction (pyruvate to acetyl CoA), the Krebs cycle (mitochondrial matrix, 2 ATP) and the electron transport chain with chemiosmosis (inner mitochondrial membrane, 34 ATP). NADH and FADH2 are the electron carriers that shuttle energy from the first stages to the ETC.
Glycolysis
The first stage of cellular respiration. It splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules. Occurs in the cytoplasm and does not require oxygen (anaerobic). Think of it as the opening act that happens outside the mitochondria.
Pyruvate
The 3-carbon end product of glycolysis. Each glucose produces two pyruvates, which then enter the mitochondria for further processing.
Transition reaction (pyruvate oxidation)
The step between glycolysis and the Krebs cycle. Pyruvate (3 carbons) is converted to acetyl CoA (2 carbons) in the mitochondrial matrix. CO2 is released and NAD+ is reduced to NADH.
Acetyl CoA
A 2-carbon molecule (acetyl group attached to coenzyme A) that enters the Krebs cycle. It is the fuel the cycle runs on.
Krebs cycle (citric acid cycle)
A series of reactions in the mitochondrial matrix that oxidises acetyl CoA to CO2, producing 1 ATP (via GTP), 3 NADH and 1 FADH2 per turn. The cycle turns twice per glucose (one for each pyruvate), so the total yield per glucose is 2 ATP, 6 NADH and 2 FADH2.
Electron transport chain (ETC)
A series of protein complexes (I through IV) embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are passed along the chain, releasing energy that is used to pump protons (H+) into the intermembrane space.
Chemiosmosis
The process by which protons flow back down their concentration gradient through ATP synthase, driving the synthesis of ATP from ADP and inorganic phosphate. This is the mechanism that produces the bulk of the ATP in cellular respiration.
Oxidative phosphorylation
The combined process of the electron transport chain and chemiosmosis. It occurs at the inner mitochondrial membrane and produces approximately 34 ATP per glucose.
ATP synthase
The enzyme complex embedded in the inner mitochondrial membrane through which protons flow during chemiosmosis. It converts ADP + Pi into ATP, functioning like a molecular turbine.
NADH
The reduced form of NAD+. It carries high-energy electrons from glycolysis, the transition reaction and the Krebs cycle to the ETC. Each NADH yields approximately 3 ATP via oxidative phosphorylation.
FADH2
The reduced form of FAD. It carries electrons to the ETC, entering at complex II (rather than complex I like NADH). Each FADH2 yields approximately 2 ATP.
Substrate-level phosphorylation
Direct transfer of a phosphate group from a substrate molecule to ADP, producing ATP without the ETC. This is how ATP is made during glycolysis and the Krebs cycle.
Aerobic
Requiring oxygen. The Krebs cycle and oxidative phosphorylation are aerobic processes.
Anaerobic
Not requiring oxygen. Glycolysis is anaerobic.
Location: cytoplasm
Oxygen required: no (anaerobic)
Input: 1 glucose (6 carbons)
Output: 2 pyruvate (3 carbons each), 2 NADH
ATP: 4 produced by substrate-level phosphorylation, but 2 are invested at the start, so the net gain is 2 ATP
Glycolysis is the universal first step, shared by aerobic respiration, anaerobic respiration and fermentation.
Location: mitochondrial matrix
Input: 1 pyruvate (3 carbons)
Output: 1 acetyl CoA (2 carbons), 1 CO2, 1 NADH
ATP: 0 directly
This happens twice per glucose (once for each pyruvate). The CO2 released here is one reason you exhale carbon dioxide.
Location: mitochondrial matrix
Input: 1 acetyl CoA (per turn)
Output per turn: 3 NADH, 1 FADH2, 1 ATP (via GTP), 2 CO2
Output per glucose (2 turns): 6 NADH, 2 FADH2, 2 ATP
The cycle fully oxidises the carbon skeleton from glucose to CO2. The energy is captured in the electron carriers NADH and FADH2, which carry it to the next stage.
Location: inner mitochondrial membrane (cristae)
Input: NADH and FADH2 from all previous stages
Process: electrons are passed along protein complexes (I, II, III, IV). The energy released pumps H+ ions from the matrix into the intermembrane space, building a proton gradient. Protons flow back through ATP synthase, which converts ADP + Pi into ATP.
Final electron acceptor: 1/2 O2, which combines with protons to form water (H2O). This is why oxygen is essential for aerobic respiration.
ATP yield: approximately 34 ATP
Glycolysis: 2 ATP (net, substrate-level phosphorylation)
Krebs cycle: 2 ATP (substrate-level phosphorylation)
ETC/Chemiosmosis: 34 ATP (oxidative phosphorylation)
Total: 2 + 2 + 34 = 38 ATP
Note: many textbooks cite 36 ATP for eukaryotes because shuttling NADH from the cytoplasm into the mitochondria costs some energy. This exam uses the 38 figure.
Understanding the ETC is central to toxicology. Cyanide poisoning works by blocking complex IV of the electron transport chain, halting ATP production and causing rapid cell death. Carbon monoxide similarly interferes with oxygen delivery to cells.
Students often say glycolysis is aerobic because it is part of aerobic respiration. Glycolysis itself does not require oxygen; it is anaerobic. It happens in the cytoplasm regardless of whether oxygen is present.
Students frequently mix up the location of each stage. Glycolysis is in the cytoplasm. The transition reaction and Krebs cycle are in the mitochondrial matrix. The ETC is on the inner mitochondrial membrane.
Students sometimes think that the 4 ATP produced in glycolysis is the net gain. It is not. Two ATP are invested at the start, so the net gain is 2 ATP.
Students often forget that the final electron acceptor is oxygen (specifically 1/2 O2). Without oxygen, the ETC stalls and oxidative phosphorylation stops.
The net ATP from glycolysis (2, not 4) is a classic trick question.
Expect a question asking where glycolysis occurs and whether it is aerobic or anaerobic. Answer: cytoplasm, anaerobic.
The final electron acceptor (1/2 O2) is a common multiple-choice item.
A long-answer question asking you to trace the full 38 ATP breakdown by stage is highly likely. Memorise the stage-by-stage yield: 2 + 2 + 34.
Know the transition reaction: pyruvate (3C) to acetyl CoA (2C), releasing CO2 and producing NADH.
Oxidative phosphorylation = ETC + chemiosmosis. Know both components and where they occur (inner mitochondrial membrane).
Fill in the blank: Glycolysis occurs in the ________ and is an ________ process. (Cytoplasm, anaerobic.)
True or False: The net gain of ATP from glycolysis is 4 ATP. (False, it is 2 ATP.)
Fill in the blank: The final electron acceptor in the ETC is ________. (1/2 O2, or oxygen.)
True or False: The Krebs cycle occurs on the inner mitochondrial membrane. (False, it occurs in the mitochondrial matrix.)
Fill in the blank: In the transition reaction, pyruvate is converted into ________. (Acetyl CoA.)
Q: What is the net gain of ATP produced specifically during glycolysis?
A: 2 ATP. Although 4 ATP are produced by substrate-level phosphorylation, 2 ATP are consumed in the energy-investment phase, leaving a net gain of 2.
Q: During the ETC, what is the final electron acceptor that combines with protons to produce water?
A: Molecular oxygen (1/2 O2). Electrons from complex IV combine with oxygen and H+ ions to form water.
Q: Explain the role of oxidative phosphorylation in cellular respiration. Where does it occur and what are its two main components?
A: Oxidative phosphorylation occurs at the inner mitochondrial membrane (cristae). Its two components are the electron transport chain, which uses energy from NADH and FADH2 to pump protons into the intermembrane space, and chemiosmosis, where protons flow back through ATP synthase to drive ATP synthesis. Together they produce approximately 34 ATP per glucose.
Q: Briefly explain the transition reaction. What are the reactants and products?
A: The transition reaction occurs in the mitochondrial matrix, between glycolysis and the Krebs cycle. The reactant is pyruvate (a 3-carbon molecule). It is oxidised to acetyl CoA (a 2-carbon molecule attached to coenzyme A). CO2 is released as a byproduct and NAD+ is reduced to NADH.
Q: Explain in detail how a total of 38 ATP is formed during cellular respiration, broken down by stage.
A: Glycolysis (cytoplasm) produces a net gain of 2 ATP via substrate-level phosphorylation, plus 2 NADH. The Krebs cycle (mitochondrial matrix) produces 2 ATP (1 per turn, 2 turns per glucose) via substrate-level phosphorylation, plus 6 NADH and 2 FADH2. The electron transport chain and chemiosmosis (inner mitochondrial membrane) use the NADH and FADH2 from all previous stages. Electrons pass through protein complexes, pumping protons into the intermembrane space. Protons flow back through ATP synthase, producing approximately 34 ATP via oxidative phosphorylation. Total: 2 + 2 + 34 = 38 ATP.
Cellular respiration connects directly to membrane biology: the proton gradient across the inner mitochondrial membrane is the same principle of selective permeability covered in the membrane notes. It also connects to genetics, because mutations affecting mitochondrial enzymes can cause metabolic diseases. Photosynthesis is essentially the reverse process, using light energy to build glucose from CO2 and water.
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