Cellular Respiration: Glycolysis to Oxidative Phosphorylation

Core Concept

Cellular respiration is the controlled, stepwise oxidation of glucose to CO₂ and H₂O, releasing free energy that is captured as ATP. It is a series of redox reactions in which electrons (carried by NADH and FADH₂) are stripped from fuel and passed, in many small steps, to the final electron acceptor oxygen. The energy released at each step is used — first to make a little ATP directly, and finally to pump protons and drive ATP synthase. The overall reaction is:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (~30–32 ATP)

Key Concepts

Redox and Electron Carriers

Oxidation is the loss of electrons; reduction is the gain of electrons (remember OIL RIG). In respiration, glucose is oxidized (loses electrons), and O₂ is reduced (gains electrons, becoming H₂O). The electron carriers NAD⁺ and FAD pick up electrons (plus protons) to become NADH and FADH₂, shuttling the high-energy electrons to the electron transport chain.

Stage 1: Glycolysis (cytosol)

Glucose (6 carbons) is split into 2 pyruvate (3 carbons each) through a series of 10 enzyme-catalyzed steps. Glycolysis does not require oxygen and yields a net of 2 ATP (by substrate-level phosphorylation) and 2 NADH per glucose. It is the ancient, universal pathway shared by nearly all organisms.

Stage 2: Pyruvate Oxidation (mitochondrial matrix)

Each pyruvate is transported into the mitochondrial matrix and converted to acetyl-CoA, releasing 1 CO₂ and producing 1 NADH per pyruvate (so 2 CO₂ and 2 NADH per glucose). Acetyl-CoA is the two-carbon fuel that enters the next stage.

Stage 3: The Citric Acid (Krebs) Cycle (mitochondrial matrix)

Acetyl-CoA (2 C) combines with oxaloacetate (4 C) to form citrate (6 C), which is then progressively oxidized, regenerating oxaloacetate. For each acetyl-CoA (two per glucose), the cycle produces 2 CO₂, 3 NADH, 1 FADH₂, and 1 GTP (≈ ATP). Per glucose: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.

Stage 4: Oxidative Phosphorylation (inner mitochondrial membrane)

This stage has two linked parts:

• Electron transport chain (ETC): NADH and FADH₂ donate electrons to a series of protein complexes (I–IV) embedded in the inner membrane. As electrons fall to lower energy levels, the complexes pump protons (H⁺) from the matrix into the intermembrane space, building a proton gradient. Finally, electrons are passed to O₂, the terminal acceptor, which is reduced to water.
• Chemiosmosis: the proton gradient is potential energy — the proton-motive force. Protons flow back into the matrix only through ATP synthase, a molecular turbine whose rotation phosphorylates ADP to ATP.

This stage produces the vast majority of the ATP (~26–28 molecules).

ATP Accounting and Why It Varies

Substrate-level phosphorylation gives a fixed 4 ATP (2 from glycolysis, 2 from the citric acid cycle). Oxidative phosphorylation gives a variable amount because (a) the number of protons pumped per NADH/FADH₂ differs slightly by cell type, (b) the NADH made in glycolysis must be shuttled into the mitochondrion (the malate-aspartate vs. glycerol-phosphate shuttle), and (c) protons are also used for other transport. Hence the commonly cited ~30–32 ATP per glucose, not a single exact number.

How It Works

(1) Glycolysis partially oxidizes glucose to two pyruvate in the cytosol, capturing a little energy. (2) In the matrix, pyruvate is decarboxylated and linked to coenzyme A; the electrons harvested become NADH. (3) The citric acid cycle fully oxidizes the acetyl carbons to CO₂, loading NAD⁺ and FAD into NADH and FADH₂. (4) These carriers hand their electrons to the ETC; as electrons descend the chain, energy is used to pump H⁺ outward, creating a gradient. (5) H⁺ flows back through ATP synthase, driving the synthesis of most of the cell's ATP. Oxygen's only role is at the very end — accepting electrons so the chain keeps running. Remove O₂ and everything backs up (which is why fermentation, covered separately, is needed to recycle NAD⁺ in the absence of oxygen).

How it works

(1) Glycolysis partially oxidizes glucose to two pyruvate in the cytosol, capturing a little energy. (2) In the matrix, pyruvate is decarboxylated and linked to coenzyme A; the electrons harvested become NADH. (3) The citric acid cycle fully oxidizes the acetyl carbons to CO₂, loading NAD⁺ and FAD into NADH and FADH₂. (4) These carriers hand their electrons to the ETC; as electrons descend the chain, energy is used to pump H⁺ outward, creating a gradient. (5) H⁺ flows back through ATP synthase, driving the synthesis of most of the cell's ATP. Oxygen's only role is at the very end — accepting electrons so the chain keeps running. Remove O₂ and everything backs up (which is why fermentation, covered separately, is needed to recycle NAD⁺ in the absence of oxygen).

Common confusions

• "Respiration is the same as breathing." Wrong — cellular respiration is the metabolic oxidation of glucose; breathing is the organism-level gas exchange that supplies O₂ and removes CO₂.
• "Glycolysis requires oxygen." Wrong — glycolysis is anaerobic and runs with or without oxygen.
• "The citric acid cycle makes most of the ATP directly." Wrong — it makes only 2 ATP (as GTP); most ATP comes later from oxidative phosphorylation.
• "Oxygen is used in glycolysis or the Krebs cycle." Wrong — O₂ is used only at the end of the electron transport chain, as the final electron acceptor.
• "Exactly 38 ATP are produced." Wrong — modern estimates are ~30–32, and the number varies by cell type and shuttle used.
• "FADH₂ yields the same ATP as NADH." Wrong — FADH₂ enters the ETC later and drives less proton pumping, so it yields less ATP.

Quick review

• Redox: glucose oxidized, O₂ reduced to water; NAD⁺/FAD are electron carriers.
• Glycolysis (cytosol): 2 pyruvate, 2 ATP, 2 NADH.
• Pyruvate oxidation (matrix): acetyl-CoA + CO₂ + NADH.
• Citric acid cycle (matrix): 2 CO₂, 3 NADH, 1 FADH₂, 1 GTP per turn.
• ETC (inner membrane): electrons → O₂; protons pumped to build gradient.
• Chemiosmosis: H⁺ flows through ATP synthase → most ATP.
• Total ~30–32 ATP per glucose, variable.