6.2 Aerobic Respiration

6.2 Aerobic Respiration

When oxygen is available, a eukaryotic cell does not stop at pyruvate. It oxidizes the fuel the rest of the way, sends the electrons to a membrane chain, and uses that electron flow to make far more ATP than glycolysis alone. This section follows the eukaryotic mitochondrion. Pyruvate oxidation and the citric acid cycle, also called the Krebs cycle, occur in the matrix. The electron-transport chain lies in the inner mitochondrial membrane, including the folds called cristae. Oxygen is the final electron acceptor. It is not a source of carbon.

Pyruvate enters the mitochondrion

Glycolysis released two pyruvate molecules in the cytosol. Each pyruvate crosses into the mitochondrion. In the matrix, pyruvate oxidation removes one carbon as carbon dioxide and attaches the remaining two-carbon unit to coenzyme A, forming acetyl-CoA. The oxidation reduces one NAD+ to one NADH. Per pyruvate the products are one acetyl-CoA, one CO2, and one NADH. Per glucose the cell processes two pyruvate, so this stage yields two acetyl-CoA, two CO2, and two NADH. It makes no ATP. The carbon in that CO2 came from pyruvate, not from oxygen gas.

Two turns of the cycle in the matrix

Each acetyl-CoA donates its two carbons to a four-carbon acceptor, oxaloacetate, and citrate forms. The two carbons that entered later leave as two CO2, and oxaloacetate is regenerated. One turn, for one acetyl-CoA, produces a fixed set of products.

Add the two CO2 already released by pyruvate oxidation and one glucose has released 6 CO2, matching its six carbons. Pyruvate oxidation plus the two turns also produce 8 NADH and 2 FADH2 in the mitochondrion, before counting the separate 2 NADH that glycolysis made in the cytosol.

The cycle's ATP is substrate-level phosphorylation. In many animal cells the immediate product is GTP, which is readily converted to ATP, so the exam count is 1 ATP or GTP per turn and 2 per glucose. The cycle does not consume oxygen. The carbon reactions run in the matrix. The enzyme that forms FADH2 sits in the inner membrane so it can pass electrons into the chain, but that one enzyme does not move the cycle onto the membrane.

The chain, oxygen, and chemiosmosis

NADH and FADH2 are oxidized as their electrons enter the chain. Membrane carriers pass the electrons downhill. Energy from that flow pumps protons out of the matrix into the intermembrane space and builds a proton gradient. Chemiosmosis uses the gradient: protons flow back into the matrix through ATP synthase, and that flow drives ATP formation. This is oxidative phosphorylation. It is not a direct handover of phosphate from a fuel molecule.

Oxygen waits at the end of the chain. Electrons, protons, and oxygen combine, and water forms. If oxygen is absent, electrons stall, the chain stops oxidizing NADH and FADH2, and the gradient collapses. The cycle slows as well, because its NAD+ and FAD are no longer regenerated by the chain. The CO2 released earlier came from the carbon skeleton of pyruvate and acetyl-CoA. Oxygen accepted electrons and became water. It was not used as a carbon source.

Why FADH2 supports less ATP than NADH

NADH donates electrons at the beginning of the chain, so its electrons drive proton pumping at more than one complex. FADH2 feeds in later, at ubiquinone, and skips the first pumping step. Each FADH2 therefore drives fewer protons than each NADH. Both carriers are oxidized, and both routes still end with oxygen.

Why texts say about 30 to 32 ATP

Substrate-level phosphorylation has a definite small total: a net of 2 ATP from glycolysis plus 2 ATP from the cycle, which is 4 ATP per glucose before the chain is considered. Most of the ATP of aerobic respiration comes from chemiosmosis as the reduced carriers are oxidized. Current introductory texts often estimate about 30 to 32 ATP per glucose. Older totals of 36 to 38 used rounder ratios, commonly about 3 ATP per NADH and 2 ATP per FADH2. They also treated the shuttle that moves reducing power from cytosolic NADH into the mitochondrion in different ways. The malate-aspartate shuttle and the glycerol-phosphate shuttle do not deliver the same proton yield, so one integer was never a measured constant of every cell.

CLEP items usually test location, oxygen's role, and which stage makes the NADH, not one universal ATP integer. Pyruvate oxidation in the matrix makes NADH, and the cycle makes still more NADH plus FADH2. Glycolysis makes the cytosolic NADH and does not use oxygen. Neither 36 nor 38 is a required single answer.

A carbon count for one glucose

Start with one glucose and oxygen available. Glycolysis in the cytosol yields two pyruvate, a net of 2 ATP, and 2 NADH. Pyruvate oxidation in the matrix yields two acetyl-CoA, 2 NADH, and 2 CO2. Two turns of the cycle yield 6 NADH, 2 FADH2, 2 ATP, and 4 CO2. The carbon dioxide total is 2 plus 4, which is 6. The mitochondrial NADH from pyruvate oxidation and the cycle is 2 plus 6, which is 8, with 2 FADH2 beside them. Electrons from the carriers reach the inner membrane, oxygen accepts them, and water forms. The large and variable ATP harvest is the chemiosmotic one. The small certain harvest, before any NADH ratio is applied, is 4 substrate-level ATP per glucose.