Cellular Respiration Explained
Every time you move, think, or breathe, your cells are burning glucose to make ATP — the universal energy currency of life. Here is exactly how that happens.
The Big Picture
Cellular respiration is the process by which cells break down glucose (C₆H₁₂O₆) and oxygen to release energy stored in chemical bonds, producing carbon dioxide and water as byproducts. The overall equation is:
But this single equation hides three distinct stages, each happening in a different part of the cell and each contributing differently to the final ATP count.
Adenosine triphosphate (ATP) is a small molecule that stores energy in its phosphate bonds. When a cell needs energy for any task — contracting a muscle, building a protein, firing a nerve — it breaks off a phosphate group from ATP, releasing a usable burst of energy and leaving ADP (adenosine diphosphate). Respiration re-charges ADP back into ATP.
Stage 1 — Glycolysis (Cytoplasm)
Glycolysis means "sugar splitting." It takes place in the fluid of the cell (the cytoplasm), requires no oxygen, and is the oldest energy-harvesting pathway on Earth — present in virtually every living organism.
One glucose molecule (6 carbons) is split into two molecules of pyruvate (3 carbons each). The net yield of this stage is:
- 2 ATP (net; 4 produced, 2 invested)
- 2 NADH — electron carriers that will donate electrons later
- 2 pyruvate molecules, which pass to the next stage
If oxygen is not available, glycolysis is as far as the cell can go. It then switches to fermentation to recycle the NADH, accepting a much lower ATP yield. When oxygen is present, pyruvate moves into the mitochondria.
Stage 2 — The Krebs Cycle (Mitochondrial Matrix)
Before entering the Krebs cycle (also called the citric acid cycle), each pyruvate is converted to acetyl-CoA, releasing one CO₂ and one NADH per pyruvate — two of each total.
Acetyl-CoA then enters a cycle of eight enzyme-controlled reactions inside the mitochondria. For each turn of the cycle:
- 2 carbons enter (from acetyl-CoA) and 2 CO₂ are released
- 3 NADH and 1 FADH₂ are produced (more electron carriers)
- 1 ATP is produced directly
Because one glucose molecule yields two acetyl-CoA, the cycle turns twice per glucose, producing a total of 2 ATP, 6 NADH, and 2 FADH₂ at this stage. The carbon from the original glucose has now all been released as CO₂.
Stage 3 — Electron Transport Chain & Oxidative Phosphorylation (Inner Mitochondrial Membrane)
This is where the bulk of ATP is made — roughly 26–28 of the total ~30 ATP per glucose. The electron carriers NADH and FADH₂ donate their electrons to a series of protein complexes embedded in the inner mitochondrial membrane.
As electrons pass down the chain, energy is released and used to pump hydrogen ions (H⁺) from the matrix into the intermembrane space, building up a concentration gradient. Those ions then flow back through an enzyme called ATP synthase — a remarkable molecular turbine — and the flow drives the production of ATP. This process is called chemiosmosis.
At the end of the chain, the electrons are accepted by oxygen (O₂), which combines with H⁺ to form water. This is why you need to breathe oxygen: it is the final electron acceptor, without which the chain stalls.
Think of NADH as a rechargeable battery charged up in glycolysis and the Krebs cycle. The electron transport chain is the device that drains those batteries to do useful work — making ATP — and oxygen is needed to dispose of the spent electrons at the end.
Total ATP Yield
The theoretical maximum from one glucose molecule under aerobic conditions is approximately 30–32 ATP. In practice, mitochondrial inefficiencies mean most cells produce around 29–30 ATP. The breakdown is roughly:
- Glycolysis: 2 ATP (direct)
- Pyruvate oxidation + Krebs cycle: 2 ATP (direct)
- Electron transport chain: ~26–28 ATP (from NADH and FADH₂)
Aerobic vs. Anaerobic Respiration
When oxygen is absent or scarce — during intense exercise, for instance — cells fall back on anaerobic respiration. Glycolysis still runs, but the pyruvate is converted to lactate (in animals) or ethanol and CO₂ (in yeast) rather than entering the Krebs cycle. The payoff is only 2 ATP per glucose, which is why sprinting is unsustainable and why yeast can make beer.
Key Terms to Know
- ATP — adenosine triphosphate; cellular energy currency
- NADH / FADH₂ — electron carriers; "charged" forms of NAD⁺ and FAD
- Pyruvate — 3-carbon product of glycolysis
- Acetyl-CoA — 2-carbon unit that feeds the Krebs cycle
- Chemiosmosis — ATP synthesis driven by ion gradient across a membrane
- ATP synthase — enzyme that makes ATP using the H⁺ gradient
Summary
Cellular respiration has three stages: glycolysis in the cytoplasm (2 ATP net), the Krebs cycle in the mitochondrial matrix (2 ATP + electron carriers), and the electron transport chain on the inner mitochondrial membrane (~26–28 ATP). Oxygen is consumed only at the final step, accepting spent electrons to form water. Without it, cells revert to inefficient anaerobic pathways. The whole system exists to recharge ATP — the universal energy molecule that powers virtually every cellular function.