How Does Cellular Respiration Work?

Every breath you take, every step you climb, every thought you think — all of it runs on a molecule called ATP, and cellular respiration is how your cells make it. Here's the whole assembly line, step by step.

Cellular respiration is the set of chemical reactions your cells use to break down glucose and release the energy stored in its bonds. That energy gets captured in ATP (adenosine triphosphate), the universal energy currency of the cell. It happens in three main stages: glycolysis, the Krebs cycle, and the electron transport chain.

What Is Cellular Respiration?

Cellular respiration is essentially the reverse of photosynthesis. Plants capture light energy and store it in glucose; your cells take that glucose and release the energy back out, capturing it as ATP. The overall equation is simple:

That's why you breathe: oxygen is the final electron acceptor at the end of the chain, and carbon dioxide is the waste gas released when glucose is broken down. Without oxygen, the whole process stalls at glycolysis.

Stage 1: Glycolysis — Splitting Glucose

Glycolysis is the oldest and most universal part of cellular respiration. It doesn't need oxygen, and it happens in the cytoplasm of every cell. One glucose molecule (6 carbons) is split into two molecules of pyruvate (3 carbons each).

Glycolysis has two phases:

1. Investment phase: The cell spends 2 ATP to phosphorylate glucose, making it reactive and trapping it inside the cell.
2. Payoff phase: The 6-carbon sugar is split into two 3-carbon molecules, which are oxidized. Each one produces 2 ATP and 1 NADH.

So the net yield is 2 ATP — not much, but it's fast. If oxygen is available, pyruvate moves into the mitochondria for the next two stages.

Stage 2: The Krebs Cycle — Full Oxidation

Before the Krebs cycle can start, pyruvate must be converted into acetyl-CoA. This transition reaction happens in the mitochondrial matrix:

1. Pyruvate loses a carbon dioxide molecule (CO₂), which you exhale.
2. The remaining 2-carbon fragment is oxidized, producing NADH.
3. It attaches to coenzyme A, forming acetyl-CoA.

Acetyl-CoA then enters the Krebs cycle (also called the citric acid cycle or TCA cycle). For each acetyl-CoA that enters, the cycle turns once and produces:

• 2 CO₂ — more waste carbon you exhale
• 3 NADH — electron carriers headed to the ETC
• 1 FADH₂ — another electron carrier
• 1 ATP — direct energy payoff

Because one glucose produces two acetyl-CoA molecules, the Krebs cycle turns twice per glucose, doubling everything:

Stage 3: Electron Transport Chain & Oxidative Phosphorylation

This is where the real energy payoff happens. The NADH and FADH₂ from glycolysis and the Krebs cycle carry high-energy electrons to the electron transport chain (ETC), a series of protein complexes embedded in the inner mitochondrial membrane.

Here's how it works, step by step:

1. Electrons are dropped off: NADH and FADH₂ donate their electrons to the ETC.
2. Electrons travel down the chain: As they move through protein complexes, they release energy.
3. Protons are pumped: That energy pumps H⁺ ions from the matrix into the intermembrane space, building a proton gradient.
4. ATP synthase spins: H⁺ ions flow back through ATP synthase, which spins like a turbine and attaches a phosphate to ADP, making ATP.
5. Oxygen accepts the electrons: At the end of the chain, oxygen picks up electrons and combines with H⁺ to form water (H₂O). Without oxygen, the chain backs up and stops.

The ETC produces roughly 26 to 28 ATP per glucose, which is why this stage matters so much more than the first two combined.

ATP Yield: Where the Numbers Come From

Here's the full accounting for one glucose molecule under aerobic conditions (using modern estimates of about 2.5 ATP per NADH and 1.5 ATP per FADH₂):

Older textbooks often cite 36–38 ATP, but modern measurements show about 30–32 ATP per glucose in most human cells. The difference comes from how many protons it actually takes to make one ATP and how much energy is lost shuttling NADH into the mitochondria.

Aerobic vs. Anaerobic Respiration

When oxygen is scarce — during intense exercise, for example — cells fall back on anaerobic respiration, also called fermentation. It's much faster but far less efficient.

Anaerobic respiration produces only 2 ATP per glucose — the same as glycolysis alone — because the Krebs cycle and ETC never run. That's why sprinting can't last long: you burn through glucose fast and build up lactic acid.

Why This Matters in Real Life

• Exercise: Your body switches between aerobic and anaerobic respiration depending on intensity — the "burn" during a sprint is lactic acid buildup from fermentation.
• Breathing: You breathe faster during exercise because your cells need more O₂ for the ETC and need to expel more CO₂ from the Krebs cycle.
• Metabolism: Cellular respiration is why eating gives you energy — food is broken down into glucose, which enters this exact pathway.
• Medicine: Many poisons and drugs work by blocking specific steps in the ETC (like cyanide, which shuts down cytochrome c oxidase).