Cellular Respiration Steps: Glycolysis, Krebs Cycle & ETC Simplified (With ATP Count)
Cellular respiration is how every living cell converts glucose into usable energy — ATP. It’s one of the most tested topics in AP Biology, college intro biology, and the MCAT. This guide walks through all four stages step by step, with a complete ATP tally, molecule tracking, and the key facts examiners love to test.
Overview: The Big Picture
Cellular respiration is a catabolic process — it breaks down glucose (C₆H₁₂O₆) to release the energy stored in its chemical bonds and package it as ATP (adenosine triphosphate), the universal energy currency of the cell.
The overall equation for aerobic cellular respiration is:
This process happens in four stages, each in a specific cellular location:
| Stage | Location | Input | ATP Yield | Key Output |
|---|---|---|---|---|
| 1. Glycolysis | Cytoplasm | 1 Glucose | 2 ATP (net) | 2 Pyruvate, 2 NADH |
| 2. Pyruvate Oxidation | Mito. matrix | 2 Pyruvate | 0 ATP | 2 Acetyl-CoA, 2 NADH, 2 CO₂ |
| 3. Krebs Cycle | Mito. matrix | 2 Acetyl-CoA | 2 ATP | 6 NADH, 2 FADH₂, 4 CO₂ |
| 4. ETC + Ox. Phos. | Inner mito. membrane | 10 NADH, 2 FADH₂ | 26–28 ATP | H₂O |
Key Terms & Molecules You Must Know
Before diving into each stage, lock down these molecules — they appear in every cellular respiration question:
Stage 1 — Glycolysis
Glycolysis literally means “sugar splitting.” It breaks one 6-carbon glucose molecule into two 3-carbon pyruvate molecules through a series of 10 enzyme-catalyzed reactions. No oxygen is required — making it the foundation of both aerobic and anaerobic respiration.
Glycolysis has two phases:
Phase 1: Energy Investment (Steps 1–5)
Phase 2: Energy Payoff (Steps 6–10)
Each step happens twice — once for each G3P molecule.
Stage 2 — Pyruvate Oxidation
Before pyruvate can enter the Krebs cycle, it must be converted into Acetyl-CoA. This transition step is sometimes called the “pyruvate decarboxylation” or “link reaction.” Because glycolysis produces 2 pyruvate molecules, this step runs twice per glucose.
Stage 3 — Krebs Cycle (Citric Acid Cycle)
The Krebs cycle — named after biochemist Hans Krebs who described it in 1937 — is a circular series of 8 reactions. Each turn of the cycle processes one Acetyl-CoA. Since glycolysis produces 2 pyruvates → 2 Acetyl-CoA, the Krebs cycle turns twice per glucose molecule.
The cycle’s main job is not ATP production — it’s harvesting electrons into NADH and FADH₂ to send to the ETC. Think of it as the “electron harvesting” stage.
Stage 4 — Electron Transport Chain & Oxidative Phosphorylation
The ETC is where most ATP is made — this is the powerhouse of the powerhouse. The NADH and FADH₂ produced in the previous stages deliver their electrons to a series of protein complexes embedded in the inner mitochondrial membrane.
The Four Protein Complexes
NADH → NAD⁺
Mobile carrier
FADH₂ → FAD
Pumps H⁺
O₂ → H₂O ★
Total ATP Count: Per Glucose Molecule
Here’s the complete accounting of every ATP, NADH, and FADH₂ produced from one molecule of glucose through aerobic respiration:
Glycolysis (net)
Pyruvate Oxidation
Krebs Cycle
ETC (est.)
| Stage | Direct ATP | NADH produced | FADH₂ produced | ATP from ETC |
|---|---|---|---|---|
| Glycolysis | 2 | 2 NADH (cytoplasmic) | — | 3–5 |
| Pyruvate Oxidation | 0 | 2 NADH | — | 5 |
| Krebs Cycle (×2) | 2 | 6 NADH | 2 FADH₂ | 15 + 3 |
| Total | 4 direct | 10 NADH | 2 FADH₂ | ~26–28 |
Grand total: approximately 30–32 ATP per glucose (the modern P/O ratio estimate). Older textbooks cited 36–38 ATP — the discrepancy comes from accounting for the cost of shuttling cytoplasmic NADH into the mitochondria (which differs by cell type) and proton leakage across the membrane.
Anaerobic Respiration vs Fermentation
When oxygen is unavailable, the ETC stalls. Cells switch to anaerobic pathways to regenerate NAD⁺ — which is required for glycolysis to keep running. These pathways produce far less ATP but allow glucose breakdown to continue.
| Pathway | O₂ required? | End product | ATP yield | Who uses it |
|---|---|---|---|---|
| Aerobic respiration | Yes ✓ | CO₂ + H₂O | 30–32 ATP | Most eukaryotes |
| Lactic acid fermentation | No | Lactate | 2 ATP (glycolysis only) | Animal muscle cells, some bacteria |
| Alcoholic fermentation | No | Ethanol + CO₂ | 2 ATP (glycolysis only) | Yeast, some plants |
| Anaerobic respiration | No | Varies | 2–several ATP | Some prokaryotes (NO₃⁻, SO₄²⁻ as acceptors) |
In lactic acid fermentation, pyruvate accepts electrons from NADH → regenerating NAD⁺ for glycolysis. This is what happens in your muscles during intense exercise when oxygen delivery can’t keep up with demand — causing the burning sensation you feel.
In alcoholic fermentation, pyruvate is decarboxylated to acetaldehyde (losing CO₂), then acetaldehyde accepts electrons from NADH → ethanol. This is the basis of bread leavening and beverage brewing — the CO₂ causes bread to rise; the ethanol is the alcohol in beer and wine.
Frequently Asked Questions
What are the 4 steps of cellular respiration in order?
The four stages are: (1) Glycolysis (cytoplasm) — splits glucose into 2 pyruvate, net 2 ATP; (2) Pyruvate Oxidation (mitochondrial matrix) — converts pyruvate to Acetyl-CoA, releases CO₂; (3) Krebs Cycle (mitochondrial matrix) — harvests electrons into NADH and FADH₂, produces 2 ATP; (4) Electron Transport Chain + Oxidative Phosphorylation (inner mitochondrial membrane) — uses electron energy to make ~26–28 ATP via chemiosmosis.
How many ATP does one glucose produce?
One glucose molecule produces approximately 30–32 ATP under modern estimates (some older textbooks say 36–38). The difference comes from the cost of shuttling NADH across the mitochondrial membrane and proton leakage. In practice, living cells extract closer to 30 ATP per glucose.
Why does the Krebs cycle run twice per glucose?
Glycolysis splits one 6-carbon glucose into two 3-carbon pyruvate molecules. Each pyruvate becomes one Acetyl-CoA (a 2-carbon molecule). Since one turn of the Krebs cycle processes one Acetyl-CoA, the cycle must run twice to fully process both Acetyl-CoA molecules from one glucose. All the figures for Krebs cycle yield in textbooks are already given “per glucose” — meaning for both turns combined.
What is the role of oxygen in cellular respiration?
Oxygen serves as the final electron acceptor in the electron transport chain. At Complex IV, oxygen picks up electrons and protons to form water (H₂O). Without oxygen, electrons back up in the chain, the ETC shuts down, chemiosmosis stops, and the cell cannot produce ATP via oxidative phosphorylation. This is why oxygen deprivation is fatal to aerobic organisms within minutes.
What is chemiosmosis?
Chemiosmosis is the movement of ions (specifically H⁺ protons) across a selectively permeable membrane down their electrochemical gradient to generate ATP. In the mitochondria, the ETC pumps protons from the matrix into the intermembrane space. The resulting proton gradient drives protons back through ATP synthase, whose rotation catalyzes ADP + Pᵢ → ATP. This concept was proposed by Peter Mitchell in 1961 — he won the Nobel Prize in Chemistry in 1978 for it.