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Overview: Glycolysis, Krebs Cycle and ETS (The Three Stages of Respiration)

Biology · Respiration in Plants · NEET

Aerobic respiration breaks glucose in three linked stages: (1) Glycolysis in the cytoplasm, (2) the Krebs cycle in the mitochondrial matrix, and (3) the Electron Transport System (ETS) on the inner mitochondrial membrane. Memory hook: "Glucose goes CY-MA-IN" — CYtoplasm, then MAtrix, then INner membrane. NEET repeats this order and its exact locations almost every year, so fixing the sequence and place of each stage is a guaranteed mark.
Three Stages of Aerobic Respiration1. GlycolysisCytoplasmGlucose to2 PyruvateNet 2 ATP, 2 NADH2. Krebs CycleMitochondrialmatrixCO2 releasedNADH, FADH2, GTP3. ETSInner mito.membraneO2 = finalacceptorMost ATP madepyruvateNADH/FADH2Order: Glucose to CYtoplasm to MAtrix to INner membrane
The three stages of aerobic respiration in order, showing where each happens and its key products. Glycolysis (cytoplasm) feeds pyruvate to the Krebs cycle (matrix), which feeds NADH and FADH2 to the ETS (inner membrane), where most ATP is made and oxygen is the final electron acceptor.

Your doubts, answered

What are the three stages of aerobic respiration in the correct order?

In order: Glycolysis, then the link reaction (pyruvate to acetyl CoA), then the Krebs cycle, then the Electron Transport System (ETS). NCERT groups these into three main phases: glycolysis, the Krebs (citric acid/TCA) cycle, and the ETS with oxidative phosphorylation. The link reaction is a short connecting step, not usually counted as a separate main stage.

Where does each stage take place?

Glycolysis happens in the cytoplasm (cytosol). Pyruvate oxidation and the Krebs cycle happen in the mitochondrial matrix. The ETS is on the inner mitochondrial membrane, and protons collect in the intermembrane space. NEET tests these locations directly as match-the-column questions, so learn them exactly.

Is glycolysis inside or outside the mitochondria?

Outside. Glycolysis occurs in the cytoplasm and needs no mitochondria or oxygen, which is why even bacteria and anaerobes can do it. Only from pyruvate onward does respiration move into the mitochondria. This is a very common trap answer.

Which stage produces the most ATP?

The ETS with oxidative phosphorylation makes by far the most ATP. Glycolysis (net 2 ATP) and the Krebs cycle (via GTP) make only a little ATP directly by substrate-level phosphorylation. Most ATP comes when NADH and FADH2 from the earlier stages are oxidised in the ETS.

What links glycolysis to the Krebs cycle?

The two pyruvate molecules from glycolysis enter the mitochondrion and are converted to acetyl CoA by pyruvate dehydrogenase (the link reaction, an oxidative decarboxylation). Acetyl CoA then combines with oxaloacetic acid (OAA) to start the Krebs cycle. Acetyl CoA is also the common entry point for fats and proteins.

What is the job of glycolysis, Krebs cycle and ETS separately?

Glycolysis splits glucose into two pyruvate and makes a little ATP and NADH. The Krebs cycle fully oxidises acetyl CoA, releasing CO2 and storing energy as NADH and FADH2. The ETS then uses those NADH and FADH2 to pump protons and make the bulk of ATP, with oxygen as the final electron acceptor.

⚠️ The NEET trap
✗ Glycolysis and the Krebs cycle both happen inside the mitochondria.
✓ Glycolysis happens in the cytoplasm; only the Krebs cycle (and ETS) are inside the mitochondria (matrix and inner membrane).
🧠 If a match-the-location question puts glycolysis in the matrix or inner membrane, it is wrong — glycolysis is always cytoplasm.

Real NEET questions

2024

Match List I with List II: List I: A. Citric acid cycle B. Glycolysis C. Electron transport system D. Proton gradient List II: I. Cytoplasm II. Mitochondrial matrix III. Intermembrane space of mitochondria IV. Inner mitochondrial membrane

A · A-II, B-I, C-IV, D-III ✓
B · A-III, B-IV, C-I, D-II
C · A-IV, B-III, C-II, D-I
D · A-I, B-II, C-III, D-IV
Solution: The citric acid (Krebs) cycle runs in the mitochondrial matrix (II), glycolysis in the cytoplasm (I), the ETS on the inner mitochondrial membrane (IV), and the proton gradient builds in the intermembrane space (III). This gives A-II, B-I, C-IV, D-III. This single question checks the location of every main stage of respiration.
2017

Which of the following cell organelles is responsible for extracting energy from carbohydrates to form ATP?

A · Lysosome
B · Ribosome
C · Chloroplast
D · Mitochondrion ✓
Solution: The mitochondrion carries out the Krebs cycle, the ETS and oxidative phosphorylation, extracting most of the energy from carbohydrates as ATP. The chloroplast builds carbohydrate in photosynthesis, the ribosome makes protein, and the lysosome does intracellular digestion, so none of them extract respiratory energy.

Solved Respiration in Plants NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 22 Respiration in Plants NEET PYQs ›
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Frequently asked

How many main stages does aerobic respiration have?

Three main stages: glycolysis, the Krebs (citric acid) cycle, and the ETS with oxidative phosphorylation. A short link reaction (pyruvate to acetyl CoA) connects glycolysis to the Krebs cycle.

Which stage does not need oxygen?

Glycolysis does not need oxygen and happens in the cytoplasm. Oxygen is only used at the very end of the ETS, where it is the final electron acceptor.

In which stage is CO2 released?

CO2 is released in the link reaction and the Krebs cycle by decarboxylation. No CO2 is released in glycolysis or in the ETS itself.

Which stage makes NADH and FADH2, and which one uses them?

Glycolysis, the link reaction and the Krebs cycle produce NADH (and FADH2). The ETS uses these NADH and FADH2 molecules to drive proton pumping and ATP synthesis.

Is this respiration pathway the same in plants and animals?

Yes. Glycolysis, the Krebs cycle and the ETS occur the same way in plant and animal cells. The NCERT chapter is on respiration in plants, but the pathway is common to nearly all living cells.

Related concepts:
What is Glycolysis? EMP PathwayLink Reaction: Pyruvate to Acetyl CoAWhat is the Krebs Cycle (TCA / Citric Acid Cycle)?What is the Electron Transport System (ETS)?Respiratory Balance Sheet: Total ATP from Glucose
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Respiration in Plants PYQs