🎓 Prepared by students from Boğaziçi University

What is Cellular Respiration?

Cellular respiration is the process cells use to break down glucose and release stored energy as ATP, the molecule that powers nearly every cellular activity. It happens in almost all living organisms, from bacteria to humans.

Short answer

Cellular respiration converts glucose and oxygen into carbon dioxide, water, and usable energy (ATP), following the equation C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP (energy).

Stages of Cellular Respiration

1
Glycolysis
Glucose is split into two pyruvate molecules in the cytoplasm, yielding 2 net ATP.
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2
Pyruvate oxidation
Pyruvate enters the mitochondria and converts into acetyl-CoA, releasing CO2.
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3
Krebs cycle
Acetyl-CoA is broken down, releasing CO2 and generating NADH and FADH2.
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4
Electron transport chain
NADH and FADH2 fuel ATP synthase, producing roughly 32-34 more ATP and water.

Step-by-step worked examples

A muscle cell runs out of oxygen during intense sprinting. How does it keep producing ATP?

Without oxygen, the electron transport chain cannot run (it needs O2 as the final electron acceptor)
The cell switches to anaerobic fermentation, converting pyruvate into lactic acid
This regenerates NAD+ so glycolysis can continue producing a small amount of ATP

A cell fully respires 1 glucose molecule aerobically. Roughly how many ATP molecules does it net overall?

Glycolysis nets about 2 ATP
The Krebs cycle nets about 2 ATP
The electron transport chain produces roughly 32-34 ATP
Total is approximately 36-38 ATP per glucose molecule

Why do muscle cells have far more mitochondria than skin cells?

Mitochondria are the site of the Krebs cycle and electron transport chain — the ATP-heavy stages of respiration
Muscle cells need large, continuous amounts of ATP for contraction
More mitochondria means a greater aerobic ATP production capacity to meet that demand

Flashcards

Quick quiz

Q1.What are the reactants of cellular respiration?

Correct answer: B. Glucose and oxygen are broken down to release energy.

Q2.Where does glycolysis take place?

Correct answer: B. Glycolysis is the only stage that occurs outside the mitochondria, in the cytoplasm.

Q3.Which stage produces the most ATP?

Correct answer: C. The electron transport chain generates roughly 32-34 of the total ~36-38 ATP.

Q4.What happens to pyruvate when oxygen is unavailable?

Correct answer: B. Without oxygen, cells ferment pyruvate to regenerate NAD+ for glycolysis to continue.

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Common mistakes

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Thinking cellular respiration only happens in animals. — Correct: Nearly all organisms — plants, fungi, bacteria — respire to release energy from glucose.

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Confusing respiration with breathing. — Correct: Breathing is gas exchange; cellular respiration is the chemical process inside cells that makes ATP.

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Believing respiration and photosynthesis are opposites happening simultaneously in a plant cell. — Correct: Plants photosynthesize only in light but respire constantly, in every living cell, day and night.

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Assuming all ATP comes from glycolysis. — Correct: Glycolysis makes only ~2 ATP directly; most ATP comes from the electron transport chain.

FAQ

What is cellular respiration?

Cellular respiration is the process cells use to break down glucose with oxygen to release energy stored as ATP.

What is the formula for cellular respiration?

C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP (energy).

What are examples of cellular respiration?

Muscle cells producing ATP during exercise, yeast fermenting sugar into alcohol anaerobically, and mitochondria in any body cell generating everyday energy are all examples.

How do you calculate the ATP yield of cellular respiration?

Add the ATP from each stage: about 2 from glycolysis, 2 from the Krebs cycle, and roughly 32-34 from the electron transport chain, totaling about 36-38 ATP per glucose.