Aerobic and anaerobic respiration

Every cell needs energy. It gets it by breaking down glucose. The first step is the same each time. In the cytoplasm, the jelly that fills the cell, glucose, a six-carbon molecule, is split into pyruvate, a three-carbon molecule. What happens next depends on oxygen:

• In yeast, without oxygen, pyruvate becomes ethanol and carbon dioxide. This is anaerobic respiration, and it happens during fermentation.
• In our muscles, when oxygen runs short, pyruvate becomes lactic acid. Its build-up causes cramps.
• With oxygen, in the mitochondria, the cell's energy centres, pyruvate becomes carbon dioxide and water. This is aerobic respiration.

The energy set free is used to make ATP. ATP is the energy currency of the cell. The aerobic kind gives a lot more energy than the anaerobic kind.

A small example

Follow the carbon atoms. Glucose has 6 of them. It splits into pyruvate, which has 3. With oxygen, each pyruvate gives 3 molecules of carbon dioxide, plus water. So all the carbon ends up in carbon dioxide. This path breaks glucose down fully, and it gives the most energy.

Where marks go

Question 2 of CBSE's Science sample paper for 2026-27 uses lime water. Lime water turns cloudy with carbon dioxide, which respiration gives off. The question shows four set-ups. It asks in which one the lime water turns cloudiest. It is a 1-mark choice question, so there is no part credit.

In a written answer, each pathway is its own point: where it happens, whether oxygen is used, and what it makes. Under step marking, each point written out is one the examiner can give marks for.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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