Aerobic Respiration

Equation for aerobic respiration

Content standard 7.2 requires you to state the word equation and balanced chemical equation for aerobic respiration: glucose + oxygen → carbon dioxide + water + energy (ATP), or C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Aerobic respiration requires a continuous supply of oxygen to break glucose down completely.

Where aerobic respiration occurs

At SPM level, aerobic respiration is described as occurring in two general locations within the cell: glycolysis, the first stage of breaking down glucose, takes place in the cytoplasm, while the later stages that use oxygen and release most of the energy take place in the mitochondrion, often described as the 'powerhouse of the cell' because it is specially structured (with folded inner membranes) to carry out this energy release efficiently.

Why aerobic respiration releases so much energy

Because oxygen is available, aerobic respiration breaks glucose down completely into carbon dioxide and water, releasing almost all of the chemical energy stored in the glucose molecule. This is why aerobic respiration produces a much larger amount of ATP compared with anaerobic respiration, which only partially breaks down glucose.

How it is examined

Exam questions frequently ask you to write or complete the word or chemical equation for aerobic respiration, to name the organelle where most of aerobic respiration occurs, or to explain why aerobic respiration releases more energy than anaerobic respiration. You may also be asked to relate the structure of the mitochondrion (such as its folded inner membrane) to its function in releasing energy efficiently.

Aerobic and anaerobic respiration compared

Worked exam-style question

Question. A table gives the approximate amount of energy (as ATP) released when one glucose molecule is broken down by aerobic respiration and by anaerobic respiration in a muscle cell. Aerobic respiration releases far more ATP than anaerobic respiration.

(a) Write the balanced chemical equation for aerobic respiration. (b) Name the organelle in which most of the energy is released, and describe one structural feature that suits it to this role.

(c) Explain, in terms of how completely glucose is broken down, why aerobic respiration releases far more energy than anaerobic respiration. (d) State one situation in a healthy person when muscle cells rely on anaerobic respiration, and name the product formed.

Model answer. (a) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. (b) Most energy is released in the mitochondrion; its folded inner membrane gives a large surface area for the reactions that release energy, so it can produce ATP efficiently.

(c) In aerobic respiration oxygen is used to break glucose down completely into carbon dioxide and water, releasing almost all the chemical energy in the glucose. Anaerobic respiration breaks glucose down only partially, so most of the energy stays locked in the end product, and much less ATP is released.

(d) During vigorous exercise, when the muscles cannot get oxygen fast enough; the product formed is lactic acid.

Practice question

Try this. A yeast cell and a human muscle cell both carry out anaerobic respiration when oxygen is short. State the different end products formed in each, and explain why aerobic respiration is described as the more efficient way to release energy from glucose.

Exam tip

Key terms

Keep these respiration terms distinct:

• Cellular respiration, the release of energy from glucose in every living cell.
• Aerobic respiration, the complete breakdown of glucose using oxygen.
• Anaerobic respiration, the partial breakdown of glucose without oxygen.
• ATP, the molecule that stores and carries the energy released.
• Organelle, a cell structure such as the mitochondrion where respiration occurs.

Source:SRC-DSKP-EN

Frequently asked questions

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