Aerobic respiration

Where it happens

Aerobic respiration takes place mainly in the mitochondria of the cell, which is why active cells such as muscle and liver cells have abundant mitochondria.

The process begins in the cytoplasm, where glucose is first split into two smaller three-carbon molecules in a stage called glycolysis. Those molecules then enter the mitochondria, where the oxygen-requiring stages take place.

The inner membrane of a mitochondrion is folded into cristae, which increase the surface area for the enzymes that carry out the final energy-releasing reactions. Every living cell that has mitochondria, in plants as well as animals, respires aerobically whenever oxygen is available.

Inputs and outputs

• Input: glucose, delivered by the blood in animals or produced by photosynthesis in plants.
• Input: oxygen, taken in by gas exchange and carried to the cell.
• Input: the enzymes of the cytoplasm and mitochondria, which are not used up.
• Output: carbon dioxide, a waste gas removed at the gas exchange surface.
• Output: water, which the cell can use or excrete.
• Output: energy, transferred to ATP, with some released as heat.
• Balanced equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

The steps

1. Glucose from food is delivered to the cell and taken across the cell membrane.
2. Oxygen from breathing reaches the cell through the blood and diffuses into the cytoplasm and then the mitochondria.
3. In the cytoplasm, glucose is split into two three-carbon molecules during glycolysis, releasing a small amount of ATP.
4. The three-carbon molecules enter the mitochondria and are broken down further, releasing carbon dioxide.
5. Hydrogen removed from the glucose fragments is passed along carriers on the cristae and finally combined with oxygen to form water.
6. The large amount of energy released at this stage is transferred to ATP for the cell to use.
7. Carbon dioxide and water are produced as waste and removed from the cell.

Why it matters and how it is controlled

Aerobic respiration supplies most of the energy that living cells need for movement, growth, active transport and keeping warm. It releases far more energy per glucose than anaerobic respiration.

The rate of aerobic respiration matches the cell's demand for ATP. During exercise, muscle cells use ATP faster, so respiration speeds up; this raises the demand for oxygen and glucose and the output of carbon dioxide.

The rise in carbon dioxide in the blood is detected by the brain, which increases the breathing rate and heart rate so that more oxygen is delivered and the extra carbon dioxide is removed. Temperature also affects the rate, because every stage is catalysed by enzymes, so respiration is slower in cold conditions and stops if the enzymes are denatured.

The energy released does not all become ATP. A large share is released as heat, which in mammals and birds maintains a constant body temperature.

This is why respiration experiments with germinating seeds in a vacuum flask show a rise in temperature.

How it is examined

You may be asked for the word equation, to compare aerobic and anaerobic respiration, or to explain the results of a respiration experiment such as one showing carbon dioxide release.

Practical-based questions use germinating seeds or small animals in a flask with lime water or bicarbonate indicator to show carbon dioxide production, or a thermos flask with a thermometer to show heat release. You must be able to identify the control (boiled seeds), explain why the seeds are washed with disinfectant, and state what change in the indicator is expected.

Comparison tables asking for site, need for oxygen, products and amount of energy released are frequent in structured questions.

Common misconceptions

Worked exam-style question

Question. A student set up two vacuum flasks. Flask P contained germinating green bean seeds; flask Q contained the same mass of seeds that had been boiled and then cooled.

Both sets of seeds were rinsed in disinfectant, and a thermometer was fitted through the cotton wool plug of each flask. After two days the temperature in flask P had risen while the temperature in flask Q stayed the same.

(a) Explain the rise in temperature in flask P. (b) State the purpose of flask Q.

(c) Explain why the seeds were rinsed in disinfectant.

Model answer. (a) The germinating seeds are living and carry out aerobic respiration, breaking down glucose using oxygen. The energy released is transferred to ATP, but part of it is lost as heat, which warms the air inside the insulated flask.

(b) Flask Q is the control: boiling kills the seeds and denatures their enzymes, so no respiration occurs and any temperature change cannot be due to the seeds themselves. (c) Disinfectant kills microorganisms on the seed coats, so that any heat produced comes from the respiration of the seeds and not from bacteria or fungi.

Source:SRC-DSKP-EN

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