Edexcel IGCSE Biology · Spec 2.34-2.38
Respiration
Aerobic and anaerobic respiration compared — the equations, the energy released, and lactic acid and fermentation.
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Respiration
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Explained
Releasing energy, with oxygen and without
Respiration is the series of chemical reactions in cells that release energy from glucose. It happens in every living cell, continuously, and it is not the same thing as breathing.
Breathing moves air into and out of the lungs. Respiration is a reaction inside cells. A plant respires and has no lungs at all.
Released, not produced
The energy was already there, stored in the chemical bonds of glucose. Respiration transfers it to a form the cell can use, a molecule called ATP, which the cell then spends on muscle contraction, active transport, building proteins and keeping warm.
Saying that respiration produces or creates energy is treated as wrong, because energy cannot be created. Releases and transfers are the words that work.
Aerobic respiration
Glucose plus oxygen gives carbon dioxide plus water, releasing energy.
The balanced equation is C6H12O6 plus 6O2 gives 6CO2 plus 6H2O.
It takes place mainly in the mitochondria, and it releases a large amount of energy per glucose molecule because the glucose is broken down completely.
Anaerobic respiration
Without oxygen the glucose is only partly broken down, so far less energy is released and a product is left over. What that product is depends on the organism.
In animal muscle: glucose gives lactic acid. This lets muscles keep working during hard exercise, but the lactic acid builds up, causing fatigue, and has to be broken down afterwards using the extra oxygen taken in during recovery. That requirement is the oxygen debt.
In yeast: glucose gives ethanol plus carbon dioxide. This is fermentation, and it is what makes bread rise and produces alcoholic drinks.
Note that the yeast version produces carbon dioxide and the animal version does not, which is a favourite point of comparison.
Comparing the two
Aerobic uses oxygen, breaks glucose down completely, releases much more energy, and produces carbon dioxide and water.
Anaerobic uses no oxygen, breaks glucose down partly, releases much less energy, and produces lactic acid in animals or ethanol and carbon dioxide in yeast.
Anaerobic respiration is also faster at supplying energy in the short term, which is why sprinters rely on it. It is the total per glucose molecule that is smaller, not the rate.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme credits respiration for releasing energy or providing ATP, and then instructs the examiner to ignore produces energy. One verb earns the mark and the other does not.
The same mark scheme shows something encouraging about how these chains are marked. On a question where candidates had to identify a food and then explain its effect, the notes say that a candidate choosing the wrong food can still score the marks for respiration, for releasing energy or ATP, and for muscle contraction.
So the reasoning marks survive a wrong first answer. If you are unsure which option to pick, pick one and then explain your reasoning fully, because the explanation is being credited separately from the choice.
Elsewhere it is strict about location, giving no credit if the mitochondrion is called the cytoplasm or a chloroplast. Respiration happens in the mitochondria; photosynthesis happens in the chloroplasts, and the two organelles are not interchangeable.
Why cells need it constantly
Every process that requires energy depends on ATP, and cells store very little of it. It has to be made continuously, which is why an interruption to the oxygen supply is dangerous within minutes rather than hours.
Spec 2.34-2.38
What you need to know
- Explain what respiration does — it releases energy as ATP
- Recall aerobic respiration and its equation
- Recall anaerobic respiration in animals and in yeast
Active recall
Quick check
Answer each question before opening the answer.
Give the word equation for aerobic respiration.
Glucose + oxygen → carbon dioxide + water (+ energy released).
Give the products of anaerobic respiration in humans and in yeast.
Humans: lactic acid. Yeast: ethanol + carbon dioxide.
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