How do cells release the energy they need from glucose, with and without oxygen?
Respiration as an exothermic series of reactions that releases energy from glucose in all living cells, the word and symbol equations for aerobic respiration, anaerobic respiration in animals (lactic acid) and yeast (ethanol and carbon dioxide), oxygen debt, and the uses of the energy released.
A focused answer to the OCR Gateway GCSE Biology A topic B1 on respiration, covering aerobic respiration and its equations, anaerobic respiration in animals and yeast, oxygen debt, and the uses of the energy released in cells.
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What this dot point is asking
OCR wants you to describe respiration as an exothermic process that releases energy from glucose in all living cells, give the equations for aerobic respiration, describe anaerobic respiration in animals and in yeast, explain oxygen debt, and state what the energy released is used for.
Respiration releases energy
Respiration is not the same as breathing. Breathing (ventilation) moves air in and out of the lungs; respiration is the cellular process that releases energy. The energy released is needed to keep the organism alive.
Aerobic respiration
Aerobic respiration uses oxygen and releases a large amount of energy per glucose molecule. It happens mainly in the mitochondria, which is why active cells have many of them.
The word equation is:
The balanced symbol equation is:
Anaerobic respiration
Anaerobic respiration happens without oxygen. It releases much less energy per glucose molecule than aerobic respiration, because the glucose is not fully broken down.
In animal cells (such as muscle during hard exercise):
In yeast (and plant cells), anaerobic respiration is called fermentation:
Fermentation in yeast is used to make bread (the carbon dioxide makes dough rise) and alcoholic drinks (the ethanol is the alcohol).
Oxygen debt
During vigorous exercise, the muscles cannot get oxygen fast enough, so they respire anaerobically and produce lactic acid, which builds up. Because less oxygen was taken in than was needed to fully respire the glucose, the body builds up an oxygen debt.
After exercise, you keep breathing deeply and quickly to take in extra oxygen. This oxygen is used to oxidise (break down) the lactic acid, removing it from the muscles and repaying the oxygen debt. The heart rate also stays high to carry the lactic acid to the liver, where it is dealt with.
Uses of the energy released
The energy released by respiration is used for:
- Muscle contraction (movement).
- Active transport (moving substances against a concentration gradient).
- Keeping warm in mammals and birds (maintaining a constant body temperature).
- Building large molecules from smaller ones, for example proteins from amino acids.
The respiration practical
A common practical investigates the rate of respiration, for example using a respirometer to measure oxygen uptake by germinating seeds or small organisms, or measuring carbon dioxide produced by yeast at different temperatures. Exam questions test the method, the control of variables and how to interpret rate data, often linking back to the effect of temperature on the enzymes that control respiration.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksCompare aerobic and anaerobic respiration in human muscle cells. In your answer refer to the use of oxygen, the products formed, and the amount of energy released.Show worked answer →
A 4-mark Compare question, so make clear paired points.
Use of oxygen: aerobic respiration uses oxygen; anaerobic respiration does not use oxygen.
Products: aerobic respiration produces carbon dioxide and water; anaerobic respiration in muscle produces lactic acid.
Energy released: aerobic respiration releases much more energy per glucose molecule than anaerobic respiration, which releases far less.
Markers reward clear comparison points (best written as "aerobic does X whereas anaerobic does Y"). A full answer also notes that aerobic respiration happens mainly in the mitochondria and that anaerobic respiration is used when oxygen cannot be supplied fast enough, for example during vigorous exercise.
OCR 20213 marksAfter a sprint, an athlete continues to breathe deeply and quickly for several minutes. Explain why, using the term oxygen debt.Show worked answer →
A 3-mark applied explanation.
During the sprint, the muscles respire anaerobically because oxygen cannot be delivered fast enough. This produces lactic acid, which builds up in the muscles. The body has taken in less oxygen than the amount needed to respire the glucose fully, creating an oxygen debt.
After exercise, the athlete keeps breathing deeply to take in extra oxygen. This oxygen is used to oxidise (break down) the lactic acid, removing it from the muscles. Reward: anaerobic respiration produced lactic acid; the extra oxygen repays the oxygen debt by breaking down the lactic acid.
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