How do cells release energy from glucose, with and without oxygen?
Compare aerobic and anaerobic respiration and state their word equations and energy yields
A focused answer to the O-Level Biology outcome on respiration. The word equations for aerobic and anaerobic respiration in humans and yeast, the difference in energy yield, and the meaning of oxygen debt.
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What this dot point is asking
SEAB wants you to compare aerobic and anaerobic respiration: their word equations, whether they use oxygen, how much energy each releases, and their products. You should know anaerobic respiration in human muscle (lactic acid) and in yeast (alcohol and carbon dioxide), and be able to explain oxygen debt.
The answer
What respiration is
Respiration is the process in all living cells that releases energy from glucose. The energy is used for movement, growth, active transport, keeping warm and all the activities of life. Respiration is not the same as breathing; breathing is the movement of air, while respiration is the chemical release of energy in cells.
Aerobic respiration
Aerobic respiration uses oxygen and releases a large amount of energy from each glucose molecule. It takes place mainly in the mitochondria.
Anaerobic respiration
Anaerobic respiration releases energy from glucose without oxygen. It releases much less energy per glucose than aerobic respiration, because the glucose is not fully broken down.
In human muscle (during hard exercise):
In yeast (used in baking and brewing):
Comparing the two
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen | Used | Not used |
| Energy released | Large amount | Small amount |
| Products (in muscle) | Carbon dioxide and water | Lactic acid |
Oxygen debt
During hard exercise, the muscles cannot get enough oxygen, so they respire anaerobically, producing lactic acid. The extra oxygen needed afterwards to break down this lactic acid is called the oxygen debt. This is why an athlete keeps breathing heavily after sprinting: to take in oxygen to remove the lactic acid and repay the debt.
Examples in context
Example 1. Yeast in bread-making. Yeast respires anaerobically, producing carbon dioxide that makes bread dough rise, and alcohol that evaporates during baking. The same process produces the alcohol in brewing.
Example 2. A long-distance versus a sprint runner. A marathon runner paces themselves so muscles get enough oxygen for aerobic respiration; a sprinter goes so hard that muscles respire anaerobically, building up lactic acid and an oxygen debt over the short, intense effort.
Try this
Q1. Write the word equation for aerobic respiration. [2 marks]
- Cue. Glucose + oxygen gives carbon dioxide + water (releasing energy).
Q2. State the product of anaerobic respiration in human muscle. [1 mark]
- Cue. Lactic acid.
Q3. Explain why aerobic respiration releases more energy than anaerobic respiration. [2 marks]
- Cue. Aerobic respiration fully breaks down glucose into carbon dioxide and water, releasing all its energy; anaerobic only partly breaks it down, so less energy is released.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marks(a) Write the word equation for aerobic respiration. (b) Write the word equation for anaerobic respiration in human muscle. (c) State two differences between aerobic and anaerobic respiration.Show worked answer →
(a) glucose + oxygen gives carbon dioxide + water (+ energy released).
(b) glucose gives lactic acid (+ a small amount of energy released).
(c) Two differences: aerobic respiration uses oxygen, anaerobic does not; aerobic respiration releases much more energy from each glucose molecule than anaerobic; aerobic produces carbon dioxide and water, while anaerobic in muscle produces lactic acid. Any two clear differences are accepted.
Markers reward both correct word equations and two valid differences, especially the use of oxygen and the larger energy yield of aerobic respiration.
Original4 marksAfter a sprint, an athlete continues to breathe heavily for several minutes. Using the idea of oxygen debt, explain why.Show worked answer →
During the sprint, the muscles could not get enough oxygen for aerobic respiration, so they respired anaerobically, producing lactic acid. This built up an oxygen debt, the extra oxygen needed to deal with the lactic acid.
After the sprint, the athlete keeps breathing heavily to take in this extra oxygen. The oxygen is used to break down the lactic acid (in the liver), removing it and repaying the oxygen debt. Breathing returns to normal once the debt is repaid.
Markers reward anaerobic respiration producing lactic acid during the sprint, the oxygen debt as the extra oxygen needed, and continued heavy breathing to take in oxygen to break down the lactic acid.
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