What is respiration, and how do aerobic and anaerobic respiration differ in humans and microorganisms?
Respiration as the release of energy from glucose, the word and symbol equations for aerobic respiration, anaerobic respiration in muscles and in yeast (fermentation), and a comparison of the two.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on respiration, covering respiration as energy release from glucose, the equations for aerobic respiration, anaerobic respiration in muscles and yeast, and how the two processes compare.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to explain that respiration releases energy from glucose, give the word and symbol equations for aerobic respiration, describe anaerobic respiration in muscles and in yeast, and compare the two.
What respiration is
The energy released by respiration is used for muscle contraction (movement), keeping warm, active transport, and building large molecules from small ones. Most respiration in animals and plants is aerobic, taking place mainly in the mitochondria.
Aerobic respiration
The carbon dioxide produced is removed by the lungs; the water joins the body's water. Cells that need a lot of energy, such as muscle and sperm cells, have many mitochondria for aerobic respiration.
Anaerobic respiration in muscle
When you exercise hard, your lungs and heart cannot supply oxygen to the muscles fast enough. The muscles then respire anaerobically (without oxygen):
This releases far less energy per glucose molecule than aerobic respiration, and the lactic acid builds up, causing muscle fatigue and cramp. After exercise you keep breathing hard to take in extra oxygen; this oxygen debt is used to break down the lactic acid.
Anaerobic respiration in yeast (fermentation)
Yeast (a microorganism) respires anaerobically by fermentation:
Fermentation is used to make bread (the carbon dioxide makes the dough rise) and alcoholic drinks (the ethanol is the alcohol). The products are different from those in human muscle: yeast makes ethanol and carbon dioxide, not lactic acid.
Comparing the two
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen used? | Yes | No |
| Energy released | A lot | A little |
| Products (muscle) | CO2 + water | Lactic acid |
| Products (yeast) | - | Ethanol + CO2 |
What the released energy is used for
The energy released by respiration is needed for many life processes, and exam questions often ask you to name some. It is used for muscle contraction (movement), active transport of substances across membranes, building large molecules from smaller ones (such as proteins from amino acids), and in mammals and birds for keeping the body warm. Respiration happens in every living cell, all the time, which is why a constant supply of glucose and (for aerobic respiration) oxygen is needed.
Investigating respiration
A simple practical shows that living things respire. Small living organisms such as germinating peas or woodlice are sealed in a tube with a substance that absorbs carbon dioxide; as they respire aerobically they use up oxygen, so the gas volume falls and coloured liquid moves towards the organisms. A boiled (dead) control is used for comparison, and the experiment is kept at a steady temperature so the comparison is fair. The faster the liquid moves, the faster the rate of respiration.
Try this
Q1. Name the gas used up in aerobic respiration. [1 mark]
- Cue. Oxygen.
Q2. Give one use of the energy released by respiration in an animal cell. [1 mark]
- Cue. Any one of: muscle contraction (movement), active transport, building large molecules, or keeping warm.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksCompare aerobic and anaerobic respiration in human muscle cells. Refer to oxygen, the products and the amount of energy released.Show worked answer →
A Unit 1 compare question worth 4 marks. Reward: aerobic respiration uses oxygen, anaerobic respiration does not (1); aerobic produces carbon dioxide and water, anaerobic in muscle produces lactic acid (1); aerobic releases much more energy per glucose molecule (1); anaerobic is used when oxygen cannot be supplied fast enough, for example during vigorous exercise (1). Markers reward clear contrasts on oxygen, products and energy yield. A common error is to give the yeast products (ethanol and carbon dioxide) for human muscle.
WJEC style3 marksWrite the word equation for aerobic respiration and state where in the cell it mainly takes place.Show worked answer →
A Unit 1 recall question. The word equation is glucose + oxygen to carbon dioxide + water (1 mark for reactants, 1 mark for products). It mainly takes place in the mitochondria (1 mark). Markers reward the correct reactants and products and the location. A common error is to reverse the equation (that is photosynthesis) or to forget that energy is released, not a product written in the equation.
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