8.9–8.10Aerobic and anaerobic respiration
Cellular respiration is an exothermic reaction which is continuously occurring in living cells. The energy transferred supplies all the energy needed for living processes.
Two words to get right at the start. Exothermic means energy is released — the opposite of photosynthesis. Continuously means in every living cell, all the time, not only during exercise and not only in animals: plants respire too, day and night.
Organisms need the energy for chemical reactions that build larger molecules, for movement, and for keeping warm.
Respiration in cells can take place aerobically, using oxygen, or anaerobically, without it.
Aerobic respiration: glucose + oxygen → carbon dioxide + water
You should recognise the symbols C₆H₁₂O₆, O₂, CO₂ and H₂O.
Anaerobic respiration in muscles: glucose → lactic acid
Anaerobic respiration in plant and yeast cells: glucose → ethanol + carbon dioxide
As the oxidation of glucose is incomplete in anaerobic respiration, much less energy is transferred than in aerobic respiration. That phrase — incomplete oxidation — is the reason, and questions ask for it.
Anaerobic respiration in yeast cells is called fermentation, and it has economic importance in the manufacture of bread and alcoholic drinks. Both products of the yeast equation are used: the carbon dioxide makes bread rise, and the ethanol is the alcohol in beer and wine.
Common mistake
Anaerobic respiration does not produce "no energy" — it produces much less. And the products differ by organism: lactic acid in your muscles, ethanol and carbon dioxide in yeast. Mixing those two equations is the commonest error in this sub-topic.
- Cellular respiration
- an exothermic reaction occurring continuously in living cells, which transfers the energy needed for all living processes
- Exothermic reaction
- a reaction in which energy is released; respiration is exothermic, the opposite of photosynthesis
- Aerobic respiration
- respiration using oxygen, in which glucose and oxygen react to produce carbon dioxide and water
- Anaerobic respiration
- respiration without oxygen; in muscles glucose becomes lactic acid, and in plant and yeast cells glucose becomes ethanol and carbon dioxide
- Fermentation
- anaerobic respiration in yeast cells, used in the manufacture of bread and alcoholic drinks
8.11Response to exercise
During exercise the human body reacts to the increased demand for energy. The heart rate, breathing rate and breath volume all increase, to supply the muscles with more oxygenated blood.
All three, and the reason: more oxygen and glucose delivered to the muscles for aerobic respiration, and more carbon dioxide taken away.
If insufficient oxygen is supplied, anaerobic respiration takes place in the muscles. The incomplete oxidation of glucose causes a build-up of lactic acid and creates an oxygen debt. During long periods of vigorous activity muscles become fatigued and stop contracting efficiently.
Higher tier only: blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose. Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells — which is why you keep breathing hard for several minutes after you stop running.
Practical skill
Investigating the effect of exercise on the body
- Measure a resting heart rate by counting the pulse for one minute, repeated and averaged.
- Exercise for a set time — step-ups at a fixed rate keeps the effort comparable between people.
- Measure the heart rate immediately afterwards, then every minute until it returns to resting.
- Repeat with different exercise durations or intensities, changing only one thing at a time.
- Plot heart rate against time and compare how long recovery takes.
Watch out: Recovery time is the more interesting measurement, not just peak rate: it is a measure of how much oxygen debt built up. Fitter people return to resting sooner, which is a fair conclusion to draw from your own data.
Common mistake
Lactic acid does not cause fatigue by "poisoning" the muscle, and it is not removed by sweating it out. It is carried in the blood to the liver and converted back to glucose, and the oxygen debt is what pays for that conversion.
- Oxygen debt
- the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells
- Lactic acid
- the product of anaerobic respiration in muscles, which builds up during vigorous exercise and causes muscle fatigue
8.9Metabolism
Metabolism is the sum of all the reactions in a cell or the body. The energy transferred by respiration in cells is used for the continual enzyme-controlled processes of metabolism that synthesise new molecules.
This section exists to tie the course together — every item in it is taught in more detail somewhere else, and here they are listed as one system.
You should be able to explain the importance of sugars, amino acids, fatty acids and glycerol in the synthesis and breakdown of carbohydrates, proteins and lipids. Read the table in both directions: digestion breaks the big molecules into those four building blocks, and metabolism builds them back up into whatever the organism needs.
Common mistake
Excess protein is not stored. Amino acids the body cannot use are broken down in the liver and the nitrogen leaves as urea — which is why protein is the one food group you cannot stockpile the way you can fat or glycogen.
- Metabolism
- the sum of all the reactions in a cell or in the body, which are continual and enzyme-controlled and use the energy transferred by respiration
Before you practise
- Respiration is exothermic and happens continuously in every living cell — it is not breathing.
- Aerobic: glucose + oxygen → carbon dioxide + water, releasing much more energy per glucose molecule.
- Anaerobic in muscle produces lactic acid; in yeast and plants it produces ethanol and carbon dioxide, which is fermentation.
- During exercise heart rate, breathing rate and breath volume all increase to supply more oxygen and glucose and remove carbon dioxide.
- Oxygen debt is the extra oxygen needed afterwards to break down the lactic acid that built up.
- Metabolism is the sum of all the reactions in the body, and the energy transferred by respiration is what drives them.