O-Level Biology · Secondary 3 and 4
Respiration in Humans
Respiration is the release of energy from food inside every living cell. This topic covers aerobic and anaerobic respiration in humans, the structure of the lungs and how the alveoli are adapted for gas exchange, how we breathe in and out, and the effects of tobacco smoke. It is different from breathing, which is only the movement of air in and out of the lungs. Respiration links to nutrition, which supplies the glucose, and to transport, which carries oxygen and carbon dioxide between the lungs and the cells.
What the syllabus expects
- Define aerobic and anaerobic respiration and state the word equations for each
- Explain oxygen debt and the effects of lactic acid during and after vigorous exercise
- Identify the parts of the respiratory system and describe how alveoli are adapted for gas exchange
- Describe the mechanism of breathing in terms of the diaphragm, intercostal muscles, volume and pressure
- Compare the composition of inspired and expired air
- Describe the effects of nicotine, tar and carbon monoxide in tobacco smoke
Aerobic and anaerobic respiration
Aerobic respiration is the release of a relatively large amount of energy from the breakdown of food substances, such as glucose, in the presence of oxygen. It takes place in the mitochondria. Glucose + oxygen gives carbon dioxide + water, with a large amount of energy released. The balanced equation is C₆H₁₂O₆ + 6O₂ gives 6CO₂ + 6H₂O. The energy is used for muscle contraction, active transport, building proteins, cell division and keeping body temperature steady. Anaerobic respiration is the release of a relatively small amount of energy from glucose without oxygen. In human muscle, glucose gives lactic acid and a small amount of energy. In yeast, glucose gives ethanol, carbon dioxide and a small amount of energy. Anaerobic respiration releases much less energy because the glucose is only partly broken down, and the lactic acid or ethanol still contains energy.
Exercise and oxygen debt
During vigorous exercise, the muscles need energy faster than the heart and lungs can deliver oxygen. The muscle cells top up their energy supply by anaerobic respiration, and lactic acid builds up. Lactic acid in muscles contributes to muscle fatigue and cramps. After the exercise ends, the person keeps breathing faster and more deeply and the heart rate stays high for some time. The extra oxygen taken in is the oxygen debt. It is needed to remove the lactic acid: the lactic acid is carried in the blood to the liver, where some is oxidised in aerobic respiration to release energy and the rest is converted back to glucose. Once all the lactic acid has been removed, the debt is repaid and breathing and heart rate return to their resting values. Fitter people build up a smaller oxygen debt.
The lungs and gas exchange
Air passes through the nose, pharynx, larynx and trachea, then the two bronchi, many smaller bronchioles and finally the alveoli. The trachea is held open by C-shaped rings of cartilage, and it is lined with mucus to trap dust and bacteria and with cilia that sweep the mucus up to the throat. Gas exchange happens in the alveoli. There are millions of them, giving a very large surface area. Each alveolar wall is one cell thick and lies against a capillary wall that is also one cell thick, so the diffusion distance is short. The inner surface is covered with a thin film of moisture in which oxygen dissolves. A dense network of capillaries carries oxygen away and brings carbon dioxide, keeping the concentration gradients steep. Oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses out.
Breathing and the effects of smoking
To breathe in, the diaphragm contracts and flattens, the external intercostal muscles contract and the internal intercostal muscles relax, so the ribs move upwards and outwards. The volume of the thoracic cavity increases, the pressure inside falls below atmospheric pressure, and air rushes into the lungs. To breathe out, the diaphragm relaxes and arches up, the internal intercostal muscles contract and the external ones relax, the ribs move down and in, the volume decreases, the pressure rises and air is forced out. Inspired air has about 21% oxygen and 0.04% carbon dioxide. Expired air has about 16% oxygen and 4% carbon dioxide, and it is warmer and saturated with water vapour. In tobacco smoke, nicotine is addictive and raises heart rate and blood pressure. Tar is carcinogenic and can cause lung cancer, and it paralyses cilia. Carbon monoxide combines with haemoglobin to form carboxyhaemoglobin, reducing oxygen transport.
Worked example
During a 400 m race, an athlete's leg muscles respire anaerobically as well as aerobically. After the race she keeps breathing quickly and deeply for several minutes. (a) Write the word equation for anaerobic respiration in her muscles. (b) Explain why she continues to breathe quickly and deeply after the race. (c) Explain why anaerobic respiration releases less energy than aerobic respiration.
- For (a), recall that human muscle makes lactic acid, not ethanol: glucose gives lactic acid + a small amount of energy.
- For (b), explain why anaerobic respiration happened: during the race, oxygen could not be supplied to the muscles fast enough to meet their energy demand.
- Lactic acid built up in the muscles, creating an oxygen debt.
- After the race, the extra oxygen taken in by deep, fast breathing is used to remove lactic acid. It is carried to the liver, where some is oxidised to release energy and the rest is converted back to glucose.
- For (c), compare how fully glucose is broken down: in anaerobic respiration glucose is only partly broken down, and much energy remains in the lactic acid.
Answer: (a) Glucose gives lactic acid + a small amount of energy. (b) She has an oxygen debt: extra oxygen is needed to remove the lactic acid that built up, by breaking it down in the liver or converting it back to glucose. (c) Glucose is only partly broken down, so much of its energy stays in the lactic acid.
Common mistakes
- Using 'respiration' and 'breathing' as if they mean the same thing. Respiration is a chemical process in cells that releases energy. Breathing is the movement of air in and out of the lungs.
- Writing that anaerobic respiration in humans produces ethanol and carbon dioxide. That is yeast. Human muscle makes lactic acid only.
- Saying energy is 'produced' or 'created' in respiration. Energy is released from glucose.
- Getting the pressure direction wrong in breathing. When the thoracic volume increases, the pressure decreases, and air moves in from the higher atmospheric pressure outside.
- Saying carbon monoxide damages the lungs. Its main effect is that it combines with haemoglobin and reduces the blood's ability to carry oxygen. Tar damages the lungs.
Exam tips
- Learn the definitions of aerobic and anaerobic respiration exactly, including 'relatively large' and 'relatively small amount of energy' and 'food substances'.
- For breathing, write the answer as a chain: muscles, ribs and diaphragm, volume, pressure, air movement. Each link is usually a mark.
- For alveoli, give the feature and the reason together, for example 'walls one cell thick, so the diffusion distance is short'.
Practice questions
1. Easy: State two differences between inspired air and expired air, with approximate values.
Show answer
Expired air contains less oxygen (about 16% compared with 21%) and more carbon dioxide (about 4% compared with 0.04%). It is also warmer and contains more water vapour.
2. Medium: Describe how air is drawn into the lungs.
Show answer
The diaphragm contracts and flattens. The external intercostal muscles contract and the internal ones relax, so the ribs move up and out. The volume of the thoracic cavity increases, so the pressure inside the lungs falls below atmospheric pressure, and air flows into the lungs.
3. Harder: A heavy smoker gets breathless quickly when climbing stairs. Use your knowledge of tobacco smoke to explain why.
Show answer
Carbon monoxide combines with haemoglobin to form carboxyhaemoglobin, so less oxygen is carried to the muscles. Smoking can also damage the alveolar walls (emphysema), reducing the surface area for gas exchange. With less oxygen reaching the muscles, less aerobic respiration takes place, so less energy is released and the smoker becomes breathless.
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