AQA · A-Level · Physical Education
Course version: 7582 · Version from Sept 2016
Respiratory system: Principles of diffusion and partial pressures.
Diffusion is the passive net movement of gas molecules from a region of higher partial pressure to a region of lower partial pressure, down a partial-pressure gradient.
Checked against the AQA 7582 specification (opens in a new tab) on .
- Point 9 of 17
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Diffusion is the passive net movement of gas molecules from a region of higher partial pressure to a region of lower partial pressure, down a partial-pressure gradient. Each gas in a mixture exerts its own partial pressure; in air, oxygen has a partial pressure of about 21.2 kPa at sea level and carbon dioxide is only about 0.04 kPa. At the alveoli, oxygen diffuses from alveolar air into deoxygenated blood because alveolar partial pressure of oxygen is higher than that in the arriving blood, while carbon dioxide diffuses the opposite way because blood partial pressure of carbon dioxide is higher than alveolar. Fick's law states that the rate of diffusion is proportional to the surface area and the partial-pressure gradient, and inversely proportional to the thickness of the respiratory membrane. A larger gradient, larger area or thinner membrane therefore increases gas exchange.
What earns the marks
- Diffusion is the passive net movement of gas molecules from higher to lower partial pressure, down a gradient.
- Each gas in a mixture exerts its own partial pressure; oxygen in air is about 21.2 kPa and carbon dioxide about 0.04 kPa at sea level.
- At the alveoli, oxygen diffuses from alveolar air into deoxygenated blood because alveolar partial pressure of oxygen exceeds that in the arriving blood.
- Carbon dioxide diffuses from blood into alveolar air because blood partial pressure of carbon dioxide exceeds alveolar partial pressure of carbon dioxide.
- Fick's law: rate of diffusion is proportional to surface area and partial-pressure gradient, and inversely proportional to membrane thickness.
- A larger partial-pressure gradient, larger surface area or thinner respiratory membrane increases the rate of gas exchange.
Common mistakes and how to put them right
Thinking that oxygen diffuses from blood to alveoli.
Put it right: Oxygen diffuses from alveolar air into blood because alveolar partial pressure of oxygen is higher than that in deoxygenated blood.
Confusing partial pressure with concentration or percentage.
Put it right: Partial pressure is the pressure exerted by one gas in a mixture, measured in kPa, and it is the gradient in partial pressure that drives diffusion.
Believing that diffusion requires energy from ATP.
Put it right: Diffusion is passive and relies on the kinetic energy of gas molecules, so no ATP is used.
Ignoring membrane thickness in Fick's law.
Put it right: Rate of diffusion is inversely proportional to thickness, so a thicker respiratory membrane slows gas exchange.
Exam tips
- Read the question carefully to identify whether it asks about direction of diffusion, partial pressures or factors affecting rate.
- When a question gives partial-pressure values, compare them explicitly to decide the direction of net diffusion for each gas.
- Use the phrase 'down a partial-pressure gradient' rather than 'from high concentration to low concentration' when the data are partial pressures.
- Check units: partial pressures are usually given in kPa, so keep the unit with the value in your reasoning.
Try the lesson
The start of the Respiratory system: Principles of diffusion and partial pressures. lesson, made from checked teaching notes for this specification point.
Have a go first
Gases move between the alveoli and the blood because of differences in their partial pressures. Which statement describes the direction in which a gas diffuses?
Listen
Diffusion is the passive net movement of gas molecules from a region of higher partial pressure to a region of lower partial pressure, down a partial-pressure gradient. Each gas in a mixture exerts its own partial pressure; in air, oxygen has a partial pressure of about 21.2 kPa at sea level and carbon dioxide is only about 0.04 kPa. At the alveoli, oxygen diffuses from alveolar air into deoxygenated blood because alveolar partial pressure of oxygen is higher than that in the arriving blood, while carbon dioxide diffuses the opposite way because blood partial pressure of carbon dioxide is higher than alveolar. Fick's law states that the rate of diffusion is proportional to the surface area and the partial-pressure gradient, and inversely proportional to the thickness of the respiratory membrane. A larger gradient, larger area or thinner membrane therefore increases gas exchange. A strong answer earns marks for these points. Diffusion is the passive net movement of gas molecules from higher to lower partial pressure, down a gradient. Each gas in a mixture exerts its own partial pressure; oxygen in air is about 21.2 kPa and carbon dioxide about 0.04 kPa at sea level. At the alveoli, oxygen diffuses from alveolar air into deoxygenated blood because alveolar partial pressure of oxygen exceeds that in the arriving blood. Carbon dioxide diffuses from blood into alveolar air because blood partial pressure of carbon dioxide exceeds alveolar partial pressure of carbon dioxide. A common slip: Thinking that oxygen diffuses from blood to alveoli. Instead, oxygen diffuses from alveolar air into blood because alveolar partial pressure of oxygen is higher than that in deoxygenated blood. Exam tip: Read the question carefully to identify whether it asks about direction of diffusion, partial pressures or factors affecting rate.
Key words
Say what each word means before you turn the card.
Diffusion
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Partial pressure
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Fick's law
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Partial-pressure gradient
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See every definition
- Diffusion
- The passive net movement of gas molecules from a region of higher partial pressure to a region of lower partial pressure, down a partial-pressure gradient.
- Partial pressure
- The pressure exerted by one gas in a mixture, measured in kPa, with each gas exerting its own.
- Fick's law
- The rate of diffusion is proportional to the surface area and the partial-pressure gradient, and inversely proportional to the thickness of the respiratory membrane.
- Partial-pressure gradient
- A difference in partial pressure between two regions, down which gas molecules move, with a larger gradient increasing gas exchange.
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