Gas exchange in the lungs and in leavesAQA A-Level Biology: Revision notes
Section 1
Gas exchange in the leaves of dicotyledonous plants
In a dicotyledonous leaf, carbon dioxide diffuses in and oxygen diffuses out through stomata, which are pores in the lower epidermis, each controlled by a pair of guard cells.
Inside the leaf, the mesophyll (palisade and spongy layers) has large air spaces so that gases can diffuse to all cells, and a large surface area of cell walls in contact with air.
There is a compromise: open stomata allow carbon dioxide in for photosynthesis, but also let water vapour out by evaporation. Guard cells close the stomata when water is scarce, or at night.
Section 2
Xerophytic plants
Xerophytes are plants adapted to dry conditions. Their adaptations reduce water loss by transpiration while still allowing carbon dioxide to enter.
- Thick waxy cuticle to reduce evaporation
- Rolled leaves, with stomata on the inner surface, trap humid air and reduce the water potential gradient
- Hairs on the leaf surface trap moist air
- Sunken stomata (in pits) trap humid air
- Reduced surface area to volume ratio of leaves (for example spines or needles)
In xerophyte answers, explain how each feature lowers the water potential gradient or the surface area, not just name it.
Section 3
The human gas exchange system
Air passes through the trachea (supported by C-shaped rings of cartilage), then the bronchi, branching into bronchioles, and ends in the alveoli inside the lungs.
The alveolar epithelium is adapted for gas exchange:
- It is one cell thick and flattened (short diffusion distance)
- There are millions of alveoli (large surface area)
- Each alveolus has a dense capillary network (blood removes oxygen, maintaining the gradient)
- Ventilation maintains the concentration gradient of oxygen and carbon dioxide
Required practical 5 (dissection): use a pair of lungs or a plant or fish gas exchange system, wearing gloves and using a sharp scalpel on a dissecting board, cutting away from the body; identify the structures, draw a labelled diagram and clean the equipment afterwards.
Section 4
Ventilation
Pressure changes in the thorax move air.
Inspiration: external intercostal muscles contract and the diaphragm contracts (flattens), raising the ribs and increasing the volume of the thorax. The pressure falls below atmospheric pressure, so air flows in.
Expiration (at rest): the muscles relax, the ribs fall and the diaphragm rises, reducing the volume and increasing the pressure, so air is forced out. In forced expiration the internal intercostal muscles contract, pulling the ribs down and in (they are antagonistic to the external intercostals).
Pulmonary ventilation rate = tidal volume × breathing rate.
Worked example: tidal volume 500 cm³ × 12 breaths per minute = 6000 cm³ min⁻¹.
Air does not get 'sucked in' by the lungs. The thorax volume increases, pressure falls below atmospheric and air flows in down a pressure gradient.
Section 5
Lung disease, smoking and pollution
Lung diseases reduce gas exchange or ventilation.
- Pulmonary fibrosis: scar tissue thickens the alveolar walls (longer diffusion distance) and makes the lungs less elastic, so less air is inhaled
- Emphysema: the alveolar walls break down, so there is a smaller surface area, and elastic tissue is lost so less air is expelled
- Asthma narrows the airways, reducing ventilation, and tuberculosis damages lung tissue
Symptoms include shortness of breath and rapid breathing. Smoking and air pollution raise the risk of lung cancer and other lung diseases. When interpreting data, describe the trend with figures, then identify the effect on gas exchange or ventilation.
Section 6
Risk factors, correlation and causation
A correlation is a relationship between two variables, such as the number of cigarettes smoked and the incidence of lung cancer. A causal relationship means that a change in one variable causes the change in the other.
- A correlation does not prove causation, because other factors may differ between groups
- Causation is supported by a plausible mechanism (carcinogens in smoke causing mutations), repeated studies and control groups
- Evidence such as this led to statutory restrictions, for example bans on smoking in public places and restrictions on pollutants, to protect people's health
When evaluating, consider sample size, control groups, length of study, other variables and possible bias.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Gas exchange in the lungs and in leaves
- Marram grass grows on sand dunes, where water is scarce and winds are strong. Its leaves are rolled, with the stomata on the inner surface, and the inner surface is lined with hairs. A bean plant, which is a typical dicotyledonous plant, grows in moist soil and has flat leaves.Explain how the stomata of the bean plant allow the plant to gain carbon dioxide while limiting the loss of water.2 marks
- A student dissects the lungs of a sheep to study the gas exchange system, and then uses a model made from a bell jar, a Y-shaped tube and a rubber sheet to show how breathing works. The student also measures the breathing of a volunteer at rest.Explain how air enters the lungs during inspiration.2 marks
- A respiratory clinic treats patients with lung diseases, including pulmonary fibrosis, in which scar tissue thickens the alveolar walls and makes the lungs less elastic. The clinic's researchers also study how smoking affects the risk of lung disease.A patient with pulmonary fibrosis has a tidal volume of 300 cm³ and breathes 24 times per minute at rest. Calculate the patient's pulmonary ventilation rate and explain why fibrosis reduces the rate of gas exchange.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).