Gas ExchangeEdexcel IGCSE Biology: Revision notes
Section 1
How do plants exchange gases?
Plants exchange carbon dioxide and oxygen by diffusion — the net movement of a gas from a region of higher concentration to a region of lower concentration. Both respiration and photosynthesis happen in plant cells, so the net exchange of gases depends on light intensity:
- In the light, photosynthesis uses more CO2 than respiration produces, and produces more O2 than respiration uses — so the net exchange is CO2 in, O2 out
- In the dark, only respiration occurs — so the net exchange is O2 in, CO2 out
Respiration happens continuously, day and night; only the net direction of gas exchange changes with light.
Do not say plants 'only respire at night' — respiration is continuous; only the net gas exchange direction changes with light.
Section 2
How is the leaf adapted for gas exchange?
The leaf is adapted for efficient gas exchange:
- Broad, flat shape — large surface area for gas exchange
- Thin — short diffusion distance
- Many stomata — allow CO2 and O2 to diffuse in and out
- Air spaces in the spongy mesophyll — increase the internal surface area exposed to gases
Stomata are small pores, mainly on the underside of the leaf, that open and close (controlled by guard cells) to regulate gas exchange and water loss.
When asked how the leaf is adapted for gas exchange, always link the feature to the function — e.g. 'many stomata allow CO2 to diffuse in for photosynthesis'.
Section 3
Investigating gas exchange using hydrogen-carbonate indicator
Hydrogen-carbonate indicator solution is orange/red at normal atmospheric CO2 levels. It changes:
- Yellow if CO2 increases (more acidic — respiration exceeds photosynthesis, e.g. in the dark)
- Purple/magenta if CO2 decreases (more alkaline — photosynthesis exceeds respiration, e.g. in bright light)
A leaf is sealed in a test tube with the indicator; tubes are placed in different light intensities and colour changes compared, with a control tube containing no leaf.
A leaf kept in bright light turns the indicator purple, because photosynthesis removes more CO2 than respiration adds.
Section 4
How do humans exchange gases in the thorax?
The thorax contains the structures for ventilation and gas exchange:
- Ribs and intercostal muscles — move the ribcage to change chest volume
- Diaphragm — a sheet of muscle below the lungs
- Trachea, bronchi, bronchioles — the airways carrying air to the lungs
- Alveoli — tiny air sacs where gas exchange occurs
- Pleural membranes — surround the lungs and reduce friction during breathing
Ventilation: contraction of the intercostal muscles and diaphragm increases chest volume, decreasing pressure and drawing air in (inhalation); relaxation reduces volume, increasing pressure and pushing air out (exhalation).
Section 5
How are alveoli adapted for gas exchange, and what does smoking do?
Alveoli are adapted for efficient diffusion of oxygen and carbon dioxide between air and blood:
- Large surface area (millions of alveoli)
- Thin walls (one cell thick) — short diffusion distance
- Moist lining — gases dissolve before diffusing
- Surrounded by a dense network of capillaries — maintains a steep concentration gradient
Smoking damages the lungs and circulatory system: tar and carcinogens damage alveoli (reducing surface area, causing emphysema), cilia are destroyed (mucus builds up, causing bronchitis), carbon monoxide reduces the oxygen-carrying capacity of blood, and nicotine increases heart rate and blood pressure, raising the risk of coronary heart disease.
When explaining alveolar adaptations, always mention surface area, wall thickness, moisture AND blood supply — each is worth a separate mark.
Must Know
- Gas exchange in plants occurs by diffusion through stomata
- Net gas exchange direction depends on light: CO2 in/O2 out in light, O2 in/CO2 out in dark, because respiration is continuous
- The leaf's broad, thin shape with many stomata and internal air spaces maximises gas exchange
- Hydrogen-carbonate indicator turns purple when CO2 falls (photosynthesis dominant) and yellow when CO2 rises (respiration dominant)
- Alveoli are adapted for diffusion by large surface area, thin walls, moisture and a good capillary blood supply
- Smoking damages alveoli and cilia and increases the risk of coronary heart disease
That's the notes covered.
Carry on to the next subtopic.