2g Gas Exchange¶

Part of 2 Structure and Functions in Living Organisms.

Gas exchange is diffusion at the scale of a whole organism: oxygen and carbon dioxide have to cross a thin, moist surface with a good supply of blood or air. In plants that surface is the leaf, and the direction of the exchange changes with the light. In humans it is the alveoli, which ventilation keeps supplied with fresh air.

What You Need to Learn¶

Further detail: Pearson Edexcel International GCSE Biology specification.

How air is moved into and out of the lungs, how the alveoli are adapted for gas exchange, and the effects of smoking, with simple breathing investigations. Biology-only students also learn how leaves exchange gases and how the balance of gases changes between day and night.

Biology-Only Content: Gas Exchange in Leaves¶

This content is required for Biology-only students and is not required for Combined Science students.

In plants, gas exchange of oxygen and carbon dioxide is needed for both photosynthesis and respiration. All movement of gases is by diffusion.

Leaf adaptations for gas exchange:

Gas Exchange in Leaves¶

Plants respire all the time because respiration happens in all living cells. Photosynthesis happens only when light is available, so the changing part through the day is the rate of photosynthesis, not the fact that respiration is occurring. Carbon dioxide diffuses into leaves for photosynthesis, while oxygen diffuses out as a product. Air spaces in the spongy mesophyll and the thin leaf structure help keep the diffusion distance short. Stomata are pores in the leaf surface; guard cells control whether they are open or closed, regulating both gas exchange and water loss.

Day and Night Gas Balance¶

• Plants respire all the time, releasing CO₂ and consuming O₂ in both daylight and darkness.
• Photosynthesis can happen only when light is available.
• During the day, photosynthesis generally runs at a rate equal to or greater than respiration, so there is net CO₂ uptake and net O₂ release from the plant.
• At night, photosynthesis stops but respiration continues, so there is net CO₂ release and net O₂ uptake from the plant.

Practical investigation of net gas exchange: A water plant is placed in a beaker of water containing hydrogen-carbonate indicator:

• Hydrogen-carbonate indicator is red at normal CO₂ concentration, purple when CO₂ falls (photosynthesis dominant) and yellow when CO₂ rises (respiration dominant).
• Moving a lamp closer increases light intensity, increasing the rate of photosynthesis and turning the indicator purple.
• Moving the lamp further away reduces photosynthesis until respiration is dominant, turning the indicator yellow.

Ventilation in Humans¶

The thorax contains the main gas exchange structures:

Alveoli and Ventilation Mechanics¶

Ventilation mechanism:

Air moves from regions of high pressure to low pressure. When the thorax volume increases (ribcage up and out, diaphragm flat), pressure inside falls below atmospheric pressure, so air is drawn in.

The mechanical movements of the ribs and diaphragm create the pressure difference. Air is not actively "sucked" in; it moves in because the pressure inside the thorax becomes lower than the pressure outside the body.

Explore ventilation mechanics¶

Use the interactive below to switch between inhalation and exhalation while keeping the same thorax diagram in view. It is most useful for turning the table above into one linked picture, so rib movement, diaphragm shape, pressure change and air movement are remembered as one pattern. Open full interactive.

This model shows quiet breathing, so use it to fix the inhalation-versus-exhalation pattern rather than to memorise exact anatomy or forced-breathing detail.

Alveolar adaptations for efficient gas exchange:

• Thin walls — each alveolus is just one cell thick, giving a very short diffusion pathway.
• Folded structure — the many alveoli provide a very large total surface area.
• Rich capillary supply — blood capillaries surround each alveolus, maintaining a steep concentration gradient by continuously removing O₂ and delivering CO₂.

Smoking and Breathing Practical Work¶

Smoking damages several parts of the gas exchange and transport systems at the same time. Tar damages cilia and irritates the airways, so mucus is cleared less effectively and infections become more likely. Long-term damage to alveolar walls reduces surface area for gas exchange, which is why emphysema causes breathlessness. Carbon monoxide from smoke binds to haemoglobin and reduces how much oxygen the blood can carry. Nicotine increases heart rate and blood pressure, increasing strain on the circulatory system and contributing to coronary heart disease.

Simple breathing investigations help connect ventilation to gas exchange. Exhaled air can be bubbled through limewater to show that carbon dioxide is being released. Breathing rate can be measured at rest and again after exercise to show that exercise increases ventilation because respiring muscles need more oxygen and produce more carbon dioxide. In class practical work, the key pattern is that exercise raises both breathing rate and depth until demand falls again during recovery.

Common Confusions¶

• Plants respire too: Plants do not only photosynthesise. They respire continuously, just like animals. The balance changes through the day because photosynthesis depends on light.
• Ventilation vs gas exchange: Ventilation moves air into and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide between the air in alveoli and the blood in capillaries.
• Breathing vs respiration: Breathing means ventilation. Respiration is the chemical process in cells that releases energy from food.
• Day vs night gas balance: Students sometimes think plants only respire at night. Plants respire continuously; the difference is that photosynthesis also occurs in the day.
• Air movement in ventilation: Air moves because of pressure differences created by changes in thorax volume, not because the lungs actively pull air in.

Check Yourself¶

1. State three features of the alveoli that make gas exchange efficient.
2. Describe what happens to the intercostal muscles, the diaphragm and the pressure in the thorax during exhalation.
3. A student says that air is sucked into the lungs by the lungs expanding. Explain what is wrong with this.
4. Explain why a plant releases carbon dioxide at night but takes it in during the day. (Biology only)
5. Explain why exercise increases the rate and depth of breathing.
6. Explain how smoking can cause breathlessness.

Key Terms¶

• Stoma: a pore in the leaf surface through which gases and water vapour move.
• Guard cell: a cell that controls the opening and closing of a stoma by changes in its water content.
• Ventilation: the movement of air into and out of the lungs by muscular action.
• Alveolus: a tiny air sac in the lungs where gas exchange occurs.
• Intercostal muscles: muscles between the ribs that help move the chest during breathing.
• Diaphragm: a dome-shaped muscle beneath the lungs whose contraction and relaxation drives ventilation.
• Pressure gradient: a difference in air pressure that causes air to move from a region of higher pressure to a region of lower pressure.
• Bronchi: the two airways that branch from the trachea and enter the lungs.
• Bronchioles: small airways branching from the bronchi that lead to the alveoli.
• Pleural membranes: membranes that lubricate the surface of the lungs and the chest wall.

Connected Pages¶

• 2e Nutrition
• 2f Respiration
• 2h Transport
• Ventilation Mechanism
• IGCSE Biology Key Terms Glossary
• Practical Investigation Reference