CIE A Level Biology Topic 9: Gas Exchange Exam Questions

About CIE A Level Biology Topic 9: Gas Exchange

Gas exchange in the Cambridge 9700 syllabus focuses on the human gas exchange system. You need to describe the structure of the trachea, bronchi, bronchioles and alveoli, and recognise these tissues in photomicrographs and diagrams. You must explain the roles of cartilage, ciliated epithelium, goblet cells, mucous glands, smooth muscle and elastic fibres, and how alveoli are adapted for efficient diffusion of oxygen and carbon dioxide. The topic links back to diffusion and surface area to volume ratio in Topic 4 and to transport in mammals in Topic 8. Examiners often ask you to compare the distribution of tissues along the airways.

What you need to know

• Airway structure The trachea has C-shaped rings of cartilage, bronchi have irregular plates, and bronchioles have no cartilage. Smooth muscle and elastic fibres are found throughout, with relatively more smooth muscle in bronchioles.
• Cartilage Cartilage supports the trachea and bronchi and prevents them collapsing when air pressure inside falls during inspiration.
• Mucus and cilia Goblet cells and mucous glands secrete mucus that traps particles and pathogens. Ciliated epithelium beats the mucus up towards the throat to be swallowed.
• Smooth muscle and elastic fibres Smooth muscle contracts to narrow the airways. Elastic fibres stretch during inspiration and recoil during expiration, helping to push air out of the alveoli.
• Alveoli Alveoli have squamous epithelium one cell thick, a large total surface area, a dense capillary network and constant ventilation, which maintain steep concentration gradients and a short diffusion distance.
• Gas exchange Oxygen diffuses from alveolar air into the blood and carbon dioxide diffuses out, down their concentration gradients, across the alveolar and capillary walls.

Common mistakes in Topic 9: Gas Exchange

• Saying cilia trap dust; mucus traps particles and cilia move the mucus.
• Stating that bronchioles contain cartilage; only the trachea and bronchi have cartilage.
• Describing elastic fibres as contracting; they stretch and recoil but cannot contract, unlike smooth muscle.
• Writing that alveoli walls are thin without explaining the effect; say this gives a short diffusion distance and faster diffusion.
• Confusing ventilation with gas exchange; ventilation moves air in and out, gas exchange is diffusion across the alveolar wall.

Try three Topic 9: Gas Exchange exam-style questions