Gas exchange

• Ring, plates, none: the cartilage tells you the region. Mucus traps and cilia move; oxygen is never actively moved — it diffuses, and breathing and blood flow keep the gradients steep.
• Ring, plates, none. Mucus traps, cilia move, up to the throat. Diffusion down gradients kept steep by ventilation and blood flow.

• 9.1.1 I can describe — describe the structure of the human gas exchange system, limited to: • lungs • trachea • bronchi • bronchioles • alveoli • capillary network
• 9.1.2 I can describe — describe the distribution in the gas exchange system of cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries
• 9.1.3 I can recognise — recognise cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries in microscope slides, photomicrographs and electron micrographs
• 9.1.4 I can recognise — recognise trachea, bronchi, bronchioles and alveoli in microscope slides, photomicrographs and electron micrographs and make plan diagrams of transverse sections of the walls of the trachea and bronchus
• 9.1.5 I can describe — describe the functions of ciliated epithelial cells, goblet cells and mucous glands in maintaining the health of the gas exchange system
• 9.1.6 I can describe — describe the functions in the gas exchange system of cartilage, smooth muscle, elastic fibres and squamous epithelium
• 9.1.7 I can describe — describe gas exchange between air in the alveoli and blood in the capillaries

Why the tubes are built differently. The trachea and bronchi are wide and must not collapse, so they carry cartilage. The bronchioles are narrow and many, and their smooth muscle can change how much air reaches each group of alveoli. The alveoli must be as thin as possible, so they carry nothing but squamous cells, elastic fibres and capillaries.

• “The bronchioles are held open by rings of cartilage.” Correct Bronchioles have no cartilage. Their wall is mostly smooth muscle. Cartilage forms C-shaped rings in the trachea and irregular plates in the bronchi, and stops there.
• “The trachea is supported by complete rings of cartilage.” Correct The rings are C-shaped, open at the back where the trachea lies against the oesophagus, and the gap is bridged by smooth muscle.
• “The cilia catch the dust and pollen.” Correct Mucus traps particles; cilia beat and move the mucus up to the throat. Nothing sticks to a cilium.
• “Smooth muscle contracts to open up the bronchioles.” Correct Contraction narrows the airway; relaxation lets it widen.
• “Elastic fibres contract to squeeze air out of the alveoli.” Correct Elastic fibres are not cells and cannot contract. They stretch as the alveoli fill and recoil afterwards.
• “Goblet cells and ciliated cells line the alveoli.” Correct The alveolar wall is squamous epithelium only. No airway is lined by squamous epithelium: the trachea, bronchi and bronchioles are lined by ciliated epithelium. (The capillary endothelium is also made of flattened cells, but it lines blood vessels, not air spaces.)
• “Oxygen is actively transported from the alveoli into the blood.” Correct Oxygen moves by diffusion, down its partial pressure gradient. Ventilation and blood flow keep that gradient steep.
• A plan diagram with cells drawn in it. Correct A plan diagram shows the outlines of the tissue layers in their correct proportions and no cells.
• “Bronchioles are supported by cartilage rings.” Repair Bronchioles have no cartilage; their wall is mainly smooth muscle.
• “The trachea has complete rings of cartilage.” Repair The rings are C-shaped, open at the back, and the gap is joined by smooth muscle.
• “The bronchi have C-shaped rings, like the trachea.” Repair The bronchi have irregular plates of cartilage.
• “The cilia catch the dust and hold it.” Repair Mucus traps particles; the cilia beat to move the mucus.
• “Cilia move mucus down into the lungs.” Repair They move it up to the throat, away from the alveoli, where it is swallowed or coughed out.
• “Goblet cells are found in the alveoli.” Repair Goblet cells are in the trachea, bronchi and larger bronchioles; the alveoli have only squamous epithelium.
• “Mucous glands are in the bronchioles.” Repair Mucous glands lie in the submucosa of the trachea and bronchi only.
• “Smooth muscle contracts to open the airways.” Repair Contraction narrows the airways; relaxation widens them.
• “Elastic fibres contract to push air out.” Repair Elastic fibres recoil after being stretched; they do not contract.
• “The alveolar wall is thin so that oxygen can pass through easily.” Repair The wall is one squamous cell thick, so the diffusion distance is short and the rate of diffusion is high. Name the mechanism.
• “The alveolar cells move oxygen into the blood by active transport.” Repair Oxygen moves by diffusion down its partial pressure gradient; no ATP is used.
• “Breathing keeps the gradient steep by bringing in blood.” Repair Ventilation refreshes the air; blood flow refreshes the blood; together they keep both gradients steep.
• Drawing cells, or shading, in a plan diagram of the trachea. Repair Tissue layers only, as outlines, in the correct proportions, with the cartilage the thickest layer.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• Say what you saw, then what it is. “Cells in lacunae, so cartilage” is an answer that can be checked. “It looks like cartilage” is not.
• Timing words only. “When you breathe in” and “when you breathe out” are used here only to say when cartilage and elastic fibres act. How the chest moves air in and out is not in the 9700 syllabus, so an answer about this outcome never needs it.
• Interleave with the chapters that use this one. Topic 10 (infectious diseases) sends airborne pathogens down these airways: when you reach it, re-answer “how do mucus and cilia keep pathogens away from the alveoli?”. Chapter 8 is the other half of 9.1.7: re-read the dissociation curve and ask why haemoglobin is nearly saturated after passing the alveoli. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 9 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A multiple-choice item on topic 9 can turn on a single row of the distribution table: which region has cartilage plates, which has goblet cells, where squamous epithelium is found, what smooth muscle does when it contracts. A structured question asks you to describe the distribution of named tissues, explain their functions, or describe how oxygen reaches the blood, and rewards the precise word: plates not rings, recoil not contract, diffusion not pumping.
• Topic 9 is examined through images: a photomicrograph of a section to identify as trachea, bronchus, bronchiole or alveoli, a tissue to recognise, an electron micrograph of cilia or of the air–blood barrier. The numbers are actual size from a scale bar or a magnification, a surface area from a model the question describes, and differences between partial pressures that the question supplies.
• In Paper 3 you may be given a slide of trachea or lung to draw as a plan diagram (layers only, no cells), to measure with a calibrated eyepiece graticule, or to sample with a grid. Topic 9 names no experiment, so the investigation in this chapter models diffusion distance with agar cubes: distance to the centre is the independent variable, the time for the colour to disappear is the dependent variable, and judging the end-point is the main error. Paper 5 may ask you to analyse measurements of the barrier.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.