B3.1 Gas Exchange — Core idea
Gas exchange is essential because cells need oxygen for aerobic respiration and must remove carbon dioxide produced by metabolism.
In multicellular organisms, gas exchange becomes harder as surface area : volume ratio decreases and the diffusion distance from outside to inner cells increases.
High-scoring exam answers should link structure to function: adaptations exist to make diffusion rapid and to maintain steep concentration gradients.
Features of effective gas-exchange surfaces
Large surface area → more space for diffusion.
Thin exchange surface → short diffusion distance.
Permeable surface → gases cross easily.
Moist surface → gases must dissolve before diffusing across cell membranes.
In animals, efficiency is increased by ventilation and blood flow.
In plants, efficiency is increased by air spaces, stomata, and a short pathway between air and photosynthesizing cells.
Maintaining concentration gradients in animals
Diffusion only stays fast if there is a steep concentration gradient.
Dense capillary networks keep blood close to the exchange surface.
Continuous blood flow removes absorbed oxygen and brings carbon dioxide to be excreted.
Ventilation constantly refreshes the air in lungs or the water passing over gills.
Exam link: large surface area + thin barrier + gradient maintenance = rapid gas exchange.
Adaptations of mammalian lungs for gas exchange
Mammalian lungs contain many alveoli, giving a very large surface area.
Alveolar walls and capillary walls are each one cell thick, creating a short diffusion pathway.
A rich capillary bed surrounds alveoli, maintaining concentration gradients.
A branched system of bronchi → bronchioles → alveoli distributes air widely through the lungs.
Surfactant reduces surface tension, helping alveoli stay open and preventing collapse.
Type I pneumocytes are very thin for diffusion; type II pneumocytes secrete surfactant.
Educational caption: This diagram shows how the alveolus is specialized for gas exchange with an adjacent capillary network. It is especially useful for linking thin walls, blood supply, and surfactant production to function. Source
Ventilation of the lungs
Ventilation refreshes air in the lungs to maintain diffusion gradients.
During inhalation:
Diaphragm contracts and flattens.
External intercostal muscles contract.
Ribs move up and out.
Thoracic volume increases.
Pressure inside lungs decreases below atmospheric pressure.
Air enters the lungs.
During exhalation (quiet breathing):
Diaphragm relaxes and becomes dome-shaped.
External intercostals relax.
Thoracic volume decreases.
Pressure increases.
Air leaves the lungs.
In forced exhalation, abdominal muscles help push the diaphragm upward and internal intercostal muscles assist in pulling ribs down and in.
Exam tip: always connect muscle action → thoracic volume change → pressure change → direction of airflow.
Educational caption: This diagram helps explain ventilation by showing how muscle contraction changes thoracic volume and therefore pressure. It is ideal for exam questions asking you to explain why air moves into or out of the lungs. Source
Lung volumes you must know
Tidal volume (TV) = volume of air moved in or out during normal quiet breathing.
Inspiratory reserve volume (IRV) = extra air that can be inhaled after a normal inhalation.
Expiratory reserve volume (ERV) = extra air that can be exhaled after a normal exhalation.
Vital capacity (VC) = maximum volume exhaled after maximum inhalation.
Practical skill: be able to identify these on a spirogram or from spirometer data.
Be precise: vital capacity is not the same as tidal volume.
Educational caption: This graph shows the standard lung volumes measured in spirometry. It is useful for interpreting practical data and for distinguishing tidal volume from vital capacity and the reserve volumes. Source
Gas exchange in leaves
Leaves need gas exchange for photosynthesis and respiration.
Waxy cuticle reduces water loss, but also means gases mainly enter and leave through stomata.
Epidermis protects the leaf and contains stomata.
Guard cells control stomatal opening and therefore regulate gas exchange and transpiration.
Spongy mesophyll has many air spaces, increasing internal surface area and allowing gases to diffuse through the leaf.
Veins supply water to leaf tissues and remove products such as sugars.
In a dicot leaf cross section, know the general distribution of upper epidermis, palisade mesophyll, spongy mesophyll, air spaces, vascular bundle, and lower epidermis with stomata.
Comparison with lungs: both have large surface area, thin surfaces, and mechanisms to maintain gradients; plants rely on stomata + internal air spaces, not ventilation by muscles.
Educational caption: This diagram shows how a leaf is organized for gas exchange while limiting water loss. It is particularly useful for linking stomata, spongy mesophyll air spaces, and veins to overall leaf function. Source
Transpiration and stomatal density
Transpiration = loss of water vapour from leaves, mainly through stomata.
It is a consequence of gas exchange because stomata must open for carbon dioxide uptake.
Factors increasing transpiration rate commonly include higher temperature, greater light intensity, lower humidity, and greater wind speed.
Trade-off: open stomata allow CO2 uptake but increase water loss.
Stomatal density = number of stomata per unit area.
Practical skill: determine stomatal density using micrographs or leaf casts.
Reliability improves through repeat counts and use of replicate fields of view.
Educational caption: These micrographs show the structure of stomata in surface view and cross section. They are useful for understanding both gas exchange and the practical task of measuring stomatal density. Source
Checklist: can you do this?
Explain why larger organisms need specialized gas-exchange surfaces.
Describe how alveoli and leaves are adapted for efficient gas exchange.
Explain inhalation and exhalation using muscles, volume, pressure, and airflow direction.
Interpret a spirogram to identify tidal volume, vital capacity, IRV, and ERV.
Draw and label a plan diagram of a dicot leaf transverse section and measure stomatal density from a micrograph or leaf cast.
HL only — Haemoglobin and oxygen transport
Haemoglobin transports oxygen in red blood cells.
Each haemoglobin molecule contains haem groups that bind oxygen.
Cooperative binding means binding of one oxygen molecule makes binding of the next oxygen molecule easier.
This produces the characteristic S-shaped oxygen dissociation curve.
Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, allowing oxygen transfer from maternal to foetal blood.
Carbon dioxide can bind allosterically to haemoglobin, affecting oxygen release.
HL only — Bohr shift and oxygen dissociation curves
Bohr shift: increasing carbon dioxide concentration causes haemoglobin to release oxygen more readily.
Therefore, in actively respiring tissues with high CO2, more oxygen is unloaded where it is needed.
An oxygen dissociation curve shows haemoglobin saturation at different oxygen concentrations.
The curve is sigmoidal (S-shaped) because of cooperative binding.
A curve shifted to the right means lower oxygen affinity and greater oxygen unloading.
Exam application: explain why muscles with high respiration get more oxygen.
Educational caption: This graph shows how haemoglobin saturation changes with oxygen concentration and why the curve is S-shaped. It is especially useful for understanding cooperative binding and the Bohr shift. Source
Exam shortcuts and common traps
Do not confuse gas exchange with cell respiration: gas exchange is movement of gases; respiration is release of energy in cells.
Surfactant does not directly speed diffusion; it mainly reduces surface tension and prevents alveolar collapse.
Stomata are the main route for gas exchange in leaves, but they also cause water loss.
When comparing lungs and leaves, always mention both similarities and differences.
In data questions, use precise terminology: concentration gradient, surface area, diffusion distance, ventilation, capillary network, transpiration, stomatal density.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.