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B3.1

Gas exchange

Theme B · Form and function · Organisms · SL and HL · plus additional higher level

Every organism must take in one gas and get rid of another, and the bigger it is, the harder that becomes. This topic compares how a mammal and a flowering plant solve the same problem — with lungs in one case and leaves in the other — and the same four properties of an exchange surface explain both. At HL it moves inside the blood, to how haemoglobin loads and unloads oxygen exactly where it is needed.

🎯What you need to be able to do

  • Explain why gas exchange becomes harder as organisms get larger.
  • State the properties of gas-exchange surfaces, and explain how concentration gradients are maintained in animals.
  • Explain the adaptations of mammalian lungs and the mechanism of ventilation.
  • Measure and define tidal volume, vital capacity, and inspiratory and expiratory reserve volumes.
  • Explain the adaptations of leaves for gas exchange, and draw a plan diagram of a dicot leaf section.
  • Explain transpiration as a consequence of gas exchange, and the factors that affect its rate.
  • Determine stomatal density from micrographs or leaf casts.
  • AHL Explain cooperative binding, the adaptations of foetal and adult haemoglobin, the Bohr shift and the shape of oxygen dissociation curves.

📚The biology

Why gas exchange gets harder with size

All organisms exchange gases with their environment: aerobic organisms take in oxygen for respiration and release carbon dioxide; photosynthesizing organisms do the reverse in the light. A very small organism can rely on diffusion across its body surface. As organisms get larger, two things make that impossible:

  • Surface area to volume ratio decreases (B2.3), so there is less surface to supply each unit of volume.
  • The distance from the centre to the outside increases, so diffusion alone would take far too long to reach inner cells.

Large organisms therefore have specialized gas-exchange organs with a large internal surface, and often a transport system to carry gases between that surface and the cells.

Properties of gas-exchange surfaces

Permeable
Oxygen and carbon dioxide must be able to pass through freely.
Thin
A thin layer of tissue gives a short diffusion distance. Rate of diffusion is inversely related to distance.
Moist
Gases dissolve in a film of water before diffusing across the cells.
Large surface area
More area allows more gas to diffuse per unit time.

Maintaining concentration gradients in animals

Diffusion only continues while there is a concentration gradient. Animals keep the gradient steep in three ways:

  • Dense networks of blood vessels close to the surface, so gases are carried away or delivered almost immediately.
  • Continuous blood flow — blood that has picked up oxygen is replaced by deoxygenated blood, keeping the oxygen concentration in the capillaries low.
  • Ventilation — replacing the medium in contact with the surface: air in lungs, water flowing over gills. Fresh air or water keeps the oxygen concentration outside high and carbon dioxide low.

Adaptations of mammalian lungs

Air enters through the trachea, which divides into two bronchi, each dividing repeatedly into narrower bronchioles, which end in clusters of tiny air sacs, the alveoli. The key adaptations:

  • A branched network of bronchioles carries air to and from hundreds of millions of alveoli throughout the lungs.
  • A very high surface area: the alveoli together provide roughly the area of a tennis court.
  • Extensive capillary beds surround every alveolus, so blood flows close to the air.
  • Surfactant, secreted by type II pneumocytes, coats the moist inner surface of the alveoli and reduces surface tension, preventing the alveoli from collapsing and sticking together when air is breathed out.
  • Very thin walls: alveolar walls are one cell thick (type I pneumocytes), and capillary walls are one cell thick, so oxygen diffuses only a very short distance into the blood (B2.3).

Ventilation of the lungs

Air moves in and out because of pressure changes in the thorax. Muscles change the volume of the thorax; a larger volume means a lower pressure, so air flows in from the atmosphere, and a smaller volume means a higher pressure, so air flows out.

Inspiration (breathing in)
Diaphragm contracts and flattens, moving down.
External intercostal muscles contract, moving the ribs up and out.
Thorax volume increases, pressure decreases below atmospheric, air flows in.
Expiration (breathing out)
Diaphragm relaxes and domes upwards.
External intercostals relax; ribs move down and in. In forced expiration, internal intercostal muscles contract to pull the ribs down, and abdominal muscles contract, pushing the diaphragm up.
Thorax volume decreases, pressure increases above atmospheric, air flows out.

At rest, expiration is mostly passive: the elastic recoil of the lungs and relaxation of the muscles are enough.

Measuring lung volumes

Lung volumes can be measured with a spirometer, or simply by breathing out into an inverted, water-filled bottle through a tube and measuring the volume of water displaced.

Tidal volume
The volume of air breathed in or out in a normal breath at rest (about 0.5 dm3 in an adult).
Inspiratory reserve volume
The extra volume that can be breathed in, by forced inspiration, after a normal inspiration.
Expiratory reserve volume
The extra volume that can be breathed out, by forced expiration, after a normal expiration.
Vital capacity
The maximum volume that can be breathed out after the deepest possible breath in. It equals tidal volume + inspiratory reserve + expiratory reserve.

Some air always stays in the lungs (the residual volume), so vital capacity is less than total lung capacity.

Adaptations of leaves for gas exchange

A leaf must take in carbon dioxide for photosynthesis and release oxygen, while losing as little water as possible.

Waxy cuticle
A waterproof layer on the upper (and lower) epidermis, reducing water loss by evaporation. Gases also cannot pass through it, so exchange happens at stomata.
Epidermis
A single layer of transparent cells that protects the leaf and lets light through to the mesophyll.
Stomata and guard cells
Pores, mostly in the lower epidermis, through which CO2 diffuses in and O2 and water vapour diffuse out. Pairs of guard cells open and close them, balancing CO2 uptake against water loss.
Spongy mesophyll
Loosely packed cells with moist walls, providing a large surface area for gases to dissolve and diffuse into cells.
Air spaces
A network between spongy mesophyll cells, allowing gases to diffuse rapidly through the leaf to and from the stomata.
Veins
Xylem supplies water, keeping mesophyll cell walls moist; phloem carries away sugars made in photosynthesis.

Tissues in a leaf: the plan diagram

A plan diagram shows the outlines of tissues, not individual cells. For a transverse section of a dicotyledonous leaf, draw from top to bottom:

  1. Waxy cuticle and upper epidermis — a thin layer.
  2. Palisade mesophyll — a band of tightly packed elongated cells, rich in chloroplasts, the main site of photosynthesis.
  3. Spongy mesophyll — a thicker region, shown with air spaces.
  4. Lower epidermis with gaps for stomata, and cuticle.

Show the midrib vein in the centre, with xylem towards the upper surface and phloem towards the lower surface. Use clear continuous lines, no shading, and label each tissue.

Transpiration

Transpiration is the loss of water vapour from the leaves and stems of a plant. It is an unavoidable consequence of gas exchange: to take in carbon dioxide, the leaf must have open stomata and moist cell walls inside, and water evaporates from those walls into the air spaces and diffuses out through the same stomata. Transpiration also drives the transport of water in xylem (B3.2).

Light intensity
Stomata open in light, so transpiration increases.
Temperature
Higher temperature increases evaporation and diffusion, and air holds more water vapour, so transpiration increases.
Humidity
Humid air reduces the water vapour concentration gradient between leaf and air, so transpiration decreases.
Wind speed
Wind removes water vapour from around the stomata, keeping the gradient steep, so transpiration increases.

Stomatal density

Stomatal density is the number of stomata per unit area of leaf surface, usually per mm2. To measure it, paint clear nail varnish onto the lower epidermis, let it dry, peel it off with clear tape (a leaf cast) and mount it on a slide — or use a micrograph. At high power, count the stomata in the field of view, and calculate the area of the field of view from its diameter.

Counts vary from one field of view to another, because biological material is variable. Repeated counts in several randomly chosen fields, and a mean, give a more reliable result.

Haemoglobin and cooperative binding AHL

Haemoglobin has four polypeptide chains, each with a haem group that can bind one oxygen molecule (B1.2). Binding is cooperative: when one oxygen molecule binds, the protein changes shape slightly, and this makes it easier for the next oxygen to bind. Conversely, when one oxygen is released, the others are released more easily.

Haemoglobin also has sites, separate from the haem groups, where carbon dioxide can bind. This is allosteric binding: CO2 attaching at one site changes the shape of the whole molecule, reducing its affinity for oxygen.

Oxygen dissociation curves AHL

An oxygen dissociation curve plots the percentage saturation of haemoglobin with oxygen against the partial pressure (concentration) of oxygen. It shows the affinity of haemoglobin for oxygen at different oxygen concentrations. The curve is S-shaped (sigmoid), and cooperative binding explains the shape:

  • At low partial pressures, the curve is shallow: it is hard for the first oxygen to bind to an empty haemoglobin molecule.
  • In the middle, the curve is very steep: once one oxygen has bound, the others bind much more easily, so a small rise in oxygen concentration produces a large rise in saturation.
  • At high partial pressures, the curve levels off as haemoglobin approaches full saturation.

The steep middle section matches the range of oxygen concentrations in respiring tissues, which is exactly where unloading is needed: a small fall in oxygen concentration releases a large amount of oxygen. In the lungs, at high partial pressure, haemoglobin is almost fully saturated.

The Bohr shift AHL

Actively respiring tissues produce a lot of carbon dioxide. Carbon dioxide binds allosterically to haemoglobin and also forms carbonic acid, lowering the pH; both reduce haemoglobin’s affinity for oxygen. As a result, at the same oxygen partial pressure, haemoglobin releases more oxygen. On a graph, the whole dissociation curve shifts to the right: this is the Bohr shift.

The benefit is that oxygen is unloaded precisely where it is needed most: the more actively a tissue is respiring, the more CO2 it produces and the more oxygen haemoglobin gives up to it. In the lungs, where CO2 is low, affinity is high again and haemoglobin loads fully.

Foetal and adult haemoglobin AHL

A foetus obtains oxygen from its mother’s blood across the placenta. Foetal haemoglobin has slightly different polypeptide chains from adult haemoglobin, which give it a higher affinity for oxygen: its dissociation curve lies to the left of the adult curve. At the partial pressure of oxygen in the placenta, maternal haemoglobin releases oxygen and foetal haemoglobin takes it up. Oxygen therefore passes from mother to foetus. After birth, foetal haemoglobin is gradually replaced by the adult form.

✏️Worked example

(a) A student made a leaf cast of the lower epidermis and counted stomata in five fields of view at high power: 18, 22, 20, 19 and 21. The diameter of the field of view is 0.4 mm. Calculate the stomatal density in stomata per mm2.
(b) Explain why five fields of view were counted rather than one.
(c) From a spirometer trace, a person’s tidal volume is 0.5 dm3, inspiratory reserve volume is 3.0 dm3 and expiratory reserve volume is 1.2 dm3. Calculate the vital capacity, and the volume of air breathed per minute at 12 breaths per minute at rest.

(a) Mean count:

\[ \frac{18 + 22 + 20 + 19 + 21}{5} = \frac{100}{5} = 20\ \text{stomata} \]

Area of the field of view, using radius = 0.2 mm:

\[ \pi r^{2} = \pi \times 0.2^{2} = 0.126\ \mathrm{mm^{2}} \]
\[ \text{stomatal density} = \frac{20}{0.126} = 159\ \text{stomata per mm}^{2} \]

(b) The number of stomata varies from one part of a leaf to another, so a single count may not be representative. Repeating the count in several fields and calculating a mean reduces the effect of this variability and makes the estimate more reliable. The spread of the counts (18 to 22) also shows how variable the leaf is.

(c) Vital capacity = tidal volume + inspiratory reserve + expiratory reserve:

\[ 0.5 + 3.0 + 1.2 = 4.7\ \mathrm{dm^{3}} \]

Volume per minute = tidal volume × breaths per minute = 0.5 × 12 = 6.0 dm3 min−1.

Check it. Typical stomatal densities on the lower epidermis of dicot leaves run from about 50 to 300 per mm2, so 159 is plausible. A vital capacity of 4.7 dm3 is typical for an adult. If you get about 40 stomata per mm2, you probably used the diameter (0.4) instead of the radius in \( \pi r^{2} \).
Using the diameter in the area formula. Field-of-view sizes are almost always given as a diameter. Halve it before squaring. Using 0.4 instead of 0.2 makes the area four times too large and the density four times too small.

📝Practise

Work through these on paper, then reveal the answer. Questions 5 and 6 are AHL.

1. Explain how concentration gradients for oxygen are maintained at the alveoli.
Ventilation constantly replaces air in the alveoli with fresh air from outside, keeping the oxygen concentration in the alveoli high (and carbon dioxide low). Continuous blood flow through the dense capillary network carries oxygenated blood away and brings deoxygenated blood, keeping the oxygen concentration in the capillaries low. Oxygen is also bound by haemoglobin as it enters red blood cells, which keeps the concentration of dissolved oxygen in plasma low.
2. Describe the mechanism of inspiration.
The diaphragm contracts and flattens, moving downwards. The external intercostal muscles contract, pulling the ribs upwards and outwards. These movements increase the volume of the thorax, which decreases the pressure inside the lungs to below atmospheric pressure. Air therefore flows into the lungs, down the pressure gradient, until the pressures are equal.
3. Explain how the structure of a leaf is adapted for gas exchange while reducing water loss.
For gas exchange: stomata allow CO2 to diffuse in and O2 out; air spaces in the spongy mesophyll allow rapid diffusion of gases through the leaf and give a large, moist surface area of cell walls where gases dissolve; the leaf is thin, giving short diffusion distances; veins supply water to keep cell walls moist. To reduce water loss: a waxy cuticle on the epidermis is waterproof; stomata are mostly on the lower surface, which is cooler and shaded; guard cells close stomata when water is scarce or in darkness.
4. Explain why the rate of transpiration increases on a warm, dry, windy day.
Warm: higher temperature increases the kinetic energy of water molecules, so more water evaporates from mesophyll cell walls and water vapour diffuses faster. Dry (low humidity): the air has a low concentration of water vapour, so the concentration gradient between the air spaces inside the leaf and the outside air is steep. Windy: moving air carries water vapour away from the leaf surface, preventing a layer of humid air building up around the stomata and so keeping the gradient steep. All three increase the rate of diffusion of water vapour out through the stomata.
5. AHL Explain the S-shape of the oxygen dissociation curve for adult haemoglobin, and the advantage of this shape.
The shape is due to cooperative binding. At low oxygen partial pressures, it is difficult for the first oxygen molecule to bind, so saturation rises slowly. Binding of the first oxygen causes a change in shape of haemoglobin that makes it easier for further oxygen molecules to bind, so over the middle range saturation rises steeply. At high partial pressures the curve levels off as haemoglobin becomes fully saturated. Advantage: haemoglobin is almost fully loaded in the lungs, and the steep section coincides with the oxygen concentrations of respiring tissues, so a small fall in oxygen concentration causes a large release of oxygen where it is needed.
6. AHL Explain the Bohr shift and its benefit to actively respiring muscle.
Actively respiring muscle produces large amounts of carbon dioxide. CO2 binds allosterically to haemoglobin (and lowers pH by forming carbonic acid), causing a change in shape that reduces its affinity for oxygen. The oxygen dissociation curve therefore shifts to the right: at any given oxygen partial pressure, haemoglobin is less saturated, so more oxygen is released. The benefit is that tissues respiring fastest, which produce the most CO2, automatically receive more oxygen, supporting continued aerobic respiration.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Physiology Web — interactive oxygen–haemoglobin dissociation curve, where you can change pH, CO2 and temperature and watch the curve shift.
  • Science & Plants for Schools (SAPS) — protocols for leaf casts, stomatal counts and measuring transpiration with a potometer.
  • Histology Guide (University of Leeds) — micrographs of alveoli and leaf sections for practising plan diagrams.