B3.1 Gas exchange

Every organism exchanges gases with its environment. Aerobic cells take in oxygen for cell respiration and release carbon dioxide; photosynthesising cells take in CO₂ and release O₂ (see C1.2 Cell respiration). This gas exchange happens by diffusion, which is passive but slow over anything more than a fraction of a millimetre.

The problem of size

A unicellular organism such as Amoeba can exchange gases across its whole cell surface. As organisms get bigger this stops working, for two reasons:

• The surface area-to-volume ratio (SA:V) falls (see B2.3 Cell specialization). Volume, and so the demand for O₂, rises with the cube of length, but surface area rises only with the square.
• The distance from the centre to the outside increases, so diffusion alone would take far too long to supply the inner cells.

Large organisms therefore have a specialized gas-exchange surface with a much larger area than their outer surface, such as the alveoli of mammalian lungs, the gills of fish or the spongy mesophyll of a leaf, often combined with a transport system.

Properties of gas-exchange surfaces

Rate of diffusion depends on area, concentration gradient and distance, so efficient exchange surfaces share four properties:

1. Permeable: O₂ and CO₂ pass through freely. They are small and non-polar, so they diffuse through cell membranes.
2. Thin: often only one layer of flattened cells, giving a short diffusion distance.
3. Moist: gases dissolve in a film of water before they diffuse through the cells. On land this moisture is also a source of water loss.
4. Large surface area relative to the volume of the organism.

Diffusion produces a net movement only while there is a concentration gradient. Left alone, gases would diffuse until the concentrations on both sides of the surface were equal, and exchange would stop. Small organisms keep their gradients simply by respiring: O₂ is used up inside and CO₂ is produced. Large animals with lungs or gills need more:

• Dense networks of blood vessels: capillaries cover the exchange surface, so blood is close to every part of it.
• Continuous blood flow: blood that has picked up O₂ and lost CO₂ is carried away at once and replaced by blood arriving from the body, which is low in O₂ and high in CO₂.
• Ventilation: the air or water next to the surface is renewed, keeping its O₂ concentration high and its CO₂ concentration low.

Ventilation with air and with water

• Lungs of mammals are ventilated with air by breathing (next sections). The flow is tidal: air moves in and out through the same airways.
• Gills of bony fish are ventilated with water. Water is pumped in through the mouth, over the gill filaments and out through the gill openings, so the flow is one-way. Blood flows through the gill lamellae in the opposite direction to the water (a countercurrent), so the blood always meets water with a higher O₂ concentration and a gradient is kept along the whole length of the lamella. This matters because water holds far less dissolved O₂ than the same volume of air.

Air reaches the lungs through the trachea, which divides into two bronchi. Each bronchus branches again and again into narrower bronchioles, which end in alveolar ducts opening into clusters of alveoli. The mammalian alveolar lung is adapted for gas exchange in four main ways:

• Branched network of bronchioles: the repeated branching carries air to every part of each lung, so ventilation reaches hundreds of millions of alveoli.
• Very large surface area: an adult’s lungs contain roughly 300–500 million alveoli, each only about 0.2–0.3 mm across, with a total surface area of around 50–100 m², dozens of times the area of the skin.
• Extensive capillary beds: each alveolus is covered by a dense network of capillaries, so the area of blood in contact with the alveoli is almost as large as the alveolar surface itself.
• Surfactant: type II pneumocytes secrete surfactant (phospholipids and proteins) into the film of moisture lining the alveoli. It lowers the surface tension of the water, so the moist walls do not stick together and the alveoli do not collapse when air is breathed out.

A very short diffusion distance

The alveolar wall is mostly made of type I pneumocytes: extremely thin, flattened cells covering about 95 % of the alveolar surface. The capillary wall is a single layer of thin endothelium. Oxygen diffusing into the blood crosses only these two thin cells (sharing a basement membrane), a total of about 0.5 µm. Red blood cells squeeze through the capillaries in single file, close to the wall. (Type I and type II pneumocytes as two cell types in one tissue are also part of B2.3 Cell specialization.)

Air can only enter or leave the thorax through the airways. Gases flow from higher to lower pressure, so to move air the body changes the volume of the thorax: increasing a volume lowers the pressure inside it, and decreasing it raises the pressure. The lungs follow the movements of the chest wall and diaphragm. Two pairs of antagonistic muscles do this work: the external and internal intercostal muscles between the ribs, and the diaphragm and the abdominal muscles in the front wall of the abdomen.

At rest, expiration is passive. When the external intercostals and diaphragm relax, the lungs, which were stretched during inspiration, recoil elastically, and the volume of the thorax decreases without any muscle contracting. During exercise, forced expiration uses the internal intercostals and abdominal muscles to push out more air, quickly. How the brainstem adjusts the ventilation rate when blood CO₂ rises is part of C3.1 Integration of body systems.

A spirometer measures the volume of air breathed in and out. A simple one is a graduated container of air over water, connected to a mouthpiece by a tube; electronic spirometers measure the flow of air and use software to calculate volumes. A nose clip makes sure all the air passes through the mouthpiece. Each measurement is repeated (at least three times) to check that the readings are consistent.

From a spirometer trace you can find:

• Tidal volume (TV): the volume breathed in or out in one normal breath. Measure from the bottom to the top of a normal breath (about 0.5 dm³ at rest).
• Inspiratory reserve volume (IRV): the extra volume that can be breathed in by forcing inhalation after a normal breath in (about 3 dm³).
• Expiratory reserve volume (ERV): the extra volume that can be breathed out by forcing exhalation after a normal breath out (about 1 dm³).
• Vital capacity (VC): the largest volume that can be breathed out after the deepest possible breath in: VC = IRV + TV + ERV (about 4–5 dm³ in a young adult; it varies with sex, height, age and fitness).
• Ventilation rate: breaths per minute, counted from the peaks in a known time (about 12–16 at rest).

Some air, the residual volume (about 1.2 dm³), always stays in the lungs and airways, so a spirometer cannot measure it. Multiplying ventilation rate by tidal volume gives the volume of air breathed per minute.

During exercise, both tidal volume and ventilation rate increase. The extra tidal volume uses part of the inspiratory and expiratory reserves, so the IRV and ERV get smaller while vital capacity stays the same.

Leaves exchange gases for photosynthesis: chloroplasts in the mesophyll need a supply of CO₂, and the O₂ they produce must be removed (at night, when respiration continues without photosynthesis, the direction is reversed). Like an alveolus, the leaf needs a large, moist surface for exchange, but on land a moist surface loses water. The dicotyledonous leaf balances these needs.

Adaptations for gas exchange

• Waxy cuticle: a waterproof layer of wax secreted by the epidermis on both surfaces. It reduces water loss, but it is also almost impermeable to gases, so gas exchange happens mainly through the stomata.
• Epidermis: a single layer of tightly fitting cells that protects the leaf, secretes the cuticle and contains the guard cells. Upper epidermis cells are transparent, so light reaches the palisade mesophyll.
• Stomata and guard cells: each stoma is a pore between a pair of guard cells, which change shape to open or close it. Stomata are open when gas exchange is needed for photosynthesis, and close at night and when the plant is short of water. In many dicots most stomata are in the lower epidermis.
• Air spaces: the stomata open into a network of air spaces that runs between the mesophyll cells, so CO₂ and O₂ can diffuse quickly through the inside of the leaf.
• Spongy mesophyll: loosely packed, rounded cells with large air spaces between them. Their moist cell walls provide a very large surface area where CO₂ dissolves and diffuses into the cells. Photosynthesis using CO₂ keeps its concentration in the cells low, maintaining the gradient.
• Veins: water evaporates from the moist mesophyll cell walls and is lost, so xylem in the veins brings water to replace it; phloem carries away the sugars made in photosynthesis (see B3.2 Transport).

Most photosynthesis happens in the palisade mesophyll, tall closely packed cells with many chloroplasts just below the upper epidermis.

A plan diagram of the tissues

A plan diagram shows where the tissues are, as outlined areas, without drawing individual cells.

Transpiration is the loss of water vapour from the leaves and stems of a plant. It is a side effect of gas exchange, not a process the plant carries out for its own sake:

1. Water evaporates from the moist walls of the mesophyll cells into the air spaces, which are usually saturated, or nearly saturated, with water vapour.
2. When the stomata are open for CO₂ to enter, water vapour diffuses out through them, down a concentration gradient to the drier air outside.
3. More water then evaporates from the cell walls, and is replaced by water from the xylem.

A leaf cannot take in CO₂ without opening its stomata, and it cannot have moist exchange surfaces without evaporation. Closing the stomata saves water but also stops CO₂ entering, so photosynthesis slows.

Factors affecting the rate of transpiration

The rate of water uptake by a leafy shoot, which is close to the rate of transpiration, can be measured with a potometer.

Stomatal density is the number of stomata per unit area of leaf surface, usually in stomata per mm² (mm⁻²). To see the stomata, use one of these methods:

• Epidermal peel: peel a thin strip of lower epidermis from the leaf and mount it in water.
• Leaf cast: paint a small area of the leaf with clear nail varnish, let it dry, lift it off with clear tape and mount it; the varnish holds an impression of the epidermis.
• Micrograph: photograph the surface through a microscope and count from the image.

Calculating the density

1. Measure the diameter of the field of view at high power with a stage micrometer (for example 0.40 mm).
2. Calculate the area of the field: area = π r² = π × (0.40 ÷ 2)² = 0.126 mm².
3. Count the stomata in the field. Count those that are more than half inside the circle, and leave out the rest.
4. Repeat in several randomly chosen fields of view, away from veins and the leaf edge, and calculate the mean.
5. Stomatal density = mean number of stomata ÷ area of field. For a mean of 19 stomata: 19 ÷ 0.126 = 151 stomata per mm².

Reliability (nature of science)

Biological material is variable: the number of stomata differs from one field of view to the next, between leaves and between plants. A single count can give a misleading result, so repeated counts are needed. The mean of several counts is a more reliable estimate, and the spread of the counts (the range or standard deviation) shows how variable the leaf is. Repeating the whole procedure on other leaves (replicate trials) shows whether the result is typical of the plant.