Why gases must be exchanged

Gas exchange is the biological process where an organism takes in gases needed for metabolism and releases gaseous waste products to the environment. The gases involved depend on the organism and the conditions: animals usually take in oxygen and release carbon dioxide, while photosynthesising plant tissues take in carbon dioxide and release oxygen in daylight.

Keep the link with metabolism clear. In human cells, aerobic cell respiration produces carbon dioxide, especially during decarboxylation reactions in mitochondria. Oxygen is needed because it acts as the final electron acceptor in the mitochondrial electron transport chain, which allows ATP production to continue. If gas exchange fails, metabolism is affected very quickly.

Diffusion and organism size

Diffusion is the passive net movement of particles from a region of higher concentration to a region of lower concentration due to random molecular motion. Gas exchange depends on diffusion, but diffusion works fast enough only over short distances and across sufficiently large areas.

Small organisms can often exchange gases across their outer body surface because every cell lies close to the environment and the surface area-to-volume ratio is high. Surface area-to-volume ratio is a comparison of the exchange area of a structure with the space it contains or supplies. As an organism gets larger, its volume increases faster than its surface area, leaving less exchange surface available per unit volume. Cube models showing that surface area-to-volume ratio falls as size increases.

Large multicellular organisms solve this access problem in two connected ways. They develop specialized exchange surfaces, such as alveoli in mammalian lungs or moist mesophyll cell surfaces in leaves. Animals also use transport systems, especially the circulatory system, to distribute gases between the exchange surface and internal cells. Diffusion still handles the final short-distance movement, while bulk transport carries materials over long distances.

Four shared features

A gas-exchange surface is a biological interface where oxygen, carbon dioxide or both diffuse between an organism and its environment. It might be a gill, an alveolus or a leaf mesophyll surface, but the same design principles keep showing up.

Gas-exchange surfaces are:

• permeable, so oxygen and carbon dioxide can pass through instead of being stopped by waterproof or thick barriers;
• thin, keeping the diffusion distance short, often only one cell layer or a few cell layers;
• moist, allowing gases to dissolve before they diffuse through cells or across membranes;
• large in surface area, so many molecules can diffuse at the same time.

Moisture shows a neat biological trade-off. A wet surface helps gases dissolve and diffuse, but on land it can also lead to water loss. Much of plant leaf structure, and the surfactant-lined surface of alveoli, makes more sense once you remember that gas exchange surfaces can’t simply be allowed to dry out.

Why gradients must be maintained

A concentration gradient is a difference in the concentration of a substance between two regions. Diffusion reduces concentration gradients. Without some way to refresh the fluids on both sides of an exchange surface, gas exchange would slow down and eventually become ineffective.

In a mammalian alveolus, oxygen diffuses from alveolar air into blood because oxygen concentration is higher in the air than in the arriving blood. Carbon dioxide moves the opposite way because its concentration is higher in the blood than in the alveolar air. The lung is not “choosing” what to move; the gradients set the direction.

Blood flow and ventilation

Animals keep gradients steep by pairing exchange surfaces with transport and ventilation. Dense capillary networks bring blood very close to the exchange surface. As blood keeps flowing, oxygen-rich blood is carried away and replaced by blood with lower oxygen and higher carbon dioxide, because body cells have been respiring.

Ventilation is the bulk movement of air or water over a gas-exchange surface. In lungs, ventilation replaces alveolar air with fresh air, so oxygen stays relatively high and carbon dioxide stays relatively low. In gills, ventilation moves water over the gill surface. In many fish, water flows one way across the gills while blood flows in the opposite direction, which helps oxygen continue to diffuse from water to blood along the exchange surface.

For animals, the full picture is short-distance diffusion across the surface, continuous blood flow beside it, and ventilation on the environmental side. Leave out any one of these, and the gradient becomes harder to maintain.

The alveolar lung

A mammalian lung is an internal respiratory organ where air moves through branching tubes to microscopic alveoli, and gases diffuse between the air and the blood. Air travels from the trachea into the bronchi, then through repeated branches of bronchioles, before reaching the alveolar ducts and alveoli.

An alveolus is a small air sac in the lung, with a thin wall and surrounding capillaries, that provides a surface for gas exchange. Don’t imagine the lung as two hollow bags. Think instead of a branching tree ending in millions of tiny air spaces, each one lying close to blood.

Adaptations for rapid exchange

Mammalian alveolar lungs have a very high total surface area because they contain huge numbers of alveoli. Bronchioles form a branched network, letting air reach the exchange surfaces efficiently. Extensive capillary beds cover the alveoli, keeping blood close to the air and helping to preserve concentration gradients.

The diffusion path is extremely short: both the alveolar epithelium and the capillary endothelium are thin. Moist walls allow gases to dissolve before they diffuse. Elastic fibres help the lung recoil during breathing, while collagen fibres provide support.

Surfactant is a surface-active mixture secreted by alveolar cells that reduces surface tension in the fluid lining the alveoli. Tiny wet sacs can collapse when the water film pulls their sides together. Surfactant helps keep alveoli open, so the high surface area is not lost during exhalation.

Pressure changes move air

Inspiration is the phase of ventilation when air enters the lungs because thoracic volume increases and pressure inside the lungs falls below atmospheric pressure. Expiration is the phase of ventilation when air leaves the lungs because thoracic volume decreases and pressure inside the lungs rises above atmospheric pressure.

Air moves from higher pressure to lower pressure. The lungs don’t suck air in like a pump; muscles change the volume of the thorax, then pressure changes do the rest.

Muscles and ribs during inspiration

The diaphragm is a sheet of skeletal muscle below the lungs that changes thoracic volume when it contracts or relaxes. During inspiration, it contracts and flattens, which increases the volume of the thorax. The external intercostal muscles contract and pull the ribs upwards and outwards. The abdominal muscles relax, so the abdominal wall can move as the diaphragm descends. Thoracic volume increases, pressure decreases, and air flows in.

Intercostal muscles are skeletal muscles between the ribs that move the ribcage during ventilation. The external intercostals are the main muscles used in normal inspiration.

Muscles and ribs during expiration

During quiet expiration, the diaphragm relaxes and becomes more domed. The external intercostal muscles relax, and elastic recoil of lung tissue helps reduce thoracic volume. Pressure rises, so air flows out.

During forced expiration, the internal intercostal muscles contract, pulling the ribs downwards and inwards. At the same time, abdominal muscles contract and push abdominal organs upward against the diaphragm. That is why forced breathing after exercise feels muscular in the abdomen, not just in the chest.

The volumes you must be able to determine

Tidal volume is the volume of air inhaled or exhaled in one normal resting breath. Vital capacity is the maximum volume of air that can be exhaled after a maximum inhalation, or inhaled after a maximum exhalation. Inspiratory reserve volume is the extra volume of air that can be inhaled after a normal inhalation. Expiratory reserve volume is the extra volume of air that can be exhaled after a normal exhalation.

Treat these as measurements, not just words to learn. In a practical, you can estimate tidal volume from normal breaths. Vital capacity comes from breathing in as far as possible and then breathing out as far as possible, or doing it the other way round. The inspiratory and expiratory reserves are the extra volumes above and below the normal tidal breath.

Measuring lung volumes safely

A simple water-displacement apparatus can measure one exhaled volume. The person breathes out through a tube into an inverted graduated vessel, and the water displaced shows the volume of air collected. Don't use this setup for repeated rebreathing, because carbon dioxide would build up.

A spirometer is an instrument that measures ventilation by recording the volume or flow of air moved into and out of the lungs. Some spirometers produce a trace, which can be used to read or calculate tidal volume, vital capacity, inspiratory reserve volume and expiratory reserve volume. With a closed spirometer used for repeated breaths, carbon dioxide must be absorbed so the subject does not rebreathe increasing concentrations of it.

Leaves as exchange organs

A leaf is adapted for gas exchange because photosynthesis needs carbon dioxide and releases oxygen. Gases have to diffuse in and out, but the leaf also has to limit water loss. That trade-off between exchange and conservation is the main story here.

The waxy cuticle is a waterproof layer secreted by epidermal cells that reduces evaporation from the leaf surface. The epidermis is an outer protective tissue layer of the leaf that covers internal photosynthetic tissues. Together, the cuticle and epidermis reduce uncontrolled water loss. They don’t work well as gas-exchange surfaces, though, because the cuticle has low permeability.

Stomata, guard cells and internal air spaces

A stoma is a pore in the leaf epidermis that allows gases to diffuse between internal air spaces and the outside air. Guard cells are paired epidermal cells that change shape to alter the aperture of a stoma. When stomata open, carbon dioxide enters and oxygen leaves; when they close, water loss falls, but carbon dioxide uptake is restricted too.

The spongy mesophyll is a photosynthetic leaf tissue with loosely arranged cells and large air spaces that increase the internal surface area for gas exchange. Carbon dioxide diffuses through stomata into the air spaces, dissolves in moisture on mesophyll cell walls, then diffuses into cells containing chloroplasts. Oxygen produced by photosynthesis diffuses the other way.

Veins

Veins are vascular bundles in leaves that contain xylem and phloem for transport. Xylem supplies water to replace water lost by evaporation and to support photosynthesis; phloem carries sugars away. So leaf gas exchange is linked to plant transport, just as lung gas exchange is linked to blood flow in mammals.

What a plan diagram shows

A plan diagram is a low-power biological drawing showing where tissues are and how they sit relative to one another, without drawing individual cells. In a transverse section of a dicotyledonous leaf, it should show tissue regions, clean boundaries and labels. Save detailed cell contents for a different type of drawing.

A typical dicotyledonous leaf transverse section has the upper epidermis at the top, often with a cuticle; palisade mesophyll underneath; spongy mesophyll with air spaces below that; vascular tissue in the veins; and the lower epidermis at the bottom, often with stomata. In the vein, xylem is usually positioned toward the upper side and phloem toward the lower side.

Use single, continuous lines when drawing, and keep the proportions faithful to the specimen. Label tissues, not individual cells, unless the question specifically asks for cells. Tissue distribution links to function: palisade mesophyll receives strong light, spongy mesophyll supports gas diffusion, epidermis protects, and veins connect the leaf to the rest of the plant.

Why gas exchange causes water loss

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. Leaf gas exchange makes this hard to avoid: mesophyll cell walls have to stay moist so carbon dioxide and oxygen can dissolve and diffuse.

Water evaporates from the moist walls of spongy mesophyll cells into the air spaces inside the leaf. When the water vapour concentration is higher inside the leaf than in the outside air, water vapour diffuses out through the stomata. Open stomata help the plant take in carbon dioxide, but they also let more water escape. That’s the plant version of a biological compromise.

Factors affecting transpiration rate

Temperature usually increases transpiration. At higher temperatures, water molecules have more kinetic energy, so evaporation from cell walls rises; warm air can also hold more water vapour before it becomes saturated.

Humidity has the opposite effect. High relative humidity makes the water vapour gradient between the leaf air spaces and the outside air smaller, so water vapour diffuses out more slowly. If the outside air is saturated, net water loss by transpiration can approach zero.

Other factors matter too, because they change evaporation or diffusion. More air movement carries humid air away from the leaf and usually increases transpiration. Higher light intensity tends to open stomata for photosynthesis, which increases transpiration. When stomata close, water loss falls, especially at night or during water stress, but carbon dioxide entry is restricted and photosynthesis may be limited.

Measuring stomatal density

Stomatal density is the number of stomata per unit area of leaf surface. You can measure it using a microscope view of a peeled epidermis, or from a cast of the leaf surface. To make a leaf cast, clear nail varnish is usually brushed onto a smooth leaf surface, left to dry, peeled away, and mounted on a slide.

Use

In school microscopy, results are often given in $\text{mm}^{-2}$, but the calculation works the same way as long as the area unit is consistent.

The practical method is simple. Focus the epidermal peel or cast under high power, count the stomata in the field of view, move to a fresh area, then repeat the count. If you know the diameter of the field of view, you can use its area in the calculation. Micrographs can be counted in exactly the same way, provided the area represented by the image is known.

Reliability and biological variation

Repeating measurements improves reliability. Leaves are biological material, not manufactured tiles: stomata are not spread perfectly evenly, and the upper and lower epidermis may differ. Several counts reduce the effect of an unusual field of view and allow a mean to be calculated.

Replicate trials should use several fields of view, ideally from several leaves and plants. Variation between repeats gives useful information; it isn’t just nuisance noise. Low variation supports confidence in the mean, while high variation suggests the sample may be patchy or the method needs tightening.

Haemoglobin and cooperative binding

Haemoglobin is a globular transport protein found in red blood cells. It binds oxygen reversibly using haem groups. Each haemoglobin molecule has four subunits, and each subunit contains a haem group, a prosthetic group with iron that can bind one oxygen molecule. So, one haemoglobin molecule can carry up to four oxygen molecules.

Cooperative binding is a binding pattern where the attachment of one ligand changes the protein, making later ligand binding easier or harder. In haemoglobin, the first oxygen molecule that binds to a haem group alters the molecule’s shape, so the remaining haem groups bind oxygen more readily. Once oxygen begins to dissociate, the reverse occurs, making further unloading easier.

This adaptation lets haemoglobin load oxygen effectively in the lungs, where oxygen concentration is high, and unload it readily in respiring tissues, where oxygen concentration is lower. As a result, blood can transport far more oxygen than plasma alone could dissolve.

Foetal and adult haemoglobin

Adult haemoglobin is the main haemoglobin type after infancy, adapted to load oxygen in the lungs and unload it in body tissues. Foetal haemoglobin is produced before birth and has a higher affinity for oxygen than adult haemoglobin.

In the placenta, maternal and foetal blood do not need to mix for oxygen transfer to happen. Oxygen can dissociate from adult haemoglobin in maternal blood and bind to foetal haemoglobin, because foetal haemoglobin has the higher oxygen affinity. This keeps diffusion and loading favourable at the oxygen concentrations found in placental tissue.

Carbon dioxide interaction

Allosteric binding is binding at one site on a protein that changes the shape and activity of another site on the same protein. Carbon dioxide can bind allosterically to haemoglobin, changing haemoglobin’s affinity for oxygen. That links oxygen transport to the carbon dioxide produced by respiring tissues, setting up the Bohr shift described next.

Carbon dioxide promotes oxygen unloading

The Bohr shift is the decrease in haemoglobin’s affinity for oxygen caused by increased carbon dioxide concentration. Active tissues produce more carbon dioxide during aerobic respiration, so haemoglobin releases more oxygen in the places where demand is highest.

Carbon dioxide changes haemoglobin’s behaviour in two connected ways. In red blood cells, some carbon dioxide is converted to hydrogen ions and hydrogencarbonate ions, which lowers pH. A lower pH reduces haemoglobin’s affinity for oxygen. Carbon dioxide can also bind directly and reversibly to haemoglobin, forming carbaminohaemoglobin; this allosteric binding reduces oxygen affinity further.

The effect is clear. In a resting tissue, haemoglobin may hold on to more oxygen. In an actively respiring muscle, the higher carbon dioxide concentration shifts haemoglobin toward oxygen unloading, increasing the diffusion of oxygen into the tissue. In the lungs, carbon dioxide concentration is lower, so haemoglobin’s affinity rises again and oxygen loading is favoured.

Reading the curve

An oxygen dissociation curve shows the percentage saturation of haemoglobin with oxygen at different partial pressures of oxygen. The x-axis is usually partial pressure of oxygen in kPa; the y-axis is percentage saturation of haemoglobin with oxygen.

Partial pressure is the pressure one gas contributes in a mixture of gases. In oxygen dissociation curves, it works as a measure of oxygen concentration. Low partial pressure means little oxygen is available. High partial pressure means oxygen is more available for binding.

Why the curve is S-shaped

The adult haemoglobin curve is S-shaped, or sigmoid, because oxygen binding is cooperative. At very low oxygen partial pressure, haemoglobin has relatively low affinity, so saturation rises slowly. After one oxygen molecule binds, affinity increases and saturation climbs steeply over a narrow range. At high oxygen partial pressure, most haem groups are already occupied, so the curve flattens near full saturation.

Curve position shows affinity. A curve further to the left means higher affinity: haemoglobin becomes highly saturated at a lower oxygen partial pressure. Foetal haemoglobin has a left-shifted curve compared with adult haemoglobin, which explains oxygen transfer in the placenta. Shift the curve to the right and affinity is lower: at the same oxygen partial pressure, haemoglobin is less saturated and has released more oxygen. High carbon dioxide causes this right shift in the Bohr effect.

Think of these curves as functional maps. In alveolar capillaries, oxygen partial pressure is high enough for haemoglobin to load oxygen. In respiring tissues, oxygen partial pressure is lower, and carbon dioxide may be higher, so haemoglobin unloads oxygen. It’s gas exchange and cell metabolism shown on a graph.