Why gases must be exchanged

Gas exchange is a biological process in which 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, carbon dioxide is produced during aerobic cell respiration, especially in 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. Every cell is 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, volume increases faster than surface area, so there is less exchange surface available per unit volume. Cube models showing that surface area-to-volume ratio falls as size increases.

Large multicellular organisms deal with 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 move gases between the exchange surface and internal cells. Diffusion still carries out the final short-distance movement, but bulk transport moves materials over the long distances.

Four shared features

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

Gas-exchange surfaces are:

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

Moisture shows one of biology’s neat trade-offs. A wet surface helps gases dissolve and diffuse, but on land it also brings the risk of water loss. Plant leaf structure, and the surfactant-lined surface of alveoli, make much 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 these gradients. If the fluids on both sides of an exchange surface are not refreshed, gas exchange slows down and can eventually become ineffective.

In a mammalian alveolus, oxygen diffuses from alveolar air into the blood because oxygen concentration is higher in the air than in the arriving blood. Carbon dioxide diffuses the other way, since its concentration is higher in the blood than in the alveolar air. The lung is not “choosing” which gas to move; the gradients determine 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, it carries oxygen-rich blood away and brings in 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 one out, and the gradient is harder to maintain.

The alveolar lung

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

An alveolus is a small air sac in the lung with a thin wall and surrounding capillaries, providing a surface for gas exchange. Don’t think of the lung as two empty bags. A better picture is a branching tree that ends in millions of tiny air spaces, each sitting 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 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 in which air enters the lungs because thoracic volume increases and pressure inside the lungs falls below atmospheric pressure. Expiration is the phase of ventilation in which 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. Instead, muscles change the volume of the thorax, then the pressure difference moves the air.

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, so the volume of the thorax increases. The external intercostal muscles contract too, pulling the ribs upwards and outwards. The abdominal muscles relax, which lets the abdominal wall 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 matter most in normal inspiration.

Muscles and ribs during expiration

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

During forced expiration, the internal intercostal muscles contract and pull 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 breathed in or out during one normal resting breath. Vital capacity is the greatest 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.

These volumes are practical measurements, not just terms to memorise. In an investigation, tidal volume can be estimated using normal breaths. Vital capacity is measured with a maximal breath in followed by a maximal breath out, or the reverse. 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. This setup should not be used for repeated rebreathing, as carbon dioxide would build up.

A spirometer is an instrument that measures ventilation by recording the volume or flow of air moving 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

Leaves are adapted for gas exchange because photosynthesis needs carbon dioxide and releases oxygen. At the same time, a leaf has to let gases diffuse without losing too much water. 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 limit uncontrolled water loss, but they make poor gas-exchange surfaces 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, and then moves into cells containing chloroplasts. Oxygen produced by photosynthesis diffuses the other way.

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

What a plan diagram shows

A plan diagram is a low-power biological drawing showing the distribution and relative positions of tissues, without drawing individual cells. In a transverse section of a dicotyledonous leaf, it should show tissue regions, clear boundaries, and labels. Don’t include detailed cell contents here.

A typical dicotyledonous leaf transverse section has the upper epidermis at the top, often with a cuticle. Below that sits the palisade mesophyll, then the spongy mesophyll with air spaces, 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 rather than individual cells, unless the question specifically asks for cells. Tissue distribution links directly 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 it unavoidable: the mesophyll cell walls have to stay moist so carbon dioxide and oxygen can dissolve and diffuse.

Water evaporates from the moist cell walls of the spongy mesophyll into the air spaces inside the leaf. When the water vapour concentration is higher inside the leaf than outside, 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 increases; warm air can also hold more water vapour before it becomes saturated.

Humidity has the opposite effect. High relative humidity reduces the water vapour gradient between the leaf air spaces and the outside air, so diffusion of water vapour out of the leaf slows down. If the air outside 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 also 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 from a microscope view of peeled epidermis, or from a cast of the leaf surface. To make a leaf cast, clear nail varnish is usually spread over a smooth leaf surface, left to dry, then peeled off and mounted on a slide.

Use Dₛ = n/A, where Dₛ is stomatal density (m⁻²), n is the mean number of stomata counted in the field of view (dimensionless) and A is the area of the field of view (m²). In school microscopy, results are often given in mm⁻². The calculation stays the same as long as the area unit is consistent.

A simple practical method works like this: focus the epidermal peel or cast under high power, count the stomata in the field of view, move to another area, then repeat. 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

Repeated measurements make the result more reliable. Leaves are biological material, not manufactured tiles: stomata are not spread perfectly evenly, and the upper and lower epidermis may differ. By repeating counts, you reduce the effect of an unusual field of view and can calculate a mean.

Replicate trials should include several fields of view, ideally from several leaves and plants. Variation between repeats is useful information, not just nuisance noise. Low variation supports confidence in the mean; high variation shows 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 one ligand attaching to a protein changes the protein’s shape, making later ligand binding easier or harder. In haemoglobin, the binding of one oxygen molecule to a haem group changes the shape of the whole molecule, so the remaining haem groups bind oxygen more readily. When oxygen begins to dissociate, the reverse occurs, and further unloading becomes 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. It is 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 have 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, altering haemoglobin’s affinity for oxygen. This connects oxygen transport with 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 where the demand for oxygen is high.

Carbon dioxide affects haemoglobin 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 benefit is easy to see. In resting tissue, haemoglobin may hold on to more oxygen. In an actively respiring muscle, the higher carbon dioxide concentration shifts haemoglobin toward oxygen unloading, so more oxygen diffuses into the tissue. In the lungs, carbon dioxide concentration is lower, so haemoglobin’s affinity rises again and oxygen loading is favoured.