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Three other answers: tracheae, gills and stomata

An insect delivers air to its cells and never bothers loading it onto blood. A fish runs water past its blood in the opposite direction and strips out four fifths of the oxygen. A leaf leaves the door open and pays in water. Each is a different reading of the same problem.

Before this Surface area to volume ratio and Fick's law · Water potential and osmosis

What you should be able to do

• Trace the path of oxygen from a spiracle to an insect's muscle cell and say why its blood plays no part.
• Explain how tracheal fluid moves during activity and why that speeds up gas exchange.
• Describe the structure of a bony fish gill from arch to lamella, and the buccal pump that drives water over it.
• Explain why counter-current flow extracts far more oxygen than parallel flow would, using the gradient along the lamella.
• Describe how a leaf exchanges gases and explain the trade-off that forces xerophyte adaptations.

Insects: air to the cell, not to the blood

An insect has a waterproof exoskeleton, which solves the problem of drying out and creates the problem of getting oxygen through it. The solution is a system of tubes that pipes air directly to the tissues, so that the blood never has to carry oxygen at all.

Air enters through spiracles, pores along the thorax and abdomen, each guarded by valves and often fringed with hairs. From each spiracle a trachea runs inwards, held open by rings of chitin in its wall in the same way the trachea of a mammal is held open by cartilage. The tracheae branch repeatedly into tracheoles, which are under a micrometre across, have no chitin lining, and run between and even into the cells. Oxygen diffuses out through the tracheole wall straight into the cytoplasm, and carbon dioxide diffuses back the same way.

The fluid movement is a favourite exam question and it depends on water potential. During sustained activity a muscle cell respires anaerobically in part, and lactate accumulates in the cytoplasm. That lowers the water potential of the cell, so water moves out of the tracheole into the cell by osmosis. Air is drawn further along the tracheole to fill the space, the final stretch of the diffusion pathway is now gas rather than liquid, and oxygen reaches the cell far faster. The system tunes itself to demand without any control mechanism at all.

Diffusion is the main mechanism, but larger and more active insects supplement it. Rhythmic contraction of the abdominal muscles squeezes the tracheae and moves air along them in bulk, which is mass transport rather than diffusion. Some insects also close alternate spiracles in a co-ordinated pattern so that air is driven through the system in one direction.

Every open spiracle is also a hole through which water vapour leaves, and an insect is small enough to have a high surface area to volume ratio, so the loss matters. The spiracles are therefore kept shut when the insect is inactive and opened only as demand requires, the cuticle is waterproofed with wax, and the hairs around each spiracle hold a layer of humid air that shallows the gradient for water loss. That last point is the same trick a xerophyte uses, and it appears again at the end of this lesson.

The limit of the whole design is worth stating, because questions ask for it. Diffusion down a tracheal tube is slow over long distances, so the tracheal system sets a ceiling on insect body size. Insects were much larger in the Carboniferous, when the atmosphere held a substantially higher proportion of oxygen than it does now.

Fish: the direction of flow is the adaptation

Water is a difficult medium to breathe. A cubic decimetre of air holds around 210 cm³ of oxygen; the same volume of fresh water at 15 °C holds about 7 cm³, roughly thirty times less. Water is also about eight hundred times denser than air and far more viscous, so moving it over a surface costs a fish real energy. A gill has to be very good indeed to make that arithmetic work.

A bony fish has four gill arches on each side of its head, covered by a bony flap called the operculum. Each arch carries two rows of gill filaments, and each filament carries stacks of thin plates, the lamellae, set at right angles to it. The lamellae are where exchange happens: each is covered by a single layer of cells, contains a capillary network, and the stacking gives an enormous area in a small space.

Water is kept moving in one direction by a two-stage pump. The fish lowers the floor of its buccal cavity with the mouth open, which increases the volume, drops the pressure and draws water in. It then closes its mouth and raises the floor, raising the pressure and forcing water over the gills and out past the operculum, which opens as a valve on the way out. Water therefore flows continuously and in one direction, and never sloshes back and forth over a surface it has already stripped.

This is the mark scheme's favourite comparison, and there is a wrong way to say the right thing. Writing that counter-current flow 'maintains a concentration gradient' earns one mark. Writing that it maintains a concentration gradient along the whole length of the lamella, so that oxygen diffuses from water to blood across the entire surface rather than only the first part of it, earns the rest. The number usually quoted is that a bony fish removes about 80% of the dissolved oxygen from the water passing over its gills, against a theoretical ceiling of 50% for parallel flow.

Notice too what counter-current makes possible that sounds impossible: blood leaves the lamella with a higher oxygen content than the water leaving beside it. There is no contradiction, because that blood met its water at the opposite end, where the water was fresh. Parallel flow can never do this, since the best it can achieve is for both to end at the same value.

Gill filament

One of the many thin projections from a gill arch, sometimes called a primary lamella.

Lamella

One of the thin plates set at right angles along a gill filament, one cell thick and richly supplied with blood; the actual exchange surface.

Counter-current flow

An arrangement in which two fluids move past each other in opposite directions, so a concentration gradient is maintained along the whole length of the exchange surface.

Operculum

The bony flap covering the gills of a bony fish, acting as a valve and part of the pump that maintains the flow of water.

A gill out of water fails for a structural reason, not a chemical one. Water supports the lamellae and holds them apart; in air they stick together and collapse into a wet mass, so the surface area falls catastrophically. A stranded fish suffocates in an atmosphere containing thirty times more oxygen than the water it came from.

Leaves: the same hole for two jobs

A plant needs carbon dioxide in during the day and oxygen in at all times, and it has no muscles, no blood and nothing that could be called ventilation. It manages with pure diffusion, which works because a leaf is thin and full of holes.

Gas enters through stomata, pores in the epidermis, mostly on the lower surface of a typical broad leaf. Inside, the spongy mesophyll is a loose network of cells with large air spaces between them, so gas diffuses freely through the leaf's interior and reaches the moist surface of every mesophyll cell. Those wet cell walls are the exchange surface, and they add up to an area several times the area of the leaf itself. No cell in a leaf is more than a fraction of a millimetre from an air space, so the diffusion distance is short and no transport system for gases is needed at all.

Each stoma is bounded by two guard cells, and unlike every other exchange surface in this unit, it can be shut. When the plant has water to spare, potassium ions are actively pumped into the guard cells. The water potential of the guard cells falls, water follows by osmosis, and the cells become turgid. Because the wall facing the pore is thicker than the outer wall, and the cellulose microfibrils are arranged in hoops around the cell, the cell lengthens by curving rather than by swelling evenly. The two cells bow away from each other and the pore opens.

Reverse the process and the stoma shuts. Potassium ions leave, water follows, the guard cells go flaccid, and the pore closes. Plants shut their stomata in darkness, when photosynthesis has stopped and carbon dioxide is not wanted, and in drought, when the hormone abscisic acid is released from roots in dry soil and triggers closure.

The trade-off is unavoidable, and it is the point of the whole section. The leaf interior is saturated with water vapour while the air outside rarely is, so a pore open enough to let carbon dioxide in is open enough to let water vapour out. That loss is transpiration, and it is the reason a plant spends water it cannot always replace in order to feed itself.

Xerophytes: making the gradient shallow

A plant living where water is scarce cannot stop exchanging gases, so it attacks the other terms of Fick's law instead. Every classic xerophyte adaptation is doing one of three things: cutting the area available for loss, lengthening the diffusion pathway, or flattening the gradient by trapping humid air next to the pore.

The mechanism behind the middle rows is one idea repeated. Water vapour leaving a stoma has to diffuse away from it, and if it cannot get away it accumulates and the air immediately outside the pore approaches saturation. The difference in water potential between the leaf interior and the air just outside then becomes small, and the rate of loss falls with it. Pits, hairs and rolled leaves are all ways of preventing the humid air near a stoma from being carried off by wind.

There is a cost, and good answers acknowledge it. Everything that slows water leaving also slows carbon dioxide entering, so a xerophyte's rate of photosynthesis is typically lower than that of a plant with unrestricted stomata. Succulents that open their stomata only at night, storing carbon dioxide as an organic acid until daylight, accept the same penalty in a different currency: the storage step costs energy and the daily supply is capped by how much acid the plant can hold.

Boards divide this content up differently, which matters when you revise. AQA 7402 examines insects, fish, leaves and xerophytes together in its gas exchange topic. OCR A H420 examines insects and fish under exchange surfaces but places stomata and xerophytes with transport in plants. Cambridge 9700 does not examine the insect tracheal system or the fish gill at all, and covers xerophyte adaptations within transport in plants. Check what your own board asks for before you spend an evening on gill arches.

In the exam

• For counter-current flow, say that the gradient is maintained along the whole length of the lamella. The bare phrase 'maintains a concentration gradient' is worth one mark out of three or four.
• In the insect fluid question, the sequence wanted is lactate builds up, water potential of the muscle cell falls, water leaves the tracheole by osmosis, air moves further in, diffusion pathway through liquid is shortened.
• Insect blood does not transport oxygen. Saying that it does is a common and expensive error.
• For guard cells, name the ion. Potassium ions are pumped in, the water potential falls, water enters by osmosis, the cells become turgid and bow apart because of the thickened inner wall.
• Xerophyte answers must say how, not what. 'Hairs reduce water loss' scores nothing; 'hairs trap humid air, so the water potential gradient between the leaf and the air outside is reduced' scores.
• Check your board. Cambridge 9700 does not examine insect or fish gas exchange, and OCR A puts stomata and xerophytes with transport in plants rather than with exchange surfaces.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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