Gas exchange in insects, fish and plants

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 in most insects the blood plays little or no part in carrying oxygen. Some aquatic larvae are the exception: the midge larvae sold as 'bloodworms' carry haemoglobin in their body fluid for life in low-oxygen mud.

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.

Spiracle

A pore in the body wall of an insect, guarded by valves, through which air enters and leaves the tracheal system.

Tracheole

One of the finest branches of an insect's tracheal system, under a micrometre across, running between and into the cells and delivering oxygen to them directly.

Describe the drawing

Two rows showing the same tracheal system. In each, a pore in the body wall leads into a wide ringed tube that branches into three fine tracheoles ending inside a block of muscle. In the upper row a thick coloured length of fluid fills most of each tracheole tip; in the lower row that fluid has shrunk to a short stub near the very end.

At rest the ends of the tracheoles are filled with tracheal fluid. The movement of that fluid 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 response follows directly from the change in water potential, without a separate control mechanism.

Diffusion is the main mechanism, but larger and more active insects supplement it. Rhythmic movements of the abdomen and thorax change the volume of the body, squeezing the tracheae and moving 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. Trapping humid air in this way is the same mechanism used by the xerophyte adaptations described below.

The design has a limit. 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, though the oxygen is one plausible contributor to their size rather than a settled single cause.

Fish gills and counter-current flow

Water is a difficult medium from which to extract oxygen. 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 ventilating a gill has a substantial energy cost. The gill must therefore extract a high proportion of the oxygen available.

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. While the buccal cavity is filling, the operculum is shut and the opercular cavity behind the gills expands, lowering its pressure and drawing water across the gills, so water flows over them almost continuously and in one direction, rather than moving back and forth over water already depleted of oxygen.

Describe the drawing

Two plots of oxygen saturation against distance along a gill lamella. In the upper plot water falls from one hundred per cent to twenty while blood rises from zero to eighty in the opposite direction, the two lines staying a constant distance apart. In the lower plot both flow the same way, the water falling and the blood rising until they meet near fifty per cent part-way along and run flat together after that.

State the comparison precisely. Counter-current flow 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. 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 ceiling of about 50% for parallel flow. The 50% assumes matched flow rates and carrying capacities on the two sides, which is how the comparison is always drawn.

Counter-current flow also allows blood to leave the lamella with a higher oxygen content than the water leaving the lamella, because at its exit end that blood was in contact with fully oxygenated water. In parallel flow the two fluids converge on the same value, so this cannot occur.

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, covered by an epithelium one cell thick with blood flowing just beneath it; 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.

Gas exchange in a leaf

A plant needs carbon dioxide in during the day, and oxygen in at night once photosynthesis has stopped; in daylight the respiring cells use oxygen the chloroplasts are making, and the net movement of oxygen is outward. 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. Woody stems and roots are covered in bark, which is almost impermeable to gases, so they exchange gases through lenticels, patches of loosely packed cells in the bark whose air spaces let oxygen diffuse to the living cells beneath and carbon dioxide diffuse out.

Each stoma is bounded by two guard cells, and, like an insect's spiracle, it can be closed. When water is available, a proton pump uses ATP to move hydrogen ions out of the guard cell, and the resulting electrochemical gradient allows potassium ions to enter through channel proteins. 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.

Describe the drawing

Two pairs of guard cells side by side. On the left the pair meets along its whole length with no gap between them. On the right the same pair has bowed apart into a lens-shaped pore, with a thicker line drawn along the wall facing the pore in both, and arrows showing carbon dioxide entering and water vapour leaving.

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 triggers closure. The hormone comes from roots in dry soil and also from the leaf's own tissues as they lose water, so the signal has a local arm as well as a long-distance one.

Water loss is a consequence of opening stomata for gas exchange. The leaf interior is saturated with water vapour while the air outside rarely is, so a pore open enough to admit carbon dioxide also allows water vapour to leave. That loss is transpiration.

Guard cell

One of the pair of cells around a stoma. The pair bow apart and open the pore when they become turgid, and close it when they become flaccid.

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.

These adaptations have a cost. Anything 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, pay in a different way: the storage step costs energy and the daily supply is capped by how much acid the plant can hold.

Xerophyte

A plant whose leaves and stems have features that reduce water loss by transpiration, found where water is scarce.

Exam-board note. 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 the current specification for the qualification being taken.