Gas exchange in animals and plantsEdexcel A-Level Biology B: Revision notes

Gases cross exchange surfaces by diffusion, so an efficient surface has:

• a large surface area (folding or many small structures)
• a short diffusion distance (thin walls, often one cell thick)
• a steep concentration gradient, kept steep by ventilation and by blood flow or by the cells using the gas
• a moist surface so that gases can dissolve

Large organisms have a small surface area to volume ratio, so they cannot rely on their body surface alone and have specialised exchange organs. Every system below is a different solution to the same four requirements.

Insects have no oxygen-carrying pigment in their body fluid. Air enters through spiracles, which are openings along the thorax and abdomen that can be opened and closed to control water loss. Spiracles lead to tracheae, air-filled tubes strengthened with rings of chitin so that they do not collapse. These branch into narrower tracheoles.

Tracheoles are thin-walled and their fluid-filled tips lie against, or even inside, the respiring cells, especially flight muscle. Oxygen therefore diffuses a very short distance, through air for most of the path, which is much faster than diffusion through liquid. Carbon dioxide diffuses out in the reverse direction. Large or active insects also use body movements, such as pumping the abdomen, to ventilate the tracheae.

Water contains much less oxygen than air, so a bony fish needs a very efficient system. Each gill is made of many filaments, and each filament carries many lamellae: thin plates with a rich capillary supply. This gives a large surface area and a short diffusion distance.

The mouth floor and the operculum (the bony flap over the gills) move to pump a continuous, one-way flow of water over the lamellae. In each lamella blood flows in the opposite direction to the water, which is counter-current flow. The blood always meets water with a higher oxygen concentration, so a gradient is kept along the whole length of the lamella and up to about 80% saturation can be reached. In parallel flow the concentrations would equalise part-way along, limiting saturation to about 50%.

Air passes through the trachea, bronchi and bronchioles to millions of alveoli. The alveolar wall is a single layer of flattened epithelial cells and is surrounded by a dense network of capillaries whose walls are also one cell thick, so the diffusion distance is tiny. The total surface area is very large and the lining is moist, so oxygen dissolves before it diffuses.

Ventilation (the diaphragm and intercostal muscles changing the volume of the thorax) renews the air in the alveoli, and blood flow removes oxygen and brings carbon dioxide, so the concentration gradients are maintained. Unlike insects, mammals need blood to transport gases because the lungs are far from most cells.

To show the tracheal system, use a dead locust (or similar insect) from a reputable supplier.

1. Remove the wings and pin the insect dorsal side up on a dissecting board.
2. With fine scissors make a shallow cut along the midline of the abdomen, from rear to front, to avoid damaging the tubes.
3. Pin back the flaps and add saline to keep the tissue moist.
4. Gently remove the gut and fat. The tracheae appear as silvery, ringed tubes.
5. Use a hand lens or microscope to follow them to the spiracles and to see branching.

Safety and ethics: use an already dead or humanely killed specimen, minimise any suffering, handle sharp instruments carefully, wear gloves if required, wash hands afterwards and dispose of waste correctly.

A leaf is thin and flat, giving a large surface area and short diffusion distance. Carbon dioxide enters and oxygen leaves through stomata, pores usually most numerous on the lower epidermis. Behind each stoma are the air spaces of the spongy mesophyll, which let gases diffuse to the moist surface of the photosynthesising cells. Carbon dioxide is used up in photosynthesis, which keeps a gradient from outside to inside.

Stomata cause water loss by diffusion of water vapour, so plants face a trade-off. Xerophytes such as marram grass reduce loss with a thick waxy cuticle, rolled leaves, sunken stomata in pits and hairs, which trap humid air and reduce the water potential gradient.

Woody stems and roots cannot use stomata. Lenticels are small raised pores in the bark through which oxygen and carbon dioxide diffuse between the air and the living tissues beneath.