BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. ENERGY UTILIZATION

9.4. Gas Exchange

Gas exchange refers to the exchange of respiratory gases between an Organism and its environment. Aerobes require oxygen from the external environment for Respiration, while both aerobes and most anaerobes release carbon dioxide (CO₂) into the environment as the end product ("waste") of respiration. The actual surface across which this exchange takes place is called the respiratory surface. In all organisms, gas exchange is accomplished through a physical process known as diffusion. For diffusion to be efficient, the respiratory surface must meet several requirements:

1) it must be permeable to allow gases to pass through;

2) the layer forming it must be thin, because diffusion is only effective over distances of no more than 1 cm;

3) the area of the respiratory surface must be large enough to allow sufficient quantities of gases to be exchanged in accordance with the organism's metabolic demands;

4) it must be richly supplied with a Blood supply (and sometimes require a ventilation mechanism as well) in organisms (specifically large animals) where blood serves as the transport medium for gases. This helps maintain a steep diffusion gradient—that is, a large concentration difference—across the Two Sides of the surface.

To understand how the maximum rate of diffusion across the respiratory surface can be achieved, we must turn to Fick's law. According to this law, The rate of gas diffusion across a respiratory surface is proportional to the following expression (see section 5.9.8, which discusses diffusion across membranes):

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Organisms obtain the oxygen they need either directly from the atmosphere or from dissolved oxygen in Water. The oxygen content of water and air differs significantly. A given unit volume of air contains vastly more oxygen (21%) than the same volume of water (0.8%). Consequently, the volume of water that aquatic organisms, such as fish, must pass over their respiratory surfaces to meet their metabolic needs is much greater than the volume of air required by terrestrial vertebrates. This also necessitates a different ventilation mechanism in aquatic animals. Furthermore, water is about 700 times denser and 100 times more viscous than air. This means that moving water across the respiratory surface requires more energy. Finally, oxygen diffuses through water 1000 times slower than through air, making it much more difficult to maintain a steep concentration gradient across the respiratory surface in an aqueous environment. It is hardly surprising, therefore, that fish have a much lower metabolic rate than lung-breathing animals.

9.4.1. Single-celled organisms, such as the amoeba

The amoeba is a single-celled organism belonging to the subkingdom Protozoa. Its body shape is irregular and changes as it moves. Its diameter is typically less than 1 mm, resulting in a high surface area-to-volume ratio, which is generally characteristic of unicellular organisms. The amoeba's respiratory surface is its outer Cell membrane. Gas diffusion occurs across the entire body surface at a rate high enough to meet all the animal's metabolic demands. Oxygen enters The Cell and CO₂ leaves the cell down their respective diffusion gradients (see section 5.9.8, which describes diffusion across membranes).