We have seen that when nutrient compounds are broken down in the process of respiration, over 90 percent of the energy yield depends on the presence of oxygen, which makes possible the complete oxidation of the compounds to carbon dioxide and water. Thus a basic problem for the great majority of living organisms is the procurement of ox¬ygen and the elimination of carbon dioxide. Monocular compound microscopes are great tools in studying the mechanics of he processes involved in gas exchange in a living cell.
It is a common misconception that oxygen procurement is a prob¬lem faced only by animals, and that gas exchange in green plants consists exclusively of intake of carbon dioxide and release of oxygen. The latter is the exchange that takes place in association with photo¬synthesis, but the carbohydrate products of photosynthesis are of little value to the plant if they cannot be respired to provide usable meta¬bolic energy. Thus plants like animals; are constantly taking in oxy¬gen and releasing carbon dioxide as they carry out the process of cellular respiration. When a green plant is exposed to bright light, both photosynthetic and respiratory gas exchange are usually taking place; since the rate of photosynthesis then greatly exceeds the rate of respiration, the net effect is one of uptake of carbon dioxide and re¬lease of oxygen. The reverse is true, of course, when the green plant is in the dark or when it has no leaves in winter. Thus respiratory gas exchange is a requirement for plants as much as it is for animals.
The Problem
Gas exchange between a living cell and its environment always takes place by diffusion across a moist cell membrane as seen using monocular compound microscopes. The gases must be in solution if they are to move across the membrane. In unicellular organisms and many small multicellular ones, particularly those that are aquatic, this requirement poses no serious problem, because each cell is either in direct contact with the surrounding medium or only a few cells removed from that medium. Hence these organisms have usually not evolved special respiratory devices.
When large body size is predominantly in two dimensions, gas ex¬change can still take place chiefly by direct diffusion between the individual cells and the surrounding medium. Some brown algae, the kelps, may grow to a length of 60 to 70 m, but the blades of even the longest kelps remain very thin. This observation was made with the help of monocular compound microscopes. As a result, no cell is far from the surface, and the total gas-exchange area is fairly large in relation to the volume of the plant.
When increase in body size involves three dimensions, as it gener¬ally does, the maintenance of a respiratory surface of adequate dimensions relative to the volume becomes a problem, because area (a square function) increases much more slowly than volume (a cube function). The problem is most acute for the more active animals, whose rapid utilization of energy demands a large amount of oxygen per unit of body volume per unit time.
An additional complicating factor is that many organisms have evolved relatively impermeable outer body coverings as evident when these organisms were examined using monocular compound microscopes. Coverings such as the waxy epidermis of the leaves of terrestrial plants, or animal skin with its derivative scales, feathers, and hair, function as protec¬tive barriers between the fragile internal tissues and organs and the often hostile outer environment, but their presence, which demands a gas-exchange surface confined to a restricted region of the body, makes the problem of adequate exchange area even more critical.
Another complication brought on by large three-dimensional size in animals is that many cells are deep within the body of the organism, far from the gas-exchange surface. Diffusion alone is incapable of moving gases in adequate concentrations across the immense number of cells that may intervene between these more distant cells and the exchange surface. In general, simple diffusion suffices for movement of substances through aqueous media only when the distances are less than one millimeter. Some other mechanism for conveying gases to the individual cells of the organism therefore becomes essential.