Biology · B Form and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
All organisms must take in one gas and get rid of another. The gases move by diffusion, so an exchange surface must be thin, moist, permeable and large in area. Large organisms have a small surface area-to-volume ratio, so they need special organs: lungs, gills or leaves.
Diffusion continues only while there is a concentration gradient. Animals keep it steep by ventilation, which brings fresh air or water, and by a continuous flow of blood through dense capillary networks.
In a leaf, gases pass through stomata into the air spaces of the spongy mesophyll. Water vapour escapes by the same route. This loss is transpiration, and it cannot be avoided while stomata are open.
Students write that air entering makes the lungs expand. The muscles enlarge the thorax first; the pressure falls, and that is what draws the air in.
Answer: 4.5 dm3
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