AS Biology Coursework
AS Biology Coursework
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AS Biology Coursework Excerpt
In biology the word contraction is used in a special sense. When we speak of the contraction of a bar of iron we mean a diminution of volume but when a muscle contracts there is no alteration of volume, but only a change of shape, whereby the muscle grows shorter and thicker. This is also true of the contraction of simple masses of protoplasm, as in the one-celled organisms. In these forms contraction is a much simpler phenomenon than in muscle and we therefore have a better prospect of analyzing it and discovering the underlying mechanism.
Motion in these cases and in plant cells may show great diversity. In plants protoplasmic streaming may range from the most irregular sort of motion to a regular flow which passes up one side of the cell and down the other in a continual cycle. In animal cells the type of irregular motion most studied is that found in the ameba, and hence called ameboid. An ameba consists of a single cell which at first sight looks like a structureless mass of protoplasm but is in reality not so simple as it appears. It creeps about by pushing out irregular projections into which part of its substance flows, while other portions of its body are simultaneously retracted. Currents of protoplasm (and corresponding countercurrents) are seen throughout the cell. The interior of the mass is liquid while the exterior is firmer, resembling a jelly. The change from liquid to jelly or the reverse apparently takes place with great ease and is a characteristic feature of the creeping movements.
An eminent biologist of the last generation said that it would be futile to seek for any simple explanation of the movements of the ameba. You might as well try, he said, to explain that the movements of a gunny sack with a man inside are due to a very simple mechanism. How far he was wrong may be judged from the fact that it is now possible to imitate the movements of an ameba to a remarkable extent by means of simple mixtures of oil and other substances. Not only does a drop of this kind, which we may call for convenience an artificial ameba, creep about, but its interior, under the proper conditions, shows currents and countercurrents. We cannot say that we have, as yet, a complete explanation of ameboid motion but we have gone far enough to make it evident that mechanical factors are ade quate to account for the observed phenomena.
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