So far, we have touched on six of the traditional properties of living organisms: eating, breathing, excreting, moving, growing and reproducing. The only one that remains is "responding to outside influences". A dog salivating at the smell of food, a flower opening in sunlight and a worm crawling towards moisture are examples of organisms responding to stimuli from their surroundings. All organisms respond to their surroundings in order to improve their chances of survival and reproduction.
Single cells also react to their environments, detecting signals and responding appropriately. Thanks to many years of research in cell and molecular biology we now understand - in considerable detail in some cases -how they do this. The principles are quite simple, though (as ever in biology) the details are complicated. We shall concentrate on the principles.
If a single-celled organism moves from place to place by swimming, it will swim towards food. It will also avoid noxious stimuli; for instance, it might swim from more acidic to less acidic water. The direction in which the organism swims is determined by a two-stage process. First, the organism must locate the food or measure the acidity of the water; that is, it must detect the relevant stimulus. Second, it must then use its swimming apparatus to move in the right direction - respond to the stimulus. Of course, the response must be precisely related to the stimulus. There must be a "connecting mechanism" between stimulus and response. If this connection were not precise, either the response would be inappropriate or there would be no response at all.
In multicellular organisms, stimulus and response are directed towards the wellbeing of the whole organism rather than the individual cell. For example, a cell that secretes a hormone (a) "knows" how much of the hormone the rest of the body needs from moment to moment and (b) makes appropriate adjustments in the amount that it produces. In this case, the stimulus is a measure of the body's need for the hormone, and the response is hormone production. The cell must ensure that the stimulus evokes the response only when required.
The cell that secretes a hormone helps to keep the whole body alive. It benefits from its altruism because it is part of the body; it would not survive if the body died. This might seem different the single-celled organism, where the stimulus (food or acidic surroundings) evokes a response (movement) that is directly relevant to the individual cell's survival. Nevertheless the underlying pattern is the same: specific stimulus, appropriate response, and a precise mechanism linking the two.
Almost all living cells respond to a wide range of stimuli. It makes no difference whether the cell is autonomous and free-living, or part of a large multicellular organism. Eukaryotic or prokaryotic, cells respond in precisely-engineered ways to stimuli that are pertinent to their needs. The stimuli might be physical (light, temperature, mechanical contact, etc.) or chemical (nutrient, toxin, specific signalling molecule from another cell, etc.). Broadly speaking, cellular responses to all stimuli, physical or chemical, follow the same basic principles; we shall focus mainly on chemical signals in this chapter.
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