A living cell can respond to stimuli. A stimulus is a change in cell's environment that can make it change its activity. The cell's change in activity is known ad its response to the stimulus. An external stimulus is a change in the cell's surrounding, while an internal stimulus is a change within the cell itself.
Irritability or sensitivity is the ability of a cell to respond to stimuli. This ability is a basic property of the protoplasm, and hence , is a characteristic of all living organisms. To survive, a living organism must respond appropriately to stimuli. For example, the amoeba that moves away from food would die of starvation eventually, and a plant whose roots grows away from water would wilt and die.
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