A cell, a tissue, an organ, or the whole organism may be involved in the act of responding to a change in the environment. The sudden appearance of snake, while walking will result in many immediate responses in human beings viz., running or any other action to avoid being attacked, faster heartbeat, faster breathing, sweating to cool the exited body, etc. All these responses occur within seconds.
A complex network of commutation is achieved which involves several life processes like movement, locomotion, transport, respiration, etc. In regulating and co-ordinating different organs systems of the animal body, both chemical and nervous co-ordination are involved. Hormones (produced by ductless glands) and neurons (structural and functional units of the nervous system and sensory system) take part in such co-ordination in case of animals.
To illustrate the need for control and co-ordination we have until now taken most of the examples from animals. Animals have nervous system and endocrine glands for greater co-ordination and control. But plants also react to specific environmental conditions. Wilting of leaves and closing of stomata in hot and dry conditions can easily be seen.