The characteristics of life

Scientists have long debated what the criteria for life are, and there is still no definite answer. Take, for example, viruses – which to this day scientists have still not agreed on whether or not they are to be considered life forms. Some examples of characteristics of life include:

  • homeostasis
  • organisation
  • metabolism
  • growth
  • adaptation
  • response to stimuli
  • reproduction

The process known as homeostasis is vital to life because it is essential that the right internal conditions are maintained within an organism, so that they are sufficient for enzymes to work efficiently, as enzymes are vital to maintaining life as they are involved in all major metabolic life reactions.


Animals are an example of a multicellular organism. There is a division of labour between different tissues and organs (and cells) which keeps the organism functioning. Stimulus and response is required, to allow the different tissues to communicate with each other.

Multicellular organisms require communication as they are differentiated and have this division of labour, in order to coordinate the functions of the different systems. It provides a link between the receptor, detecting a stimulus, and the effector being triggered to carry out an appropriate response.

Stimulus and response

The body must respond to external stimuli (such as changes in temperature, light, sound, taste, etc) as well as internal stimuli (such as changes in body temperature, blood sugar levels and presence of pathogens). The communication which takes place between the detection of a stimulus and the implementation of the response relies on two systems: the nervous system and the endocrine system (hormonal system).

Nervous communication uses signals carried by nerves throughout the central nervous system (CNS), in the form of an electrical impulse. The endocrine system uses hormones, which travel around in the bloodstream and trigger a response when they bind with target cells or target tissues.


The term homeostasis describes the process of maintaining a constant internals environment despite external changes which may be taking place.

Negative feedback

One mechanism, called negative feedback, operates by detecting the external change (stimulus), communicating with other cells, and reversing the change (response). The diagram outlines negative feedback response for a change in temperature: