Biology is the science that deals with living things. Sometimes it is necessary to make a difference between organisms that are alive, and other things that are not alive. This might not be as easy as it seems, but in general:
- Living things react to stimuli.
- Living things interact with their environment, which includes members of the same and other species.
- Living things have a metabolism: they take in food which they convert to the energy they need.
- Living things reproduce: they give birth to others of the same species. This is not true of all individual organisms. In eusocial organisms, some castes cannot reproduce. But, since the sterile workers are all the produce of a single queen, they are one collective.
Many things that appear to be one organism are in fact several living together. An example is lichen. Lichen is a symbiosis between a blue-green alga and a fungus. Organisms that live together may not reproduce together, but their life processes are bound up together. They help each other to live.
Modern ideas [change]
Cells and macromolecules [change]
All life processes on Earth use the chemistry of carbon compounds. In particular, all life uses long-chain molecules such as proteins and nucleic acid. With water, which is essential, the long molecules are wrapped inside membranes to form cells. This is true of all known life.
Systems theory [change]
Living organisms are open systems. The main idea is that the processes serve to keep them alive by homeostasis. They are always changing, but always staying within certain limits as long as they live. 
They do this by exchanging materials and information with their environment. They undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli and reproduce. All of this preserves the individual and its race or species.
Related pages [change]
- Cannon W.B. 1932. The wisdom of the body. London.
- Reiner J.M. 1960. The organism as an adaptive control system. Prentice-Hall, Englewood Cliffs, New Jersy.