Living things are biological structures that respond to changes in the environment or within their own entities. This includes animals, plants, fungi and the single-celled organisms known as bacteria. Living things have complex biochemical organizations that allow them to process substances and utilize energy in order to respond to changes around them.
According to New Mexico Tech, all living things display the seven characteristics of life. The first of these is that they are comprised of cells. Single-celled organisms have only one cell that completes all of the necessary tasks of life, while multicellular organisms have many specialized cells. Living things are also complexly organized, both on the molecular and cellular level. In multicellular organisms, cells are organized into tissues, and tissues are organized into organs.
In order to be considered a living thing, a structure must take in energy and use it not only to respond to the environment, but also to grow and maintain itself. Living things grow in several ways; cells divide to form new cells, cells become larger, and an organism increases in size as its number of cells increases. The sixth characteristic of life is the ability to reproduce, and the seventh is the ability to adapt to the environment. Though individual organisms may not satisfy these final two characteristics, groups of similar organisms must in order to be considered "living things."