The basic unit of life is the cell. All living organisms are made of cells. The simplest life forms, bacteria, have correspondingly simple cells that do not contain a nucleus. Higher single-celled plants and animals have more complicated cells, containing numerous organelles, the most important of which is the nucleus. The nucleus contains the genetic material, DNA and RNA, and this is another characteristic of life. All living things contain DNA, which carries the blueprint for making the organism and for reproduction. Even those mysterious viruses, which are not considered to be fully alive, have DNA.
Before DNA was discovered, life was defined by the presence of the following characteristics: movement, growth, reproduction and the presence of such abilities as the ability to make food (plants) or to ingest food (animals) and the ability to respond to the environment. There are some non-living things that can do some of these things. Crystals can grow for instance, but they cannot eat or reproduce or respond to their environment, unlike even the simplest bacteria, which can do all these things. Bacteria eat, grow and reproduce. They respond to their environment and they can move.
Higher plants and animals also show these characteristics. All green plants can use chlorophyll to produce their own foods by photosynthesis. Most plants do not obviously move, except perhaps to sway in the breeze, but they do grow and that growth is a form of movement. As a plant grows, its roots spread out below the ground and its branches and leaves fill the spaces in the air. The movements are slow but they are there. Plants don’t have the sensory organs that animals are blessed with but plants can still respond to their environment, growing towards the light, or sending roots out to find water sources. And of course, plants reproduce. So they fulfill all the characteristics of life: they have DNA which is held in nuclei within their cells. They can grow, move and reproduce.
Animals are the same, having DNA, cells and the ability to grow, reproduce and move. Because they do not make their own food, they are much more mobile than plants and have developed sophisticated senses and to senses in order to find their food and escape from predators. So movement is more obvious in animals, as is their ability to respond to changes in their environment.
Plants and animals are obviously alive and the differences between them and non-living things like rocks are pretty obvious. However as we developed microscopes and looked at smaller and smaller organisms, we found that the boundaries between life and non-life were not so obvious. Viruses are considered to be on the boundary of life for several reasons. They do not have a cellular structure and they cannot exist outside of a host. However they consist of strands of the self-replicating molecules DNA and RNA and within a host, they can reproduce, sometimes in the billions. What the viruses show is that life evolved from self replicating molecules and they are evidence for this transition that first occurred billions of years ago.
The diversity of life on Earth is mind boggling. Millions of species live in virtually every ecosystem and habitat on the planet. Life comes in every possible shape, size and life-style but they all share these characteristics: They are made of cells which contain the self-replicating molecules, DNA and RNA, which are used to pass on the characteristics of that species from generation to generation in the form of reproduction. In addition, all living organisms show the abilities to grow, move and respond to their environment.
