What is life? Life can be called the absence of death, but that does not shed light on what life is, only on what life isn’t. This isn’t an easy question; to start we need to ask what organisms that “live” have in common.
The basic unit for all living organisms is the cell. Organisms can consist of a single cell (bacteria for example), or many cells. In more complicated organisms, cells are organized to form more complicated structures such as organs, for example. In each of these cells, unique forms of deoxyribonucleic acid (DNA) molecules are present. DNA can form into genes, which are used to make the proteins and other molecules a cell needs to survive and reproduce.
To survive, organisms need to get energy. Energy is captured in many ways, photosynthesis (using sunlight for energy) being one of them. The whole set of chemical reactions that cells use to receive energy is called metabolism.
Organisms also need to reproduce (the production of offspring by one or more parents). There are two basic types of reproduction – asexual, in which the cell simply splits in half, is common in single-cell organisms. More complicated organisms reproduce by sexual reproduction by combing sex cells (gametes) from both parents. This produces an offspring with a combination of characteristics from both parents. These offspring have a greater chance of survival in changing conditions than asexually reproduced organisms because sexually reproduced offspring can adapt while asexual production are simply clones.
Mutation is also an important characteristic of living organisms. Mutations are random changes in the structure or composition of DNA, which can be caused by mistakes in reproduction or external events, such as UV damage. Mutations can be beneficial, harmful, or neutral. Beneficial mutations help an organism adapt. Organisms with beneficial mutations have a greater chance of survival, so over time they become prevalent. This process is called evolution. If enough changes occur an entire new species can be formed! Other organisms which cannot adapt decline and may even become extinct. This ongoing extinction and evolution have lead to a great amount of diversity on earth.