What Makes Something Alive?
What's the difference between a rock and a rabbit? It seems obvious, but what specific things make the rabbit alive and the rock not alive? Biologists have identified several key characteristics that all known living organisms share. Anything that possesses all of these characteristics is considered biotic, or living. Things that do not are abiotic, or non-living.
The Characteristics of Life
1.Organization: All living things are made of one or more cells. The cell is the basic unit of life. Some organisms, like bacteria, are just a single cell. Others, like humans, are made of trillions of cells organized into tissues, organs, and organ systems.
2.Metabolism: Living things use energy. Metabolism is the sum of all chemical reactions that build up or break down materials. Think of it as how an organism manages its energy—getting it, using it, and storing it.
3.Homeostasis: Living organisms must maintain a stable internal environment, even when the external environment changes. This process is called homeostasis. For example, your body sweats to cool you down and shivers to warm you up, keeping your internal temperature around 98.6°F (37°C).
4.Growth and Development: All living things grow. Single-celled organisms mostly just increase in size. Multicellular organisms go through development, a process of change, to become mature adults.
5.Reproduction: Organisms make more of their own kind through reproduction. This can be asexual (one parent) or sexual (two parents). Reproduction ensures that a species survives.
6.Response to Stimuli: Living things react to changes in their environment. A change that causes a reaction is a stimulus (plural: stimuli). A plant turning toward sunlight is responding to the stimulus of light.
7.Evolution: As a group, living things change over time. This is called evolution. These changes, driven by natural selection, allow a species to adapt to its environment over many generations.
The Cell Theory: A Foundation of Biology
The idea that all life is made of cells is one of the most important concepts in biology. It's summarized in the Cell Theory, which was developed by several scientists in the mid-1800s.
The three main points of the Cell Theory are:
1.All living organisms are composed of one or more cells.
2.The cell is the basic unit of structure and function in all living organisms.
3.All cells arise from pre-existing cells.
This theory fundamentally tells us that you can't have life without cells, and you can't get a cell without another cell making it. It connects every living thing on Earth.