Definitions of Life

Biology is the study of living things. One individual living entity is referred to as an organism. You are likely already familiar with some organisms: you are an organism, any one of your friends is an organism, and any one cat or dog are an organism. Things that are not alive, such as rocks, air, asteroids, and planets, are not organisms. This may all be incredibly obvious to you — you knew that dogs and cats were alive, and you might know that plants and fungi are alive – but can you come up with a definition of life?  It might surprise you to know that biologists, scientists who study biology, are unable to agree on a definition of life. Instead, most biologists define life as anything that exhibits these seven traits: Growth and development, reproduction, heredity, homeostasis, metabolism, cells, and response to stimuli.  

If anything exhibits these seven traits, then it is considered to be alive, and an individual unit of this living thing is referred to as an organism. A brief description of each of these seven traits is given here: 

1. Growth and development: living things change over time. A young organism, like a baby cactus or baby human, will eventually grow up into an adult human or adult cactus. Even tiny single-celled organisms like bacteria undergo changes between their inception and their maturity.  

2. Reproduction: all living things have parents. Some organisms, like humans, mice, and octopi have two parents. Others, like most bacteria and some plants, have one parent.  

3. Heredity: all living things pass on some or all of their traits to their offspring. Humans can only give birth to human babies, and bacteria can only have bacteria offspring. Heredity in living things is controlled by the molecule DNA.  

4. Homeostasis: all living things maintain near-constant conditions inside of the body of the organism. The human body maintains a near-constant temperature around 37 degrees Celsius. If you eat something salty, your kidneys will work hard to pump out excess salt in order to maintain near-constant concentration of sodium inside your body.  

5. Metabolism: all living things rely on chemistry inside the body of the organism in order to get energy. Some organisms, like plants, use chemistry to harvest the energy from sunlight. Other organisms, like animals, need to eat in order to harvest their energy. Energy is required in order to grow and to reproduce, and all living organisms need chemistry to harvest this energy.  

6. Cells: all living things, by definition, are made out of cells. Cells are the basic unit of life, and contain structures like the cell membrane, proteins, and DNA. Furthermore, each cell is capable of cellular reproduction. Some organisms, like bacteria, are composed of just one cell. An adult human has over 40 trillion cells!  

7. Response to stimuli: all living things are able to measure and respond to their environment. If you accidentally touch a hot stovetop, you might flinch, jump back, or scream as a response. If a bacterium senses a harmful chemical in its environment, it might swim away.