Life is easy to recognise, but hard to define. The study of life is called biology and people who study life are biologists. A lifespan is the average length of life in a species. All life is directly or indirectly powered by solar energy. Without energy from the sun no life could exist.
All known life on Earth is based on the chemistry of carbon compounds. In particular, all life uses long-chain molecules such as proteins and nucleic acid. With water, which is essential, the long molecules are wrapped inside membranes to form cells. This may or may not be true of all possible forms of life in the Universe: it is true of all life on Earth today. In English there are many interjections (sayings) related to the word life including "you only live once so live life to the fullest" or "life of the party".
Living organisms are open systems. They are always changing, because they exchange materials and information with their environment. They undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli and reproduce.
Through natural selection, they adapt to their environment in successive generations. More complex living organisms can communicate through various means. Many life forms can be found on Earth. The properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon and water-based cellular form with complex organization and heritable genetic information.
At present, the Earth is the only planet we have detailed information about. The question of whether life exists elsewhere in the Universe is open. There have been a number of claims of life elsewhere in the Universe. None of these have been confirmed so far. The best evidence of life outside of Earth is fossil evidence of possible bacterial life on Mars.
Definitions of life[change]
One explanation of life is called the cell theory. The cell theory has three basic points: All living things are made up of cells. The cell is the smallest living thing that can do all the things needed for life. All cells must come from pre-existing cells.
Something is often said to be alive if it:
- takes in food, uses the food for energy, and passes waste products (see metabolism);
- moves, meaning it must either move itself, or have movement inside of itself;
- reproduces, either sexually (with another living thing) or asexually, by creating copies of itself; and
- reacts to its surroundings
However, not all living things fit every point on this list.
- mules cannot reproduce, and neither can worker ants
- viruses and spores are not actively alive (metabolising) until the conditions are right.
They do, however, fit the biochemical definitions: they are made of the same kind of chemicals.
Gallery of images of life[change]
An Adult citrus root weevil is an example of an insect