In multicellular organisms, similar cells form tissues. All living organisms are made up of one or more cells, which are considered the fundamental units of life.Įven unicellular organisms are complex! Inside each cell, atoms make up molecules, which make up cell organelles and structures. Living things are highly organized, meaning they contain specialized, coordinated parts. Although nonliving things may show some of these characteristic traits, only living things show all of them. To make this separation, we must come up with a list of properties that are, as a group, uniquely characteristic of living organisms.īiologists have identified various traits common to all the living organisms we know of. Because of this, many definitions of life are operational definitions-they allow us to separate living things from nonliving ones, but they don’t actually pin down what life is. However, it’s surprisingly hard to come up with a precise definition of life. What is it that defines life? How can we tell that one thing is alive and another is not? Most people have an intuitive understanding of what it means for something to be alive. If you were to burn the wood in a fire, the fire would not be alive either. The parts of a chair that are made of wood were once alive, but they aren’t any longer. The computer you’re using to read this article is not alive, and neither is a chair or table. However, snow falling from the clouds is not alive. The dog I can hear barking is alive, and so is the tree outside my window. What is life? What does it mean to be alive?
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