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DIVIDING UP L IVING THINGS

We can divide up living things into different groups based on the features they have in common. For example, plants, animals and bacteria are particular groups. Large groups can be divided into smaller ones where more detailed similarities exist—plants can be divided into flowering and non-flowering varieties and animals into those with backbones and those without. The basic unit used in the classifying is the species . All living things are made up of cells . A cell is the simplest unit of life. Living things can be divided up according to the type of the cells they have. The simplest cells are those of bacteria.

The smallest bacteria can be one-10,000th of a millimeter across. There are many other one-celled, or unicellular, life forms. These are called protists and they have a more complex structure than bacteria. Then there are still more complex life forms, in which groups of cells act together as a team to their mutual advantage. Human beings are such a life form and are made up of thousands of millions of cells. Identifying the simplest life form of all is difficult. Viruses exist in the borderland between the living and the non-living worlds. A hundred times smaller than a bacterium, they cannot do anything except reproduce themselves. They cannot do this without first infecting a living cell and taking over its normal processes. The combination of cell plus virus may be said to be alive, but outside a cell a virus is lifeless.

What is the difference between living and non-living? This car production line is fully automatic and run by robots. It seems to do some of the things that living things do, such as taking simple materials and making them into something more complex. Yet we would not say that it is alive. It cannot reproduce itself or adapt readily to change, for example. Perhaps one day we may build machines that can do these things. Will they be alive?

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