living things can be found within a relatively thin layer on or near the
surface of the earth. Apart from the sun's energy, all their needs are
supplied by the small proportion of the earth's resources contained in
this layer. If the water, oxygen, and other elements necessary for life
were only used once, they would soon run out. This is why many of nature's
processes work in cycles. There is a constant exchange of the elements
between air, earth, water, plants, and animals. Most biological processes,
even those with very complicated pathways, wind up back where they started.
These recycling processes ensure that all living things are able to live
and grow. In a continuous cycle, plants and animals exchange the chemicals
necessary for energy and building materials.
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