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WORDS TO UNDERSTAND

Amino acid —naturally occurring chemicals that are used by living organisms to build proteins. Plants and many micro-organisms can make all the amino acids they need but animals must get them from food. Element —a substance that cannot be broken down into a simpler substance by chemical means. There are 92 natural elements, including oxygen, hydrogen, nitrogen, carbon, iron and uranium. Fossil —the remains or traces of an organism that existed in the past. Usually, only the hard parts, such as bones, shells and teeth, become fossils, though occasionally whole animals may be found, such as the mammoths that have been uncovered frozen in the ice caps of Siberia and North America. Fossils generally occur where rocks have formed from the sediments of river, lakes or sea-beds.

Molecule —the smallest part of a chemical compound. Molecules consist of atoms of different elements.

Nucleic acid —a complex chemical compound found in living cells. There are two types: DNA and RNA.

Natural selection —according to Charles Darwin, this is the process by which new species appear. The offspring of a plant or animal that are best suited to their environment have the best chance of survival. The survivors pass on their advantages, which may come about through mutation, to their offspring. This results in a gradual change in the characteristics of the organism, eventually leading to the evolution of a new species. Ozone layer —ozone is a form of oxygen that is created by the action of ultraviolet radiation. It is found in tiny quantities in the atmosphere, especially between 20 and 25 km above the Earth’s surface. This is the ozone layer. It absorbs most of the ultraviolet radiation from the Sun, protecting living organisms on Earth from its harmful effects. This ozone shield is being damaged by air pollution. Photosynthesis —a process by which green plants and some types of bacteria convert simple molecules into complex compounds. It is the direct or indirect source of energy for nearly all forms of life. The energy of sunlight is used to drive a chemical reaction in which carbon dioxide gas from the atmosphere is combined with water to produce glucose, the simplest type of sugar, and oxygen. Almost all the oxygen in the atmosphere today has been produced by photosynthesis.

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