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CHAPTER 2 THE CHEMICAL CONTEXT OF LIFE Introduction Nature is not neatly packaged into the individual life sciences. While biologists specialize in the study of life, organisms and the world they live in are natural systems to which the basic concepts of chemistry and physics apply. Biology is a multidisciplinary science, drawing on the insights from other sciences. Life can be organized into a hierarchy of structural levels. At each successive level additional emergent properties appear. A. Chemical Elements and Compounds 1. Matter consists of chemical elements in pure form and in combinations called compounds Organisms are composed of matter . Matter is anything that takes up space and has mass. An element is a substance that cannot be broken down into other substances by chemical reactions. There are 92 naturally-occurring elements. Each element has a unique symbol, usually from the first one or two letters of the name, often from Latin or German. A compound is a substance consisting of two or more elements in a fixed ratio. Table salt (sodium chloride or NaCl) is a compound with equal numbers of chlorine and sodium atoms. While pure sodium is a metal and chlorine is a gas, their combination forms an edible compound, an emergent property. 2. Life requires about 25 chemical About 25 of the 92 natural elements are known to be essential for life elements . Four elements - carbon (C), oxygen (O), hydrogen (H), and nitrogen (N) - make up 96% of living matter.
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Most of the remaining 4% of an organism’s weight consists of phosphorus (P), sulfur (S), calcium (Ca), and potassium (K). Trace elements are required by an organism, but only in minute quantities. Some trace elements, like iron (Fe), are required by all organisms. Other trace elements are required only by some species. For example, a daily intake of 0.15 milligrams of iodine is required for normal activity of the human thyroid gland. B. Atoms and Molecules 1. Atomic structure determines the behavior of an element Each element consists of unique atoms. An atom is the smallest unit of matter that still retains the properties of an element. Atoms are composed of even smaller parts, called subatomic particles. Two of these, neutrons and protons , are packed together to form a dense core, the atomic nucleus, at the center of an atom. Electrons form a cloud around the nucleus. Each electron has one unit of negative charge. Each proton has one unit of positive charge. Neutrons are electrically neutral. The attractions between the positive charges in the nucleus and the negative charges of the electrons keep the electrons in the vicinity of the nucleus. A neutron and a proton are almost identical in mass, about 1.7 x 10-
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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