Tyson JJ, chen kc & novak B (2003) sniffers, buzzers, toggles and blinkers: dynamics of regulatory and signaling pathways in the cell. Curr. Opin. Cell Biol. 15, 221–231.
venter Jc, Adams md, myers eW et al (2001) The sequence of the human genome. Science 291, 1304–1351.
It is at first sight difficult to accept the idea that living creatures are merely chemical systems. Their incredible diversity of form, their seemingly purposeful behavior, and their ability to grow and reproduce all seem to set them apart from the world of solids, liquids, and gases that chemistry normally describes. Indeed, until the nineteenth century animals were believed to contain a Vital Force—an “animus”—that was responsible for their distinctive properties.
We now know that there is nothing in living organisms that disobeys chemical or physical laws. However, the chemistry of life is indeed special. First, it is based overwhelmingly on carbon compounds, the study of which is known as organic chemistry. Second, cells are 70% water, and life depends largely on chemical reactions that take place in aqueous solution. Third, and most important, cell chemistry is enormously complex: even the simplest cell is vastly more complicated in its chemistry than any other chemical system known. In particular, although cells contain a variety of small carbon-containing molecules, most of the carbon atoms present are incorporated into enormous polymeric molecules—chains of chemical subunits linked end-to-end. It is the unique properties of these macromolecules that enable cells and organisms to grow and reproduce—as well as to do all the other things that are characteristic of life.
The ChemiCal ComponenTs of a Cell
Living organisms are made of only a small selection of the 92 naturally occurring elements, four of which—carbon (C), hydrogen (H), nitrogen (N), and oxygen (O)—make up 96.5% of an organism’s weight (Figure 2–1). The atoms of these elements are linked together by covalent bonds to form molecules (see Panel 2–1, pp. 90–91). Because covalent bonds are typically 100 times stronger than the thermal energies within a cell, they resist being pulled apart by thermal motions, and they are normally broken only during specific chemical reactions with other atoms and molecules. Two different molecules can be held together by noncovalent bonds,
The ChemiCal ComponenTs  of a Cell
CaTalYsis anD The Use of eneRGY BY Cells
Figure 2–1 The main elements in cells, highlighted in the periodic table. When ordered by their atomic number and arranged in this manner, elements fall into vertical columns that show similar properties. atoms in the same vertical column must gain (or lose) the same number of electrons to attain a filled outer shell, and they thus behave similarly in bond or ion formation. Thus, for example, mg and Ca tend to give away the two electrons in their outer shells. C, n, and o occur in the same horizontal row, and tend to complete their second shells by sharing electrons.
The four elements highlighted in red constitute 99% of the total number of atoms present in the human body. an additional seven elements, highlighted in blue, together represent about 0.9% of the total. The elements shown in green are required in trace amounts by humans. it remains unclear whether those elements shown in yellow are essential in humans. The chemistry of life, it seems, is therefore predominantly the chemistry of lighter elements. The atomic weights shown here are those of the most common isotope of each element.
1000 10,000 kJ which are much weaker (Figure 2–2). We shall see later that noncovalent bonds are important in the many situations where molecules have to associate and dissociate readily to carry out their biological functions.
Water is held Together by hydrogen Bonds