Life as a Biochemical Process

Beginning with the highest levels of taxonomy, we have taken a quick tour of the varieties of organisms, and have briefly seen some of their important parts. So far, this account has been entirely descriptive. Because of the tremendous diversity of living systems, descriptive accounts are a crucial underpinning to any more explanatory theories. In order to understand how biological systems work, one has to know what they are. Knowledge of cells and tissues makes possible the functional accounts of physiology. For example, knowing that the cells in the bicep and in the heart are both kinds of muscle helps explain how the blood circulates. However, at this level of description, the work that individual cells are able to do remains mysterious. The revolution in biology over the last three decades resulted from the understanding cells in terms of their chemistry. These insights began with descriptions of the molecules involved in living processes, and now increasingly provides an understanding of the molecular structures and functions that are the fundamental objects and actions of living material.

More and more of the functions of life (e.g. cell division, immune reac tion, neural transmission) are coming to be understood as the interactions of complicated, self-regulating networks of chemical reactions. The substances that carry out and regulate these activities are generally referred to as bio molecules. Biomolecules include proteins, carbohydrates, lipids-all called macromolecules because they are relatively large and a variety of small molecules. The genetic material of the cell specifies how to create proteins, as well as when and how much to create. These proteins, in turn, control the flow of energy and materials through the cell, including the creation and transformation of carbohydrates, lipids and other molecules, ultimately ac complishing all of the functions that the cell carries out. The genetic material itself is also now known to be a particular macromolecule: DNA.

In even the simplest cell, there are more than a thousand kinds of biomol ecules interacting with each other; in human beings there are likely to be more than 100,000 different kinds of proteins specified in the genome (it is unlikely that all of them are present in any particular cell). Both the amount of each molecule and its concentration in various compartments of the cell determines what influence it will have. These concentrations vary over time, on scales of seconds to decades. Interactions among biomolecules are highly non-linear, as are the interactions between biomolecules and other molecules from outside the cell. All of these interactions take place in parallel among large numbers of instances of each particular type. Despite this daunting complexity, insights into the structure and function of these molecules, and into their interactions are emerging very rapidly.

One of the reasons for that progress is the conception of life as a kind of information processing. The processes that transform matter and energy in living systems do so under the direction of a set of symbolically encoded in structions. The “machine” language that describes the objects and processes of living systems contains four letters, and the text that describes a person has about as many characters as three years’ worth of the New York Times (about 3×109). In the next section, we will delve more deeply into the the chemistry of living systems.

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