6.1Components of Life


Homeostasis of Body Temperature

Life originated in water on the earth’s surface and has lived on the earth’s surface throughout its long history. Fig. 6-1 shows a comparison between the elemental composition of the earth’s surface (earth’s crust) and living organisms. A weight ratio is shown because a comparison using the number of atoms results in a higher composition ratio of lighter elements. Two characteristics can be ascertained from this observation. First, limited varieties of lighter elements comprise living organisms. Second, although silicon is abundant in the earth’s crust, carbon is abundant in living organisms. Carbon produces a broad range of organic compounds; however, its most important property in the creation of life is production of functional macromolecules.

Fig. 6-1 Constituent Elements of the Human Body and the Earth’s Crust (by %Weight)

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Molecules That form Life

Table 6-1 shows the composition of compounds that comprise living organisms. Although Escherichia coli has been cited as an example in this table, it should be understood that other cells are essentially similar.

Table 6-1 Bacterial Cell Constituents

Water comprises 70% of an organism. Water is a polar molecule that dissolves most ions and other polar molecules. Except for lipids, all other biomolecules and macromolecular compounds are polar molecules having a high affinity for water. Despite having a low molecular weight, hydrogen bonds of a water molecule join those of neighboring molecules, thus resulting in compounds with a high melting point, boiling point, and specific heat compared with other low-molecular-weight compounds. With these properties, water effectively functions to safely allow the creation of organisms in the thermal environment of the earth’s surface and maintain life after its emergence.

Other than water, most molecules are in the form of organic compounds, which are carbon-containing compounds. More information on the essential organic compounds that constitute organisms will be presented shortly, but as shown in Table 6-1, organic compounds that constitute organisms are mostly macromolecules, with a relatively few low-molecular-weight organic compounds.

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Organic Compounds Forming Life

The four main organic compounds forming organisms are proteins, lipids, carbohydrates, and nucleic acids. You may be familiar with the three essential nutrients (proteins, lipids, and carbohydrates) required for survival. The reason nucleic acids are not essential nutrients despite being important compounds is that the required amount of nucleic acids can be synthesized internally using carbohydrates and amino acids. The nutritionally important trace organic compounds called vitamins cannot be synthesized internally in the required amounts and must be ingested.

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