Biochemistry - Chemical Reactions in Living Cells, Volume 1 - D. Metzler 1980

The Molecules We Are Made Of
Chemical Composition of Cells

What is the precise amount of Proteins, Nucleic Acids, CARBOHYDRATES, Lipids, and other substances present in a living Cell? There is no single definitive answer to this question, as Cells vary dramatically from one another. For instance, certain specialized cells accumulate large quantities of triglycerides or carbohydrates, whereas glandular cells are frequently distinguished by an unusually high Protein and RNA content. Furthermore, body composition changes over the course of development. For example, a pig embryo consists of 97% Water, but by birth this value drops to 89%; for an unfed 45 kg pig, it stands at 67%, whereas for a very obese 135 kg animal, it does not exceed 40%.

A straightforward approximate analysis of tissue composition is performed as follows. The tissue is thoroughly dried under vacuum at low Temperature, and lipids are extracted from the resulting solid material using appropriate Solvents. After evaporating the solvent, the remaining lipids are weighed. This method demonstrates that young green vegetables contain 2–5% lipids on a dry-weight basis. Even in very lean meat, the lipid content ranges from 10–30%. Protein content is frequently estimated by measuring nitrogen content and multiplying the resulting value by 6.25. In young green plants, protein may account for 20–30% of the dry weight, whereas very lean meat can contain up to 50–70%.

The ash weight obtained after incinerating a tissue sample at high temperatures serves as a measure of its inorganic component content. Typically, ash constitutes 3–10% of the tissue weight; however, this percentage is significantly higher in specialized Tissues such as bone.

Within the framework of such an approximate analysis, carbohydrate content is estimated by subtracting all other components from 100%. Young green plants contain up to 50–60% carbohydrates, whereas typical animal tissues contain only 2–10%. In exceptional cases, however, carbohydrate levels in animal tissues can be substantially higher—for instance, Glycogen makes up 28% of an oyster.

The nucleic acid content of cells ranges from 0.1% in Yeast and 0.5–1% in Muscle and bacterial cells to 15–40% in the Thymus and sperm cells. Obviously, in these latter two cases, the exceptionally high nucleic acid concentration invalidates the standard method of estimating protein content by multiplying the nitrogen percentage by 6.25.

Class="center">Table 2-10 Approximate composition of metabolically active Cells and Tissues

^a Watson J. D., Molecular Biology of the Gene, 3rd ed., p. 69, Benjamin, New York, 1976. The percentages given for amino acids, nucleotides, carbohydrates, and lipids include the contributions of their respective precursors present within the cells.

^b Burton B. T., Human Nutrition, 3rd ed., p. 505, McGraw-Hill, New York, 1976.

^c Long C., ed., Biochemists’ Handbook, pp. 677–679, Van Nostrand-Reinhold, Princeton, New Jersey, 1961.

Table 2-10 presents a comparative composition of a bacterium (E. coli), a green plant (spinach), and a metabolically active animal tissue (rat liver).