Chemical composition of living tissues

Living things and the rocks of the earth's crust are made of the same elements, but in very different amounts. Living matter has far more carbon and hydrogen: in the human body carbon is about 18.5% and hydrogen 9.5%, against only 0.03% and 0.14% in the crust.

How do scientists find out what a tissue contains? They grind it in trichloroacetic acid and filter it. What passes through (the filtrate) is the acid-soluble pool: thousands of small compounds. What stays behind (the retentate) is the acid-insoluble fraction. If they instead burn dried tissue, the ash that remains holds the inorganic elements, like calcium and magnesium. Weighing tissue before and after drying shows how much is water.

Amino acids are the building blocks of proteins. Each has an amino group and a carboxyl group on the same carbon (the α-carbon), plus a hydrogen and a variable group called R. The R group decides which amino acid it is. Twenty kinds are found in proteins: glycine (R is just hydrogen), alanine (R = CH₃), serine (R = CH₂OH)... They can be acidic (glutamic acid), basic (lysine), neutral (valine) or aromatic, with a ring (tyrosine, phenylalanine, tryptophan). In solution an amino acid can carry both a positive and a negative charge at once: a zwitterion.

Lipids are the fats and oils, like the oil used to fry puris or the butter on a paratha. They are built from fatty acids and glycerol. A fatty acid is a carbon chain: palmitic acid has 16 carbons, arachidonic acid 20. Fatty acids are saturated (no double bonds) or unsaturated (with double bonds). Glycerol joined to one, two or three fatty acids makes mono-, di- or triglycerides: fats and oils. Phospholipids, such as lecithin, also contain phosphate; they build cell membranes.

Nitrogen bases are adenine, guanine, cytosine, uracil and thymine. A base joined to a sugar is a nucleoside (adenosine, guanosine...). Add a phosphate and it becomes a nucleotide (adenylic acid...). DNA and RNA are long chains of nucleotides.