Four elements make up 96 percent of your body's mass
Oxygen, carbon, hydrogen and nitrogen account for almost all of a human body by weight. From those few ingredients, living things assemble proteins, sugars, fats and genetic material, and they do it in strikingly similar ways across the whole tree of life, from bacteria to blue whales to oak trees.
Biologists call this sameness the biochemical universals, or the material unity of living beings. The same kinds of molecules and many of the same metabolic pathways turn up in organisms that otherwise look nothing alike. It ranks alongside cell theory and evolution as one of the big unifying ideas of biology, and it is why discoveries in yeast or mice so often apply to people.
Genetic material shows both reliability and flexibility. DNA mostly sits in its familiar double helix, where C pairs with G and A with T; that stable pairing is what makes it dependable for storing information. RNA uses U instead of T and behaves more like a protein, folding into intricate three-dimensional shapes such as transfer RNAs, ribosomes and ribozymes. Nucleotides, the building blocks of both, also serve as energy carriers like ATP and as signalling molecules inside cells.
Proteins are built in layers of structure. The sequence of amino acids comes first, set by the organism's genes. Local stretches coil into alpha helices or lie flat in beta sheets, the chain folds into a compact overall shape, and some proteins combine several chains; haemoglobin, for example, joins two alpha and two beta chains. Many enzymes stay inactive until a cofactor, such as a metal ion or a heme group, locks in. Usually just twenty amino acids go into proteins, but a few organisms add two extras. Selenocysteine is inserted where the code normally says stop, and pyrrolysine appears in some methane-producing microbes.
Sugars and fats round out the picture. Single sugars like glucose and fructose link into pairs such as sucrose and lactose, or into long chains like starch, glycogen and cellulose. Lipids, with water-loving heads and water-shy tails, form the membranes around every cell and store energy as triglycerides. Lignin, the stiffening material in wood and the second most abundant biopolymer after cellulose, is unusual: it forms through random radical reactions, so it lacks the consistent handedness found in most biological molecules.
Source: Biomolecule
