Metabolites, macromolecules and proteins

Some chemicals in living things have a known role in normal life processes: amino acids, sugars and so on. These are primary metabolites. Plants, fungi and microbes also make many other chemicals whose role in the organism itself is often not clear, but which are very useful to people: secondary metabolites. Examples: alkaloids (morphine), toxins (ricin), lectins (concanavalin A), drugs (vinblastin), pigments, essential oils, rubber, gums, antibiotics, and spices like curcumin (from turmeric).

The acid-soluble pool holds small molecules weighing about 18 to 800 daltons: the micromolecules. The acid-insoluble fraction holds proteins, nucleic acids and polysaccharides, which weigh ten thousand daltons and more: these are macromolecules, long chains (polymers) of smaller units. Lipids also end up in the insoluble fraction, but only because they form membrane vesicles when tissue is ground; their molecules are small, so they are not strictly macromolecules.

What a cell is made of, by amount: water comes first (70-90%), then proteins (10-15%), nucleic acids (5-7%), carbohydrates (3%), lipids (2%) and ions (1%).

Proteins are polypeptides: chains of amino acids joined by peptide bonds. Since they use up to 20 different amino acids, a protein is a heteropolymer (a chain of different units). Some amino acids our body cannot make; these essential amino acids must come from food.

Proteins do many jobs: collagen forms the ground substance between cells; trypsin is an enzyme; insulin is a hormone; antibodies fight infection; receptors pick up signals; GLUT-4 carries glucose into cells. Collagen is the most abundant protein in animals; RuBisCO, the photosynthesis enzyme, is the most abundant protein in the whole biosphere.