text: Primary metabolites as defined by Kossel are components of basic metabolic pathways that are required for life.  They are associated with essential cellular functions such as nutrient assimilation, energy production, and growth/development.  They have a wide species distribution that span many phyla and frequently more than one kingdom.  Primary metabolites include the basic building blocks of life: carbohydrates, lipids, amino acids, and nucleic acids.
  Primary metabolites that are involved with energy production include respiratory and photosynthetic enzymes. Enzymes in turn are composed of amino acids and often non-peptidic cofactors that are essential for enzyme function. The basic structure of cells and of organisms are also composed of primary metabolites.  These include cell membranes (e.g. phospholipids), cell walls (e.g. peptidoglycan, chitin), and cytoskeletons (proteins).
  Primary metabolite enzymatic cofactors include members of the vitamin B family. Vitamin B1 as thiamine diphosphate is a coenzyme for pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase, and transketolase which are all involved in carbohydrate metabolism.  Vitamin B2 (riboflavin) is a constituent of FMN and FAD which are necessary for many redox reactions.  Vitamin B3 (nicotinic acid or niacin), synthesized from tryptophan is a component of the coenzymes NAD and NADP which in turn are required for electron transport in the Krebs cycle, oxidative phosphorylation, as well as many other redox reactions. Vitamin B5 (pantothenic acid) is a constituent of coenzyme A, a basic component of carbohydrate and amino acid metabolism as well as the biosynthesis of fatty acids and polyketides. Vitamin B6 (pyridoxol, pyridoxal, and pyridoxamine) as pyridoxal 5′-phosphate is a cofactor for many enzymes especially transaminases involve in amino acid metabolism. Vitamin B12 (cobalamins) contain a corrin ring similar in structure to porphyrin and is an essential coenzyme for the catabolism of fatty acids as well for the biosynthesis of methionine.
  DNA and RNA, which store and transmit genetic information, are composed of nucleic acid primary metabolites.
  First messengers are signaling molecules that control metabolism or cellular differentiation. These signaling molecules include hormones and growth factors in turn are composed of peptides, biogenic amines, steroid hormones, auxins, gibberellins etc.  These first messengers interact with cellular receptors which are composed of proteins. Cellular receptors in turn activate second messengers are used to relay the extracellular message to intracellular targets.  These signaling molecules include the primary metabolites cyclic nucleotides, diacyl glycerol etc.