GLOSSARY

Biosynthesis is production of substances by living systems.

A pathway is a reaction sequence; a network links pathways through branches. An intermediate may serve several functions, so pathway names are not walls separating fuels. Cycles can regenerate components while supplying biosynthesis. During exercise, simultaneous fuel use and tissue demands matter rather than imagining one switched-on pathway with all others off. A diagram simplifies organization without removing the interactions or time-dependent control found in living cells.

A substrate enters a reaction and a product emerges; one step's product can feed another. A precursor can be converted, without guaranteeing complete conversion. Amount, enzymes, conditions and pathway demand determine outcomes. Extra precursor does not necessarily produce proportionally more desired product, and a physiological role alone does not establish a supplement requirement. Assess the actual intervention rather than treating a pathway arrow as proof of nutritional benefit.

Cells and organs do not share one uniform pool; cytosol, mitochondria and blood have different roles. Membranes and carriers determine direct passage versus shuttle requirements. Blood measurements do not fully reveal conditions inside muscle. Compartmentation prevents interpreting one sample or supplement as a guaranteed energy change in every cell. Distinguish local processes from whole-body averages rather than assuming a circulating level represents all intracellular reactions.