Carbohydrates
Carbohydrates can provide fuel, energy storage, and structural support. Starch stores glucose in plants; glycogen is a highly branched glucose storage polymer in animals and many other organisms. Cellulose uses glucose subunits linked in a different orientation, producing straight chains that associate into strong fibers. A cell needs enzymes that recognize the particular linkage to hydrolyze a carbohydrate efficiently. Monomer identity alone is therefore insufficient to predict function. Branching creates additional ends where some enzymes can access a storage polymer.
Humans digest starch efficiently but lack enzymes that hydrolyze cellulose’s beta-1,4 linkages. Both polymers contain glucose; their linkage geometry gives them different structures and biological roles.
A carbohydrate is not automatically a short-term energy molecule. Cellulose is a structural carbohydrate, and glycogen is stored fuel.