By John Brennan
Updated Mar 24, 2022
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When you think of starch, you likely picture the staple foods of the world—corn, potatoes, rice. Starch is the primary carbohydrate reserve in green plants, while animals, including humans, rely on glycogen for their energy storage needs.
Both starch and glycogen are efficient carbohydrate storage polymers. Plants store glucose as starch, whereas animals store it as glycogen.
Starch and glycogen both serve as energy reservoirs. In plants, starch is synthesized from glucose to support future growth; seeds, roots, and tubers pack significant amounts to fuel germination and early development. In animals, glucose from the diet is converted to glycogen in the liver and muscle tissues for quick mobilization during activity or fasting.
Both molecules are glucose polymers. Starch exists as two distinct polysaccharides: amylose (linear chains) and amylopectin (highly branched). Glycogen shares a branching pattern more akin to amylopectin, with branch points every 8–12 glucose units, enabling rapid release of glucose when needed.
Glucose can adopt several isomeric forms. Starch and glycogen are composed exclusively of α‑D‑glucose, where the anomeric hydroxyl group (C1) is positioned trans to the C6 hydroxyl group. This arrangement allows α‑1,4 and α‑1,6 glycosidic linkages that form the characteristic branched architecture.
Digestive enzymes in humans readily hydrolyze both starch and glycogen into glucose, making them valuable dietary energy sources. In contrast, cellulose—a β‑glucan with β‑1,4 linkages—forms rigid, insoluble chains that the human gut cannot digest, passing through as fiber. This structural distinction underscores why starch and glycogen are nutritionally beneficial, while cellulose provides bulk without caloric contribution.
