Simple sugars like glucose are usually introduced with a straight, open-chain structure, but in actual solution the molecule spontaneously curls around and closes into a ring almost all of the time, only a tiny fraction exists as the open chain at any given moment. Understanding carbohydrate metabolism, how cells actually break glucose down for energy and how they link sugar units together into larger molecules like starch or cellulose, depends specifically on that ring form's chemistry rather than the simpler open-chain structure most textbooks draw first.
Glucose's own carbonyl and hydroxyl groups react with each other to close the ring
Glucose's open-chain form carries a reactive carbonyl group at one end and several hydroxyl groups along its length, and in solution one of those hydroxyl groups reacts directly with the carbonyl group on the same molecule, closing it into a six-membered ring. That ring-closing reaction can happen in two slightly different orientations, producing two related ring forms, and which specific orientation, alpha or beta, a given sugar unit adopts turns out to matter enormously once sugar units start linking together into longer chains.
The alpha-versus-beta linkage decides whether an organism can actually digest the resulting chain
Starch is built from glucose units joined by alpha linkages, while cellulose is built from the same basic glucose unit joined by beta linkages instead, and that single structural difference is exactly why humans can digest starch easily but can't digest cellulose at all, human digestive enzymes are shaped to recognise and break alpha linkages specifically, and simply can't act on the differently angled beta linkage even though the underlying sugar unit is identical in both molecules.
What we're still unsure about
That glucose predominantly exists as a ring in solution, and that the alpha-versus-beta linkage distinguishes digestible starch from indigestible cellulose, are well established, extensively documented biochemistry. What's genuinely more a matter of evolutionary history than settled chemical necessity is why humans, and most animals, never evolved their own enzymes capable of breaking beta linkages despite cellulose being extremely abundant in their diet, organisms that do digest cellulose, cattle and termites among them, generally rely on gut microbes that carry the right enzymes rather than having evolved the capability themselves, and exactly why that enzyme never independently evolved in the host animal's own genome across so many separate lineages remains more an open evolutionary question than a chemical one.
This sits inside Carbohydrate Structure & Metabolism, one of seven topics in Biochemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.