Chapter 19 · Biochemistry
19.3Carbohydrates: Sugar, Starch, and Fiber
10 min · two checks
Predict
What is the relationship between glucose, sucrose, and starch?
The idea
- Classify mono-, di-, and polysaccharides.
- Contrast starch and cellulose.
Carbohydrates are polyhydroxy aldehydes or ketones, or chains of them. Monosaccharides include glucose, fructose, and galactose, typically C₆H₁₂O₆ isomers. In water they curl into rings. A disaccharide joins two monosaccharides with loss of water: sucrose is glucose plus fructose, lactose is glucose plus galactose, maltose is two glucoses. Polysaccharides are long. Starch (amylose and amylopectin) and glycogen store glucose. Cellulose builds plant cell walls.
The link geometry is the whole difference between starch and cellulose. Human enzymes hydrolyze the starch link and cannot hydrolyze the cellulose link, so cellulose is fiber: it adds bulk and is not a calorie source for us. Cows host microbes that can. Sugar in the diet is the small carbohydrates; “complex carbohydrate” usually means starch. Both ultimately supply glucose, at different speeds.
Keep these
- Mono: one sugar. Di: two. Poly: many.
- Glucose, fructose, and galactose are isomers with different structures.
- Starch is digestible glucose storage. Cellulose is not, for humans, because the link differs.