Carbohydrates

Biomolecules are the molecules of life: carbohydrates, proteins, vitamins, nucleic acids and hormones. Carbohydrates come first: the sugars and starches that give us energy.

Chemically, carbohydrates are optically active polyhydroxy aldehydes or ketones (many –OH groups plus a C=O), or compounds that give these when hydrolysed. Three groups:

• Monosaccharides cannot be split into simpler sugars: glucose and fructose.
• Oligosaccharides give two to ten monosaccharides on hydrolysis. The disaccharides give two: sucrose (cane sugar), maltose (malt sugar) and lactose (milk sugar).
• Polysaccharides are long chains of many monosaccharide units: starch, cellulose and glycogen.

A sugar that can reduce Fehling's solution or Tollens' reagent is a reducing sugar: it has a free aldehyde or ketone group. All monosaccharides are reducing sugars.

Glucose, C₆H₁₂O₆, is the most important. Its open-chain structure was worked out from its reactions:

• With HI, it gives n-hexane: so its six carbons are in a straight chain.
• With bromine water (a mild oxidant), the CHO becomes COOH, giving gluconic acid: so it has an aldehyde group.
• With nitric acid, both ends are oxidised, giving saccharic acid: so the other end carries a primary alcohol group, –CH₂OH.
• With acetic anhydride, it forms a penta-acetate: so it has five –OH groups.

But some facts did not fit an open chain. Glucose does not give Schiff's test or react with NaHSO₃, as an aldehyde should. The answer: glucose exists mainly as a six-membered ring. The –OH on carbon 5 adds to the aldehyde group, forming a cyclic hemiacetal. This makes a new –OH on carbon 1, which can point two ways, giving two forms, α-glucose and β-glucose. Only a tiny fraction is ever in the open-chain form, which is why some aldehyde tests fail.

In the polysaccharides, the difference between α and β matters a lot:

• Starch, the plants' energy store, is made of α-glucose. It has two parts: amylose (unbranched chains, soluble in water) and amylopectin (branched, insoluble).
• Cellulose, the main material of plant cell walls, is made of β-glucose in long straight chains. Humans cannot digest it.
• Glycogen is the animal store of glucose, kept in the liver and muscles. It is like amylopectin but even more branched, which is why it is called animal starch.