Simple sugars and disaccharides

Carbohydrates range from the sugar in a soft drink to the tough fibers of wood. The small ones are sugars, and the large ones are long chains of sugars linked together.

What makes a carbohydrate

Organic molecules made of carbon, hydrogen, and oxygen, in or near the proportions of the formula Cₙ(H₂O)ₙ (where n is a whole number), are carbohydrates. The formula reads like “carbon plus water,” which explains the name: carbo- for carbon and -hydrate for water.

Most of a carbohydrate’s carbons carry a hydrogen atom and a hydroxyl group (–OH). These polar –OH groups form hydrogen bonds with water, which is why sugars dissolve so easily. Some carbohydrates carry other functional groups too, such as amino or sulfate groups.

Monosaccharides: single sugars

Small carbohydrates that usually taste sweet are sugars. The simplest sugars are monosaccharides (“single sugars”), the monomers of carbohydrates. The most common monosaccharides have either five carbons (pentoses) or six (hexoses):

• Ribose (C₅H₁₀O₅) is the pentose in RNA.
• Deoxyribose (C₅H₁₀O₄), with one fewer oxygen atom than ribose, is the pentose in DNA.
• Glucose (C₆H₁₂O₆) is the most common hexose.

Glucose is the sugar that circulates in your blood. It dissolves in blood and other body fluids and is carried into cells. Inside a cell, enzymes break glucose into smaller molecules, releasing energy. The cell uses that energy to make ATP, a molecule that works like ready cash for powering many cell processes.

A glucose molecule can exist as a straight chain or as a ring. By convention, its carbons are numbered 1 through 6. The ring forms when the oxygen attached to carbon 5 bonds to carbon 1. Dissolved in the watery fluids of living things, glucose is almost always in ring form.

Isomers of glucose

Glucose has several isomers, and cells can tell them apart. Picture the glucose ring lying flat on a table, with each –OH group pointing either above or below the ring. Moving a single –OH, or flipping the whole molecule into its mirror image, gives a different isomer:

These differences look tiny, but enzymes notice them. Enzymes that use D-glucose, for example to build starch, bind L-glucose poorly, and L-glucose is rare in living cells. The α/β difference matters just as much: chains of α-glucose form the energy stores starch and glycogen, while chains of β-glucose form cellulose.

Disaccharides: two sugars joined

Two monosaccharides joined together make a disaccharide (“two sugars”). They’re linked by a dehydration reaction: one sugar gives up an –OH, the other gives up an H, and these leave as a water molecule. The two sugars end up connected through an oxygen atom, and this covalent bond between two sugars is a glycosidic bond.

Splitting a disaccharide into its two monosaccharides is a hydrolysis reaction: a water molecule is added back across the glycosidic bond. Digesting the lactose in milk, for instance, splits it into galactose and glucose.

Check yourselfWhy do D-glucose and L-glucose behave so differently in cells, even though they contain exactly the same atoms?

They’re enantiomers, mirror images with their atoms arranged differently in space. Enzymes bind sugars by fitting their shape, so an enzyme built to hold D-glucose fits L-glucose poorly.

Check yourselfWhat kind of reaction splits sucrose into glucose and fructose, and what does it use up?

A hydrolysis reaction. It uses one water molecule to break the glycosidic bond, reversing the dehydration reaction that formed sucrose.