Alpha and beta glucose

Alpha glucose (α-glucose) and beta glucose (β-glucose) are the two ring forms of glucose, C₆H₁₂O₆. They are isomers: the same atoms, arranged differently in one place. In alpha glucose the OH group on carbon 1 points below the ring; in beta glucose it points above it. That one change decides what the sugar builds: alpha glucose makes starch and glycogen, and beta glucose makes cellulose.

Glucose structure

Glucose is a monosaccharide, a single sugar unit, and the monomer from which larger carbohydrates are made. It is a hexose sugar: it has six carbon atoms. Its molecular formula is C₆H₁₂O₆ and its relative molecular mass is 180.

In water, glucose is mostly found as a ring. Five of the carbons and one oxygen atom make a six-membered ring, and carbon 6 sticks out above the ring as a CH₂OH group. The carbons are numbered from 1, the carbon to the right of the ring oxygen, round to 5, then 6 in the CH₂OH group. Biologists usually draw the ring flat, as below, with the H and OH groups pointing up or down from each carbon.

Alpha glucose structure

Look at carbon 1, on the right next to the ring oxygen. In alpha glucose its OH group points down and its H points up. The OH on carbon 1 is on the opposite side of the ring to the CH₂OH group on carbon 5.

Beta glucose structure

In beta glucose the H and OH on carbon 1 swap: the OH points up and the H points down. Everything else is the same. A quick way to remember it: ABBA, alpha below, beta above.

The difference between alpha and beta glucose

Check your understanding.

The glycosidic bond

Two monosaccharides join by a condensation reaction: an OH group on one sugar reacts with an OH group on the other, a molecule of water is removed, and the two sugars are left joined through an oxygen atom. This C-O-C link is a glycosidic bond.

The bond is named after the carbons it joins. Carbon 1 of one glucose joined to carbon 4 of the next is a 1,4 glycosidic bond. If the carbon 1 was in the alpha form, it is an α-1,4 glycosidic bond; if beta, a β-1,4 glycosidic bond. A bond from carbon 1 to carbon 6 is a 1,6 glycosidic bond, and it makes a branch.

The reverse reaction is hydrolysis: adding a water molecule breaks the glycosidic bond and separates the sugars again. In the body, enzymes such as amylase and maltase catalyse these hydrolysis reactions during digestion.

Disaccharides

A disaccharide is two monosaccharides joined by one glycosidic bond. The three you need to know:

Why alpha and beta glucose matter: starch, glycogen and cellulose

Polysaccharides are made by condensing many glucose units. Which isomer is used changes the whole shape of the polymer.

• Starch (plants) is made of alpha glucose. Amylose is an unbranched chain joined by α-1,4 bonds that coils into a helix, making it compact. Amylopectin also has α-1,6 bonds, so it branches.
• Glycogen (animals and fungi) is also alpha glucose, with α-1,4 and α-1,6 bonds, but it is more branched than amylopectin. The many ends can be hydrolysed at once, so glucose is released quickly.
• Cellulose (plant cell walls) is made of beta glucose joined by β-1,4 bonds. To line up the OH groups on carbons 1 and 4, every other beta glucose is turned upside down. The result is long, straight chains that lie side by side and are held together by many hydrogen bonds, forming strong microfibrils.

Starch and glycogen are good stores because they are compact, insoluble so they don't affect water potential, and easy to hydrolyse back into glucose. Cellulose is good for cell walls because its straight, hydrogen-bonded chains are strong. Humans can digest starch but not cellulose, because our enzymes can break α-1,4 bonds but not β-1,4 bonds.

Worked examples

Example 1: the formula of maltose

Maltose is made from two glucose molecules, and one water molecule is removed:

Its relative molecular mass is 2 × 180 − 18 = 342.

Example 2: a short chain

Three glucose molecules join in a chain by condensation. How many water molecules are removed, and what is the formula of the product?

Three units need two glycosidic bonds, so 2 water molecules are removed. The formula is 3 × C₆H₁₂O₆ − 2 × H₂O = C₁₈H₃₂O₁₆, with a relative molecular mass of 540 − 36 = 504. In general, a chain of n glucose units has n − 1 glycosidic bonds.

Example 3: naming a bond

A bond joins carbon 1 of a beta glucose to carbon 4 of the next glucose. It is a β-1,4 glycosidic bond, the bond found in cellulose.

Common mistakes

• Moving the wrong OH. Only the OH on carbon 1 changes between alpha and beta. Draw the rest of the ring the same.
• Losing the water. Condensation removes a water molecule; hydrolysis adds one. Don't mix them up.
• Saying cellulose is a chain of alpha glucose. Cellulose is beta glucose; starch and glycogen are alpha glucose.
• Calling the glycosidic bond a peptide bond or ester bond. Peptide bonds join amino acids and ester bonds join fatty acids to glycerol.
• Forgetting the CH₂OH. Carbon 6 sits outside the ring, above carbon 5.
• Writing glucose + glucose for sucrose. Sucrose is glucose + fructose; maltose is glucose + glucose.

Practise with a short timed paper

Carbohydrates are part of the first topic of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day.

Read next

Biology · Biological moleculesStarch, glycogen and cellulose: structure and functionThe structure of starch, glycogen and cellulose: alpha and beta glucose, glycosidic bonds, branching, microfibrils, and how each fits its function.