Monosaccharides and Glucose Isomers

A monosaccharide is a single sugar unit. Monosaccharides can act as monomers: units that join together to form larger carbohydrates.

Glucose, galactose and fructose are common monosaccharides. Each has the molecular formula $$\ce{C6H12O6}$$, but their atoms are arranged differently. Molecules with the same molecular formula but different arrangements are called isomers.

The two ring forms of glucose

Glucose can form a ring in two forms, α-glucose and β-glucose. These are isomers that differ in the positions of the hydrogen and hydroxyl ($$\ce{-OH}$$) groups attached to carbon 1.

In the conventional ring representation:

• In α-glucose, the hydroxyl group on carbon 1 points down and the hydrogen points up.
• In β-glucose, the hydroxyl group on carbon 1 points up and the hydrogen points down.

The two forms have the same molecular formula; the difference is the arrangement of these groups at carbon 1.

Disaccharide Formation

A disaccharide is a sugar made from two monosaccharides joined together. The monosaccharides join in a condensation reaction: a bond forms between them and a molecule of water is released. The bond joining the sugar units is called a glycosidic bond.

Different pairs of monosaccharides form different disaccharides:

• Maltose: glucose + glucose
• Sucrose: glucose + fructose
• Lactose: glucose + galactose

The linkage can be reversed by hydrolysis. In this reaction, water is used to break the glycosidic bond, separating the disaccharide into its two monosaccharides.

Starch

Starch is a storage polysaccharide in plants: it stores many glucose units in a form that can later be broken down to release glucose. It is made from many $$\alpha$$-glucose molecules joined by glycosidic bonds. Each bond forms in a condensation reaction, which releases water.

Structure of starch

Starch contains two polysaccharides, amylose and amylopectin:

• Amylose is an unbranched chain of $$\alpha$$-glucose units joined by $$\alpha$$-1,4 glycosidic bonds. The chain coils into a compact shape.
• Amylopectin is a branched chain. Its glucose units are joined by $$\alpha$$-1,4 glycosidic bonds along the chains, with $$\alpha$$-1,6 glycosidic bonds at the branch points.

The numbers in the bond names identify the carbon atoms joined in neighbouring glucose units.

Starch as a storage molecule

Starch is compact, so many glucose units can be stored in a small space. It is also insoluble: it does not substantially affect the cell’s water potential and does not diffuse out of the cell.

When glucose is needed, the glycosidic bonds in starch can be broken by hydrolysis, using water. The branches of amylopectin provide many chain ends where breakdown can occur, allowing glucose to be released more rapidly.

Glycogen

Glycogen is a storage polysaccharide in animal cells: it stores many glucose units in a form that can be broken down when glucose is needed. It is formed when many $$\alpha$$-glucose molecules join through condensation reactions, producing glycosidic bonds.

Within each chain, the glucose units are joined by $$\alpha$$-1,4 glycosidic bonds. At branch points, $$\alpha$$-1,6 glycosidic bonds join one chain to another. Glycogen is highly branched, even more so than amylopectin.

Glycogen: Structure and storage

Glycogen’s compact structure allows many glucose units to be stored in a small space. It is also insoluble, so it does not substantially affect the cell’s water potential or diffuse out of the cell.

The many branches provide many ends at which glycosidic bonds can be broken. This allows glycogen to be broken down rapidly, making glucose available when it is needed.

Cellulose

Cellulose is a structural polysaccharide: a large carbohydrate made from many $$\beta$$-glucose units joined together. It forms strong fibres that help support plant cell walls.

Cellulose chains and microfibrils

The glucose units join by condensation reactions, which release water as bonds form. Each bond connects carbon 1 of one glucose unit to carbon 4 of the next. This is called a $$\beta$$-1,4 glycosidic bond.

Alternate glucose units are rotated by $$180^\circ$$ relative to one another. This arrangement produces straight, unbranched cellulose chains. Neighbouring chains lie alongside one another and form many hydrogen bonds between them. Together, these bonds hold the chains in strong bundles called microfibrils.

Cellulose in plant cell walls

Cellulose microfibrils give plant cell walls tensile strength: resistance to being stretched. When water enters a plant cell, the cell contents press against the wall. The strong wall resists this pressure, helping to prevent the cell from bursting.

Benedict's Tests for Sugars

Benedict's solution is used to test for reducing sugars. It is blue before the test. When it is heated with a reducing sugar, its colour changes and a solid precipitate can form.

Testing for a reducing sugar

Add Benedict's solution to the sample, then heat the mixture in a hot water bath. A negative result remains blue. A positive result changes from blue through green, yellow or orange to a brick-red precipitate, depending on the amount of reducing sugar present. The further the colour change progresses towards brick-red, the greater the amount of reducing sugar.

Testing for a non-reducing sugar

A non-reducing sugar does not give a positive Benedict's test until it has been broken down. First test the original sample for reducing sugars. If the result is negative, use a separate portion of the sample for the non-reducing sugar test:

1. Heat the portion with dilute hydrochloric acid. This hydrolyses the sugar, breaking it down into reducing sugars.
2. Cool the mixture, then neutralise the acid, for example with sodium hydrogencarbonate. Benedict's test needs alkaline conditions.
3. Add Benedict's solution and heat the mixture in a hot water bath.

If the original sample remained blue but the treated portion gives a positive result, the original sample contained a non-reducing sugar. If the treated portion also remains blue, the test gives no evidence of a non-reducing sugar.

Iodine Test for Starch

The iodine test uses a colour change to show whether starch is present in a sample. The test solution contains iodine dissolved in potassium iodide solution.

Add iodine solution to the sample and observe the colour:

• Blue-black: starch is present; the result is positive.
• Yellow-brown: the iodine solution retains its original colour, indicating a negative result for starch.