Structure of Carbohydrates

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Types of Monosaccharides

Carbohydrates are made of carbon, hydrogen and oxygen atoms. They are made from monosaccharides, which are simple sugars containing three to seven carbon atoms.

Examples of monosaccharides

Monosaccharides are simple sugars. Examples include:
- Glucose.
- Galactose (found in milk).
- Fructose (found in fruit).

Glucose

Glucose is a hexose sugar that has the chemical formula C₆H₁₂O₆.
Glucose is an important source of energy in humans.
During cellular respiration, the energy released from glucose helps to make adenosine triphosphate (ATP).

Alpha vs beta glucose

Alpha- and beta-glucose are isomers. Isomers have the same molecular formula but a different arrangement of atoms in space.
The carbon atoms are numbered from 1 – 6 and the OH (hydroxyl) groups are in a different orientation around C₁.

Disaccharides and Polysaccharides

When two monosaccharides join via a condensation reaction, they form a disaccharide. When more than two disaccharides join together, they form a polysaccharide chain.

Examples of disaccharides

Glucose + glucose → maltose.
Glucose + fructose → sucrose.
Glucose + galactose → lactose.

Functions of disaccharides

Sucrose is common table sugar.
Lactose is the sugar found in milk.
- Lactose intolerance is a common problem where the body is unable to digest lactose.

Polysaccharides

Polysaccharides are made up of three or more monosaccharides joined together by glycosidic bonds.
The chain may be branched or unbranched.
The chain may contain different types of monosaccharides.
Starch, glycogen, cellulose and chitin are examples of polysaccharides.

Benedict's Test for Sugars

Benedict’s solution (also known as Benedict's reagent or the Benedict’s test) can be used as a test for reducing and non-reducing sugars.

Reducing sugars

All monosaccharides are reducing sugars.
- E.g. Glucose, galactose and fructose.
Some disaccharides are reducing sugars.
- E.g. Lactose and maltose.

Test for reducing sugars

Benedict's solution can be reduced by reducing sugars.
Benedict's solution is a clear blue liquid that changes colour and gives a precipitate depending on how much it is reduced.
- Step 1: Place 2 ml of the substance in a boiling tube (substance must be in liquid form).
- Step 2: Add 10 drops of Benedict's solution.
- Step 3: Place in a boiling water bath for 3-5 minutes.

Results of the Benedict's test

Blue solution → no reducing sugar.
Green/yellow precipitate → traces of reducing sugar.
Orange/red precipitate → moderate amounts of reducing sugar.
Brick red precipitate → large amount of reducing sugar.

Non-reducing sugars

Non-reducing sugars will show a negative result to the Benedict’s test. A second test is needed to determine if non-reducing sugar is present.
Sucrose is a non-reducing sugar. It is a disaccharide made up of glucose and fructose joined by a glycosidic bond.

Test for non-reducing sugars

Step 1: Boil in dilute HCl (to hydrolyse the non-reducing sugar).
Step 2: Neutralise the solution by adding sodium hydrogen carbonate.
Step 3: Repeat the Benedict’s test.
- The result will now be positive if a non-reducing sugar is present.
- If the solution remains blue, then no sugar is present.