Heterotrophic nutrition - nutrients (cont'd)
Monacia Williams, Contributor

Last week we began our study of the macronutrients, starting with carbohydrates. The lesson ended with a promise that we would look a little closer at one of the properties of all monosaccharides, the fact that all are reducing sugars. Please note also that some disaccharides are also reducing sugars, except sucrose, which is a non-reducing sugar.

A reducing sugar is a compound which, when heated with Benedict's solution, changes the clear-blue solution of Benedict's to a brick-red precipitate. It does this by reducing the Cu2+ of copper sulphate to Cu+ of copper oxide. Copper oxide is insoluble and forms a red precipitate.

Why is it important?

Why is it important for you to know this? It is important because this is the reducing sugar test. Many examination questions will require you to know this fact. Benedict's, which contains the copper sulphate, is the test reagent for reducing sugars.

Remember I also said that sucrose, although a disaccharide, is a non-reducing sugar? A non-reducing sugar is one which will not change the clear-blue solution of Benedict's reagent to a brick- red precipitate. How then will we be able to identify sucrose? We do so by carrying out what is known as the non-reducing sugar test. What does this involve? Let us see if we can work this one out using what we already know.

Do you remember how the disaccharide was formed? Did you answer "by the joining together of two monosaccharide units with the elimination of one molecule of water"? If you did, then you are correct! What do you think would happen if we were to replace the water that was removed? We would regain our monosaccharide units! So, we have now converted our disaccharide to monosaccharide and we can confirm this by testing for the presence of monosaccharide, can't we? Yes, you agree, but how can the water be put back? We do this by adding dilute hydrochloric acid to the solution of the disaccharide. The process by which the water is replaced is called hydrolyzing and, at the end of the process, the disaccharide is hydrolyzed, that is, it is split into its constituent units. The addition of the hydrochloric acid makes the solution acidic and Benedict's will not give a positive result in acid conditions, therefore, sodium hydroxide is added to the mixture to neutralise the acid.

The identification of starch is carried out using the starch test. The reagent for this test is iodine in potassium iodide (I2/KI), simply called iodine. This reacts with starch to give a blue-black colour.

Proteins

Proteins are also large molecules which contain the elements carbon, hydrogen, oxygen and nitrogen. Some proteins also contain sulphur. They are made up of small molecules called amino acids. When two amino acids join together, one molecule of water is lost. This means that, like disaccharides and polysaccharides, protein molecules are formed by dehydration synthesis or condensation reaction. There are about 20 naturally occurring amino acids and these can be arranged in any order to form many different types of proteins.

Characteristics and functions of proteins

• Some proteins are soluble, for example, haemoglobin, which is the red pigment in blood.
• Some are insoluble, for example, keratin, which is used to form hair and nails.
• Proteins are used in growth and repair to form new cells and to repair damaged tissues.
• Some proteins form antibodies which are used in the body's defence system.
• Some form enzymes which catalyse the reactions, without which the body cannot function.
• Some form hormones which are used in the coordination of the activities of the body.
• Proteins can be identified with the Biuret test. The test reagents are:
• five per cent potassium hydroxide (KOH)
• one per cent copper sulphate (CuSO4).

The reaction is positive for the presence of protein if a mauve colour develops.

Next week, we will look at the last macronutrient - the lipids - as well as the micronutrients.

Members of the St Andrew Technical High School team that defeated McGrath High in TVJ's Schools' Challenge Quiz, last week. From left, Richard Beckford, Corey Jospeh, Shaniece Wauchope (captain) and Nicholas Brown. - Contributed

Monacia Williams teaches at Glenmuir High School.