Carbohydrates

The chemical structure of carbohydrates

Carbohydrates are organic compounds, like most biological molecules. This means they contain carbon and hydrogen. In addition, carbohydrates also have a third element: oxygen.

The molecular structure of carbohydrates

Carbohydrates are composed of molecules of simple sugars - saccharides. Therefore, a single monomer of carbohydrates is called a monosaccharide . Mono- means 'one', and -sacchar means 'sugar'.

Monosaccharides can be represented with their linear or ring structures.

Types of carbohydrates

There are simple and complex carbohydrates.

Simple carbohydrates are monosaccharides and disaccharides . Simple carbohydrates are small molecules composed of only one or two molecules of sugars.

• Monosaccharides are composed of one molecule of sugar.

  • They are soluble in water.

  • Monosaccharides are building blocks (monomers) of larger molecules of carbohydrates called polysaccharides (polymers).

  • Examples of monosaccharides: glucose , galactose , fructose , deoxyribose and ribose .

• Disaccharides are composed of two molecules of sugar (distance for 'two').
  • Disaccharides are soluble in water.
  • Examples of the most common disaccharides are sucrose , lactose , and maltose .
  • Sucrose is composed of one molecule of glucose and one of fructose. In nature, it is found in plants, where it is refined and used as table sugar.
  • Lactose is composed of one molecule of glucose and one of galactose. It is a sugar found in milk.
  • Maltose is composed of two molecules of glucose. It is a sugar found in beer.

Complex carbohydrates are polysaccharides . Complex carbohydrates are molecules composed of a chain of sugar molecules that is longer than simple carbohydrates.

• Polysaccharides (poly- means 'many') are large molecules composed of many molecules of glucose, ie individual monosaccharides.
  • Polysaccharides are not sugars, even though they are composed of glucose units.
  • They are insoluble in water.
  • Three very important polysaccharides are starch , glycogen and cellulose .

The main function of carbohydrates

The main function of carbohydrates is to provide and store energy.

Carbohydrates provide energy for important cellular processes, including respiration. They are stored as starch in plants and glycogen in animals and are broken down to produce ATP (adenosine triphosphate), which transfers energy.

There are several other important functions of carbohydrates:

• Structural components of cells: cellulose, a polymer of glucose, is essential in the structure of cell walls.

• Building macromolecules: Carbohydrates are vital parts of biological macromolecules, nucleic acids such as DNA and RNA. Nucleic acids have simple carbohydrates deoxyribose and ribose, respectively, as part of their bases.

• Cell recognition: Carbohydrates attach to proteins and lipids, forming glycoproteins and glycolipids. Their role is to facilitate cellular recognition, which is crucial when cells join to form tissues and organs.

How do you test for the presence of carbohydrates?

You can use two tests to test the presence of different carbohydrates: Benedict's test and the iodine test.

Benedict's test

Benedict's test is used to test for simple carbohydrates: reducing and non-reducing sugars. It is called Benedict's test because Benedict's reagent (or solution) is used.

Test for reducing sugars

All monosaccharides are reducing sugars, and so are some disaccharides, for instance, maltose and lactose. Reducing sugars are so-called because they can transfer electrons to other compounds. This process is called reduction. In the case of this test, that compound is Benedict's reagent, which changes colour as a result.

To perform the test, you need:

• test sample: liquid or solid. If the sample is solid, you should dissolve it in water first.

• test tube. It should be completely clean and dry.

• Benedict's reagent. It is blue in colour.

Steps:

1. Place 2cm3 (2 ml) of test sample into a test tube.

2. Add the same amount of Benedict's reagent.

3. Add the test tube with the solution to a water bath and heat for five minutes.

4. Observe the change, and record the change in colour.

You might come across explanations claiming that reducing sugars are present only when the solution turns red / brick-red. However, this is not the case. Reducing sugars are present when the solution is either green, yellow, orange-brown or brick red. Take a look at the table below:

Fig. 1 - Benedict's test for reducing sugars

Test for non-reducing sugars

The most common example of non-reducing sugars is the disaccharide sucrose. Sucrose does not react with Benedict's reagent as reducing sugars do, so the solution would not change colour and would remain blue.

In order to test for its presence, the non-reducing sugar needs to be hydrolysed first. After it is broken down, its monosaccharides, which are reducing sugars, react with Benedict's reagent. We use dilute hydrochloric acid to perform hydrolysis.

For this test you need:

• test sample: liquid or solid. If the sample is solid, you should dissolve it in water first.

• test tubes. All test tubes should be completely clean and dry before use.

• dilute hydrochloric acid

• sodium hydrogen carbonate

• pH tester

• Benedict's reagent

The test is carried out as follows:

1. Add 2cm3 (2ml) of sample into a test tube.

2. Add the same amount of dilute hydrochloric acid.

3. Heat the solution in a gently boiling water bath for five minutes.

4. Add sodium hydrogen carbonate to neutralize the solution. Since Benedict's reagent is alkaline, it won't work in acidic solutions.

5. Check the pH of the solution with a pH tester.

6. Now carry out Benedict's test for reducing sugars:

   • Add Benedict's reagent to the solution that you just neutralized.

   • Place the test tube into a lightly boiling water bath again and heat for five minutes.

   • Observe the colour change. If there is any, it means reducing sugars are present. Refer to the table with results and meanings above. Therefore, you can conclude that a non-reducing sugar is present in the sample, as it was successfully broken down into reducing sugars.

Iodine test

The iodine test is used to test for starch , a complex carbohydrate (polysaccharide). A solution called potassium iodide solution is used. It is yellow in color.

The test is carried out as follows:

1. Add 2 cm3 (2ml) of the test sample into a test tube.

2. Add a few drops of the potassium iodide solution and shake or stir.

3. Observe the change in color. If the solution turns blue-black, starch is present. If there is no change and the solution remains yellow, it means there is no starch present.

This test can be performed on solid test samples too, for instance adding a few drops of potassium iodide solution to a peeled potato or grains of rice. They would change the color to blue-black as they are starchy foods.