Structures of carbohydrates, proteins and lipids

Carbohydrates

Carbohydrates in our diet include sugars and starch.

The glucose molecule is small enough to be absorbed directly through the walls of the digestive system but starch is a polymer of glucose. Starch must be broken down first by carbohydrase enzymes called amylase into maltose and then into glucose molecules by another carbohydrase.

Proteins

Proteins are made up of amino acids.

Proteins are big molecules that are too large to pass through the gut wall. They must first be broken down into amino acids by protease enzymes. These amino acids can then be used to make proteins in the body.

Lipids

Lipids are made of fatty acids and glycerol.

In a lipid, there may be 1, 2 or 3 fatty acids combined with glycerol - fatty acid 1, 2 and 3 may be the same or different.

Lipid molecules are too large to pass through the gut wall and must be digested first by lipase enzymes.

Digestive enzymes are used to break down food in the gut into small, soluble molecules that can be absorbed through the gut wall.

Specified practical 1.3B

Qualitative identification of starch (iodine), glucose (Benedict's reagent) and protein (biuret)

Use qualitative reagents to test for a range of carbohydrates, lipids and proteins

Qualitative tests for foods

There are several qualitative tests for food chemicals. These can be used to detect the presence of food chemicals but not how much is present.

Test for sugars

Method

Results

It may go through stages - green, yellow, orange, red or brown - depending on how much glucose is present.

Sugars classed as reducing sugars will react with Benedict's solution on heating for a few minutes. Glucose is an example of a reducing sugar.

Reducing sugars give a red/brown precipitate with Benedict's solution. The precipitate takes a while to settle in the tube. The colour you’ll see is likely to be simply red or brown. If there's not much glucose present, the final colour may be green or yellow, or orange if there's a little more.

Hazards

• Wear safety goggles.
• Benedict's solution is an irritant.
• Avoid contact with skin and eyes.

Test for starch

Add iodine solution to the food being tested.

Foods containing starch will turn a blue-black colour.

The iodine test can also be used with a microscope to stain starch grains in plant cells.

Hazards

• Wear safety goggles.
• Iodine solution is an irritant.
• Avoid contact with skin and eyes.

Test for proteins

The biuret test is used to detect proteins.

Biuret reagent is available as a single solution.

Method

1. Add 1 cm³ of biuret solution A to the food solution.

Results

Hazards

• Wear safety goggles.
• Biuret solution is corrosive.
• Avoid…