Carbohydrates, lipids and proteins

Food contains three main kinds of large molecule: carbohydrates, proteins and lipids. Each is built from smaller units, and enzymes in the gut break them back down so that they can be absorbed. Simple chemical tests show which of them a food contains.

Key facts

  • Carbohydrates contain carbon, hydrogen and oxygen. Simple sugars such as glucose join to make large carbohydrates: starch (stored in plants), glycogen (stored in animals) and cellulose (plant cell walls).
  • Proteins are chains of amino acids folded into particular shapes. They contain nitrogen as well as carbon, hydrogen and oxygen. Enzymes, antibodies and many hormones are proteins.
  • Lipids (fats and oils) are made of glycerol and fatty acids: a triglyceride has one glycerol and three fatty acids. Lipids store energy and insulate the body.
  • Food tests: iodine turns from orange-brown to blue-black with starch; Benedict's solution, heated in a water bath, turns from blue to green, yellow or brick-red with reducing sugars; Biuret reagent turns from blue to purple with protein; Sudan III stains lipids red.
  • Carbohydrases such as amylase break carbohydrates into simple sugars; proteases break proteins into amino acids; lipases break lipids into glycerol and fatty acids.
  • Joining two units releases a molecule of water (a condensation reaction); splitting the bond uses one (hydrolysis). A chain of n units has n − 1 bonds: glycosidic bonds in carbohydrates, peptide bonds in proteins and ester bonds in lipids.

Worked examples

Example 1

A food sample is tested. Iodine turns blue-black, Benedict's solution stays blue, Biuret turns purple and Sudan III shows no red layer. What does the food contain?

  1. Iodine blue-black: starch is present.
  2. Benedict's stays blue: no reducing sugar.
  3. Biuret purple: protein is present.
  4. No red layer with Sudan III: no lipid.

Answer: starch and protein, but no reducing sugar and no lipid

Example 2

Describe how to test a sample of milk for protein.

  1. Put about 2 cm³ of milk in a test tube.
  2. Add the same volume of Biuret reagent and shake gently.
  3. Blue to purple (lilac) means protein is present; staying blue means none.

Answer: add Biuret reagent; if it turns from blue to purple, protein is present

Example 3

Thirty amino acids join to make a polypeptide. How many peptide bonds form, and how many water molecules are released?

  1. A chain of n units has n − 1 bonds between them. 30 − 1 = 29
  2. Each bond forms in a condensation reaction that releases one water molecule.
  3. So 29 water molecules are released.

Answer: 29 bonds and 29 water molecules

Example 4

Why does food need to be digested before it can be used by the body?

  1. Starch, proteins and lipids are large molecules that cannot pass through the gut wall.
  2. Enzymes break them into sugars, amino acids, glycerol and fatty acids.
  3. These small, soluble molecules are absorbed into the blood and used to build new molecules or release energy.

Answer: large insoluble molecules must be broken into small soluble ones that can be absorbed into the blood