Nutrition & Digestion

Almost everything in your food (and in your own body) is built from three families of large biological molecules:

All three are organic molecules because they are based on carbon. Proteins are the only one of the three that always contains nitrogen, which is why plants need nitrate ions from the soil to build them.

Carbohydrates

• Glucose (C₆H₁₂O₆) is a single simple sugar molecule.
• Maltose is made when two glucose molecules join together.
• Starch, glycogen and cellulose are all very long chains made from many glucose molecules joined together.
  • Starch is how plants store glucose (in roots, tubers, seeds)
  • Glycogen is how animals store glucose (in the liver and in muscles)
  • Cellulose is what plant cell walls are made of
• These long chains are insoluble in water, which is why they make good storage molecules: they stay where the cell puts them and do not affect the cell's water balance.

Proteins

• Proteins are long chains of amino acids joined together.
• There are about 20 different amino acids, and they can be arranged in any order. A different sequence makes a different protein, which is why there are hundreds of thousands of different proteins doing different jobs in the body.
• The amino acid sequence determines the shape of the protein. The shape, in turn, determines what the protein does. Examples:
  • Enzymes (catalysts that speed up chemical reactions)
  • Haemoglobin (the oxygen-carrying protein in red blood cells)
  • Keratin (the protein in hair, nails and skin)
  • Antibodies (used by the immune system)

Lipids

• Lipids are built from glycerol and fatty acids. One glycerol plus three fatty acids gives a single triglyceride, the most common lipid in food.
• Solid lipids at room temperature are called fats (butter, lard).
• Liquid lipids at room temperature are called oils (olive oil, sunflower oil).