Digestion

• Carbohydrates, proteins, lipids (or fats) are major nutrients that we need in large quantities.
• We get these by eating them. They are broken down first and then reassembled into our own carbohydrates, proteins and lipids. This is because:

  • most of the molecules in food are too large to pass through the absorbing surface of the gut wall
  • the carbohydrates, proteins and lipids are reassembled in the form required, rather than other animal or plant versions
• Nutrient: Carbohydrates MAjor Function: Source of energy, glucose is the main respiratory substrate Major sources - Starch: potatoes, rice and wheat products, bread, cereals and pasta. Sugars: fruit, smoothies, fizzy drinks, chocolate and sweets
• Nutrients: Proteins Function: Growth and Repair Major sources: Meat, eggs, cheese, beans, nuts and seeds
• Nutrients: Lipids (Fats) Function: Energy storage, insulation, cell membranes Major sources: Butter, oils, fatty foods like crisps, cakes and biscuits
• Carbohydrates

  Carbohydrates in our diet include sugars and starches.

  
• The glucose molecule is small enough to be absorbed directly through the walls of the digestive system.

  Starch is a polymer of glucose. It must be broken down into glucose molecules – it is too large to pass through the gut.
• Cellulose is also made up of glucose molecules. It makes up plant cell walls. It is therefore a fundamental part of our diet. It cannot be broken down by the digestive system, so is egested from the gut.

  Once absorbed by the body, glucose molecules are transported to cells and:

  • used for respiration
  • reassembled into the storage form of carbohydrates in animals - glycogen
• In plant metabolism, the glucose produced by photosynthesis is converted into starch for storage, and cellulose, for cell wall synthesis.
• 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.

  Once inside the body, the amino acids are reassembled into the proteins the individual requires – the process of protein synthesis.

  Excess amino acids are broken down in the liver.
• Lipids

  Lipids are esters of fatty acids and glycerol.

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

  In the body's cells, they are reassembled into the lipids the cell needs, for instance, for the cell membranes.
• The liver does not secrete any enzymes, but it plays an important role in digestion, and then processing food molecules that have been absorbed.
• The liver produces bile.

  Bile:

  • Emulsifieslipids, breaking them up physically into tiny droplets. Tiny droplets have a much larger surface area, over which lipases can work, than larger pieces, or drops of lipid.