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Diet and digestion

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Chemical digestion

The most important function of physical digestion is to increase the surface area of the food you eat. This makes it easier for enzymes to act on the large insoluble food molecules and break them down into small soluble ones.

 

The lock and key model

An enzyme is a protein molecule. Each type of enzyme catalyses a particular reaction or type of reaction. They can do this because the long chains of amino acids which make up the enzyme are folded into a particular shape known as the active site. The substrates of the reaction fit into the active site of the enzyme. This works like a lock and key. Once the substrate is in place in the active site, it binds to the enzyme and the reaction then takes place rapidly. The products of the reaction are released from the surface of the enzyme.

 

The lock and key model of how enzymes work.

 

The effect of temperature and pH on enzyme action

Enzymes are affected by anything which changes the shape of the active site. Temperature and pH can both affect the active site and so alter the activity of the enzyme.

The effect of temperature on enzyme action

The effect of pH on enzyme action

The chemical breakdown of food in the body depends on the action of enzymes. The digestive system has a number of adaptations which help in this chemical breakdown.

 

The production of enzymes

Enzymes in the digestive system speed up the breakdown of the food. Enzymes are made in many places along the digestive system.

  • Carbohydrases - enzymes which break down carbohydrates into simple sugars
  • Proteases - enzymes which break down proteins, ending up with amino acids
  • Lipases - enzymes which break down lipids into fatty acids and glycerol

Enzymes are named according to the type of food molecule that they break down. This is fairly logical but sometimes you may find older names like amylase (carbohydrase), pepsin and trypsin (protease).

 

Place produced Enzyme Substrate (what it digests)
  Mouth   Amylase (carbohydrase)   Starch
  Stomach   Pepsin (protease)   Protein
  Pancreas   Carbohydrase
  Protease
  Lipase
  Carbohydrates
  Proteins
  Lipids
  Ileum (small intestine)   Carbohydrase
  Protease
  Lipase
  Carbohydrates
  Proteins
  Lipids

 

The production of stomach acid

The stomach is a very harsh environment. Specialised cells in the stomach wall produce large amounts of hydrochloric acid and this means that the stomach is about pH 2. The low pH provides an environment that allows the protease enzyme pepsin to be active. This starts the digestion of proteins as the food is churned around in the stomach.

The acid in the stomach also helps to kill any bacteria that may be on the food. It is not foolproof though because some microorganisms that cause food poisoning are adapted to be able to survive in the stomach acids.

The acid can sometimes cause stomach ulcers if the protective layer on the inside of the stomach wall is damaged.

 

The production of alkaline bile

Bile is produced in the liver and stored in the gall bladder. As food leaves the stomach, bile is squirted along the bile duct and into the duodenum. It is extremely important as it helps to neutralise the acidic contents of the stomach as they enter the top of the small intestine. This prevents the acid from damaging the small intestine which is kept at a pH of about 7.4 (just slightly alkaline). This is the optimum pH for the carbohydrases, proteases and lipases made in the pancreas and in the small intestine itself to work.

 

Alkaline bile raises the pH of the acidic stomach contents when they come into the small intestine.