Chapter 3 Digestion

A summary of the 3rd chapter (Digestion) from the AQA Human Health and Physiology textbook

  • Created by: R_Hall
  • Created on: 12-06-12 09:40

3.1 Physical and chemical breakdown of food

  • We physically digest food by breaking large pieces into smaller pieces, which increases the surface area. Most physical breakdown occurs in mouth, by the teeth (fat is broken down by bile)
  • Starch, protein and fat (large insoluble molecules) are broken down by enzymes into small and soluble molecules, able to pass into bloodstream
  • Carbohydrate+ carbohydrase= glucose
  • Protein+ protease= amino acids
  • Lipids (fats)+ lipase= fatty acids
  • We have 4 types of teeth- incisors, canines, pre-molars and molars
  • Enamel is the hard outer layer, but is dissolved in acid. If it is eroded, the sensitive dentine may be damaged and expose the pulp, which results in pain
  • Saliva is  an alkaline liquid which help neutralise acids that are eaten or that bacteria make when they feed on plaque. It also contains amylase to digest starch
  • You bite with incisors, then add saliva and chew with molars to crush it. Your tongue mixes the food and saliva to break it down, and then it is swallowed.
  • When you swallow, the tongue pushes the food back, and the mucus in saliva helps the food slip down the oesophagus
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3.2 The human digestive system

  • The salivary gland, pancreas and gut lining contain specialised cells with produce enzymes
  • The digestive juices (with enzymes) pass into the gut and are mixed with food, the gut wall contracts and relaxes (peristalsis) to help the process of mixing
  • Each digestive enzymes best works in a small range of pH. A different pH could denature the enzymes
  • The stomach holds the food for a few hours (so we can eat regularly). The acid activates a protease enzyme and kills microbes. The stomach cells are protected by a layer of mucus
  • Food moves from the stomach to the small intestine. Bile (made in liver) is released via the bile duct and contains alkaline salts to neutralise the acid so pancreatic enzymes can work
  • Bile also breaks fats into small droplets- emulsification
  • In the small intestine, carbs, protein and fats are broken down. It is designed to absorb soluble food; the inner surface is covered with villi (which increases surface area) which allow small molecule to diffuse into capillaries underneath
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3.2 The human digestive system cont.

  • Following absorption, the food is assimilated. Surplus glucose is taken to liver and muscles and turned into fat or stored as glycogen. Fatty acids and glycerol are used to build cell membranes or hormones. Lipids are stored in fat cells
  • Amino acids are made into proteins for cell building, or for enzymes and haemoglobin. Excess aa cannot be stored, they are broken down by liver (deamination)
  • In the large intestine, water in reabsorbed and the remaining faeces are egested
  • Cystic Fibrosis is an inherited disorder that cause the production of thick mucus. This blocks the pancreatic ducts so digestive enzymes cannot be released. The mucus also covers villi, to prevent absorption of nutrients
  • Coeliac disease is a condition triggered by gluten (protein in cereals). The gluten causes the immune system to attack the villi, and reduce the surface area
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