Nutrition and Digestion

• Components of a balanced diet


  • Proteins
  • Carbohydrates
  • Lipids
  • Vitamins and minerals
  • Fibre
  • Water
• Proteins


  Broken down to make amino acids, which are then used to make enzymes and other proteins
• Carbohydrates


  Needed for energy release in all cells
• Lipids


  Deposited as fat just below the skin for insulation and as a store of energy when the diet does not contain enough
• Vitamins and minerals


  Needed in tiny amounts for the correct functioning of the body
• Fibre


  Adds bulk to food so that it can be moved along the digestive system by peristalsis and prevents constipation, and probably bowel cancer
• Water


  Necessary for all life processes. Water is continually being lost through excretion and must be continually being replaced
• A balanced diet contains all of these nutrients in the correct proportions to stay healthy because we need more of one kind of nutrient than others
• People need a different balance of nutrients at different times in their lives. Children need more protein because they are still growing rapidly. Pregnant women need more iron than usual to supply the growing baby with blood cells
• If we consume more energy than our bodies need the extra is deposited as stores of fat. This can lead to obesity, heart disease and diabetes
• Eating food involves


  1. Ingestion - Taking food into the body (by mouth in humans)

  2. Digestion - Breakdown of large food molecules in to smaller ones

  3. Absorption - Digested food particles enter the blood stream

  4. Assimilation - Converting food molecules into other molecules for use in the body

  5. Egestion - Removal of undigested material (faeces) from the body
• Parts of the digestive system


  • Mouth
  • Oesophagus (gullet)
  • Stomach
  • Liver
  • Gall bladder
  • Pancreas
  • Small intestine (duodenum + ileum)
  • Large intestine/colon
  • Rectum
  • Anus
• What happens in the mouth


  1. Teeth and tongue break food down into smaller pieces

  2. Saliva from salivary glands moistens food so it easily swallowed and contains amylase to begin digestion of starch
• What happens in the oesophagus


  Each lump of swallowed food called a bolus is moved from the mouth to the stomach by waves of muscle contraction called peristalsis
• What happens in the stomach


  1. Food enters through a ring of muscle known as a sphincter

  2. Acid and protease enzymes are secreted to begin protein digestion

  3. The acid also kills germs

  4. Movements of the muscular wall churn up food turning into the liquid chyme

  5. The partially digested food passes little by little into the duodenum through another sphincter
• What happens in the liver


  1. Liver cells make bile which emulsifies fats

  2. Excess glucose is converted into glycogen and stored in liver cells

  3. Amino acids not used are broken down to form urea in the liver and passed to the kidneys for excretion
• What happens in the gall bladder


  1. Stores bile, made by the liver

  2. Bile passes from the gall bladder along the bile duct into the duodenum to neutralise the acidic chyme
• What happens in the pancreas


  Secretes amylase, lipase and protease enzymes and sodium hydrogen carbonate into the small intestine
• What happens in the small intestine

  1. The secretions from the gall bladder and pancreas along with carbohydrase, protease and lipase enzymes secreted by the small intestine wall complete digestion

  2. Digested food is absorbed into the blood through the villi
• What happens in the large intestine/colon


  Water and some nutrients are absorbed from the remaining material
• What happens in the rectum


  The faeces consisting of indigestible food, dead cells from the alimentary canal lining and bacteria is compacted and stored
• What happens at the anus


  Faeces are egested through a sphincter
• Peristalsis


  The muscles in the wall of the alimentary canal contract pushing food further along. Fibre in food is important for keeping food bolus soft and bulky making peristalsis easier
• Types of digestion


  • Physical digestion - occurs mainly in the mouth where food is broken down into smaller pieces by the teeth and tongue
  • Chemical digestion - the breakdown of large food molecules into small ones
• Some molecules are already small enough to pass through the gut wall into the blood stream without being digested (water, glucose, vitamins and minerals)
• Chemical digestion of starch

  1. Amylase breaks down starch into maltose

  2. Maltase breaks down maltose into glucose
• Chemical digestion of proteins

  Protease breaks down proteins into amino acids
• Chemical digestion of lipids


  Lipases break down lipids into glycerol and fatty acids
• Enzymes involved in digestion

  • Amylase - breaks down starch to maltose
  • Maltase - breaks down maltose to glucose
  • Pepsin - breaks down proteins to peptides
  • Trypsin - breaks down proteins to peptides
  • Peptidases - break down peptides to amino acids
  • Lipase - breaks down lipids to glycerol and fatty acids
• Hydrochloric acid


  Secreted by cells in the stomach lining, necessary for providing the optimum pH for pepsin enzyme action and kills microorganisms
• Bile


  Produced by liver cells, stored in the gall bladder until required and then passes along the bile duct into the duodenum. Bile is highly alkaline and neutralises the food from the stomach and makes the chyme slightly alkaline. Bile also emulsifies fats by breaking them up into smaller droplets increasing the surface area for digestion to happen faster
• Villi


  • Finger-like projections that cover the surface of the ileum to increase the surface area for absorption
  • Villi are covered in microvilli which further increase the surface area
  • Villi have a thin wall 1 cell thick so food particles do not have far to travel to be absorbed
  • Villi have a rich blood supply to carry away absorbed molecules quickly
  • Villi contain lacteals that carry fat droplets separate from the rest of the food particles
• Absorption


  When the food has been digested and is small enough, it crosses the cell membranes of the intestine and blood vessels into the blood where it is taken around the body to wherever it is needed
• Assimilation


  1. Most nutrients absorbed from the intestines travel in the blood to the liver

  2. Vitamins A and D are stored in the liver

  3. Insulin from the pancreas stimulates the liver to store excess glucose as glycogen, and convert any further excess to fat

  4. Amino acids are used by active cells to build proteins, and any excess is broken down in the liver and converted to glucose or other compounds

  5. The 'amino' part is converted to urea which is excreted in the urine