Human

• A balanced diet includes appropriate proportions of carbohydrates, protein, lipids, vitamins, minerals, fibres and water
• Carbohydrates:

  • Can be in the form of simple sugars (glucose, lactose, fructose, sucrose) or in the form of polysaccharides (starch, glycogen)
• Too much carbohydrates can lead to tooth decay or obesity
• Lipids: Makes cell membranes, insulation, organ protection and energy stores
• Protein: helps with growth and repair of tissues
• Minerals:

  • Calcium: makes teeth and bones
  • Iron: makes haemoglobin in red blood cells
• Vitamin A: maintains retina in the eye

  Vitamin C: makes connective tissues

  Vitamin D: helps bones absorb calcium
• Too little vitamin C can cause scurvy
• Too little vitamin D can cause rickets
• Fibre:

  • Non-digestible
  • Helps push food through our small intestines
  • Mostly made from cellulose
• Too little fibre causes constipation

  Too much fibre causes diarrhoea
• Water:

  • Helps us stay hydrated
  • Maintains osmosis correctly in cells
  • Provides solvent for enzymes and food molecules
  • Important for blood plasma
• Energy requirements depend on activity levels, age and pregnancy
• 6 stages of the digestive system:

  1. Mouth
  2. Oesophagus
  3. Stomach
  4. Duodenum
  5. Ileum
  6. Large intestine
• Mouth:

  • Mechanical digestion: teeth breaks food down and increases the surface level for enzymes
  • Chemical digestion: amylase in saliva breaks down starch into maltose
• Oesophagus:

  • Chewed food is passed through from mouth to stomach
• Stomach:

  • Food held for hours
  • Mechanical digestion: wall churns food
  • Chemical digestion: hydrochloric acid kills bacteria (prevents food poisoning), pepsin breaks proteins into amino acids
  • Sphincter (ring of muscle) holds food until ready to be released into duodenum
• Duodenum:

  Pancreas: releases pancreatic juice which contains lots of enzymes like amylase, trypsin and lipase

  Liver: produces bile which is stored in the gall bladder, it emulsifies large lipids which increases the surface area of oils for lipase to work
• Bile and pancreatic juices are alkali so that they can neutralise stomach acid
• Bile is not an enzyme
• Ileum:

  • Food is ready to be absorbed
  • Large surface area due to the big number of villi and microvilli
• Villus:

  • Network of blood vessels and lacteal
  • Blood vessels form hepatic portal vein going to the liver
  • Lacteal absorb products of fat digestion into the lymphatic system
  • Food molecules are transported in the blood to the tissues where they get assimilated into cells
• Large intestine:

  Colon: absorbs water

  • left with undigestible fibres (faeces)

  Rectum: stores faeces

  Anus: expells faeces
• Peristalsis:

  • Occurs in the gut
  • The gut has 2 layers of muscle: circular and longitudinal
  • Pushes food by contraction
• Adaptation for absorption:

  Small intestines:

  • Very long: longer absorption time
  • Covered with villi: increases surface area

  Villi

  • One cell thick layer (epithelium): shortens diffusion distance for blood vessel/lacteal
  • A dense network of blood capillaries: transports absorbed food quickly, maintaining a steep diffusion gradient
• Amylase:

  • Found in salivary glands and the pancreas
  • Breaks down starch
  • Produces maltose
• Maltase:

  • Found in the small intestine wall
  • Breaks down maltose
  • Produces glucose
• Pepsin:

  • Found in the stomach

  Trypsin:

  • Found in the pancreas

  Both breaks down proteins and produces amino acids
• Lipase:

  • Found in the pancreas
  • Breaks down lipids
  • Produces glycerol and fatty acids