Digestive

• Digestion

  The process of breaking down of carbohydrate, protein and fat molecules into products small enough to be absorbed into the blood and into the cells
• Mechanical digestion

  • The physical breakdown of food particles into smaller pieces to increase the surface area
  • Teeth cutting, tearing and grinding food
  • Churning action in the stomach
  • Gall bladder releasing bile salts into small intestine to break down fat into smaller droplets
• Chemical digestion

  • Uses enzymes to break down large, complex molecules into smaller, simpler molecules that can be absorbed into bloodstream
  • Carbohydrates split into monosaccharides (glucose)
  • Proteins split into peptides and amino acids
  • Lipids split into fatty acids and glycerol
  • Nucleic acids split into nucleotides
• Alimentary canal


  The continuous tube that runs from the mouth to the anus, the lining of the alimentary canal is the surface through which nutrients are absorbed
• Mouth


  • Intake of food (ingestion)
  • Food is chewed (mastication)
  • As the food is chewed it is mixed with saliva, which contains mucus to lubricate the food and a digestive enzyme –> salivary amylase
  • Mechanical digestion begins with action of jaws and teeth
  • Tongue shapes food into a bolus and pushes it into the pharynx
• Oesophagus


  • Tube connecting pharynx to stomach
  • Has circular and longitudinal muscle layers
  • Peristalsis moves food through the oesophagus
• Peristalsis

  1. Circular muscle behind the ball of food contracts
  2. Circular muscle around and in front of food relaxes
  3. Assisted by the secretion of mucus that lubricates the inner lining
• Stomach

  • Oesophagus opens into the stomach, enlarged section of alimentary canal
  • Food undergoes mechanical and chemical digestion
  • Nutrients not absorbed due to thick mucus layer
  • Stomach is acidic pH around 2
• Small intestine

  • Longest part of alimentary canal, around 6-7m
  • Duodenum = extends from bottom of stomach in a curve around the pancreas, most chemical digestion occur here
  • Jejunum = middle section, lining allows effective absorption of carbohydrates and proteins
  • Ileum = absorption of B12, bile salts, remaining digestion products are absorbed
  • Digestion continues under influence of pancreatic juice, bile, intestinal juice
  • Mechanical digestion continues through segmentation
• Absorption of nutrients

  • Products of digestion, vitamins, minerals, water absorbed through small intestine wall into blood
  • Lining of small intestine has folds, villi and microvilli to maximise absorption of nurtrients
• Villi

  • Tiny projections that line small intestine
  • Single layer of cells cover
  • Inside villi is a lymph capillary called a lacteal, which is surrounded by a network of blood capillaries
  • Absorption via simple diffusion (higher concentration of nutrients inside small intestine than outside villos cells)
  • Active transport to bring in nutrients agaisnt concentration gradient (low->high)
• Large intestine

  • About 1.5m long, includes caecum, colon, rectum, anus
  • No villi, no digestive juices secreted, but mucus secreted
  • Slow movement of material, 18-24 hours during this time remaining water is absorbed to become more solid
  • Bacteria breaks down remaining organic compounds and produces some vitamins which are then absorbed. Mineral nutrients are also absorbed
  • Remaining material forms faeces
• Constipation

  Reduced movement of large intestine, contents remain too long, faeces become dry and hard, defecation difficult and painful, often caused by lack of dietary fibre
• Diarrhoea

  Irritation of small or large intestine increases peristalsis, contents move through before adequate water absorption, can be caused by bacteria, virus, parasite, cancer, coeliac disease, lactose intolerance
• Bowel cancer

  Uncontrolled growth of cells in large intestine wall, may be linked to diet high in red meat and low in fibre, also linked to high alcohol consumption and smoking
• Coeliac disease

  Inability to tolerate gluten, immune system damages/destroys villi in small intestine, preventing nutrient absorption
• Chyme

  When food chunks are converted to a thick, soupy liquid= chyme
• Pancreatic juices

  • Contains enzymes
  • Pancreatic amylase -> breaks down starch
  • Pancreatic protease -> breaks down proteins into small chains of amino acids
  • Ribonuclease + deoxyribonuclease -> digests RNA and DNA
  • Pancreatic lipases ->breaks down fat into fatty acids + glycerol
• Intestinal Juices

  Digestion of carbohydrates, lipids and proteins

  • Lipase= will break down lipids into fatty acids + glycerol
  • Peptidase= will break down proteins into amino acids
  • Sucrase, lactase, maltase= will break down sucrose, lactose, maltose into glucose + fructose + galactose
• Bile


  Produced by liver and stored in gall bladder, it does not have digestive enzymes but has bile salts which break down fat into tiny dropelts which increase surface area for lipases to act on. Bile salts are a form of mechanical digestion. Bile is relased through the common bile duct
• Absorption of amino acids in the villi

  Amino acids are absorbed by active transport into the blood capillaries
• Absorption of fatty acids in the villi

  Fatty acids and glycerol are absorbed by simple diffusion into the cells of the villi. Fatty acids and glycerol then recombine to form fats, these tiny fat droplets enter lacteals.
• Absorption of water in the villi

  Water and water-soluble vitamins are absorbed into the blood capillaries by diffusion.
• Absorption of simple sugars in the villi

  Simple sugars such as glucose are absorbed by active transport. They passed through the cells on the outside of the villi and into the blood capillaries.