6 Digestion

• Mouth
  • Physically breaks up food along with enzymes
• Oesophagus
  • carries food from the mouth to the stomach
• Stomach
  • muscular sack
  • inner layer produces enzymes
  • stores food
  • breaks down proteins with enzymes
• Ileum
  • long muscular tube
  • food is digested by enzymes
  • inner wall is folded into villi (SA)
  • villi have their own microvilli (SA)
• Large intestine
  • absorbs water
• Rectum
  • faeces are stored before egestion

Chemical

• All digestive enzymes carry out hydrolysis reactions to break down food
• They do this by adding water to the food molecule splitting it
• Carbohydrases
• Lipases
• Proteases
• Usually, some enzymes turn large molecules into smaller chunks and then other enzymes create the monomers from the smaller chains

Physical

• Teeth and mastication (chewing)
• Increases surface area
• Walls of the stomach churn food up

• Amylase is produced by the salivary glands in the mouth and in the pancreas
• Amylase creates maltose from starch
• Maltase then creates glucose from maltose
• Sucrose is found in many natural foods
• Lactose is found in milk and dairy products
• Sucrase produces glucose and fructose
• Lactase produces glucose and galactose

1) Saliva enters the mouth and is mixed with the food during chewing

2) Salivary amylase starts hydrolysing starch into maltose. It also contains mineral salts which maintain a neutral pH

3) Food is swallowed and the amylase is denatured by the acid preventing the breakdown of starch

4) Food enters the ileum where it mixes with pancreatic juice

5) Pancreatic juice contains pancreatic amylase continuing the hydrolysis of starch into maltose

6) Alkaline salts are produced from the pancreas and the wall of the ileum which takes the pH back to neutral to prevent the denaturing of the enzymes

7) Muscles in the intestine push food along the ileum

8) Maltase is produced in the epithelial lining - it is a membrane-bound disaccharidase

• Peptidases (proteases) break down proteins
• Endopeptidases
  • hydrolyse the peptide bond between amino acids in the middle of a long polypeptide chain creating smaller chains of polypeptides
• Exopeptidases
  • hydrolyse the peptide bonds between amino acids at the terminal end of a polypeptide creating dipeptides and single amino acids
• Dipeptidases
  • hydrolyse the peptide bond between  two amino acids (dipeptides)
  • membrane-bound in the epithelial cells in the ileum

• Hydrolysed by lipases
• Produced in the pancreas
• Hydrolyse the ester bonds in triglycerides
• Produces monoglycerides and fatty acids
• Lipids are split into smaller droplets called micelles by bile salts
• Bile salts are produced by the liver
• This is called emulsification
• This increases the surface area of lipids

• Folded into villi that are 1mm long
• Each villus has its own microvilli to increase the surface area
• Rich blood supply
• Increased surface area for diffusion
• Thin-walled reducing the diffusion distance
• Contain muscle for movement - this maintains the diffusion gradient
• As products of digestion are produced, new nutrient-rich material replaces it
• Rich blood supply maintains the concentration gradient as more molecules can be carried away

• Amino acids and monosaccharides
  • diffusion and co-transport (topic 4.5)
• Triglycerides
  • monoglycerides and fatty acids stay associated with the bile salts
  • these are called micelles and are only 4-7nm in diameter
  • micelles come into contact with epithelial cells on the villi
  • here, micelles break down releasing the monoglycerides and fatty acids which easily diffuse across the membrane as they are polar
  • once inside the cells, they are transported to the endoplasmic reticulum where they recombine and form triglycerides
  • they then go to the Golgi apparatus where the triglycerides associate with cholesterol and lipoproteins to form chylomicrons
  • chylomicrons are special particles adapted for the transport of lipids
  • they leave the cells by exocytosis into lymphatic capillaries called lacteals found at the centre of each villus
  • then there go from the lymphatic system into the bloodstream
  • chylomicrons are hydrolysed by enzymes in endothelial cells in blood capillaries  where they can diffuse into cells