Digestion 5

• Define digestion


  The process by which large molecules in the food we eat are hydrolysed into smaller more soluble molecules that can be absorbed into the body
• Define Physical breakdown


  where large food particles are mechanically, broken into smaller ones by the action of the teeth, and then by churning food in the stomach
• Define chemical breakdown


  Where larger, insoluble molecules are broken down into smaller soluble ones by the actions of enzymes
• Define hydrolysis


  Adding water to break the bonds
• Outline the digestion of starch

  Carbohydrase enzymes hydrolyse starch into molecules of maltose, and then to molecules of glucose
• How is starch digested in the mouth?

  Salivary glands, secrete saliva, which contains amylase. This catalyses, the hydrolysis of starch into maltose, breaking the glycosidic bonds
• How is starch digested in the small intestine lumen?

  Starch digestion continues in the duodenum. Pancreatic amylase is secreted from the pancreas. This catalyses the hydrolysis of any undigested starch molecules into maltose.
• How is maltose digested?

  Because maltose is a disaccharide, it is still too large to be absorbed into the cells of the small intestine and into the bloodstream. so maltose is digested by an enzyme called Maltase which is bound to the cell membranes of cells lining the small intestine. Maltase catalyses the hydrolysis of maltose in molecules of glucose. Glucose is small enough to be absorbed into the bloodstream.
• What enzymes hydrolyse proteins?

  Peptidases
• Where does protein digestion begin and continue?

  Begin: Stomach Continue: Small intestine
• Define endopeptidases


  They hydrolyse the peptide bonds between amino acids in the central region of the protein molecule, forming smaller, peptide molecules
• Where are endopeptidases produced?

  Pancreas And stomach
• Define exopeptidases


  They hydrolyse the peptide bonds between terminal amino acids of the protein molecule to release dipeptides or single amino acids
• Where are exopeptidases produced or found?

  pancreas and can be found on the brush border of the small intestine wall
• Why is it beneficial to digest polypeptides in the middle to produce smaller peptides?

  Increased surface area and provide more ends for exopeptidases to carry out hydrolysis of external peptide bonds faster
• Define dipeptidases


  Type of exopeptidases that hydrolyse peptide bonds holding two amino acids of a dipeptide. They are membrane bound and are found on the cell surface membrane of small intestine epithelial cells
• Where does lipid digestion begin and end?

  Begin: duodenum End: Small intestine (Ileum)
• What does lipid digestion involve?

  Chemical changes by enzymes and physical changes by emulsification
• What happens when lipids are mixed with water?

  They form large droplets
• Where are bile salts produced?

  Liver
• What do the bile salts do to the large droplets?

  They can break these large droplets up and form smaller droplets by process called emulsification, and this increases the surface area of the lipids in order to speed up the action of lipases
• How are lipids hydrolysed?

  Lipase enzymes. Lipids are enzymes produced in the pancreas that hydrolyse the Ester bonds found in triglycerides to form fatty acids and glycerol. Also monoglycerides can sometimes form (just one fatty acid and glycerol)
• Which small molecules can be absorbed through the stomach wall?

  Water and ethanol
• The small intestine is highly adapted for absorption. The lining is highly folded for forming structures called villi. Each individual cell also has many microvilli.
• How does the presence of microvilli increase absorption?

  Increases the surface area for absorption as more channels and carriers can be present in the membrane. There is more area for the enzymes to be embedded in the membrane for digestion.
• Why do epithelial cell lining the small intestine need lots of mitochondria?

  To produce ATP needed for active transport of sodium ions
• How are amino acids and monosaccharide like glucose absorbed from the gut?

  By co-transport, this means absorption is coupled with the absorption of another substance in this case, sodium ions
• There are specific molecules called co transport proteins in the membrane of the microvilli that takes both sodium ions and molecules, such as glucose into the cytoplasm together
• How does a co-transport protein recognise sodium ions, glucose molecules, but nothing else?

  Sodium ions and glucose have a complimentary shape to a binding site on the protein which has a specific tertiary structure
• Describe what happens during Co-transport

  1. Sodium ions actively transported from ileum cell to blood
  2. this maintains/forms a diffusion gradient for sodium ions to enter cells from gut
  3. Sodium ions, enter by facilitated diffusion with glucose/cotransport
  4. Glucose leaves the cell and enters the blood by facilitated diffusion
• How are amino acids, absorbed?

  By co-transport with Na+ ions
• What are the products of digestion of lipids?

  Fatty acids and glycerol and monoglycerides
• Bile salts that are used for emulsification stay associated with these molecules and they all come together to form tiny droplets called micelles
• Micelles are soluble in water and transport their contents to the epithelial cells at the small intestine. When micelles come into contact with the epithelial cells, they breakdown and release the glycerol, fatty acids and monoglycerides. As these are non-polar molecules, they easily diffuse across the cell surface membrane by simple diffusion.
• What happens once inside the cells?

  The triglyceride molecules reform, forming structures called chylomicrons and are transported out of the cell by exocytosis. They first are absorbed into vessels called lacteals (part of lymphatic system), but ultimately are transported into the blood.
• What are chylomicrons?


  Lipoprotein, formed by golgi body by adding protein to triglycerides