Digestion And Absorption

• • The large biological molecule (starch, proteins) in food are too big to cross cell membranes
  • This means they can't be absorbed from the gut into the blood
  • During digestion, these large molecules are broken down into smaller molecules (glucose, amino acids), which can move across cell membranes
  • This means they can be easily absorbed from the gut into the blood, to be transported around the body for use by the body cells.
• • Most large biological molecules are polymers which can be broken down into smaller molecules using hydrolysis reactions
  • Hydrolysis reactions break bonds by adding water
  • During hydrolysis, carbohydrates are broken down into disaccharides and then monosaccharides.
  • Fats are broken down into fatty acids and monoglycerides.
  • Proteins are broken down into amino acids
• • Digestive enzymes are used to break down biological molecules in food.
  • A variety of different digestive enzymes are produced by specialised cells in the digestive systems of mammals
  • These enzymes are then released to mix with food
  • Since enzymes only work with specific substrates different enzymes are needed to catalyse the breakdown of different food molecules
• Amylase:

  • Amylase is a digestive enzyme that catalyses the breakdown of starch
  • Starch is a mixture of two polysaccharides, each made from long chain of alpha-glucose molecules
  • Amylase works by catalysing hydrolysis reactions that break the glycosidic bonds in starch to produce maltose
  • Amylase is produced by the salivary glands, which release amylase into the mouth and also by the pancreas which release amylase into the small intestine
• Membrane-bound disaccharidases

  • Membrane-bound disaccharides are enzymes that are attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine)
  • They help to break down disaccharides into monosaccharides
  • Again this involves the hydrolysis of glycosidic bonds
• The digestion of lipids:

  • Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids.
  • This involves the hydrolysis of the ester bonds in lipids
  • Lipase are mainly made in the pancreas - they're then secreted into the small intestine where they act
• The digestion of lipids:

  • Bile salts are produced by the liver and emulsify lipids - this means they cause the lipids to form small droplets
  • Although bile salts are not enzymes they are really important in the process of lipid digestion
  • Several small lipid droplets have a bigger surface area than one large droplet for the same volume of lipid - more surface area for lipase to work on
  • Once the lipid has been broken down by lipase, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles.
  • Micelles help the products of lipid digestion to be absorbed
• • Proteins are broken down by a combination of different peptidases
  • These are enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amino acids.
• Endopeptidases act to hydrolyse peptide bonds within a protein
• Exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules. They remove single amino acids from proteins.
• Dipeptidases are exopeptidase that work specifically on dipeptides. They act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them. Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine
• The products of digestion are absorbed across the ileum epithelium into bloodstream
• Monosaccharides:

  • Glucose is absorbed by active transport with sodium ions via a co-transporter protein.
  • Galactose is absorbed in the same way using the same way using the same co-transporter protein
  • Fructose is absorbed via facilitated diffusion through a different transporter protein
• Monoglycerides and Fatty acids:

  • Micelles help to move monoglycerides and fatty acids towards the epithelium
  • Because micelles constantly break up and reform they can release monoglycerides and fatty acids, allowing them to be absorbed - whole micelles are not taken up across the epithelium.
  • Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane
• Amino acids:

  • Amino acids are absorbed in a similar way to glucose and galactose.
  • Sodium ions are actively transported out of the epithelial cells into the ileum itself.
  • Then they diffuse back into cells through sodium-dependent transporter proteins in the epithelial cell membranes carrying the amino acids with them
• The movement of Na+ out the cell allows the absorption of glucose into the cell lining of the ileum by maintaining a diffusion gradient which Na+ moves in via co transport bringing glucose with it as it enters via facilitated diffusion
• Describe and explain two features you would expect to find in a cell specialised for absorption.

  • Folded membrane/microvilli so larger surface area for increased absorption
  • Large number of co-transporter / carrier / channel proteins so faster rates of absorption
  • Large number of mitochondria to release energy for active transport
  • Membrane-bound digestive enzymes so maintains a concentration gradient for faster absorption
• Advantages of the lipid droplet and micelle formation

   1.      Droplets increase surface areas (for lipase / enzyme action);

  2.      (So) faster hydrolysis / digestion (of triglycerides / lipids);

  3.      Micelles carry fatty acids and glycerol / monoglycerides to / through membrane / to (intestinal epithelial) cell;
• How is the Golgi important in the role of lipid absorption

  •  Modifies / processes triglycerides;
  • Combines triglycerides with proteins;
  •  Packaged for release / exocytosis

  OR

  Forms vesicles;
• Describe the role of enzymes in the digestion of proteins in mammals

  1. Hydrolysis of peptide bonds
  2. Endopeptidase act in the middle of protein/polypeptide and produces short(er) polypeptides/ increase number of ends;
  3. Exopeptidases act at end of protein/polypeptide and produces produces dipeptides/amino acids
  4. Dipeptidase acts on dipeptide/between two amino acids and  produces (single) amino acids;
•  Describe the mechanism for the absorption of amino acids in the ileum.

  • Facilitated diffusion of amino acid (into cell when higher concentration in lumen)
  • Co-transport;
  • Sodium ions actively transported from cell to blood/capillary/tissue fluid
  • Creating sodium ion concentration/diffusion gradient
  • Facilitated diffusion of amino acid into blood/capillary;