Digestion and Absorbtion

• Digestion - Large, insoluble biological molecules in food, which cant pass through cell membranes, are hydrolysed into smaller, soluble molecules so they can be absorbed across cell membranes to take in nutrients.
• Enzymes are essential for digestion as they catalyse the hydrolysis reactions which break down the biological molecules
• Biological molecules must be digested before they can be absorbed into the blood stream because they are large and insoluble molecules so they cant pass through cell membranes.
• Carbohydrates are broken down by amylase and membrane bound disaccharides
• Amylase catalyses hydrolysis reaction of the glycosidic bonds in starch to make maltose.

  Membrane bound disaccharides are attached to cell membranes in the ileum, breaking down into monosaccharides.
• Lipids are broken down by lipase, with the help of bile salts.
• Lipase hydrolyses the ester bond and the bile salts emulsify causing large fat molecules to get smaller.
• Proteins are broken down by protease or peptidases.
• Protease hydolyses the peptide bond in proteins and releases amino acids and this is completed by endopeptidases and exopeptidases.
• Endopeptidases- outside of cell
• Endopeptidases act to hydrolyse peptide bonds within a protein
• Exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules removing single amino acids.
• Carbohydrates

  outside of the gut:

  starch -- amylase--> maltose

  Inside the epithelial cells of the small intestine:

  maltose -- maltase --> glucose
• Lipids

  lipid --bile salts --> fat droplets --> glycerol and fatty acids
• Proteins

  Outside of the gut:

  protein -- endopeptidase --> polypeptide

  Inside cell surface membrane:

  polypeptide -- exopeptidase --> amino acids
• Explain the difference in roles of endopeptidase and exopeptidase

  Endopeptidase acts to hydrolyse peptide bonds within a protein and exopeptidase acts to hydrolyse peptide bonds at the ends of protein molecules removing amino acids.
• Glucose is absorbed by active transport with sodium ions via co-transporter proteins.
• Micelles help to move monoglycerides and fatty acids. Monoglycerides and fatty acids are lipid soluble, so can diffuse directly across membranes.
• Amino acids are co-transported with sodium ions in sodium dependant transporter proteins.
• The digestion products of lactose, glucose and galactose, are absorbed across the epithelial cells of the ileum ,which is found at the start of the small intestine, by active transport with sodium ions via co-transporter proteins.