digestion and absorption

• large biological molecules (e.g. starch, proteins) in food are too big to cross cell membranes

  means they cannot be absorbed from the gut into the blood

  during digestion these large molecules are broken down into smaller molecules e.g. glucose, amino acids which can move across the cell membrane

  means they can be easily absorbed from the gut into the blood, to be transported around the body for use by body cells
• most large biological molecules are polymers which can be broken down into smaller molecules (monomers) using hydrolysis reactions
• hydrolysis reactions break bonds between monomers by adding water
• during hydrolysis:

  • carbohydrates broken down into disaccharides and them monosaccharides
  • fats broken down into fatty acids and monoglycerides
  • proteins broken down into amino acids
• Digestive enzymes are used to break down biological molecules in food - a variety of digestive enzymes are produced by specialised cells in digestive system of mammals - enzymes are then released to mix with food
• since enzymes only work with specific substrates, different enzymes needed to catalyse breakdown of different food molecules
• Amylase:

  • digestive enzyme that catalyses breakdown of starch
  • starch is a mixture of 2 polysaccharides - each made from long chains of alpha glucose molecules
  • amylase works by catalysing hydrolysis reactions that break the glycosidic bonds in starch to produce maltose (a disaccharide)
  • amylase is produced by the salivary galnds which release amylase into the mouth
  • also by the pancreas - releases amylase into the small intestine
• membrane-bound disaccharidases are enzymes that are attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine)

  • help to break down disaccharides into monosaccharides
  • again involves the hydrolysis of glycosidic bonds
• sucrose - sucrase = glucose + fructose
• maltose - maltase = glucose + glucose
• lactose - lactase = glucose + galactose
• monosaccharides can be transported across the epithelial cell membranes in the ileum via specific transporter proteins
• Digestion of lipids - lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids

  involves the hydrolysis of the ester bonds in lipids

  mainly made in the pancreas - then secreted into small intestine where they act
• 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 very important in the process of digestion
• several small lipid droplets have a larger surface area than a single large droplet (for the same volume of lipid). So the formation of small droplets greatly increases the surface area of lipid that is available for lipases 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 (proteases). 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
• trypsin and chymotrypsin are both endopeptidases - they are synthesised in the pancreas and secreted in the small int
• pepsin is another endopeptidase - releases into the stomach by cells in the stomach lining. Pepsin only works in acidic conditions - these are provided by hydrochloric acid in the stomach
• exopeptidases act to hydrolyse peptide bonds at the ends of protein molecules. they remove single amino acids from proteins
• dipeptidases are exopeptidases that work specifically on dipeptides. they separate the 2 amino acids that make up a dipeptide by hydrolysing the peptide bond between them. they are often located in the cell-surface membrane of epithelial cells in the small-int
• 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 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
  • bc micelles constantly break up and reform they can 'release' monoglycerides and fatty acids, allowing them to be absorbed
  • micelles are not taken up across the epithelium
  • monoglycerides and fatty acids are lipid soluble and so can diffuse directly across the epithelial cells membrane
• amino acids:

  • absorbed in a similar way to glucose and galactose
  • sodium ions are actively transported out of the epithelial cells into the ileum itself
  • they then diffuse back into cells through sodium-dependent transporter proteins in the epithelial cell membranes carrying the amino acids with them
• visking tubing is partially permeable - it allows small molecules like water to pass through but not larger molecules like proteins