Digestion

• What is digestion?

  The process in which large, insoluble molecules are hydrolysed by enzymes to produce smaller, soluble molecules that can be absorbed and assimilated.
• What is chemical digestion?

  Hydrolysis reactions (water is added to break bonds within a molecule)
• What is mechanical digestion?

  The physical breakdown of large molecules into smaller sections (e.g. actions of the teeth and stomach churning)
• Why are different digestive enzymes produced in different parts of the digestive system?

  They require different optimum pHs.
• What occurs in starch digestion?

  Chewing stimulates the secretion of amylase from the salivary gland.

  In the mouth, the glycosidic bonds in starch are broken in hydrolysis reactions catalysed by amylase, to form maltose (disaccharide).

  In the small intestine, the glycosidic bond in maltose is then broken in a hydrolysis reaction catalysed by maltase (a membrane-bound dissacharidase) to form alpha-glucose. This glucose is soluble and can be absorbed across the gut wall into the blood via co-transport.
• What are the two types of amylase and what does this mean?

  Salivary - starch digestion occurs in the mouth

  Pancreatic - starch digestion occurs in the small intestine
• Where is maltase found?

  The cell membranes of epithelial cells in the small intestine.
• Where and how is sucrose digested?

  Sucrose is released from the cells of many natural foods by the mechanical action of the teeth. In the small intestine, the glycosidic bond is broken in a hydrolysis reaction catalysed by sucrase (found in the epithelial cell membranes), forming alpha-glucose and fructose.
• Where and how is lactose digested?

  In the small intestine, the glycosidic bond within lactose (the sugar found in milk) is broken down in a hydrolysis reaction catalysed by lactase (found in the epithelial cell membranes), forming alpha-glucose and galactose.
• What is an endopeptidase?

  An enzyme that hydrolyses peptide bonds within a polypeptide chain to form smaller polypeptides.
• What is an exopeptidase?

  An enzyme that hydrolyses peptide bonds at the ends of a polypeptide chain to form dipeptides or amino acids.
• What is a dipeptidase?

  An enzyme that hydrolyses peptide bonds in dipeptides to form amino acids.
• Where are exopeptidases and dipeptidases found?

  Membrane-bound on the surface of small intestine epithelial cells.
• What is pepsin and how is it produced?

  Pepsin is an endopeptidase with an optimum pH of 2, so works in the stomach to digest proteins.

  The stomach secretes pepsinogen from chief cells in gastric pits, which is then activated to pepsin by HCl (secreted by parietal cells in the stomach).
• What is trypsin and how is it produced?

  Trypsin is an endopeptidase with an optimum pH of 8, so works in the small intestine to digest proteins.

  The pancreas secretes trypsinogen which is then activated to trypsin in the small intestine.
• Why are pepsin and trypsin first released in an inactive form?

  So that they don't digest proteins within the cells that produce them.
• What is the role of HCl in the stomach?

  . To activate pepsinogen to pepsin

  . To provide optimum pH conditions (acidic) for enzyme activity

  . To kill some pathogens as part of primary defence
• What is the role of sodium hydrogen carbonate that is produced by duct cells of the pancreas into the small intestine (duodenum)?

  To neutralise stomach acid (as an alkali), providing optimum pH for small intestine enzymes (e.g. trypsin).
• How are small intestine epithelial cells adapted for their function of digestion?

  . Many microvilli (large surface area)

  . Many ribosomes (protein synthesis - enzymes)

  . Many mitochondria (high energy release - active transport/protein synthesis)

  . Many membrane-bound exopeptidases and dipeptidases - digestion of polypeptide chains to smaller molecules for absorption
• How are proteins digested?

  . Endopeptidases (e.g. pepsin in stomach/trypsin in small intestine), forming smaller polypeptides

  . Exopeptidases on small intestine surface, forming dipeptides and amino acids

  . Dipeptidases on small intestine surface, forming small, soluble amino acids that can be absorbed into the blood.
• What is co-transport?


  The simultaneous transport of two substances across a cell membrane.
• How is a concentration gradient established for sodium ions in co-transport?

  Sodium ions are actively transported (requiring energy release from ATP hydrolysis) out of the ileum epithelial cells and into the blood using a carrier protein in the membrane. This establishes a conc. gradient for the facilitated diffusion of sodium ions into the cell from the small intestine lumen. This doesn't require energy but requires a complementary shaped carrier protein.
• How does glucose co-transport with sodium ions?

  The carrier protein involved has two receptor sites - one complementary to sodium, and one to glucose. Both need to bind for the transport to occur. Due to the sodium concentration gradient established, glucose is therefore able to be transported via co-transport against its own concentration gradient, into the epithelial cells from the lumen. The glucose can then move into the blood via facilitated diffusion using a complementary shaped carrier protein.
• As well as glucose, what other molecule can be co-transported?

  Amino acids
• Where does lipid digestion occur?

  Small intestine
• How is the optimum pH for lipid digestion obtained?

  Bile made in the liver and stored in the gall bladder is released in the duodenum. The bile (sodium hydrogen carbonate) is alkaline and neutralises the stomach acid to provide the optimum pH for lipase to act at.
• What are the five stages of lipid digestion?

  . emulsification

  . digestion/hydrolysis

  . absorption into small intestine epithelial cells

  . re-synthesis and production of chylomicrons

  . absorption into lacteal within villi
• Why is emulsification required in lipid digestion?

  Triglycerides are non-polar and therefore insoluble so form large lipid globules with a small SA:V ratio and a small surface area for lipase to act on, slowing the rate of hydrolysis.
• What are bile salts?

  Found in bile, they have hydrophobic tails and negatively-charged hydrophilic heads.
• What occurs in emulsification during lipid digestion?

  Mechanical digestion in the in the stomach breaks down large fat globules and the bile salts attach to the smaller droplets, their hydrophobic tails within and their hydrophilic heads on the outside, associating with water. The negative charge of the bile salt heads repels droplets from other, so they do not re-join. This gives the small droplets a larger surface area (and SA:V) for lipase to act on, so there is a faster hydrolysis of the triglycerides.
• What occurs in the hydrolysis of lipid digestion?

  Lipase catalyses the hydrolysis reaction that breaks ester bonds within the triglyceride to produce 2 fatty acids and a monoglyceride.
• How are the products of lipid hydrolysis absorbed into the ileum (small intestine) epithelial cells?

  The fatty acids and monoglycerides/glycerol interact with bile salts to form micelles. The bile salts give greater solubility and the micelle holds a high concentration of fatty acids and monoglycerides that they transport to the surface of the ileum epithelial cells. Here the micelles break down and fatty acids and monoglycerides/glycerol diffuse into the epithelial cells through the bilayer via simple diffusion.
• How are triglycerides re-synthesised and transported in the ileum epithelial cells?

  Fatty acids and monoglycerides move into the smooth ER where triglycerides are then formed in a condensation reaction. The carboxylic group of the fatty acid reacts with the OH group of glycerol to form an ester bond. The triglycerides are transported via vesicles to the Golgi body where they are combined with proteins and cholesterol to form chylomicrons. Vesicles containing chylomicrons form and move to the cell surface.
• How are chylomicrons absorbed?

  Vesicles containing chylomicrons fuse with the epithelial cell membrane, releasing them via exocytosis. The chylomicrons enter the lacteal of a villi and are transported through the lymphatic system. The lymphatic system drains into the subclavian vein located in the neck, where the triglycerides then enter the blood.
• How is the small intestine adapted for its function?

  . many villi/microvilli (SA - carrier/channel proteins)

  . capillary network + close blood supply (SCG)

  . wall of lumen one cell thick (SDP)

  . muscle layer - contract to move food along

  . epithelial cells with many ribosomes (protein synthesis - enzymes etc.)

  . epithelial cells with many mitochondria (energy - protein synthesis, active transport, co-transport)