3.3 Digestion

• What is digestion?

  The hydrolysis of large and insoluble substances into smaller/soluble substances.
• Give three examples of physical digestion

  Teeth, Muscular movements and peristalsis
• What is peristalsis?

  The involuntary constriction and relaxation of the muscles of the intestine or another canal
• Give an example of chemical digestion

  Enzymes
• How is the gut wall adapted to absorb products of digestion in the ileum

  It has many microvilli, a single layer of epithelial cells and many capillaries
• What reaction does digestion always use?

  Hydrolysis reactions
• What are the 3 enzymes involved in digestion?

  Endopeptidase, exopeptidase and dipeptidase.
• Describe the role of endopeptidase

  Hydrolyses specific bonds between amino acids in the middle of the polypeptide to produce shorter polypeptides. Increasing the number of polypeptide ends for exopeptidase to hydrolyse.
• Describe the role of exopeptidase

  Hydrolyses bonds between specific amino acids at polypeptide ends to produce dipeptides or individual amino acids.
• Describe the role of dipeptidase

  Hydrolyses peptide bonds between specific amino acids within the dipeptide producing individual amino acids.
• Where is endopeptidase located?

  Stomach and small intestine
• Where is exopeptidase located?

  Small intestine
• Where is dipeptidase located?

  Epithelial cell membranes of the small intestine ileum.
• What happens when dipeptidase breaks down dipeptides

  It hydrolyses them into amino acids where amino acid absorption occurs, by co-transport, through channel proteins and then into the blood
• Why do rabits eat their poop

  It gives the intestine chance to digest undigested protein so that more amino acids are absorbed, because protein/food passes through the ileum again.
• Describe the process of amino acid uptake by co-transport in the ileum.

  With each amino acid one Na⁺ ion is absorbed through a carrier protein by co-transport into the epithelial cells. Amino acids then pass through a carrier protein by facilitated diffusion into the blood. Na⁺ is actively transported out of the epithelial cells into the blood; this maintains a diffusion gradient for the initial co-transport.
• What's the equation for the digestion of a polysaccharide in the mouth? What enzyme hydrolyses the bond?

  Starch + water --> Many maltose molecules.

  Amylase hydrolyses this bond
• Where is amylose made?

  Salivary gland
• What's the equation for the absorption of a monosaccharides in the ileum? What enzyme hydrolyses the bond?

  Maltose + water --> alpha-glucose + alpha-glucose.

  Maltase hydrolyses this bond
• Give two enzymes that can hydrolise glycosidic bonds?

  Amylase, Maltase, Lactase, Sucrase, etc.
• What are disaccharidases?

  An enzyme that hydrolyses the glycosidic bond in disaccharides.
• Name two disaccharidases.

  Maltase, Lactase, Sucrase.
• Where can disaccharidases be found?

  They can be found membrane-bound to the ileum epithelial cell membranes.
• Describe and explain features you would expect to find in a cell specialised for absorption. [3]

  ○ Microvilli as they increase surface area to increase the points at which diffusion can take place.

  ○ Lots of mitochondria to provide the energy required for absorption.

  ○ Specialized transport proteins embedded into the cell membrane (carrier or channel proteins) to facilitate the transport of substances across the membrane.
• Give the equation for the digestion of a large hydrophobic lipid aggregate

  large lipid aggregate --> Bile salts --> emulsified droplets - these are soluble
• How does our stomach turn fat into aggregates?

  Our stomach churns fat to turn them into aggregates.
• What are bile salts produced by and where are they stored?

  Produced in the liver and stored in gall bladder
• How does bile aid in the hydrolysis of large lipid balls?

  Bile breaks down large lipid balls to increase surface area for faster hydrolysis of the lipids by lipase in the ileum.
• Once hydrolysed what do monoglycerides and fatty acids stick to and what does this form?

  They stick to the bile salts to form micelles
• How do micelles help with absorption?

  micelles help with absorption by moving fatty acids and glycerol or monosaccharides towards the intestinal epithelium
• What do micelle enable fatty acids to diffuse through

  They allow fatty acids to diffuse through the phospholipid bilayer into the epithelial cells and then out of these cells into the lymph capillary.
• What is exocytosis?

  The phospholipid bilayer merges with the chylomicron membrane to release products into the lymphatic system.
• A carrier protein in the cell lining of the gut actively exchanges one sodium ion into the cell in exchange for one proton out of the cell, use your knowledge of transport across cell membranes to explain how the carrier protein does this.

  This is done by co-transport which uses ATP. sodium and hydrogen ions bind to the protein and the protein changes shape to move the ions across the membrane.
• High salt diets can lead to a higher than normal concentration of salt in the blood plasma entering capillaries, potentially leading to a build up of tissue fluid. Explain how.

  Higher salt levels result in a lower water potential of the tissue fluid, due to this less water returns to the capillary by osmosis.