Describe the digestion of a buttered piece of bread from the time it is ingested until it is absorbed in the body. (20mks)

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Mouth

  • Mechanical digestion where food is chewed and broken into small pieces; for easy swallowing and to increase surface area for enzymatic action. 
  • Food is mixed with saliva which has salivary amylase, salivary amylase breaks down some starch into maltose; saliva softens,dissolves, lubricates the food.
  • Tongue rolls the food into a bolus.
  • The bolus is then swallowed through the oesophagus/ gullet to the stomach. 
In the stomach,
  •  the presence of the bolus in the stomach stimulates the secretion of gastrin; a hormone that stimulates the secretion of gastric glands to secrete gastric juice; 
  • Gastric juice contains dilute hydrochloric acid; pepsin and rennin; Dilute HCl creates an acidic medium for enzyme pepsin; also kills; germs present in the food bolus.
  • Proteins are digested to polypeptides.
In the duodenum; 
  • The acidic chyme stimulate the walls of the duodenum to secrete secretin and cholecytokin which stimulate the secretion of pancreatic juice and bile respectively.
  • The pancreas; produces pancreatic juice; which contains pancreatic amylase; pancreatic lipase; and trypsin enzymes; 
  • Pancreatic amylase breaks down the remaining starch to maltose,pancreatic lipase breaks down the lipids into fatty acids and glycerol; trypsin breaks down proteins and polypeptides to peptides. 
  • The liver produces bile; that contains bile salts; which emulsify; fats into small fatty droplets increasing their surface area for enzymatic action;
  • Bile salts neutralize the acidic chyme and create an alkaline medium for enzymatic action. 
  • In the ileum secretory cells secrete intestinal juice; that contains maltase; sucrase lactase, peptidase and lipase;
  • Maltase breaks down maltose to glucose and lipase breaks down fatty and oils to fatty acids and glycerol; peptidase breaks down peptides to amino acids.
  • Glucose and amino acids are absorbed into the ileum wall; to the blood stream; fatty acids and glycerol are absorbed by the lacteals.
20 marks

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