Digestion

9.2 Digestion
 
Types of Digestion
  • Digestion is the process that breaks down large and complex pieces of food into smaller and simple pieces that can be dissolved for easy absorption.
  • Digestion is made up of two parts, that is, physical digestion and chemical digestion.
 
 
 
Digestion of Carbohydrates in The Mouth
  • The digestive process begins in the mouth.
  • The presence of food in the mouth stimulates the secretion of saliva from the salivary glands.
  • Saliva contains salivary amylase that hydrolyses starch to maltose.
  • The pH of the saliva ranges between 6.5–7.5, which is suitable for salivary amylase to act at its optimum.
  • Saliva helps food to form bolus and makes it easier to be swallowed.
  • When swallowing, the epiglottis will close the trachea opening to prevent food from entering the trachea.
  • In the oesophagus, the food bolus is moved by peristalsis.
  • Peristalsis is the rhythmic contraction and relaxation of muscles along the alimentary canal.
  • Peristalsis pushes the bolus through the oesophagus until it enters the stomach.
 
Digestion of Protein in The Stomach
  • The surface of the stomach wall is lined with epithelial cells that have undergone adaptations in structure and function to form gastric glands.
  • These epithelial cells are chief cells, parietal cells and mucous cells.
  • Chief cells secrete pepsinogen.
  • Parietal cells secrete hydrochloric acid.
  • Mucous cells secrete mucus.
  • Pepsinogen is an inactive enzyme that is activated by hydrochloric acid to become pepsin.
  • Pepsin then hydrolyses proteins into polypeptides.
  • The functions of hydrochloric acid are to:
    • prepare a medium with a suitable pH (pH 1.5–2.0) for pepsin to act.
    • stop the enzymatic action of salivary amylase.
    • kill bacteria in food.
  • The function of mucus is to protect the stomach wall from the reaction of hydrochloric acid and digestive enzymes.
  • The food in the stomach is mixed with gastric juice made up of hydrochloric acid and pepsin.
  • Food is churned by the peristaltic action of the stomach wall muscles for a few hours.
  • The contents in the stomach finally change to a semifluid called chyme.
  • Chyme enters the duodenum slowly when the sphincter muscle relaxes.
 
 
 
Digestion of Carbohydrates, Proteins and Lipids In The Small Intestine
  • The small intestine consists of duodenum, jejunum and ileum.
  • Duodenum is the first part of the small intestine which receives chyme from the stomach.
  • Duodenum also receives bile produced by the liver and pancreatic juice secreted by the pancreas.
  • The pancreas secretes pancreatic amylase, trypsin and lipase into the duodenum through the pancreatic duct.
  • Liver produces bile.
  • The gallbladder stores bile.
  • The bile flows into the duodenum through the bile duct.
  • Functions of bile:
    • neutralise the acidic chyme.
    • prepare an alkali condition (pH 7.6–8.6) for enzyme action in the duodenum.
    •  emulsify lipids by breaking down lipids into tiny droplets to increase surface area for lipase activity.
  • In duodenum: 
    • Pancreatic amylase hydrolyses starch to maltose.
    • Trypsin hydrolyses polypeptides into shorter peptides.
    • Lipase hydrolyses lipids into fatty acids and glycerols.
  • Glands on the ileum wall secrete mucus and intestinal juice that contains maltase, sucrase, lactase, lipase and erepsin.
  • The alkali medium in the ileum allows enzymes to act at its optimum.
Carbohydrate Digestion
  • Maltase hydrolyses maltose into glucose.
  • Sucrase hydrolyses sucrose into glucose and fructose.
  • Lactase hydrolyses lactose into glucose and galactose.
Lipid Digestion
  • Lipase hydrolyses lipids into fatty acids and glycerols.
Protein Digestion
  • Erepsin hydrolyses peptides into amino acids.

Digestion

9.2 Digestion
 
Types of Digestion
  • Digestion is the process that breaks down large and complex pieces of food into smaller and simple pieces that can be dissolved for easy absorption.
  • Digestion is made up of two parts, that is, physical digestion and chemical digestion.
 
 
 
Digestion of Carbohydrates in The Mouth
  • The digestive process begins in the mouth.
  • The presence of food in the mouth stimulates the secretion of saliva from the salivary glands.
  • Saliva contains salivary amylase that hydrolyses starch to maltose.
  • The pH of the saliva ranges between 6.5–7.5, which is suitable for salivary amylase to act at its optimum.
  • Saliva helps food to form bolus and makes it easier to be swallowed.
  • When swallowing, the epiglottis will close the trachea opening to prevent food from entering the trachea.
  • In the oesophagus, the food bolus is moved by peristalsis.
  • Peristalsis is the rhythmic contraction and relaxation of muscles along the alimentary canal.
  • Peristalsis pushes the bolus through the oesophagus until it enters the stomach.
 
Digestion of Protein in The Stomach
  • The surface of the stomach wall is lined with epithelial cells that have undergone adaptations in structure and function to form gastric glands.
  • These epithelial cells are chief cells, parietal cells and mucous cells.
  • Chief cells secrete pepsinogen.
  • Parietal cells secrete hydrochloric acid.
  • Mucous cells secrete mucus.
  • Pepsinogen is an inactive enzyme that is activated by hydrochloric acid to become pepsin.
  • Pepsin then hydrolyses proteins into polypeptides.
  • The functions of hydrochloric acid are to:
    • prepare a medium with a suitable pH (pH 1.5–2.0) for pepsin to act.
    • stop the enzymatic action of salivary amylase.
    • kill bacteria in food.
  • The function of mucus is to protect the stomach wall from the reaction of hydrochloric acid and digestive enzymes.
  • The food in the stomach is mixed with gastric juice made up of hydrochloric acid and pepsin.
  • Food is churned by the peristaltic action of the stomach wall muscles for a few hours.
  • The contents in the stomach finally change to a semifluid called chyme.
  • Chyme enters the duodenum slowly when the sphincter muscle relaxes.
 
 
 
Digestion of Carbohydrates, Proteins and Lipids In The Small Intestine
  • The small intestine consists of duodenum, jejunum and ileum.
  • Duodenum is the first part of the small intestine which receives chyme from the stomach.
  • Duodenum also receives bile produced by the liver and pancreatic juice secreted by the pancreas.
  • The pancreas secretes pancreatic amylase, trypsin and lipase into the duodenum through the pancreatic duct.
  • Liver produces bile.
  • The gallbladder stores bile.
  • The bile flows into the duodenum through the bile duct.
  • Functions of bile:
    • neutralise the acidic chyme.
    • prepare an alkali condition (pH 7.6–8.6) for enzyme action in the duodenum.
    •  emulsify lipids by breaking down lipids into tiny droplets to increase surface area for lipase activity.
  • In duodenum: 
    • Pancreatic amylase hydrolyses starch to maltose.
    • Trypsin hydrolyses polypeptides into shorter peptides.
    • Lipase hydrolyses lipids into fatty acids and glycerols.
  • Glands on the ileum wall secrete mucus and intestinal juice that contains maltase, sucrase, lactase, lipase and erepsin.
  • The alkali medium in the ileum allows enzymes to act at its optimum.
Carbohydrate Digestion
  • Maltase hydrolyses maltose into glucose.
  • Sucrase hydrolyses sucrose into glucose and fructose.
  • Lactase hydrolyses lactose into glucose and galactose.
Lipid Digestion
  • Lipase hydrolyses lipids into fatty acids and glycerols.
Protein Digestion
  • Erepsin hydrolyses peptides into amino acids.