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

Lipid Digestion

Protein Digestion