Digestion in Humans

What is Digestion?

• Digestion is the process whereby large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells.

Digestion in the mouth

The salivary glands in the mouth secrete saliva, which is mixed with the food by the tongue. The saliva contains mucus which softens the food. Salivary amylase digests starch to maltose.

The chewing action of teeth breaks up larger pieces of food into smaller pieces (physical digestion). The tongue rolls the food into small, slippery, round masses or boli. Peristalsis in the walls of the oesophagus and gravity push the bolus into the stomach.

Digestion in the stomach

The presence of food in the stomach stimulates the gastric glands to release gastric juice.Peristalsis in the walls of the stomach mixes the food with the gastric juice. Gastric juice is a dilute solution of hydrochloric acid (pH 2), mucus and pepsin.

Dilute hydrochloric acid:

1) Stops the action of salivary amylase by denaturing it

2) Changes inactive forms of enzymes pepsinogen to active form pepsin

3) Provide slightly acidic conditions suitable for the action of pepsin

4) Kills potentially harmful microorganisms in the food

Pepsin digests proteins to polypeptides.

Food remains in stomach for 3-4 hours. Partly digested food becomes liquefied, forming chyme. Pyloric sphincter relaxes to release the chyme from stomach into duodenum.

Digestion in the small intestine

Chyme enters the duodenum and stimulates the release of:

– pancreatic juice by the pancreas

– bile by the gall bladder

– intestinal juice by the small intestine

These alkaline fluids neutralise the acidic chyme. The alkaline medium is needed for the action of intestinal and pancreatic enzymes.

The pancreas secretes pancreatic juice, which contains the enzymes pancreatic amylase, pancreatic lipaseand trypsin.

The gall bladder releases its stored bile. The bile passes through the bile duct into the duodenum.

The intestinal juice secreted by the intestinal glands contains the enzymes sucrase, maltase,lactase, intestinal lipase and peptidases. These enzymes digest food molecules.

Pancreatic juice	Action of Pancreatic Amylase: starchà maltose   Action of Pancreatic Lipase: fats à fatty acids and glycerol   Action of Pancreatic Trypsin: protein à polypeptides
So Many Lazy Little People Intestinal juice	Action of Intestinal Sucrase: sucroseà glucose and fructose   Action of Intestinal Maltase: maltoseà glucose   Action of Intestinal Lactase: lactose àglucose and galactose   Action of Intestinal Lipase: fats àfatty acids and glycerol   Action of Intestinal Peptidase: polypeptides àamino acids
Bile	Bile salts emulsify fats.This causes the fats to break into tiny fat droplets suspended in water, forming an emulsion. Emulsification increases the surface area to volume ratio of the fats, speeding up their digestion by lipase. Emulsified fats are digested by lipases (pancreatic andintestinal lipases) to fatty acidsand glycerol.