Stomach has a major part in digestion. Do you know what stomach digests? In this lesson, you will learn about digestion in the stomach and small intestine.
Explanation
What is a stomach?
Stomach is a muscular bag which keeps the food inside it for 3 to 4 hours.
Physical digestion
It is done through the churning movement of stomach walls. It means the muscles mix the food with digestive juices and change the boli ( food ) into chyme. Chyme is a semi solid paste, mixture of food and digestive juices. Boli is changed into chyme so the surface area for enzyme reaction would increase which would fasten the rate of digestion.
Chemical digestion
In the stomach, only proteins are broken down into polypeptide chains. Gastric walls of the stomach secrete gastric juice which contains pepsin, hydrochloric acid and mucus. Mucus is lined on the gastric walls so the corrosive effect of acid wouldn't affect the stomach walls.
Functions of HCl
Production of HCl makes the environment acidic ( almost 2 pH) so pepsin can work better. Pepsin works best in acidic environments. HCl activates the inactive form of pepsinogen to pepsin and denatures the salivary amylase. It also disinfects the food by killing all the pathogens.
An enzyme, pepsin, is used to break the protein into polypeptide chains. Partial breakdown occurs.
Protein are broken down into polypeptides through pepsin (proteases).
Digestion in Small Intestine:
The food is then transported to the small intestine through the pyloric sphincter (to know what is pyloric sphincter, click here.) When the food enters the small intestine's first part (duodenum) digestion of the other food particles begins. The juices released by pancreas contain salivary amylase, pancreatic lipase and pancreatic proteases (trypsin). These enzymes enter the duodenum with the pancreatic duct. Bile enters the duodenum via the bile duct. Enzymes like, maltase, lipases and peptidases (erepsin) are released by the epithelial cells of the small intestine.
Carbohydrates, fats, proteins are digested as followed:
Carbohydrates:
Starch ---amylase---- maltose
Maltose ---maltase--- glucose
Sucrose ---sucrase---- glucose + fructose
Lactose ---lactase---- glucose + galactose
2. Proteins:
Proteins ---trypsin----polypeptides
Polypeptides ---erepsin---- amino acids
3. Fats
Bile salts in the bile reduces the surface tension of fats. It is only a physical digestion. Bile reduces the attractive forces between the fat molecules. This increases fats surface area to volume ratio for the lipases to efficiently act on them.
Fats ---lipase--- fatty acids + glycerol
Digestion ends in the small intestine. Absorption of the digested food molecules and assimilation of the food molecules will be discussed in the next lesson.
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