Question

What are the $4$ stages of digestion?

Answer 1

Digestion is the process performed by a group of organs by which the complex molecules of food are broken down into soluble and simpler chemical compounds, which are then absorbed as nutrients by the body. The unbroken molecules of food or leftover parts are then excreted as faeces. The food is digested chemically (through enzymes) or physically (through mouth). The process happens in the digestive tract.

Complete answer
There are four stages of digestion, namely
1. Ingestion and Propulsion:
2. Digestion.
3. Absorption of Nutrients.
4. Egestion:
Ingestion and propulsion is that, processing of food starts with a bite. Egestion is elimination of wastes.
Ingestion and propulsion is the first process in digestion. The mouth is the starting point of the digestive tract. It is the entry point of food into the body and it moves into the alimentary canal.
Digestion starts in the mouth itself as the meal is eaten immediately. Chewing by the teeth leads to breaking the food macromolecules into smaller particles, which can be more easily digested. They are aided by saliva, which mixes with food to break them down into a form the body can absorb easily and use. Amylase in the saliva breaks down starch into simple carbohydrates. The food particles travel to the oesophagus through pharynx. Then the food particles travel from oesophagus to the stomach through the peristaltic movement. The acid and enzymes present in the stomach help in breaking down the food molecules further. Stomach contains gastric juices, which constitutes protease enzyme. It helps in breaking down proteins. Pancreatic enzymes and bile from the liver enter into duodenum part of small intestine, where they mix with food. Bile aids in lipid digestion. Pancreatic enzymes aid in breaking down proteins, carbohydrates and lipids.
Absorption takes place in the jejunum and ileum parts of the small intestine. The food particles that are indigestible enter the large intestine. Large volume of water absorption occurs in the large intestine. Then, the waste enters the rectum in stool form and comes out from the anus as faecal matter.

Note:
Digestion is how the body converts the consumed food into nutrients, which can be used to restore energy, growth, and cells. A winding tube, which begins in the mouth and ends at the anus is the gastrointestinal or digestive tract. There are four stages in the digestive process: intake, chemical and mechanical degradation of food, nutrients’ absorption, and removal of indigestible food. Mechanical degradation of food is done by mouth, while chemical degradation is performed by stomach.