Digestion of Food: Definition & its Process

Digestion of Food: Do you know what happens to the food that enters from the mouth? Does the large bite of food that we take in through our mouths remain the same inside the stomach? A large bite of food undergoes several processes that are discussed in this article. And collectively, this process of breaking down large, insoluble molecules of food into smaller, water-soluble molecules, which can then be readily absorbed by the body, is called the digestion of food.

Digestion of Food- Definition

Digestion of food refers to the breakdown of food into its simpler form to get nutrients that can be utilised by the body for growth, repair and maintenance of the tissues and energy.

Process of Digestion of Food

Digestion of food and obtaining nutrition and energy from food is a multi-step process. These are:-
Ingestion: It is the process of taking food into the alimentary tract through the mouth, i.e. eating and drinking. This is a voluntary action.

Digestion

Types of Digestion

a. Mechanical Digestion: It includes mastication, swallowing or deglutition and churning of food.
Mastication includes chewing of food. Swallowing involves taking food into the oesophagus from the mouth. The churning of food includes the peristaltic movement in the stomach.
b. Chemical Digestion: It involves the digestion of food with the help of enzymes.

Fig: Mechanical and Chemical Digestion on Venn diagram

With taking in food, we start chewing it with the help of our teeth. This makes the larger food particles break into smaller ones. This is physical digestion or mechanical digestion. With chewing, food is being mixed with saliva with the manipulation of the tongue. This mixing of food with saliva makes the food slippery bolus structure which can pass down the alimentary canal easily. 

In the mouth itself, digestion of food starts as salivary amylase is present in the saliva so digestion of carbohydrates starts in the mouth, and almost 30% of it is digested there. The action of salivary amylase on food can be called chemical digestion. But digestion is incomplete here.

Fig: Physical and Chemical Digestion of Food

The food bolus is swallowed, which is a voluntary action and from here till the act of defecation, involves involuntary action.
The bolus travels down the oesophagus by the process of peristalsis. It involves the contraction and expansion of the circular muscles of the oesophagus. This makes the food bolus travel down the food pipe. This is an involuntary action. Propulsion includes both voluntary, i.e. swallowing, and involuntary, i.e. peristalsis, movements.

Now, food enters the stomach, which needs to get digested further. In the stomach, there are gastric glands that secrete gastric juices (contains HCl, mucus, pepsin enzyme), which helps in the digestion of food further. HCl makes the environment acidic, which helps in killing the germs, if any, present in the food bolus coming from the mouth. Also, an acidic environment is needed for the enzymes present in the gastric juices to become active.

Here carbohydrates are not digested. Mainly digestion of protein takes place by the enzyme pepsin in the stomach. The peristaltic movement of the stomach mixes and churns the food with the gastric juices and is called chyme. Then this chyme passes further to the duodenum of the small intestine.

In the duodenum, there is the secretion of bile, pancreatic juices, and intestinal juices. Bile helps in the emulsification of fat and makes the environment alkaline so that enzymes present in pancreatic juice become active to digest the food further.
Enzymes present in the intestinal juice complete the digestion process of food, i.e., complex food gets broken down into completely simpler molecules.
Carbohydrates—-> Monosaccharides or simple sugars like glucose
Proteins—-> Amino acids
Fats—-> Fatty acids and glycerol

Fig: Simpler Forms of Macromolecules

Absorption: Now, these simpler molecules need to be absorbed. Absorption takes place in the small intestine itself. The small intestine (in jejunum and ileum) is adapted to absorb the digested food. It has numerous finger-like projections called villi. These villi increase the surface area of the intestine, thereby giving a large area for the absorption of digested food. Digested food is absorbed through villi and enters the bloodstream. Lacteal absorbs fat nutrients and carries them to the circulatory system.

Fig: Small Intestine (Duodenum, Jejunum, Ileum)

Assimilation: Blood then carries these simpler foods to all parts of the body where it is assimilated. These assimilated foods are used by the cells to get energy, repair body tissues, for growth, etc. Like glucose, when absorbed by the cells, it gets oxidised to produce ATP. This ATP, when hydrolysed, releases energy. Simpler forms of food that are not going to be assimilated are stored in the liver. Like glucose can be stored in the form of glycogen which can be used in the future.

Egestion: Now, after absorption, some foods remain undigested and unabsorbed and need to be egested out of the body. These undigested food travels from the small intestine to the wider tube, large intestine (hence named so), and is pushed along by the peristaltic movement of the large intestine.

Here bacteria is present, which helps in fermenting undigested carbohydrates to produce a short-chain fatty acid. These are used as energy sources for the bacteria and other cells of the large intestine. Vitamin K is also produced by this bacterial action. Here compaction of undigested material occurs. Further absorption of extra water and electrolytes takes place.

Then left material is stored in the rectum for some time and is then egested out of the body through the anus regulated by the anal spincter. The act of egestion of waste material out of the body through the anus is called defecation. The matter which is defecated is called “stool”. This is a voluntary action.

Fig. Mechanical and Chemical Digestion

Summary

Food that we eat goes through different processes. It includes ingestion, digestion, absorption, assimilation, and egestion. We take in food through our mouth, which is called ingestion. Then food enters the stomach through the food pipe where digestion occurs, i.e., complex molecules get simplified into smaller molecules. However, digestion starts from the mouth itself. Semi-digested food enters the intestines, where further digestion takes place.

In the small intestine, absorption of digested food takes place. Undigested food is taken to the rectum through the large intestine, where it is stored for some time and is then ejected out of the body.

Frequently Asked Questions (FAQs) on Digestion of Food

Q.1. How is food digested step by step?
Ans: We take in food through our mouth, which is called ingestion. Then food enters the stomach through the food pipe where digestion occurs, i.e., complex molecules get simplified into smaller molecules. However, digestion starts from the mouth itself. Semi-digested food enters the intestines, where further digestion takes place. In the small intestine, absorption of digested food takes place.

Q.2. What are the four stages of digestion?
Ans: The four stages of digestion are-
a. Ingestion
b. Digestion
c. Absorption
d. Egestion

Q.3. How does digestion take place in the human body?
Ans: Digestion starts from the mouth itself. Different enzymes are secreted from the stomach, pancreas, and intestines. Different enzymes help in simplifying the food. Like pepsin in the stomach and trypsin in the intestine helps in the digestion of proteins to amino acids. Similarly, amylase enzymes help in breaking carbohydrates into simple sugars. These simplified forms of food are absorbed in the small intestine. And then, unabsorbed food is ejected out of the body through the anus.

Q.4. What is the difference between digestion and absorption?
Ans: Digestion is the process in which complex food molecules are broken down into simpler forms, while in absorption, simplified food is taken in or absorbed through villi in the small intestine to the bloodstream.

Q.5. Why do we digest food?
Ans: We digest food to get the food absorbed so that we can get energy from food, get nutrients to grow, and repair damaged tissues.