Absorption and Assimilation of Digested Products and Egestion: Do you know how we get energy for performing various activities? We get energy from the food we eat. The food gets digested and absorbed in the body, which in turn provides us energy for the growth and development of the tissues. The process of breakdown of food into simpler substances in which they get absorbed in the body is known as digestion. Digested food gets absorbed in the body, which gets assimilated in the body. Unabsorbed food gets excreted out of the body, the process known as egestion. Read this article to know more about absorption, assimilation, and egestion of the food.

What is Absorption?

Absorption is the process by which the digested products pass through the intestinal mucosa into the blood or lymph. It occurs by two processes, namely passive diffusion and active or facilitated transport mechanisms. Nutrients that get absorbed in the body are glucose, amino acid, vitamins, minerals, water, etc. Maximum absorption of food takes place in the small intestine except for water. Absorption of water takes place in the large intestine.

Absorption of Monosaccharides

Absorption of carbohydrates occurs in the form of monosaccharides like glucose, galactose, etc.  Their absorption takes place by an active transport process, which is against the concentration gradient. The passage of these substances into the blood takes place from an area of low concentration to an area of high concentration. This process requires energy. Absorption of carbohydrates occurs in the stomach and jejunum (part of the small intestine). However, some of the substances like fructose are absorbed with the help of the carrier ions like Na+. This mechanism is called facilitated transport.

Absorption of Amino Acids

Absorption of amino takes place mainly by active transport, but some amino acids are absorbed by facilitated diffusion. Absorption of amino acids takes place in the duodenum and jejunum.

Absorption of Water

The transport of water depends upon the osmotic gradient. Water is mainly absorbed in the large intestine from an area of high concentration to an area of low concentration. 

Absorption of Electrolytes (salts)

Absorption of electrolytes like salt, various ions like sodium, potassium, calcium, chloride, etc., takes place in the small intestine by means of active transport. Absorption of iron and calcium ions takes place in the duodenum and bile salts in the ileum.

Absorption of Water-soluble Vitamins

Water-soluble vitamins such as vitamin B and vitamin C are reabsorbed into the blood by the simple diffusion process.

Absorption of Alcohol

Absorption of alcohol begins in the stomach, which continues in the small intestine. The surface area of the small intestine is much greater because of the presence of villi in it, so when alcohol passes into the duodenum, it gets readily absorbed.

Absorption of Fatty Acid and Glycerol

Fatty acids and glycerol are insoluble in water. Hence they cannot be absorbed directly into the blood. They are at first converted into small droplets called micelles, which further move into the intestinal mucosa. Micelle is an aggregate of many molecules. They are further broken down into very small fat globules called chylomicrons. These chylomicrons are transported into the lymph vessels called lacteals which are present in the villi. These lymph vessels then release the absorbed substances into the bloodstream.

Difference Between Micelles and Chylomicrons

Micelle	Chylomicrons
The products of digested fat are first incorporated into small droplets called micelles.	Micelles get further broken down into smaller globules called chylomicrons.
Digested fat is absorbed in the intestinal mucosa in the form of micelles.	Digested fat is released from the intestinal mucosa in the form of chylomicrons

Absorption of Nutrients in Different Parts of the Digestive System

Absorption of food occurs in different parts of the alimentary canal like the mouth, stomach, small intestine, and large intestine. However, maximum absorption occurs in the small intestine.

Mouth	Stomach	Small intestine	Large intestine
Certain drugs coming in contact with the mucosa of the mouth and lower side of the tongue are absorbed into the blood capillaries lining them	Absorption of water, simple sugars, alcohol, etc. takes place	It is the main organ for the absorption of nutrients. It is the site for complete digestion, and the final products of digestion are absorbed through the mucosa into the bloodstream and lymph.	In the large intestine, absorption of water, minerals, and drugs take place.

Assimilation of Food

Absorbed food is transported by blood and lymph. Blood transports different food materials to different parts of the body, where they get assimilated to provide energy for the growth, development, and repair of the cells of the body.

Therefore, assimilation means the incorporation of the absorbed food materials into the tissue cells as their integral and homogeneous components.

1. Proteins: Amino acids are not stored directly in the body but are taken up by the cells in connection with the synthesis of proteins. Proteins are required for the growth, development, repair, etc., of the body. Excess amino acids can be converted into glucose and then to fat, and thus, they are stored in the body. During the conversion of amino acids into glucose, the amino acids are deaminated.
Deamination is the process of removal of amino groups (–NH2). The liver is the chief site for deamination.

2. Carbohydrates: Monosaccharides like glucose, fructose, and galactose reach the liver by the hepatic portal vein. They are converted into glycogen in the liver, the process is known as glycogenesis and are stored for supply at the time of need. Some amount of glucose is also stored in the muscles as glycogen. Some amount of glucose is converted into fat and is stored as such.

3. Fats: The absorbed fat is stored in the fat deposits of the body, such as the subcutaneous layer, mesenteries, and adipose tissue. Fats are used as fuel. They act as insulators in the body, thereby protecting the body from shock. In the liver cells, the fats are converted into amino acids and carbohydrates whenever required.

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Egestion

Food is taken in through the mouth. Intake of food is known as ingestion. Food is taken in the form of macromolecules, which get broken down into simpler molecules with the help of enzymes in our body. Food is mainly in the form of carbohydrates, proteins, fats, and nucleic acid. Food gets digested and absorbed in the body. After the absorption of digested products, undigested food that reaches the large intestine contains a large amount of water. The walls of the large intestine absorb water and the remaining inorganic nutrients like calcium and iron.  As the water is absorbed, the semi-solids that remain of the unabsorbed food are called faeces. Faeces can remain in the large intestine (colon part of the large intestine) for a long time before they are passed to the rectum for egestion.
Therefore, egestion or defecation means the elimination of faeces from the alimentary canal.

Mechanism of Egestion

Unabsorbed food passes from the small intestine to the large intestine with the help of peristaltic movement. This undigested material that passes into the large intestine is known as chyme. Water and electrolytes are absorbed in the large intestine. After the absorption of water, chyme gets converted into semi-solid faeces.

Escherichia coli bacteria live inside the colon, which produces vitamin B1, vitamin B2, vitamin B12, and vitamin K. These vitamins get absorbed in the intestinal wall of the colon, and in turn, these bacteria feed on the undigested food present there. As the pellets of faeces enter into the rectum, an extension of the rectal wall induces the feeling of defecation due to the reflex called ‘ defecation reflex.’

This reflex initiates peristalsis in the last part of the colon that is known as the sigmoid colon and the rectum. As a result of this reflex, the faeces move towards the anus. As the faeces reaches the anus, involuntary relaxation of the internal anal sphincter and voluntary relaxation of the external anal sphincter causes the defecation process. In infants, defecation occurs by reflex action without voluntary control over the external anal sphincter.

Faeces contain a number of substances such as:

1. Undigested food mainly consists of cellulose and other plant fibres.
2. Mucus and dead mucosal cells from the gut wall
3. Bile pigments and their derivatives give colour to faeces.
4. Smell-producing indole, skatole, thiols, and hydrogen sulfide gases are produced due to the bacterial decay of food.

Summary

Food provides us with energy for the various activities performed by the body. The process of breakdown of food into simpler substances in which they get absorbed in the body is known as digestion. Digested food gets absorbed in the body, which gets assimilated in the body. Absorption is the process by which the digested products pass through the intestinal mucosa into the blood or lymph. Thus, assimilation means the incorporation of the absorbed food materials into the tissue cells as their integral components. Unabsorbed food gets excreted out of the body, the process known as egestion.

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Frequently Asked Questions

Q.1. Define absorption.
Ans: Absorption is the process by which the digested products pass through the intestinal mucosa into the blood or lymph.

Q.2. Define assimilation.
Ans: It means the incorporation of the absorbed food materials into the tissue cells as their integral components.

Q.3. What are micelles?
Ans: Micelles are the small aggregates of molecules that are during the absorption of fat. These are the small molecules of the digested products of fats.

Q.4. What are chylomicrons?
Ans: Micelles get broken down into smaller globules called chylomicrons.

Q.5. Define egestion.
Ans: It is defined as the process of excretion of unabsorbed and unwanted food out of the body.

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