Absorption of Digested Food - Importance, methods, Work

Digestion starts from the movement we put our food in our mouth. The food is broken down using salivary enzymes and the process of absorption is further carried out in the entire alimentary canal. The food gets absorbed at different locations in the alimentary canal such as the stomach, small intestine and large intestine. Digestion and Absorption is one of the important topics in Biology from the chapter Digestion and Absorption.

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This Story also Contains

1. The Digestive System and Absorption
2. Functions of each part of the Digestive System
3. The Small Intestine
4. Mechanism of Absorption

The Digestive System and Absorption

It is an elaborate duct which acts as a pathway that transports food substances in the digestive system to become nutrients that can be absorbed by the body. The digestive tract is the mouth, oesophagus, stomach, small intestine and large intestine and all of them have importance.

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Functions of each part of the Digestive System

• Mouth: Also involves mechanical digestion by churning and since it contains enzymes it also involves chemical digestion.

• Esophagus: Moves food through the mouth to the stomach with the help of peristalsis.

• Stomach: Swallowed food is digested even further by gastric juices in the stomach producing a semi-liquid substance called chyme.

• Small Intestine: The main place where the micronutrients pass through and are actively absorbed with the assistance of enzymes and bile. As for the villi, and microvilli, the structure enhances the surface area for better absorption of the nutrients in the bloodstream.

• Large Intestine: Excretes water and electrolytes and synthesizes and eliminates solid waste.

Altogether, such components allow the extraction and absorption of nutrients from foods, responding to the energy and nutrient demands of the body.

Site of Absorption of Digested Food

The main site of absorption of the digestive food begins in the small intestine. It is the only site where absorption of food begins when the digested food molecules that pass through the wall of the small intestine reach into the bloodstream and when the food particles reach the bloodstream they are ready to be transported to the different parts of the body wherever they are needed.

Diagram of the Digestive System

The Small Intestine

The small intestine is the primary site of absorption

Anatomy of the Small Intestine

The duodenum is the first part of the small intestine with a length of approximately 25 cm, the second part is the jejunum, which is fairly long, around 252 cm, and the third part is the ileum which has a length of 152 cm.

• Duodenum: As stated, the small intestine comprises the first section in which most of the chemical digestion takes place. The organ also gets chyme from the stomach, bile from the liver, and digestive enzymes from the pancreas.

• Jejunum: The middle part which is the bulb mainly acts as a nutrient sponge.

• Ileum: The last part that acts as the primary absorptive site of vitamin B12 as well as bile salts, and every nutrient that is not absorbed in the jejunum.

Structure of the Small Intestine Wall

On the side of the small intestine, the lining is made of some protrusions known as the villi and microvilli.

• Villi: Pin-like structures which give the walls a larger area on which the nutrients may be absorbed. Every villus has a network of capillaries as well as lacteal which are vessels that transport absorbed nutrients in the bloodstream and lymphatics respectively.

• Microvilli: Much even tinier structures on the epithelial cells of the villi also have a brush border that enhances the surface area and contains enzymes that help in the last steps of digestion and absorption of nutrients.

Diagram of the Small Intestine

Mechanism of Absorption

The methods of nutrient uptake within the small intestine include several different means by which substances are allowed to pass through the wall of the small intestine into either the bloodstream or the lymphatic system.

Simple Diffusion

It lets small, non-polar molecules like gases and lipids pass across the cell membrane from a region where there were many to a region of few without using energy.

Passive Transport

As with simple diffusion, this one is also a passive process and transports water and other small molecules across the membrane down their concentration gradient.

It includes two types:

Diffusion

• Water and specific ions that do not require any carrier proteins to move across the cell membrane.

Facilitated diffusion

• This process involves carrier proteins or channels in the cell membrane through which large or polar molecules such as glucose and amino acid diffuse across the membrane against their concentration gradient without consuming energy.

Active Transport

While in passive transport, molecules move through the selectively permeable membrane naturally, in active transport, molecules need a form of energy usually ATP to move from where they are in high concentration to where they are low. This process is useful in transporting nutrients such as glucose and amino acids when their concentration is lesser on the side of the intestinal cells as compared to the concentration in the cells lining the intestine.

Sodium-Potassium Pump

A specific kind of transport in which the molecule ATP is utilised in moving sodium ions out of the cell as well as potassium ions into the cell. Earlier, such a pump is used to maintain the cells’ electrochemical potential essential for the uptake of nutrients and overall cellular processes.

Endocytosis

When large particles or whole amounts of fluids are taken right inside the cell, it is referred to as this type of transport. This mechanism is very important in engulfing large molecules and particles that cannot diffuse through the cell membrane.

Diagrams showing passive and active transport mechanisms

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