Digestion and Absorption

What is Digestion?

Digestion: Digestion could be described as the process by which the food we eat passes through our bodies and are directed towards the purposes of providing the body with energy or building new cellular components and tissues, such as muscle or fat. The parts of food that the body cannot use and also other wastes from the body are eliminated in the form of excrement. 

Digestion can also be referred to as a form of catabolism that can be divided into two processes based on how food is broken down. These are mechanical digestion and chemical digestion respectively. 

Mechanical digestion can be defined as the physical breakdown of large pieces of food into smaller pieces which can be accessed by digestive enzymes. In chemical digestion, enzymes break down food particles into the small molecules that the body can absorb. 


Mechanism of Digestion

As the food enters the mouth it undergoes mechanical digestion by the action of mastication (chewing) and it also becomes when it is in touch with saliva. Saliva contains salivary amylase which starts the digestion of starch in the food.

After the food undergoes mastication and starch digestion, it will be in the form of a small, round slurry mass called a bolus. It travels down the oesophagus and into the stomach by the action of peristalsis. 

In the stomach, gastric juices initiate protein digestion. These juices mainly contain pepsin and hydrochloric acid. As the HCl may damage the stomach wall, mucus is secreted by the stomach which provides a slimy layer that acts as a shield against the damaging effects of the acid. 

Protein digestion takes place at the same time via mechanical mixing by peristalsis allowing the mass of food to mix with the digestive enzymes.

After some time a thick liquid called chyme is produced which enters the duodenum where it mixes with digestive enzymes from the pancreas and bile juice from the liver and then passes through the small intestine. After the chyme is completely digested, it is absorbed into the blood. 

It should be noted that 95% of nutrient absorption occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the large intestine where the pH is slightly acidic about 5.6 ~ 6.9. Some vitamins, such as vitamin K  and biotin are produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material exits from the rectum during defecation.

[Image will be Uploaded Soon]


What is Absorption?

Absorption: 

The absorption of nutrients takes place by diffusion through the wall of the small intestine which is a part of the gastrointestinal tract between the stomach and the large intestine.

The main function of the small intestine is the absorption of minerals and nutrients found in food. Digested nutrients pass across the blood vessels in the wall of the intestine through a process of diffusion. 

The mucosa or the inner wall of the small intestine is lined with columnar epithelial tissue. The absorbed minerals and nutrients are transported by blood vessels to different parts of the human body where they are used to build proteins, carbohydrates and lipids required by our body. The food that remains undigested and unabsorbed passes into the colon or the large intestine.


Digestion and Absorption of Carbohydrates

Carbohydrate is one of the primary building blocks of life and the source of energy. The following is a brief summary of how digestion and absorption of carbohydrates take place.


Digestion of Carbohydrates

There are three types of carbohydrates which are absorbed by the small intestine; these are

  1. Glucose

  2. Galactose

  3. Fructose

The digestion of starch starts in the mouth mainly facilitated by salivary amylase. The majority of carbohydrate digestion is carried out in the stomach and the duodenum. The main enzyme that plays a role in digestion is pancreatic amylase, which yields disaccharides from starch by digesting the alpha 1-4 glycosidic bonds. The disaccharides produced (alpha-dextrinase, sucrose and maltase) are all converted to glucose by brush border enzymes.

Disaccharides occurring naturally in food do not require amylase to break them down. Brush border enzymes such as lactase, sucrose and trehalase hydrolyse these compounds into molecules of glucose, fructose and galactose.


Absorption of Carbohydrates

Glucose and galactose are absorbed by the apical membrane with the help of the secondary active transport (along with Na+) through the Sodium-Glucose co-transporter (SGLT1). Both of these carbohydrates exit the cell via GLUT2 receptors across the basolateral membrane into the blood. Another carbohydrate fructose enters the cell by facilitated diffusion via GLUT5 and is transported into the blood via GLUT2 receptors.

[Image will be Uploaded Soon]


Digestion and Absorption of Proteins

Protein is another important component of various cellular components. The following is a brief summary of how digestion and absorption of carbohydrates take place.


Digestion of Protein

Protein digestion initiates in the stomach with the action of pepsin. It breaks down protein into amino acids and oligopeptides. The process of protein digestion is completed in the small intestine with brush border enzymes and pancreatic enzymes. They split oligopeptides into amino acids, di and tri-peptides.


Protein Absorption

Amino acids are absorbed via a sodium co-transporter, in a similar mechanism to the monosaccharides. The absorbed amino acids are then transported across the basolateral membrane via facilitated diffusion. Di and tri-peptides are absorbed via separate H+ dependent co-transporters and once they are inside the cell, these peptides are hydrolysed to amino acids.

[Image will be Uploaded Soon]


Digestion and Absorption of Lipids

Lipids or fats function as energy storage units. Fat digestion and absorption take place in a different manner than the other two building block of cellular components. The following is a brief summary.


Lipid Digestion

Lipids are hydrophobic in nature and that is why they are poorly soluble in the aqueous environment of the digestive tract. Their digestion is initiated by lingual and gastric lipases. However, these enzymes only digest 10% of ingested lipids.

The remainder part of the lipids is digested in the small intestine. The bile assists in digestion by emulsifying fat goblets into smaller chunks called micelles.

Pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase are the three major enzymes involved in lipid digestion. They hydrolyse the micelles and break them down into fatty acids, cholesterol, monoglycerides and lysolecithin.


Absorption of Lipids

The products from the digestion of lipids are released at the apical membrane and diffuse into the enterocyte. Inside the cell, these products are re-esterified to form the original lipids, triglycerides, phospholipids and cholesterol. The lipids are then packaged inside apo-proteins to form a chylomicron. The chylomicrons are too large to enter circulation, so they enter the lymphatic system via lacteals.

[Image will be Uploaded Soon]

FAQ (Frequently Asked Questions)

Q1: What are the Differences Between Absorption and Assimilation?

Absorption 

Assimilation

The process of taking digested simple molecules into the bloodstream or lymph via the microvilli and intestinal villi.

This process includes synthesising new compounds from the absorbed molecules.

It takes place in the small intestine.

It takes place in the liver.

During this process, nutrients are added to the bloodstream.

Nutrients are taken out of the bloodstream.

Q2: Where is Protein Digested?

Protein digestion takes place in the stomach and duodenum. Pepsin, which is secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas, breaks down food proteins into polypeptides. These polypeptides then are further broken down by various exopeptidases and dipeptidases into amino acids. 


The digestive enzymes however are generally secreted as their inactive precursors known as  zymogens. For example, trypsin is secreted by the pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides.

Q3: Explain the Process of Digestion of Carbohydrates Fats and Proteins

Please have a look at the respective sections in the article.

Q4: How is Protein Digested?

Please have a look at the digestion of the protein part of the article.