mechanism of food digestion

Mechanism of Food DIGESTION

Digestion of Carbohydrates :

The diet of most of the animals including man consists of carbohydrates.

Depending upon the complexity, carbohydrates are of three types : polysaccharides, disaccharides and monosaccharides.

During the process of digestion, both poly and disaccharides are broken down to monosaccharides and in this form they can be absorbed into the body.

Some of these complex carbohydrates are starch and cellulose present in cereal grains, potato, fruits and tubers; sucrose present in cane sugar; lactose present in milk etc.

Enzymes that act on carbohydrates are collectively known as carbohydrases.

Pancreatic juice and intestinal juice also contain carbohydrate digesting enzymes.

Pancreatic juice contains pancreatic amylase that acts on starch to digest it into maltose, isomaltose and dextrin.

The secretions of the brush border cells constitute the intestinal juice or succus entericus.

This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases etc.

StarchMaltose + Isomaltose + Dextrin

Maltose + Isomaltose + DextrinGlucose

SucroseGlucose + Fructose

LactoseGlucose + Galactose

Digestion of Proteins:

Proteins are complex organic compounds made up of smaller units called amino acids. So in the process of digestion, all proteins are broken down to amino acids.

Enzymes that hydrolyse proteins are collectively known as proteases or peptidases.

Many of these enzymes are secreted in their inactive form or proenzymes as their active form would hydrolyse cellular and extracellular proteins of the organism itself.

Inactive enzymes are converted to their active form only at the site of action.

Saliva as such does not contain any protein digesting enzyme, but it can denature the uncooked natural proteins like the ones present in raw egg, unboiled milk or uncooked germinating seeds.

However, this is not a process of hydrolysis as in digestion.

Action of Gastric Juice:

The gastric glands of stomach produce a light coloured, thin and transparent gastric juice.

It contains water, hydrochloric acid (0.3%) and inactivated enzymes prorennin and pepsinogen.

The presence of HCl makes the medium highly acidic (pH = 1 or 2) so that pepsin can act on proteins to convert them into peptones and proteoses.

However, there is no pepsin in invertebrates.

Both prorennin and pepsinogen are converted to their active forms in the presence of HCl.

Pepsin and rennin can also do the same function once they are formed.

HCl also helps to kill bacteria and other harmful organisms that may be present along with the food.

Rennin acts on the casein protein of milk and converts it into paracasein which in the presence of calcium ions forms calcium paracaseinate (curdling of milk).

The function of rennin is then taken over by pepsin and other milk-coagulating enzymes. Pepsin then acts on it.

These reactions are summarized below:

Prorennin (inactive)Rennin (active)

Pepsinogen (inactive)Pepsin (active)

Milk CaseinParacasein

Paracasein + Ca⁺⁺

Calcium paracaseinatePeptones and proteoses

Action of Pancreatic and Intestinal Juice:

Both pancreatic juice and intestinal juice (succus entericus) are poured into small intestine.

Pancreatic juice contains trypsinogen, chymotrypsinogen, procarboxypeptidases, lipases, amylases or amylopsin, DNAases and RNAases.

All these enzymes of pancreatic juice can act only in the alkaline medium.

This change in the medium of food, from acidic to alkaline, is done by the bile juice.

Therefore, bile juice acts on the food before the action of pancreatic juice.

All these actions are given below :

Trypsinogen (inactive)Trypsin (active)

Peptones and proteosesPeptides

Chymotrypsinogen (inactive) is activated to chymotrypsin by trypsin itself.

Chymotrypsin is another important milk coagulating enzyme and can hydrolyse casein into paracasein which then coagulates to form calcium paracaseinate.

However unlike rennin, it acts in the alkaline medium.

Chymotrypsin can act on other proteins also.

Digestion of Fats:

Fat digestion starts only when the food reaches the stomach.

Some amount of gastric lipase is present.

Gastric lipase is of little importance except in pancreatic insufficiency.

Most of the fat digestion begins in the duodenum, pancreatic lipase being one of the most important enzymes involved.

Bile juice contains bile salts that break down the bigger molecules of fat globules into smaller droplets by reducing the surface tension of fat droplets.

This process is known as emulsification of fats.

Lipase is the chief enzyme that acts on emulsified fats.

It is present both in the pancreatic juice and intestinal juice.

Pancreatic lipase (steapsin) is the principal enzyme involved in fat digestion.

Lipase converts emulsified fats into diglyceride and monoglycerides releaSing fatty acids at each step.

At the end of digestion, all fats are converted into fatty acids, glycerol and monoglycerides.