Process of Digestion

 Process of Digestion

Introduction

The major nutrients required by the living body can be classified as carbohydrates, fats, proteins, vitamins minerals and water. Of these carbohydrates, fats and proteins generally cannot be absorbed in their natural forms through the gastrointestinal mucosa and, therefore, are required to be digested in absorbable forms with preliminary digestion.

The processes by which carbohydrates, fats, and proteins are broken into small enough compounds with the help of digestive enzymes for absorption and the mechanisms by which the digestive end products, as well as water, electrolytes, and other substances, are absorbed are called digestion and absorption.

Digestion of the Various Foods

Hydrolysis of Carbohydrates

Almost all the carbohydrates of the diet are either large polysaccharides or disaccharides, which are combinations of monosaccharides bound to one another by condensation. This means that a hydrogen ion (H+) has been removed from one of the monosaccharides, and a hydroxyl ion (−OH) has been removed from the next one. The two monosaccharides then combine with each other at these sites of removal, and the hydrogen and hydroxyl ions combine to form water (H2O).

When carbohydrates are digested, this process is reversed and the carbohydrates are converted into monosaccharides. Specific enzymes in the digestive juices of the gastrointestinal tract return the hydrogen and hydroxyl ions from water to the polysaccharides and thereby separate the monosaccharides from each other. This process, called hydrolysis, is the following (in which R″-R′ is a disaccharide):

Hydrolysis of Fats

Almost the entire fat portion of the diet consists of triglycerides (neutral fats), which are combinations of three fatty acid molecules condensed with a single glycerol molecule. During condensation, three molecules of water are removed. Digestion of the triglycerides consists of the reverse process: the fat-digesting enzymes return three molecules of water to the triglyceride molecule and thereby split the fatty acid molecules away from the glycerol. Here again, the digestive process is one of hydrolysis.

Hydrolysis of Proteins

Proteins are formed from multiple amino acids that are bound together by peptide linkages. At each linkage, a hydroxyl ion has been removed from one amino acid and a hydrogen ion has been removed from the succeeding one; thus, the successive amino acids in the protein chain are also bound together by condensation, and digestion occurs by the reverse effect: hydrolysis. That is, the proteolytic enzymes return hydrogen and hydroxyl ions from water molecules to the protein molecules to split them into their constituent amino acids.

Chemistry of digestion

Therefore, the chemistry of digestion is simple because, in the case of all three major types of food, the same basic process of hydrolysis is involved. The only difference lies in the types of enzymes required to promote the hydrolysis reactions for each type of food. All the digestive enzymes are proteins which act at various places of digestive tract.