Our diet consists of proteins, carbohydrates and fats that have to be digested and absorbed by the intestines in order to nourish the body. While some of these substances are macromolecules, most of them have complex structures that cannot be directly absorbed by intestinal cells. The digestive process consists of breaking down these food materials into smaller molecules that can be absorbed.
Hydrolysis, the process by which chemical bonds are broken by the addition of a molecule of water, is the basic chemical reaction by which these molecules are split into their constituent, smaller molecules. Interestingly, although proteins, carbohydrates and fats are vastly different in their structures and make up, it is the same chemical reaction, hydrolysis, that is required for their break down during the chemical digestion process.
Protein Hydrolysis
Proteins are macromolecules with complex structures made of units called amino acids linked together. There are 20 amino acids and they combine in varying combinations and proportions to form different proteins. Each amino acid comprises a central carbon atom linked to a hydrogen atom (–H), an amine group (–NH2), a carboxyl group (– COOH) and a variable group, different in different amino acids.
Amino acids link with each other by peptide linkages to form proteins and polypeptides. A peptide linkage is formed by the removal of a molecule of water (H2O) or condensation. That is, this bond is formed by removing a hydrogen ion (– H) form one amino acid and a hydroxyl ion (–OH) from the next.
So when such a bond has to be broken, the hydrogen and hydroxyl ions have to be replaced. This is done by hydrolysis and regularly occurs in the digestive process as the bonds between amino acids are broken to split proteins. This reaction occurs in the presence of protein digesting enzymes.
Hydrolysis of proteins initially yields peptones and proteoses, which are further broken down into dipeptides and tripeptides and finally into individual amino acids.
Hydrolysis of Carbohydrates
Carbohydrates are made of sugar units called monosaccharides being strung together. A monosaccharide contains 5 or 6 carbon atoms with hydrogen and oxygen in the ratio of 2:1, arranged to form a ring. When two monosaccharides combine, one hydrogen ion (–H) is removed from one monosaccharide molecule and a hydroxyl ion (–OH) is removed from the next with the two molecules combining at the sites of removal.
So when the bond between two monosaccharides has to be broken, these ions have to be replaced. This is done by hydrolysis in the presence of specific carbohydrate-digesting enzymes or amylases. Hydrolysis of complex carbohydrates and polysaccharides yields disaccharides and finally monosaccharides, mainly glucose.
Fat Hydrolysis
Fats are mostly made of carbon and hydrogen (hydrocarbons) and very little oxygen. Most fats, especially those in our diet, are triglycerides or neutral fats which are molecules consisting of three fatty acids joined by one molecule of glycerol. These fatty acids are carbon-hydrogen chains with a –COOH group at the end. Fatty acids, just like vegetable oils, maybe saturated or unsaturated.
The formation of triglycerides from glycerol and three fatty acids involves condensation with the removal of three molecules of water. So the digestion of triglycerides will require the replacement of three molecules of water by hydrolysis. Fat hydrolysis occurs in the presence of enzymes called lipases.
Hydrolysis is the main chemical reaction in the digestive process and regularly occurs during chemical digestion as fats, proteins and carbohydrates are broken down into smaller molecules for absorption by the intestine.
Sources
Murray R.K., Granner D.K., et al, “Harper’s Illustrated Biochemistry” (McGraw Hill; 26th edition, 2003)
Guyton and Hall, “Textbook of Medical Physiology” (Saunders; 10th edition, 2003)
Lakshmi Ananth - Lakshmi Ananth is a practising ENT, head and neck surgeon and has faculty position at a medical college. She is specially interested in ...
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