Sulav Indra Paul

Digestion is the mechanical and chemical breaking down of

food into smaller components, to a form that can be

absorbed, for instance, into a blood stream. Digestion is a

form of catabolism; a break-down of macro food molecules

to smaller ones.

The prefix “Macro” is from the Greek and means big or large, used because

macronutrients are required in large amounts.

There are three broad classes of macro-nutrients. Such as:

1. Proteins: Molecules contain nitrogen, carbon, hydrogen and oxygen. They act

as biological catalysts (enzymes), and act as molecules of immunity.

2. Carbohydrates (saccharides): Molecules consist of carbon, hydrogen and

oxygen atoms. A major food source and a key form of energy for most

organisms.

3. Fats (Lipids): Molecules consist of carbon, hydrogen, and oxygen atoms. The

main constituents are Vitamins A, D, E and K, cholesterol.

The Process of protein digestion in fish are given below-

1.1 Mastication: Protein digestion begins in fish mouth, where chewing

combines saliva with food, also called mastication, and it breaks down into

smaller particles.

1.2 The Stomach (In Carnivorous fish): The stomach produces HCl, which is

needed to activate protein digestion.

1.3 Enzymes: The stomach secretes an enzyme called protease that removes the

bonds holding together the long chains of amino.

1.4 The Pancreas: The pancreas releases trypsin, another digestive enzyme, into

the small intestine. The protein molecules are reduced in size until they can pass

through the intestinal wall.

1.5 The Bloodstream: Once the protein molecules are reduced to their smallest

component parts, they are ready to enter the bloodstream

The process of carbohydrates digestion in fish body are given below-

2.1 Mouth: Carbohydrate digestion begins in the mouth. The salivary glands in the

mouth secrete saliva, which helps to moisten the food, which begins the process of

breaking down the polysaccharides in the carbohydrate food.

2.2 Stomach ( In Carnivorous fish ): After chewing into smaller pieces and mixed

with salivary amylase and other salivary juices, it is swallowed and passed through

the esophagus. The mixture enters the stomach where it is known as chyme. The

stomach produces acid which destroys bacteria in the food and stops the action of the

salivary amylase.

2.3 Pancreas and Intestine: In response to chyme being in the duodenum, the

pancreas releases the enzyme pancreatic amylase, which breaks the polysaccharide

down into a disaccharide. The small intestine then produces enzymes called lactase,

sucrose and maltase, which break down the disaccharides into monosaccharaides.

The monosaccharaides are single sugars that are then absorbed in the small intestine.

3.1 Fat Emulsification – First Step in the Digestion of Fats:

The problem with fat digestion is that the digestive enzymes are water-soluble and so

cannot dissolve into globules of fat to act on the triglycerides they contain.

Fat droplets have to first be broken down into much smaller sizes so that these enzymes

have plenty of surface area to work on. This process is called emulsification of fat.

Fat globules are somewhat fragmented when they get churned with other food in the

stomach. But for the most part, emulsification of fat is achieved by the phospholipid

lecithin and bile salts, both secreted by the liver in bile.

Bile salts and lecithin have both water soluble and fat soluble portions. The fat soluble

portions dissolve in the fat globules while their water soluble portions protrude from the

surface of the globules, soluble in the watery fluids in the lumen of the small intestines.

Lipases are the enzymes required in the breakdown of lipids and are secreted by various

parts of the digestive system:

Lingual lipase – secreted in the mouth by the lingual glands.

Pancreatic lipase – secreted in pancreatic juice.

Enteric lipase – present in small quantities in enterocytes or intestinal cells.

Pancreatic lipase acts on emulsified fat splitting the triglycerides into free fatty acids and 2-

monoglycerides by a process called hydrolysis.

3.2 Lipase Enzymes and Lipid Digestion:

After the enzymatic action and hydrolysis of the triglycerides, bile salts have yet another

role to play in the fat digestion process.

Bile salts help by forming what are called ‘micelles’. (Micelles are tiny globules, each 3

to 6 nanometers in diameter, made up of about 20 to 40 molecules of bile salts).

Bile salts have both fat and water soluble parts – a fat-soluble sterol nucleus and a

water-soluble polar group.

So a number of these molecules collect around the products of fat digestion and form

micelles, the sterol nuclei in the centre and the polar groups on the outside.

The micelles dissolve easily in the digestive fluids in the intestines.

So not only do these micelles help in quickly removing products of fat digestion from

the area of their production, they also help transport them to the intestinal cells which

absorb them.

3.3 Bile Salts and ‘Micelle’ Formation in Fat Digestion:

The main function of macronutrients is to provide energy,

counted as calories. While each of the macronutrients provides

calories, the amount provided by each varies. Carbohydrate

provides four calories per gram, protein also four while fat

provides nine.