Post on 01-Jul-2015
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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.