Physiology of digestion

Digestion

Digestion is the process by which the complex food molecules are broken down
with the help of digestive enzymes into smaller and simpler ones, which then
become ready for absorption and assimilation.
Digestive system
The digestive system is one kind of system to digest food materials so that they can
be absorbed and utilized in the metabolic process of the fish and to remove toxic
particles of food substances.it includes digestive tract and digestive gland.
Types of natural food of fish

The natural food of fishes can be divided into-

 Main or basic food consumed by the fish under favorable condition
 Occasional or secondary food which is eaten by the fish in small quantities
when available
 Incidental food which enters the gut along with other items
 Emergency food or obligatory food when is ingested in the absence of basic
food and on which the fish is able to survive.
Types of fish categories
1. Carnivores- The fishes in contrast to herbivore have shorter gut, the intestine
is straight, very little coils are present. Some of the carnivores possess
intestinal caecae. They prey on small organisms and consume high
percentage of animals such as copepods, Daphnia and insects.The examples
of carnivorous fishes are Walla go attu, Mystus seenghala, Mystus
cavassius, Mystus vittatus, Channa striatus, Channa marulius, Channa
punctatus, Notopterus chitala, Rita rita, etc.

 Piscivores- fish eaters.

 Benthophages- eat animals living in or on sediments.
 Zooplanktivores- eat planktonic animals, eg- Daphnia, Diaptomus

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 2. Omnivores- eat both plant and animal food. Omivorous fishes like Cyprinus
carpio, Cirrhina mrigala, Puntius etc. are consuming both plants and
animals. The rotifers, mud and sand are also found in the alimentary canal.
Their gut length intermediates between carnivorous and herbivorous fishes
3. Herbivores - eat plant food, either large algae and water plants or
phytoplankton (Euglena, microsystis) and microalgae. They consume about
70% unicellular algae, filamentous algae and aquatic plants. In addition to
plant material these fishes also consume 1-10% of animal food and mud.
The common examples are Labeo species Osphronemus goramy,
Oreochromis etc. Herbivorous fishes have long and coiled intestine
4. Detritivores- They consume detritus along with zooplanktons and
phytoplanktons.

The fishes can also be named on the basis of modification of buccopharynx:

(1) Predator - they possess well developed grasping and holding teeth, e.g.
sharks, pike and gars, etc)
(2) Grazers - they take the food by bite. These fishes feed on plankton and on
bottom organisms, e.g., bluegill (Lepomis macrochirus), parrot fish and
butterfly fish.
(3) Strainers - they have efficient straining or filtering adaptation due to the
arrangement of gill rakers forming sieve for straining the food material.
They are plankton feeders.
(4) (4) Suckers - the fishes have inferior mouth and sucking lips. The response
depends upon stimulus of touch e.g., sturgeons, Osteochilus , etc.
(5) Parasitic - amongst fishes, the deep sea eel (Simenchelys parasiticus) is
parasitic in nature. Lamprey and hagfish are parasitic.
Various parts of the alimentary canal

 Mouth
 Buccal cavity/ Mouth cavity
 Pharynx
 Oesophagus
 Stomach- it has two parts a) Proximal /cardiac part b) Distal/ pyloric part
 pylorus- contain pyloric ceaca

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  Intestine
 Rectum and
 Anus.
Herbivore and detritivores fishes have longer intestine than carnivore fishes
 Carnivore fishes generally have stomach and a part of the digestion takes
place in the stomach. Digestion of food of animal origin is easier than plant
foods which have harder cell.
 Herbivore fishes generally have no stomach so most of the digestion takes
place in intestine
 Detritivore fishes consume non-living organic matter. Food of detritivore
fishes consists of very high percentage of non food materials of the total gut
contents
Digestion of different Macronutrients in fish.
There are three broad classes of macro-nutrients. Such as:
1. Proteins: Molecules contain nitrogen, carbon, hydrogen and oxygen atoms.
A major food source and a key form of energy for most organisms.
2. Carbohydrates (saccharides): Molecules consist of carbon, hydrogen and
oxygen atoms. A major foor 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.
Digestion Process
The materials which enter into the intestine after leaving the stomach, known as
chyme, is acidic in condition (the mixture of food, mucus and gastric juices
became a slurry called chyme) But intestinal enzymes work in alkaline condition.
In intestine, alkaline condition is created due to :
- Bile, the secretion of liver which is stored in gall bladder
- Bicarbonate buffering compounds from pancreas, which neutralize the
acidic chyme
After coming out of the stomach and upon enteting the small intestine, the food is
acidic due to the action of the HCl. However, the enzymes that react with the food
in the small intestine need an alkali medium. Bile does this; it neutralises the food
and then provides an alkali medium for the various enzymes to act. Additionally,
bile also emulsifies fat.

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Bile is not acid. It's an alkaline fluid consisting of bile salts, bile pigments,
cholesterol and lecithin. It is produced by the liver, stored in the gallbladder and
released intermittently into the duodenum, the upper part of the small intestine,
when needed to digest fat.
The pancreatic juices and bile that are released into the duodenum, help the body to
digest fats, carbohydrates, and proteins.
Intestine
When acidic chyme enter into intestine, the secretin and cholecystokinin (CCK)
hormones stimulate the secretion of pancreatic enzymes and bile from liver.
The secretin and CCK secreted from intestinal epithelial cell wall and is
transported by blood stream to pancreas, gall bladder and liver.
Function of secretin: increases the secretion of pancreas and bile from liver
Function of CCK: increases the pancreatic secretion and cause contraction of gall
bladder to secrete bile
In intestine the source of enzymes are as follows:
- Pancreas (secrets zymogens of enzymes and bicarbonate)
- Secretory cell or intestinal wall
- Intestinal microflora
Digestion mechanism of different classes of food
Food substances of fish
a) protein, carbohydrate and fat
-digested first and then absorbed
b) Vitamins , minerals and water
-directly absorbed.
A protein precursor, also called a pro-protein or pro-peptide, is an inactive
protein (or peptide) that can be turned into an active form by post-translational
modification, such as breaking off a piece of the molecule or adding on another
molecule. The name of the precursor for a protein is often prefixed by pro-.
Examples include proinsulin and proopiomelanocortin, which are both
prohormones.
Protein precursors are often used by an organism when the subsequent protein is
potentially harmful, but needs to be available on short notice and/or in large
quantities. Enzyme precursors are called zymogens or proenzymes. Examples are
enzymes of the digestive tract in humans.
Some protein precursors are secreted from the cell. Many of these are synthesized
with an N-terminal signal peptide that targets them for secretion. Like other
proteins that contain a signal peptide, their name is prefixed by pre. They are thus

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called pre-pro-proteins or pre-pro-peptides. The signal peptide is cleaved off in
the endoplasmic reticulum. An example is preproinsulin.

Protein Digestion

1. Digestion of protein in fish body

The process of protein digestion in fish are given below:
1.1 Mastication: Protein digestion begins in a fish mouth. Where chewing
combines mucous (saliva may or may not present) with food, also called
mastication and it breaks down into smaller particles
1.2 The stomach(In carnivorous fish): The stomach produces HCI, which is
needed to protein digestion
1.3 Enzymes: The stomach secrets an enzyme called protease that removes the
bonds holding together the long chains of amino acid
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.

Enzymes involved in protein digestion are called proteolytic enzyme. The

proteolytic enzymes are two types
-Endopeptidases
-Exopeptidases

Endopeptidases : The enzymes which breakdown the central peptide bonds and
form smaller peptide chains are called endopeptidases
eg- pepsin, tripsin, chymotripsin.

Pepsin- a major proteolytic enzyme in fish stomach. Active at pH 1.5-3.0

Exopeptidases : The enzymes which breakdown the terminal peptide bonds and
separate amino acid are called exopeptidases.
Eg- Carboxypeptidases.

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 Some intestinal proteolytic enzymes (Proteases)

Intestinal Sources Zymogen form End Types of

proteases product of enzymes
digestion
Trypsin Pancreas Trypsionogen Shorter Endopeptidas
chain e
peptide
Chymotripsin Pancreas Chymotripsinogen Shorter Endopeptidas
chain e
peptide
Carboxypeptidas Pancreas Procarboxypeptidas Single Exopeptidase
e e amino
acid
Aminopeptidase Epitheliu Proaminopeptidase Splits off Exopeptideas
m cells of individual e
intestine A.A. from
amino
terminal
end of
peptide
chain
Dipeptidase Epitheliu Breakdow Exopeptideas
m cells of n e
intestine dipeptides
into
constituen
t AA

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Enzymes Zymogen
Pepsin Pepsinogen
Trypsin Trypsinogen
Chymotriopsin Chymotripsinogen
Carboxypeptidase Procarboxypeptidase
Aminopeptidase Proaminopeptidase
Dipeptidase Prodipeptidase
Lipase Prolipase
Esterase Proesterase
Zymogen: Inactive form of enzyme is called zymogen

 A zymogen, also called a proenzyme, is an inactive precursor of an enzyme.

 A zymogen requires a biochemical change (such as a hydrolysis reaction
revealing the active site, or changing the configuration to reveal the active
site) for it to become an active enzyme.
 The biochemical change usually occurs in Golgi bodies, where a specific
part of the precursor enzyme is cleaved in order to activate it.
 The inactivating piece which is cleaved off can be a peptide unit, or can be
independently folding domains comprising more than 100 residues.
 Although they limit the enzyme's ability, these n-terminal extensions of the
enzyme or a “prosegment” often aid in the stabilizing and folding of the
enzyme they inhibit.
 The pancreas secretes zymogens partly to prevent the enzymes from
digesting proteins in the cells in which they are synthesised.
 Enzymes like pepsin are created in the form of pepsinogen, an inactive
zymogen. Pepsinogen is activated when chief cells release it into the gastric
acid, whose hydrochloric acid partially activates it.
 Another partially activated pepsinogen completes the activation by removing
the peptide, turning the pepsinogen into pepsin. Accidental activation of
zymogens can happen when the secretion duct in the pancreas is blocked by
a gallstone resulting in acute pancreatitis.

Zymogen form of some enzymes:

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Carbohydrate digestion
2. Digestion of carbohydrate in fish body

The process of protein digestion in fish are given below:

2.1 Mouth: Carbohydrate digestion begins in a fish mouth. Where chewing
combines mucous (saliva may or may not present) with food, also called
mastication and it breaks down into smaller particles
2.2 Stomach: After chewing into smaller pieces it passed through the
esophagus into stomach where it is known as chime. Stomach produces
acid that destroy bacteria in the food.
2.3 Pancreas and intestine: The pancreas releases the enzyme pancreatic
amylase which breaks down the polysaccharide into disaccharide. The
small intestine then produces enzymes called lactase, sucrose and maltase,
which breaks down the disaccharides into monosaccharide. The
monosaccharide are single sugars that are absorbed in the intestine.
The enzymes associated with carbohydrate digestion are called
carbohydrases.
The common carbohydrates in fish food are:
Glycogen- storage carbohydrate of animals
Starch- storage carbohydrate of plants
Chitin – structural carbohydrate of animals
Cellulose- structural carbohydrate of plants.

Starch and Glycogen

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Amylase- hydrolyze the starch and glycogen
Amylase are 2 types
α Alpha amylase
β Beta amylase
Digestive amylase of animals is generally alpha amylase which acts on α (1,4)
glucosidic linkage of both starch and glycogen and converts into a mixture of
glucose and maltose.
Starch contain- amylose and amylopectin [contain both α (1,4) and α (1,6) linkage]
Glycogen is structurally similar to amylopectin but their only difference is that
glycogen contain more branch than amylopectin (eg- contain more α (1,6) linkage).
Chitin
Chitin is breakdown by the chitinase enzyme. This enzyme is found particularly in
fish species that eat crustaceans or zooplankton
-secreted by stomach and pancreas. Additional source, bacteria (living within the
intestine of fish)
Chitin → di and trimers of N-acetylglucosamine → Monomer ( N- acetyl
glucose amine).
Cellulose
Cellulose is broken down by the enzyme cellulase. Fishes generally can not
produce cellulose (probably derived from intestinal bacteria). Among cellulase
producing bacteria vibrio and Aeromonas are usually dominant in fish.
Cellulase breakdown the (1,4) linkage of cellulose and produce glucose

Cellulose → Glucose

Other Source Substrate End product

cabohydrases
Maltase Intestinal Maltose Glucose
epithelium
Sucrase Intestinal Sucrose Glucose and
epithelium Fructose
Lactase Intestinal gland Lactose Galactose and
Glucose
Cellobiase Cellobiose Glucose

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Fat digestion
At first large fat molecules are converted into smaller globules by the action of bile
and then they can be hydrolyzed by lipolytic enzymes.This process known as
emulsification. Lipase and esterases (called lipolytic enzyme) are involved in fat
digestion, secreted from pyloric caeca and intestinal mucosa and pancreas.
No fat digestion- occurs in mouth
Very little digestion – in stomach
Most of the fat is digested in the intestine (at pH 6.8-7.6)
Fat Emulsification
Bile acids play their first critical role in lipid assimilation by promoting
emulsification. As derivatives of cholesterol, bile acids have both hydrophilic and
hydrophobic domains (i.e. they are amphipathic). On exposure to a large aggregate
of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid,
with the hydrophilic domains remaining at the surface. Such coating with bile acids
aids in breakdown of large aggregates or droplets into smaller and smaller droplets.

Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished

predominantly by pancreatic lipase. The activity of this enzyme is to clip the fatty
acids at positions 1 and 3 of the triglyceride, leaving two free fatty acids and a 2-
monoglyceride
Lipase is a water-soluble enzyme, and with a little imagination, it's easy to
understand why emulsification is a necessary prelude to its efficient activity.
Shortly after a meal, lipase is present within the small intestine in rather huge
quantities, but can act only on the surface of triglyeride droplets. For a given
volume of lipid, the smaller the droplet size, the greater the surface area, which
means more lipase molecules can get to work.

As monoglycerides and fatty acids are liberated through the action of lipase, they
retain their association with bile acids and complex with other lipids to form
structures called micelles. Micelles are essentially small aggregates (4-8 nm in
diameter) of mixed lipids and bile acids suspended within the ingesta. As the
ingesta is mixed, micelles bump into the brush border of small intestinal
enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up
into the epithelial cells.

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Absorption
Transfer of digested food materials from the digestive tract into blood stream and
lymphatic system
Types of absorption:
i) Passive diffusion
ii) Active transport
iii) Pinocytosis

Passive diffusion Active transport Pinocytosis

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  It does not  It requires  Intake of food
require expenditure of materials by fusion and
expenditure of energy constriction of the
energy  Occurs against portion of the
 Occurs in concentration membrane of
response to gradient absorbing cell
concentration  Digested food  Fairly large molecules
gradient materials are are absorbed
 Digested food transported by
materials are not carrier system.
transported by Carriers are
carrier system highly specific for
 Relatively smaller their substances
food molecules  Relatively smaller
are absorbed food molecules
are absorbed

Absorption site
-Occurs mainly in intestine, also stomach, rectum can be the absorption site.
The enterocytes are the absorptive cell of the intestinal epithelium.
-The enterocytes form villi and microvilli which provide large surface area for
absorption
Villi
Are tiny finger like structures that protrude from the wall of the intestine and have
additional structure. Extensions called microvilli (singular microvillus) which
protrude from epithelial cells lining villi.They increase the absorptive area of the
intestinal wall. Digested nutrients pass into the villi.Cirrculating blood then carries
these nutrients away

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 Figure: Lumen of small intestine
Digestibility
Of the food ingested, only a part is absorbed and used for metabolic processes. The
non-absorbed portion of the diet leaves the body as faeces

Fish anatomy

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Digestive system of fish

• Gall bladder = store & concentrate bile. Distributed to small intestine to aid
digestion
• Pyloric caeca = enlargement of stomach surface, increase absorptive area of
GI (gastrointestinal) tract
• Spleen = Blood filter, important organ for the immune system

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