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Science -Std9
DIGESTIVE SYSTEM
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Digestion : It is the process which gives us energy to perform various activities.
Digestion is defined as the process during which complex, non-diffusible food
substances are converted into simple, diffusible form by enzymes.
Enzymes are proteins which help to speed up chemical reactions. They play a very
important role in digestion.
Characteristics of enzymes :
1. They are proteins and thus get denatured or destroyed by heating.
2. They act within a narrow range of temperatures between 35°C and 40°C. known as
optimum temperature.
3. They act only at a specific pH i.e at a particular acidity or alkalinity. Eg : Pepsin
requires an acidic medium while trypsin requires an alkaline medium
4. They act only on one substrate i.e they are substrate specific. Eg Amylopsin acts
only on the starch in the duodenum
5. They speed up the chemical reactions without interfering in them.
Constituents of food :
1. Carbohydrates : They include sugars, starch, cellulose , etc. Carbohydrates breakdown
to form simple sugars.
a. Sugars- They are of 3 types.
i) Monosaccharides – They have a general chemical formula C6H12O6 and are the simple
sugars
Eg.Glucose, Fructose, Galactose.
ii) Disaccharides : They have a general chemical formula C12H22O11. Eg : Maltose, Lactose,
Sucrose.
iii) Polysaccharides : They are complex sugars made of long chains of monosaccharides.
Eg .Cellulose, Starch, Glycogen.
2..Proteins : Proteins are made up of chains of nitrogen containing building blocks called
amino acids.Proteins are broken down in the stomach during digestion by enzymes known as
proteases into smaller polypeptides to provide amino acids for the body, including the
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essential amino acids that cannot be biosynthesized by the body itself. Proteins are used for
building and growth of body cells. Eg. Casein- Milk protein, Albumin- Egg protein
3. Fats (Lipids) : Fats are composed of carbon, hydrogen, nitrogen, fatty acids. Fats are
broken down by the enzymes to form fatty acids and glycerol.
4. Vitamins : They are required in small amounts. Eg : Vitamins A, B,C,D,E,K
Digestive system :
Human digestive consists of
i)
Alimentary canal ii) Digestive glands
Alimentary canal is a long tubular structure varying in diameter starting from mouth to
anus and is 9m long. Whereas the digestive glands include the salivary glands, liver and
pancreas.
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Alimentary Canal
It shows the following parts :
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1.MOUTH : It is the outermost opening of the alimentary canal. It is covered by lips. Mouth
leads to a buccal cavity which has a tongue and teeth. The roof of the buccal cavity is the
hard palate.
Function : Mouth is used for ingestion of food. Muscular tongue helps in mixing saliva with
food, swallowing and in speech. Tongue shows presence of various taste buds which are used
to detect various tastes of food.
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TEETH: Teeth cut and break the food into smaller pieces. The process of crushing
and grinding of food in the mout is called as mastication. There are total 20 teeth in
the buccal cavity of a child and 32 in an adult human being which form the permanent
set of teeth.
Structure of tooth :
Premolar
Canine
Each tooth is made up of 2 parts
a) The root is embedded in the jaw bone
b) The crown which is the visible portion.
Enamel is the shiny coating over the teeth. Dentine is below the Enamel and has
canals which radiate outwards .Cementum fixes the root in its position. Pulp is the
soft connective tisuue in the central space (pulp cavity) of tooth
There are 4 types of teeth in the permanent set with different functions.
Teeth in 1 jaw :
i)Inscisors (4) : They are the 4 teeth in the front row of the jaw.They are used for biting or
cutting into food.
ii) Canines (2) : they are a pair of sharp teeth with pointed edges used for tearing into the
food. They are present on either sides of incisors.
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iii) Premolars (4): They are two on each side of the jaw. They are used for crushing and
grinding of the food.
iv) Molars (6) : They are three on each side of the jaw. They are also used for crushing and
grinding.
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Salivary glands :They secrete saliva. Secretion of saliva increases with the vision
taste and smell of food.
 Composition of saliva :
1. Saliva consists of 99 percent water, mucus , salts and enzyme salivary amylase (ptyalin)
Function : Salivary amylase converts the starch in the food to maltose
Salivary amylase
Starch
Maltose
2.Saliva mixes with the food to form a bolus which is swallowed.
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Tongue : Structure : It is a fleshy organ. It has sensory receptors called taste buds.
Function :They help in detecting the taste of various foods. Tongue helps to mix saliva with
the food. It presses against the hard palate and pushes the bolus into the pharynx.
There is a flap called epiglottis present at the beginning of the larynx.It closes the opening
of the larynx when food is being swallowed so that it enters only the oesophagus and not the
larynx.
The soft ball of food formed in the mouth after chewing is called bolus.
2. OESOPHAGUS :Structure :It is a long thin tube connecting the buccal cavity to the
stomach. It pierces through the diaphragm to join the stomach.
Function : No digestion occurs in the oesophagus. The food (bolus) passes through the
oesophagus by a wave of alternate contraction and relaxation of the circular muscles of the
oesophagus. This is known as peristalsis.
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3.STOMACH :Structure - Stomach is a muscular bag. It has 3 parts i) Cardiac stomach
which has the cardiac sphincter ii) Fundus –The middle part. iii) Pyloric stomach – The part
which opens into the duodenum and has the pyloric sphincter. Sphincters prevent backward
movement of food. Food stays in the stomach for 3 hours.
Function : Stomach churns the food. Churning helps to break the food into smaller particles
and also helps in mixing food with the gastric juice. Walls of the stomach secrete gastric
juice.
Compositon of gastric juice: Water, Mucus, dilute Hydrochloric acid, and enzyme pepsin in
form of its precursor pepsinogen and rennin in the form of its precursor prorennin.
Gastric juice kills the germs prsent in the food and HCl in it provides the acidic medium
required to convert pepsinogen to pepsin.
HCl
Pepsin
Pepsinogen
Pepsin
Proteins
Peptides
(Inactive)
(Active)
HCl
Rennin
Prorennin
Rennin
Casein
Paracasein
(Inactive)
(Active)
After digestion in the stomach the food forms a pulp called chyme.
4. SMALL INTESTINE :Structure : It is 7m long tube and is highly coiled.
Small intestine has 3 parts :
i) Duodenum : It is U shaped, the common bile duct opens into it.
ii) Jejunum : Next part of the small intestine and is narrower than the duodenum. No
chemical digestion takes place here.
iii) Ileum : It is highly twisted and coiled.
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 Villi :The inner layer of the small intestine is highly folded to form numerous
projections called villi. Each villus is lined by columnar epithelial cells. The core of
the villus has a lymph vessel called lacteal. There are capillaries present inside the
villus for absorption.
Structure of villus
The common bile duct is formed by the union of hepatic duct coming from the liver and the
cystic duct coming from the gall bladder. This common bile duct is later joined by the
pancreatic duct and finally opens into the duodenum.
 Digestion in duodenum :
The acidity of the food reaching the small intestine stimulates the walls of the duodenum to
secrete some hormones which stimulate the pancreas to secrete pancreatic juice and the gall
bladder to squeeze out bile.
Bile and cystic duct
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Bile :Bile is secreted by the liverand is stored in the gall bladder. The bile is yellowish green
in colour and flows through the common bile duct to the duodenum.
Composition : Bile contains Pigments Bilirubin and Biliverdin which give it the colour,
Sodium carbonate, bile salts , water and cholesterol.
Bilirubin and biliverdin are produced due to the breakdown of dead and worn out R.B.C’s in
the liver,
Function : Bilirubin and biliverdin give colour to the faecal matter.
Bile salts break down fats into tiny droplets i.e emulsify them.
Sodium carbonate create an alkaline medium for the action of intestinal enzymes since they
act only in an alkaline environment.
 Pancreatic juice : Compostion : Pancreatic juice consists of water, sodium salts and
enzymes such as Amylopsin, Trypsin and Steapsin.
i) Amylopsin: It digests leftover starch to form Maltose
Amylopsin
Starch
Maltose
ii) Trypsin: Inactive trypsinogen in the pancreatic juice is first converted to active trypsin by
enzyme enterokinase secreted by the walls of the small intestine. Trypsin then acts on
proteins proteoses and peptones to fom polypeptides.
Enterokinase
Trypsinogen
Trypsin
Trypsin
Proteins + Peptones + Proteoses
Peptides
iii) Steapsin : It acts on emulsified fats to form Fatty acids and glycerol.
Steapsin
Emulsified fat
Fatty acids + Glycerol
 Digestion in the Ileum :
The walls of the Ileum secrete the intestinal juice. It contains the enzymes Erepsin, Maltase,
Sucrase, Lactase, Lipase.
i) Erepsin – Converts proteoses, peptones and peptides to amino acids.
ii) Maltase : Acts on maltose to form glucose.
iii) Sucrase : Converts sucrose into glucose and fructose
iv) Lactase : Converts lactose (milk sugar) into glucose and galactose.
v) Lipase : Breaks down emulsified fat to form fatty acids and glycerol.
The completely digested liquid in the small intestine is called chyle.
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5. LARGE INTESTINE :The undigested food material moves from the small intestine by
peristalsis to the large intestine.
Structure :Large intestine is around 1.5m long. It is divided into 3 parts
i) Caecum – It is the junction of opening of small intestine into the large intestine. The
vermiform appendix arises from it.
ii) Colon – It begins from the caecum and travels up as the ascending colon as horizontally as
the transverse colon and down as the descending colon.
iii) Rectum – The final 15cms of the gut form the rectum. It is bag like and ends into the
anus. The anal canal is guarded by sphincter muscles. They control the elimination of faecal
matter through the anus.
Function :1. Colon does not secrete any enzyme but promotes water absorption.
2. The bacteria present help to produce and facilitate the absorption of various important
chemicals into the body
3. Formation of faeces takes place takes place in the colon.
 Faeces : The undigested food material from the large intestine forms the faeces.
Composition of faeces: It consists of 75% water and 25% solid matter. Out of the 25% solid
matter 30% dead bacteria 10-20% fats, 2-3% proteins and 30% roughage.
 ABSORPTION AND ASSIMILATION :
Absorption is the process by which the end products of digestion pass through the intestinal
mucosa into the blood or lymph. The absorbed food material becomes a part of the cells, this
is called assimilation. Absorption of nutrients takes place in 2 ways :
a. Glucose and amino acids having small molecular size are absorbed into the blood through
the blood vessels of the villi in the intestine. They first reach the liver through the hepatic
portal vein. Here excess glucose is stored as glycogen. Amino acids are reassembled to form
the proteins required by the body. Excess amino acids are broken down as urea and excreted
along with urine.
b. Fatty acids and glycerol being insoluble in blood are absorbed by the lacteal of the villi and
enter the lymphatic system. From there they ultimately enter the blood stream. Some fats are
used in synthesis of certain compound required by the cells, excess fat is stored under the
skin in the adipose tissue.
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Summary of stages in digestion :
Part of
Alimentary
Secretion
Enzyme
secreted
Substrate
Product of
digestion
Mouth
Saliva
Ptyalin
Starch
Maltose
Oesophagus
--
--
--
--
Pepsin (Adults)
Proteins
Peptides
Gastric juice,
Stomach
Rennin(Children) Caseinogen
Paracasein
a. Bile
--
Fats
Emulsified fats
b. Pancreatic
juice
1. Amylopsin
Starch
Maltose
2. Trypsin
Proteins, Proteoses,
peptones
Peptides
3. Steapsin
Fats
Fatty acids
1.Erepsin
Proteins, Peptides
Amino acids
2.Maltase
Maltose
Glucose
3. Sucrase
Sucrose
Glucose, Fructose
4. Lactase
Lactose
Glucose,Galactose
HCl
Small intestine
i. Duodenum
ii. Ileum
Intestinal
juice
Colon
--
--
--
Absorbs water and
mineral salts
Rectum
--
--
--
Temporary storage
of faeces.
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