Grade 12 Zoology Note


Digestive system:

The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body. Food passes through a long tube inside the body known as the alimentary canal or the gastrointestinal tract.

The digestive system consists of alimentary canal and digestive glands. Alimentary canal is complete and has well defined regions. It is concerned with ingestion, digestion, absorption and egestion of waste material.


Alimentary canal:

Fig: parts of alimentary canal of human.

It is a long coiled tube of about 8 to 10 meters long. It is of various diameters at various parts. It starts from mouth and ends in anus. It consists of following parts they are as;




It is a transverse slit like opening bounded by movable upper and lower lip. It opens into a small gap called vestibule. Vestibule is a space between lips and jaw. It opens not buccal cavity.


Buccal cavity:

It is known as oral cavity or mouth cavity also. It is bounded by upper and lower jaws. The upper jaw is fixed and lower jaw is only movable up and down and sidewise. The jaws are provided with teeth in a row. At the floor, there is tongue. The roof of buccal cavity is made by palate. 

                   Fig; buccal cavity of human



Human beings and most of other mammals are heterodont. They have different types of teeth with different functions.

1.     incisors -help to cut large piece into small pieces

2.     canines - help in tearing flesh

3.     premolars – help in mastication

4.     molars – help in mastication

They are diphyodont. They have two sets of teeth, milk teeth and permanent teeth. 

Dental formula – it is expression of number and type of teeth on right or left side (one side) of jaw.

Dental formula of milk teeth i 2/2 c 1/1 pm 0/0 m2/2 5x2/5x2 = 20

Dental formula of permanent teeth i 2/2 c1/1 pm 2/2 m 3/3 8x2/8x2 = 32

i stands for incisor

c stands for canine

pm stands for premolar

m stands for molar

the numerator is for number of teeth in upper jaw

the denominator is for number of teeth in lower jaw


milk teeth start to drop out at about age of 5 or 6. the 3rd molar teeth are also known as wisdom teeth and appear at about the age of 17 to 21. teeth in females appear earlier than in males.


Structure of tooth: 

It has 3 regions.     


Fig; L.S. of an incisor and a molar tooth.

1.Crown – part which project above gum

2.Neck - part surrounded by gum

3.Root - part embedded in bone, the incisor and canine and lower premolar have one root, upper premolar and lower molar have 2 roots and upper molar have 3 roots.

Tooth consists of enamel which is the hardest part of human body. It covers the dentine of crown. Dentine has many canaliculi that pass radially from the pulp cavity. Cement covers root of tooth. Periodontal membrane covers cement and fixes tooth in socket (thecodont).


Inside tooth, there is pulp cavity containing mass of cells, blood vessels and nerve constitute pulp. It is for growth of tooth. Dentine forming odontoblast and enamel forming ameloblast cells are also present.




                               Fig; Structure of human tongue


It is highly muscular organ attached at the floor of buccal cavity by a fold called frenulum. The upper surface is provided with numerous papillae containing taste buds. The taste buds are sensitive to taste of food. Types of papillae

1.Filliform- smallest, most numerous, conical, mostly found at center of tongue, white in color.

2.Fungiform - less in no. red and rounded, found at tip and margin of tongue.

3.Vallate papillae- large in size, about 5 to 12 in no. arranged in inverted v shape at the base of tongue.

4.Foliate – leaf like, not developed in man, found at sides of tongue.

Tip of tongue – sweet

Sides of tongue – sour

Posterior end of tongue – bitter.


Functions of tongue:

1.Detects taste

2.Helps in chewing, mix saliva

3.Aids in swallowing

4.Cleans teeth and gum

5.Plays role in speech


The roof of buccal cavity is called palate. Anterior part is called hard palate. It bears transverse ridges called rugae. The posterior part is smooth and called as soft palate. The hinder part freely hangs down as a small flap called uvula. Buccal cavity receives saliva from salivary gland.



It is wide opining at back of mouth cavity. It leads to two openings : gullet and glottis. There is a muscular flap called epiglottis which closes glottis when food is swallowed. There are 2 openings of internal nares above and two openings of Eustachian tubes at the sides. It is the only part common to digestive and respiratory system.



It is a long narrow muscular tube which connect mouth to stomach. It is about 25 cm long. It pierces diaphragm to open into stomach. It undergoes peristalsis to carry down food and water or fluid.



It is a large muscular elastic bag situated below diaphragm on left side. It has four parts

1.Cardiac: it is so called because it lies near heart. In between oesophagus and cardiac part of stomach there is cardiac sphincter. 

2.Fundus: it extends superiorly from the cardiac part. It is usually filled with air.

3.Body: it is main part of stomach. 

4.Pyloric part: it is distal part of stomach. it opens into duodenum. It opens and closes several times. At the time of opening, a small amount of partially digested food(chime) is passed into duodenum.

Gastric gland secretes gastric juice.


Small intestine:

It is divisible into 3 parts.

1.Duodenum – it is c shaped and about 25 cm long. It receives bile juice and pancreatic juice through common bile duct.

2.Jejunum - it is about 2.5 meter in length. It is coiled part.

3.Ileum – it is about 3.5 meter long. It is highly coiled part. Both jejunum and ileum are suspended by mesenteries. The inner wall of ileum has number of folds called villi. It is mainly for digestion and absorption.


                           Fig; Internal structure of ileum


Large instestine:

                                   Fig; parts of large intestine


It is about 1.5 meter long and divisible into following parts;

1.Caecum: It is pouch like structure about 6 cm long. There is ileocaecal valve preventing back flow. Attached to caecum is a slender vermiform appendix of about 10 cm long. It is vestigial in man but functional in herbivores. The inflammation of appendix is called appendicitis at the time of infection.

2.Colon: It is inverted U shaped and divisible into 

Ascending colon: It is the first part on right side. It moves upward from caecum.

Transverse colon: It is horizontal part.

Descending colon: It moves down at left side. It descends down

Pelvic colon: It is s shaped or sigmoid. Undigested material can remain in colon for about 36 hours.

Colon is for absorption of water mainly.

Rectum: small muscular region, straight, about 15 cm long. It opens to the exterior through anus. Undigested material remains here for a short time. The anus is guarded by 2 sphincters Throughout the alimentary canal, mucous glands secrete mucus.


Digestive glands:

Associated with digestion, there are following digestive glands which secrete enzymes serving the chemical digestion of food.

1.Salivary glands:                        

There are three pairs of salivary glands. There is one pair of Parotid gland. It is the largest of the salivary glands. It is situated near the ears. Stenson’s duct of Parotid gland opens near the upper 2nd molar. One pair of sublingual glands is small and situated beneath tongue. Ducts of Rivinus open into floor of oral cavity. One pair of submandibular glands are medium sized and situated at angle of lower jaw. Warten’s duct open near the lower central incisors.
 Salivary glands secrete saliva into oral cavity. Saliva is slightly acidic pH 6.8. About 1 to 1.5 Liter of saliva is secreted daily. Saliva contains starch digesting enzyme salivary amylase also called ptyalin.



It is the largest gland. It lies in the upper right side of abdominal cavity just below diaphragm. It is reddish brown and weighs about 1.4 to 1.5 Kg. It has two lobes right and left separated by falciform ligament. The right lobe is further divisible into right lobe proper, quadrate lobe and caudate lobe. Gall bladder is a pear shaped sac like structure. It stores bile juice. The right and left hepatic ducts join to form common hepatic duct. It joins the cystic duct which arises from GB. The cystic duct and other hepatic duct join to form bile duct. It runs down to join the pancreatic duct to form hepatopancreatic duct. It opens into duodenum. The liver lobes are made up of small lobules called hepatic lobules. Hepatic lobules are separated by Glission’s capsule. Liver cells are arranged radially and separated by hepatic sinusoids (wide capillaries). Attached to liver cells there are kupffer’s cells which are phagocytic in nature.

                                          Fig; Ducts of liver

Functions of liver:

1.Liver is main organ of Bile production. Bile contains bile pigment and salts. Bile pigments are Bilirubin(yellow) and biliverdin(green) used in coloration of bile and faeces. Salts of Sodium and Potassium bicarbonate neutralize acids. Sodium glycocholate and taurocholate help in emulsification.

2.Deamination: NH2 amino group is removed from amino acid resulting in the production of ammonia. Ammonia is converted into urea.

3.Glycogenesis: it is conversion of excess glucose into glycogen with the help of insulin. 

4.Glycogenolysis:  conversion of glycogen into glucose by liver cells with the help of glucagon.

5.Lipogenesis: conversion of excess glucose and amino acid into fats.

6.Gluconeogenesis: formation of glucose or glycogen from non carbohydrate sources( amino acid, fatty acid and glycerol). It also occurs in kidney and muscle.

7.Detoxification: converts toxic substance into harmless substance eg harmful prussic acid is neutralized and rendered harmless.

8.Haemopoises: formation of blood corpuscles, produces RBC in embryo.

9.Destruction of RBC: old and worn out RBC broken down in liver.

10.Synthesis of Blood protein: Prothrombin and fibrinogen are synthesized.

11.Secretion of Heparin ( anticoagulant)

12.Synthesis of vitamin A: synthesis of vitamin A from Beta carotene (carrot).

13.Phagocytosis: Kupffer’s cell engulf microorganisms and dead cells.

14.Production of heat:  high metabolic activities, enough heat produced, maintain optimum body temperature, maintain homoestasis.

15.Osmoregulation: produce angiotensinogen(protein) helping maintain body fluid.

16.Storage:  glycogen, fat, vitamins A D E K, comps of Fe Cu K etc.

17.Lymph formation: important seat for lymph formation.


It is soft, lobulated, greenish pink gland. It weighs about 60 grams, 2.5 cm wide and 12 to 15 cm long. It located posterior to stomach. It comprises head, body and tail. Head lies in the curve of duodenum, body behind the stomach and tail reaches the spleen near left kidney. The main pancreatic duct is formed by smaller ducts within pancreas. The main pancreatic duct open into hepatopancreatic ampulla.


Internal structure: 

It has two parts, Exocrine part and Endocrine part. Exocrine part consists of rounded lobules(acini) which secrete alkaline pancreatic juice pH 8.4 about 500 to 800 ml of pancreatic juice secreted every day. The pancreatic juice contains 3 proenzyme, trypsinogen, chymotrypsinogen and procarboxypeptidase and some enzymes like pancreatic amylase, DNase, RNase, Lipase etc  Endocrine part consists of islet of Langerhans. About one million islets are found mostly in tail.

Physiology of digestion:

The main objective of digestion is to convert complex food molecules in to simple. Which can easily absorbed by the body cells. This process include ingestion, digestion, absorption, assimilation and egestion or defaecation.


Digestion in mouth:

In mouth, the food is mixed with the saliva from salivary gland. Saliva contains the enzyme called salivary amylase. Salivary amylase acts on carbohydrate and changes some of carbohydrate into maltose, isomaltose and limit dextrine. Food remains in mouth for a short time so digestion is not complete. 


Enzyme substrate products

Salivary amylase + starch ------------- maltose + isomaltase+ limit dextrine


Digestion in stomach:

In the stomach, the gastric juice is secreted by gastric gland. Gastric juice contains pepsiongen, HCl and water. Peptic cells or zymogenic cells secrete pepsinogen. Parietal cells secrete HCl. Prorenin is found in infants.


Functions of HCl:

1.converts inactive pepsinogen into active pepsin

2.provides acidic medium

3 stops growth of Bacteria

Pepsin changes inactive prorenin into active rennin. Pepsin acts on protein and changes it into peptones and proteoses. Rennin acts on milk casein. Milk casein is changed into paracasein. Paracasein combines with Calcium ion to form Calcium paracaseinate. It is then acted by pepsin and changed into peptones and proteoses. In the stomach, gastric lipase is also secreted. It acts on fats to change into fatty acids and glycerol.


Enzyme substrate products

Pepsin + protein --------- peptones + proteoses

Rennin + milk casein ------- paracasein 

Paracasein + Ca ++ -- --- Calcium paracaseinate

Pepsin + Calcium paracaseinate --- peptones + proteoses

Lipase + fats -------------------- fatty acids and glycerol



Digestion in duodenum:

The partially digested food called chime is passed on to the duodenum slowly. It is mixed with bile juice and pancreatic juice. The bile juice contains bile pigments and bile salts. The pancreatic juice contains enzymes like trypsinogen, amylase and lipase etc. The inactive trypsinogen is converted into active trypsin by enterokinase. Trypsin then acts on protein, peptones and proteoses and change into polypeptides. Amylase acts on starch and changes into maltose. Lipase acts on emulsified fat and change into fatty acids and glycerol.


The functions of bile:

 1.It neutralizes acids

2. It provides alkaline medium

3. It emulsifies fats 

Enzyme substrate products

Trypsin + protein, proteoses, peptones ------ polypeptides

Amylase + starch ----------------------------- maltose

Lipase + emulsified fat ---------------------- fatty acids, glycerol


Digestion in jejunum and ileum: 

Here, intestinal juice is produced by crypts of Lieberkuhn. The intestinal juice contains amino peptidase, dipeptidase, maltase, lactase, sucrase etc. the amino peptidase acts on polypeptides and changes into smaller peptides. Dipeptidase acts on dipeptides and changes into amino acids. Maltase acts on maltose. It is changed into glucose. Lactase acts on lactose to change that into galactose and glucose. Sucrase acts on sucrose and changed into fructose and glucose. 


Enzyme substrate products

Amino peptidase + polypeptides -------------------- smaller peptides 

Dipeptidase + smaller peptides --------------- amino acid

Maltase + maltose ------------------------- glucose

Lactose + lactose --------------------------- glucose and galactose

Sucrase + sucrose -------------------------- glucose and fructose


Absorption :

Absorption takes place in the ileum mainly. For this, the internal lining of ileum is provided with large no. of finger like projections called villi. The villi increase the absorptive surface. These are provided with blood vessels and lymph. Glucose, amino acid etc are absorbed into the blood vessel. Blood takes them to liver for storage of glucose. The fatty acids and glycerol come into the lymph which is taken into adipose tissue for storage. 



Glucose, fatty acids and glycerol are used for the liberation of energy in the body. Amino acids are used to produce protoplasm in the body for growth and repair of damaged tissue. 



Undigested materials are passed into large intestine. Water is absorbed here. Then these are carried down. Lastly undigested materials are discharged out from anus. It is called egestion.

Go Top