Digestive system, Various Parts of Digestive System, Absorption, Defecation

Zoology Assignment Help >> Digestive system, Various Parts of Digestive System, Absorption, Defe >> Digestive system, Various Parts of Digestive System, Absorption, Defecation

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Digestive System

Digestion: It is a process of nutrition by which the larger and complex food materials are converted into smaller and smaller food particles which are easy to absorb. In this process the covalent bonds of nutrition are cleaved by the hydrolysing enzymes. In protozoans and in lower animals the cells engulfs the food particles (endocytosis) and then they are digested in the protoplasmic food vacuoles. This type of digestion is intracellular digestion. When the digestion is outside the cell then such type of digestion is called as extracellular digestion.

The digestive system: It includes the alimentary canal and the associated glands. The intake of food is ingestion whereas the passing out of the unabsorbed food material is egestion. The terminal part of soft palate hanging into the oropharyngeal isthmus is called uvula. Both respiratory or nasal passage and buccal cavity are connected together by nesopalatine 'duct. These ducts are connected with the Jacobson's organ.

Floor of the buccal cavity is occupied by a tongue. Anterior free part of the tongue is attached basally to oral cavity by a fold of the mucous membrane called frenulum linguae. Upper part of the tongue is full of lingual papillae. These are -

1. Fungiform papillae

2. Foliate papillae

3. Circumvallate papillae

4. Filliform papillae

All except the filliform papillae bear taste buds.

Man is a diphydont, heterodont animal. First set of teeth is called milk set. This set is replaced by permanent teeth set. It contains following four types of teeth-

1. Incisor,

2. Canine,

3. Premolar,

4. Molar.

Milk set does not have molar teeth.

Last molar is also called wisdom teeth. Premolar and molar both are called cheek teeth.

Dental formula:

Dental Formula

Teeth is formed by odontoblast cells.

Crown of the teeth is covered by the hardest substance of the body called "enamel".

Salivary glands-Four pairs of salivary-glands are found. They secrete saliva which contains enzyme ptylin.

These are -

1. Sublingual glands

2. lntra-orbital or zygomatic glands

3. Parotid glands

4. Submaxillary or submandibular glands.

The inflammation of parotid salivary glands is called mumps. Parotid glands are also supposed to secrete enzyme ptylin.


Buccal cavity is continuous with oropharynx. Each lateral wall of oropharynx bear small lymphoid nodes called tonsil.

The posterior wall of pharynx has two openings-dorsal gullet or oesophagus and ventral glottis or larynx. Epiglottis is found hanging over glottiS and closes it at the time of swallowing food.


It bores through diaphragm and enters to abdominal cavity and then opens into stomach by cardiac orifice. It is surrounded by cardiac sphincter.


It is having three parts¬

(a) Cardiac part

(b) Fundic part

(c) Pyloric part


The small intestine is divisible into¬

(a) Proximal-Duodenum

(b) Middle-Jejunum

(c) Distal-Ileum

The walls of jejunum and ileum are having circular or spiral folds. Ileum leads into a rounded bulb like structure called sacculus rotundus. This leads into large intestine which comprises of-

(a) Caecum,

(b) Colon,

(c) Rectum. (Histological structure of alimentary canal has been already described in the chapter 'Histology of mammalian organs')


1. In mouth: Food is broken down, masticated and mixed with saliva. Salivary amylase converts starch into maltose. Another enzyme maltase converts maltose into glucose.

2. In stomach: Digestive gastric juice contains water, mucin, salt, HCI and three enzymes. These are -

(a) Pepsin - Converts protein -> Peptones

(b) Rennin - Converts milk protein -> Caseinogen

(c) Lipase-Converts fats -> Glycerol and fatty acid

Pepsin and rennin are secreted in their zymogen form. They are converted into active form by HCI

3. Digestion in small intestine¬- Three types of juice are mixed in small intestine. These are -

(a) Pancreatic juice:

1. Trypsin-Proteins -> Proteose + Peptones

2. Chymotrypsin - Proteins -> Peptides

3. Carboxypeptidase - Peptides -> Amino acids

4. Amylase - Starch -> Maltose

5. Lipase - Fat -> Fatty acid + Glycerol

6. Maltase - Maltose -> Glucose

7. Ribonuclease - R.N.A. Nucleotides

(b) Intestinal juice-

1. Erepsin - Peptones + Proteose -> Amino acids

2. Maltase - Maltose -> Glucose

3. Sucrase - Sucrose -> Glucose + Fructose

4. Lactase - Lactose -> Glucose + Galactose

5. Peptidase - Peptides -> Amino acids

6. Lipase - Fat -> Glycerol + Fatty acids

Not - Erepsin is a group of enzymes.

(c) Bile juice - Bile contains no enzyme. It only emulsifies the fat and provides suitable medium for the intestinal digestion.

4. In the large intestine-No digestion occur but to a smaller extent the cellulose is digested in colon in herbivores.


In man, a little amount of water, alcohol and mineral salts are absorbed in the stomach but the major absorption occur in small intestine.

Absorption is done by -

1. Active transport

2. Passive transport

Glucose is diffused through intestinal membrane and taken into blood-stream. Fat globules are encased in protein coat and are called chylomicron. They are transported into lymph capillaries which appear to be milky white and called lacteals. This milky white suspension is called chyle.


Egestion of undigested part of the food is called defecation. Its colour is due to the presence of stercobillin and urobillin.

FOOD Classification-

(a) According to their chemical composition

(a) Proteins

(b) Carbohydrate

(c) Fats

(d) Vitamins

(e) Water

(f) Mineral salts

(2) According to their function in body

(a) Body builders-e.g. Proteins, water etc.

(b) Energy producers-e.g. carbohydrate, fats etc.

(c) Regulators -e.g. vitamins, salts etc.

Proteins: Mulder (1839) proposed the term "protein". They are called body builders. They are made up of amino acids.

Proteins are classified variously.

Classification based on chemical composition is given here -

(a) Simple proteins-Formed only of amino acids. e.g. albumin, histones etc.

(b) Conjugated proteins-Amino acids along with some other non-proter part. e.g. nucleoproteins, lipoproteins etc.

(c) Derived proteins-They are derived by original proteins by heating enzymatic reaction etc. e.g. metaprotein.

Carbohydrates: They are made up of carbon, hydrogen and oxygen. All carbohydrates have hydrogen-oxygen ratio 2:1.

They are -

1. Monosaccharides-only of one sugar molecule. e.g.

(a) Triose - Glyceraldehyde

(b) Tetrose - Erythrose

(c) Pentose - Xylose, arabinose

(d) Hexose - Glucose, fructose

(e) Heptose

2. Disaccharides-Bimers of monosaccharide. e.g. maltose, lactose etc.

3. Oligosaccharides-Made up of 2-10 monosaccharide molecules. e.g. sucrose, verbascose etc.

Fats or Lipids-1 Simple lipids - e.g. waxes, adipose tissue.

2. Complex lipid - e.g. phospholipids, glycolipids, amino lipids.

3. Derived lipids -e.g. cholesterol, sterols.

The lipids are insoluble in water but soluble in organic solvents. They provide bulk to the body and are emergency energy providing substances.