Digestive Organs
· Frogs are heterotropic, holozoic, carnivorous and predaceous.
· Digestive system consists of the alimentary tract and accessory organs of digestion.
· Digestive system consists of the alimentary tract and accessory organs of digestion.
· Extends from mouth to cloacal aperture.
A. Mouth:
· Transverse opening bounded by immovable upper jaw and movable lower jaw.
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| Structure of tooth |
B. Buccopharyngeal cavity:
· Space that is enclosed between the upper and lower halves of the head.
· Fusion of buccal cavity and pharynx.
a. Teeth:
· Upper jaw: three sets of teeth:
a. Vomerine teeth
· Upper jaw: three sets of teeth:
a. Vomerine teeth
b. Maxillary teeth
c. Pre-maxillary teeth.
· Teeth: small sharp-pointed, hook-like and directed backwardly.
· Meant for preventing the captured prey from escaping.
b. Dentition:
· Homodont – All teeth similar
· Polyphyodont – Replaced many times
· Acrodont – Not lodged in the socket but fixed by cement directly to the bone.
c. Pre-maxillary teeth.
· Teeth: small sharp-pointed, hook-like and directed backwardly.
· Meant for preventing the captured prey from escaping.
· NO MASTICATORY FUNCTION.
· Homodont – All teeth similar
· Polyphyodont – Replaced many times
· Acrodont – Not lodged in the socket but fixed by cement directly to the bone.
· Crown has an inner thick dentine layer and an outer enamel layer.
· A pulp cavity is present inside the tooth filled with pulp.
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| A well-labelled diagram of the alimentary canal of frog |
c. Tongue
· Thick fleshy, arising from front of lower jaw i.e. anterior end is fixed.
· Bifurcated at the tip (posterior end), sticky, protrusible and fleshy.
· Used for catching the prey. Secretions of lingual glands and intermaxillary glands make the tongue sticky.
· It is supported by hyoid bone.
d. Posterior region of the buccopharyngeal cavity
· Opening of Eustachian tube (that leads to middle ear cavity) at the sides of the roof.
· Opening of vocal sac on the floor in male, near the angles of the lower jaw.
· Slit-like opening called Glottis at the floor leads to the laryngotracheal chamber.
· Medially placed wide gullet leads to the oesophagus.
e. Sub-rostral fossae and prelingual elevations
· Three median elevations at the tip of the lower jaw known as prelingual elevations get fixed to the tip of the upper jaw known as sub-rostral fossae to close the mouth tightly during the gaseous exchange.
f. Opening of internal nares: for gaseous exchange.
g. Bulging of eyeballs: Behind the internal nares.
C. Oesophagus: It is also called food pipe, highly distensible and Short due to the absence of the neck. Insensibly passes into the stomach with no external demarcation between them.
D. Stomach: 5.7 cm in length, wide anteriorly, narrow posteriorly.
· Two parts:
a. Anterior cardiac part: Separated from the oesophagus by cardiac sphincters.
b. Posterior pyloric part: Separated from duodenum by pyloric constriction.
· Inner surface is thrown into a number of longitudinal folds that converge posteriorly.
· Opens posteriorly into the duodenum through the pyloric sphincter.
· Note – The fundic part in the stomach is absent.
E. Small intestine: It is the longest part of the alimentary canal suspended by the mesentery.
· Two parts:
a. Anterior cardiac part: Separated from the oesophagus by cardiac sphincters.
b. Posterior pyloric part: Separated from duodenum by pyloric constriction.
· Inner surface is thrown into a number of longitudinal folds that converge posteriorly.
· Opens posteriorly into the duodenum through the pyloric sphincter.
· Note – The fundic part in the stomach is absent.
E. Small intestine: It is the longest part of the alimentary canal suspended by the mesentery.
· It consists of two parts: Duodenum and Ileum.
a. Duodenum
· U-shaped narrow tube 3 – 5 cm in length.
· Receives the opening of the hepatopancreatic duct or common bile duct.
· Consists of finger-like projections called villi.
a. Duodenum
· U-shaped narrow tube 3 – 5 cm in length.
· Receives the opening of the hepatopancreatic duct or common bile duct.
· Consists of finger-like projections called villi.
b. Ileum
· It is a coiled tube 20-25 cm in length. The ileum is longer in tadpole than an adult.
· Villi are more numerous and extensive.
· It is the main site of absorption of digested food.
· Large intestine and Cloaca
· Short (~ 4.5 cm) but a wide tube.
· Large intestine opens into a short chamber called the cloaca through the anus, guarded by anal sphincter.
· Helps in the absorption of water and minerals from undigested food.
· Cloaca: common passage for faeces, urine and reproductive products ( sperm and ova)
· Cloaca opens to the exterior through the cloacal aperture.
Histology of Alimentary Tract
· It consists of Four layers from within to outwards:- Mucosa
- Submucosa
- Muscularis
- Serosa
· Oesophagus: Columnar cells are ciliated. Muscularis mucosa is absent.
· Stomach: Epithelium is sunk down in under-lying connective tissue forming gastric glands. They consist of
· Goblet cell or Mucous neck cells – Secrete mucus
· Parietal cells/Oxyntic cells – Secrete HCl
· Peptic/Chief cells – Secrete pepsinogen.
· Small intestine(SI): mucosa is folded into finger-like projections called Villi. Goblet cells are present. NO digestive glands.
· Large intestine(LI): Goblet cells numerous.
2. Submucosa: Consists of coarse connective tissue containing principal nerve fibres, blood vessels, lymphatics.
3. Muscularis: Inner circular and outer longitudinal muscle layers. Most developed in the stomach.
4. Serosa: Visceral layer of peritoneum composed of flattened cells.
Accessory Organs of Digestion
1. Liver
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| T.S. of Liver |
· The largest gland.
· Reddish-brown in colour.
· Consists of three lobes.
· Between the two lobes, there is a thin-walled round greenish gall bladder.
· Liver secretes bile which is transported to the gall bladder for storage via hepatic ducts.
· Consists of three lobes.
· Histologically, it consists of small secretory units called tubule or lobule which branch and anastomoses with each other forming a complex network.
· Hepatocytes are polyhedral in shape.· Between the two lobes, there is a thin-walled round greenish gall bladder.
· Liver secretes bile which is transported to the gall bladder for storage via hepatic ducts.
· Bile gives a greenish colour to the gall bladder.
· Bile is alkaline due to the presence of NaHCO3.
· Bile is alkaline due to the presence of NaHCO3.
· Bile does not contain any digestive enzyme.
· Cystic duct, the duct of gall bladder receives the same hepatic ducts directly and descends towards the duodenum as the bile duct, which receives the pancreatic duct to form hepatopancreatic duct.
· Cystic duct, the duct of gall bladder receives the same hepatic ducts directly and descends towards the duodenum as the bile duct, which receives the pancreatic duct to form hepatopancreatic duct.
· Ornithine cycle or urine production takes place in the liver.
· Liver also produces heparin.
2. Pancreas
· Liver also produces heparin.
· Second largest gland.
· Yellowish in colour.
· Lies in mesentery between duodenum and stomach.
· Histologically consists of large no. of lobules with clusters of secretory cells called acini → Secrete digestive enzymes.
· Also contains a compact group of cells called Islets of Langerhans → Secrete hormones: insulin (β or Beta cells), glucagon (𝝰 or alpha cells).
· Histologically consists of large no. of lobules with clusters of secretory cells called acini → Secrete digestive enzymes.
· Also contains a compact group of cells called Islets of Langerhans → Secrete hormones: insulin (β or Beta cells), glucagon (𝝰 or alpha cells).
· Remember,
- Alpha Cells: Glucagon
- Beta cells: Insulin
· Pancreatic duct opens into the common bile duct.
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| T.S. of pancreas |
Physiology of Digestion
· A frog is an insectivorous animal.· The prey is caught with its sticky tongue and ingested and swallowed whole without mastication.
· No digestion occurs in the oesophagus.
A. Digestion in the Stomach:
· Mechanical churning, mixing of food, killing of prey if still alive.
· Gastric juice:
· HCl ⟶ makes an acidic medium, activates enzymes, denatures proteins.
B. Digestion in the Intestine:
a. Bile
· Secretion is activated by cholecystokinin hormone
· Makes the medium alkaline
· Bile salts ⟶ emulsify fat droplets
b. Pancreatic juice
A. Digestion in the Stomach:
· Mechanical churning, mixing of food, killing of prey if still alive.
· Gastric juice:
· HCl ⟶ makes an acidic medium, activates enzymes, denatures proteins.
· Pepsinogen ⟶ Converted to pepsin and then Pepsin acts on proteins to give Peptones and Proteoses.
(i) Trypsinogen is a proenzyme that is activated by enterokinase present in intestinal juice to trypsin that converts proteases and peptones to polypeptides.
· Trypsinogen (inactive) + Enterokinase ⟶ Trypsin (Autocatalyst)
· Trypsin + Proteoses + Peptones ⟶ Polypeptides
(ii) Amylopsin or amylase converts carbohydrate (polysaccharide) to maltose, lactose and sucrose (disaccharides).
(iii) Lipase converts fats into fatty acids and glycerol.
· Trypsinogen (inactive) + Enterokinase ⟶ Trypsin (Autocatalyst)
· Trypsin + Proteoses + Peptones ⟶ Polypeptides
(ii) Amylopsin or amylase converts carbohydrate (polysaccharide) to maltose, lactose and sucrose (disaccharides).
(iii) Lipase converts fats into fatty acids and glycerol.
B. Digestion in the Intestine:
a. Bile
· Secretion is activated by cholecystokinin hormone
· Makes the medium alkaline
· Bile salts ⟶ emulsify fat droplets
b. Pancreatic juice
· Secretion is activated by secretin.
· Consists of:
· Consists of:
- Trypsinogen
- Chymotrypsinogen
- Carboxypeptidase
- Nuclease – DNAase and RNAase
- Amylase
- Lipase
c. Intestinal juice or Succus entericus:
Consists of:
Consists of:
- Enterokinase
- Erepsin
- Peptidase
- Maltase, Sucrase, Lactase
- Lipase
- Nucleosidase
· The Enterocrinin hormone stimulates them to secrete the intestinal juice (succus entericus).
· Enzymes present in intestinal juice act as [(E) ⟶ Enzymes]
(i) Polypeptidase (E) + Polypeptide ⟶ Amino acids
(ii) Maltase (E) + Maltose ⟶Glucose + Glucose
(iii) Lactase (E) + Lactose ⟶ Glucose + Galactose
(iv) Sucrase (E) + Sucrose ⟶Glucose + Fructose [Which are monosaccharides].
(v) Lipase acts upon the fat emulsion to break it down into fatty acid and glycerol.
· Enzymes present in intestinal juice act as [(E) ⟶ Enzymes]
(i) Polypeptidase (E) + Polypeptide ⟶ Amino acids
(ii) Maltase (E) + Maltose ⟶Glucose + Glucose
(iii) Lactase (E) + Lactose ⟶ Glucose + Galactose
(iv) Sucrase (E) + Sucrose ⟶Glucose + Fructose [Which are monosaccharides].
(v) Lipase acts upon the fat emulsion to break it down into fatty acid and glycerol.
Also, Read our other Notes Related to Frog for Entrance Exams: