Nutrition and Digestion: Intake of food, digestion of food, absorption ofdigested food materials and elimination of undigested waste materials are included in nutrition.
Toad is a carnivorous animal. The main food of toad is live worms, insects, earth worms, snails etc. With the sticky tongue it catches the prey and swallows it. Like all vertebrate animals, in the food of toad protein, carbohydrate, fat, salt, vitamins and water components are present. These six types of components can be obtained from the bodies of worms and insects. It is not required to digest salt, vitamins and water. The cells of the body of toad can directly absorb these three types of food. But protein, carbohydrate and fat are complex organic substances; hence the body cells cannot absorb them directly. In the presence of enzymes these insoluble, complex and non absorbable food substances take part in organic chemical reactions and transforms into simple and absorbable liquid components. These changes of food substances are called digestion.
After the intake of the food, there are no chemical changes inside the buccal cavity. The food substances become slippery by the mucus secreted from the walls of the buccal cavity. Food substances reach the stomach through the oesophagus where digestion begins.
Digestion in stomach: Inner walls of the stomach are glandular. After the foodsubstances reach the stomach a hormone is secreted from its walls. This hormone is called gastrin. By the influence of gastrin, digestive juice is secreted from the gastric glands. The digestive juice contains mucin, hydrochloric acid and pepsin. Pepsin is a kind of enzyme.
Mucin: It is not an enzyme. It makes the food substance slippery.
Hydrochloric acid: It destroys the germs coming in with the food substances,and creates the acidic medium to activate the pepsin. In this situation the action of pepsin hastens.
Pepsin: From the gastric gland inactive papsinogen is secreted. Hydrocloricacid transform this inactive pepsinogen enzyme into active pepsin. Pepsin converted the protein into proteose and peptone.
The stomach continuously contracts and expands. Thus food materials in the stomach are transformed into semi liquid substances. This half-digested liquid is called chyme.
Digestion in duodenum: When the chyme reaches the duodenum from thestomach, it comes in contact with the bile, secreted from the liver and pancreatic juice from pancreas. Bile converts the fat food into smaller droplets and makes it suitable for reaction with enzyme. Besides, it neutralizes the acidity of the food and brings them to alkaline medium. As a result, action of pancreatic juice hastens. There is no enzyme in the bile. The name of the digestive juice secreted from the pancreas is pancreatic juice.
Three enzymes are present in this juice, e.g. Trypsin, Amylase and Lipase.
Trypsin: Trypsin converts the proteose and peptone into soluble and absorbable amino acid.
Lipase: This enzyme breaks the fat droplets and converts it into fatty acid andglycerol.
Amylase: This enzyme converts the carbohydrates or starchy food into glucose. The reaction is as follows:
Digestion in Ileum: When the digested food substance reaches the ileum,enzymes are secreted from its walls. There are Protein splitting trypsin, carbohydrates spliting amylase, moltase, sucrase, lactase and fat splitting lipase enzymes. As a result, rest of the undigested protein, carbohydrate and fat are converted into amino acid, glucose (monosaccharide) and fatty acid and glycerol respectively.
Absorption: Digested liquid and simple food i.e. glucose and amino acids areabsorbed by the capillaries of the villi of the small intestine and the fatty acid and glycerol being absorbed by the lacteal ducts of the villi. Excess glucose reaches the liver through hepatic portal vein. Here glucose is transformed into glycogen and is stored. Excess amino acid being transformed into urea is discharged as excreta through the kidney and the fatty food stored at different places of the body as fat.
Egestion: After the absorption of digested food, the undigested part of the foodreaches the large intestine from the small intestine. Here water and mineral salts are absorbed. The rest of the waste material is stored in the rectum. Afterwards these are thrown out of the body through the cloacal aperture as faeces.