Nutrition in animals Holozoic nutrition:
It is a method of nutrition that involves the ingestion of liquid or solid organic material.
It involves different steps namely, ingestion, digestion, absorption, assimilation and egestion.
Human beings exhibit holozoic mode of nutrition involving five basic steps.
Digestion:
Digestion is the process by which complex food is broken down into simple absorbable form.
Digestion of food starts from mouth and ends in small intestine
Digestive system is made up of alimentary canal and associated glands.
Digestive system in human beings:
Digestive system in human beings is formed by alimentary canal and digestive glands.
Parts of alimentary canal:
It comprises different parts like mouth, oesophagus, stomach, small intestine and large intestine. It starts with mouth and ends with anus.
ingestion
The process of taking in food through mouth is called as ingestion.
Mastication
Teeth help in the process of mastication. Mastication is the process of mixing chewed food with saliva.
Digestion of food starts in the mouth with the help of salivary amylase present in the saliva.
Salivary amylase is the enzyme which digests starch into glucose for absorption
Epiglottis
Pharynx is the common channel for food and air. When we swallow food, a flap-like valve called the epiglottis closes the windpipe. Epiglottis prevents the entry of food particles into respiratory tract.
Oesophagus
Oesophagus also called as food pipe helps in conveying the food from buccal cavity to stomach. The oesophagus is also known as the gullet. It is about 25 centimetres long.
Peristaltic movements
Peristaltic movements are the alternate contractions and relaxations of oesophageal wall which bring about movement of food from buccal cavity to the stomach.
Food conveyed to the stomach is called as bolus as it is round in shape.
Stomach
Stomach is the widest part of the alimentary canal. It is a J-shaped muscular organ divided into three parts namely, cardia, fundus and pylorus regions.
Stomach as a whole can hold at about two litres of food.
Stomach secretes a fluid called as digestive juice called gastric juice .
Gastric juice
It is made up of hydrochloric acid, mucous and enzymes like pepsin and rennin.
Hydrochloric acid kills the bacteria present in the food and softens the molecules of food.
Mucous
Mucous protects inner lining of the stomach from the action of hydrochloric acid.
chyme
Digestive enzymes partially digest some nutrients like proteins and fats. Stomach churns the food into a milky paste. This partially digested food is called as chyme.
Chyme is conveyed to small intestine for further digestion. Chyme is acidic in nature.
Small intestine
Small intestine is made up of three regions namely duodenum, jejunum and ileum.
• Acidic chyme from the stomach is received by the duodenum for further digestion.
• Duodenum also receives bile form the liver. Bile reduces the acidity of chyme.
• Bile also provides alkaline environment to activate some enzymes which bring about digestion of certain nutrients in the food.
• Duodenum also receives pancreatic secretions which help in the digestion of food.
• Duodenum also secretes some enzymes on its own.
• All these substances bring about digestion of food in the intestine.
• The inner walls of small intestine are thrown into many folds which have millions of small finger like projections called villi.
Villi increase
• Villi increase the surface area for digestion as well as absorption of digested food by eight times.
Small intestine also helps in the process of absorption and assimilation.
Large intestine
• Undigested food is sent into large intestine.
Large intestine comprises of colon and rectum. Large intestine receives undigested food from small intestine.
Water from the food is reabsorbed to a great extent in the large intestine.
Semi-solid undigested waste is stored in the rectum for defecation.
Anus is the opening of the alimentary canal to the exterior. This helps in the elimination of faeces by the process of egestion.
Digestive glands:
These glands are also called as associated glands. These are also considered to be exocrine glands which have ducts to drop their secretions into the target organ directly. The secretions of the digestive glands help in the process of digestion. These glands include salivary glands, gastric glands, intestinal glands, liver and pancreas.
Salivary glands are present inside the buccal cavity. They secrete saliva. Saliva helps in lubrication of food . This saliva plays an important role in breaking down complex components like starch into simple sugars. It brings about partial digestion of starch.
Gastric glands are microscopic glandular cells present in the inner lining of the stomach. Gastric glands secrete gastric juice comprising HCl, pepsin and prorennin.
Gastric juice helps in the digestion of proteins. Gastric juice helps in emulsification of fats.
Intestinal glands are present in the inner lining of small intestine. These secrete various enzymes which aid in the process of digestion of all the components of food.
Liver is the largest gland in our body. The liver secretes a yellowish green watery fluid called bile. It is temporarily stored in a sac called the gall bladder.
Bile provides an alkaline environment for many enzymes to get active. It also reduces the acidity of chyme. Bile plays an important role in the digestion of fats.
Bile is sent into duodenum through a narrow tube-like structure called the bile duct.
Bile breaks the larger fat molecules into tiny droplets, thereby increasing their surface area, which helps in the digestion of fats easily.
Pancreas is the mixed gland. It acts as both endocrine and exocrine gland. The pancreas secretes the pancreatic juice that helps to digest carbohydrates, proteins and fats. The pancreatic juice converts carbohydrates into simple sugars and glucose, proteins into amino acids, and the lipids into fatty acids and glycerol.
Absorption: The process of allowing simple absorbable nutrients into blood capillaries through surface of the villi is called as absorption. Inner wall of small intestine comprises many finger like projections called as villi. Villi increase the surface area for absorption of food.
Each villus is made up of central structure called as lacteal which mainly absorbs simple fats and transports them into lymphatic system.
Lacteal is surrounded by a network of fine blood capillaries. Blood capillaries absorb glucose molecules and amino acids and transport them in the blood. Vitamins and minerals get readily absorbed into the blood.
Assimilation:
The process of utilisation of absorbed food, such as glucose, amino acids, fatty acids and glycerol is called as assimilation. Energy needed for various activities is obtained from glucose.
Glucose is broken in the cells in the presence of oxygen to syntheise energy in the form of ATP.
Amino acids are used for building and repairing body parts. Fatty acids and glycerol are stored in the adipose tissue and under the skin for future use.
Egestion:
It is the process by which undigested food is passed to exterior through an opening called as anus. Rectum stores undigested waste in the form of faeces. Faeces are sent out through anus.
Solved questions Class 7 Science Nutrition in Animals
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