Nutrition in animals
Animals exhibit heterotrophic mode of nutrition. As animals cannot synthesise their own food, they depend on plants or other smaller animals for food.
Types of animals
Based on the food they consume, animals are classified into herbivores, carnivores and omnivores.
Holozoic nutrition: It is a process by which animals take in their food. 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 food is broken down into simple absorbable substances. Digestion of food takes place in the digestive system. Digestive system is made up of alimentary canal and associated glands. Digestion in man is an extracellular process.
Digestive system in human beings: Digestive system in human beings is formed by alimentary canal and digestive glands.
Parts of alimentary canal: It is also called as digestive tract. It comprises different parts like mouth, oesophagus, stomach, small intestine and large intestine. It starts with mouth and ends with anus.
Mouth is guarded by upper lip and lower lip. The process of taking in food through mouth is called as ingestion.
Buccal cavity is the inner region of the mouth. It encloses teeth and tongue. The buccal cavity leads into pharynx.
a) Teeth are of different types namely, incisors, canines, premolars and molars.
• Incisors are used for cutting and biting food. There are 4 incisors in each jaw. They are also called as biting teeth.
• Canines are sharp and pointed and are used to pierce or tear food. There are 2 canines in each jaw.
• Premolars have broad grinding surfaces. Hence they help in chewing and grinding the food. There are 4 premolars in each jaw.
• Molars are the principal grinders which help to chew and grind the food. There are 6 molars in each jaw.
Teeth help in the process of mastication.
• Mastication involves proper chewing of food and mixing it with saliva.
• Food is broken into small pieces by biting, grinding and crushing.
• 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, a form of carbohydrate present in the food.
Structure of a tooth
b) Tongue is a muscular organ attached to the floor of the buccal cavity at the back.
• It is free at one end and helps in pushing the food inside while chewing.
• It helps in mixing of food with saliva and swallowing it.
• It has many taste buds on it which can taste different types of food. Taste buds are sensory receptors which give a sense of taste.
• Taste receptors can taste sugar, salt, sour, bitter and one another taste called as umami.
• Tongue also helps in speech.
Pharynx is at the back of buccal cavity.
• The pharynx is the common channel for food and air.
• When you swallow food, a flap-like valve called the epiglottis closes the windpipe.
• Epiglottis prevents the entry of food particles into respiratory tract.
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.
• Food inside the oesophagus reaches the stomach by peristalsis 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 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. Digestive juice is made up of hydrochloric acid, mucous and some enzymes.
• Hydrochloric acid kills the bacteria present in the food and softens the molecules of food.
• Mucous protects inner lining of the stomach from the action of hydrochloric acid.
• 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 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 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.
• 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.