Nutrition in Animals

"Animal nutrition includes nutrient requirement, mode of intake of food and its utilization in the body."

All organisms including humans require food for growth, repair and functioning of the body. Plants can prepare thier own food, but animaks cannot prepare thier own food. They get ready made food from plants or from animals that eat plants. Thus, animals show heterotrophic mode of nutrition. Nutrition is a complex process involving various steps. They are - (1) Ingestion (2) Digestion (3) Absorption (4) Assimilation (5) Egestion

• Ingestion: Ingestion is the process by which food is taken in by the organisms. It varies in different animals. There are different ways of ingestion.

• Cockroaches have special mouth parts for sucking, catching and cutting the food. A spider weaves a web to catch its food. A butterfly and bee use its feeding tube to suck nectar from flowers. The mosquitoes suck the blood through their feeding tube. Snakes like the python swallow the animals. Humming-birds suck the nectar of plants. Frogs use their tongue to catch food. Infants of human and many other animals feed on mother's milk. Mammals have well-developed structures to take in food. In our case, ingestion occurs in the mouth and we use our hands for this purpose. Well-developed jaws and teeth are present for cutting and grinding the food. Some aquatic animals filter tiny food particles floating nearby and feed upon them.
• Starfish feeds on animals covered by hard shells of calcium carbonate. After opening the shell, the starfish pops out its stomach through its mouth to eat the soft animal inside the shell. The stomach then goes back into the body and the food is slowly digested.
  

• Digestion: Digestion is the process of breaking down complex food molecules into simpler molecules.
• Absorption: The process by which digested food molecules are taken up (absorbed) by the intestine wall and sent to the circulatory system is called absorption.
• Assimilation: The absorbed food is then incorporated into living cells and used by the body for various purposes. This is called assimilation.
• Egestion: This is the process by which undigested food is removed from the body.
  

1.0Nutrition in Amoeba

Amoeba is a microscopic, unicellular organism found in pond water. It does not have a mouth or a digestive system. Amoeba has a cell membrane, a rounded, dense nucleus and many small bubble-like vacuoles in its cytoplasm. Amoeba constantly changes its shape and position. Food of an Amoeba consists of microscopic organisms like bacteria and minute algae. The mode of nutrition in Amoeba is holozoic and it is omnivorous.

(i) When Amoeba comes in contact with food particles, it sends out pseudopodia (false feet) which engulf the prey by forming a food cup. This process is ingestion.

(ii) When the tips of encircling pseudopodia touch each other, the food is encaptured into a bag called food vacuole. The food vacuole serves as a temporary stomach secreting digestive juices.

(iii) The digested food gets absorbed and diffuses into the cytoplasm and then assimilated. The absorbed substances are used for growth, maintenance and multiplication.

(iv) The egestion of undigested food takes place at any point on the surface of the body by the vacuole.

2.0Human Digestive System

Human digestive system consists of an alimentary canal and associated glands. The process of digestion involves mastication, swallowing, digestion of food, absorption and elimination of undigested matter. Digestion is brought about with the help of special molecules called enzymes. Enzymes break down food particles into simpler molecules through chemical reactions. Enzymes are generally proteinous in nature.

Building Concepts

Why do we need a digestive system?

Explanation

All animals need food to survive, grow and function properly. Often the food eaten is solid. Solid food cannot be absorbed by the body cells as it is. That's why we need a digestive system. Our digestive system has two basic jobs to do with the food we take in. (a) The first job is to break down large food particles so that they can be carried through the body. (b) The second job of the digestive system is to transform the molecules of food into nutrients which can be utilized by our body.

3.0Alimentary Canal

It is a long, muscular and coiled tube. It starts from the mouth and ends at anus. The different organs of the alimentary canal are as follows: (1) Mouth (3) Oesophagus (gullet or food pipe) (5) Small intestine (7) Rectum (2) Mouth cavity (buccal cavity) (4) Stomach (6) Large intestine (8) Anus

Associated with the alimentary canal, there are some glands that secrete digestive juices. These glands are - (i) Salivary glands (ii) Liver (iii) Pancreas

Mouth

Mouth is the anterior part or opening of alimentary canal. It opens into buccal cavity. Food is taken into the body through the mouth.

Buccal Cavity

The process of digestion starts in the buccal cavity. It contains the teeth, tongue and salivary glands. Food is chewed (masticated) by the teeth. The chewed food gets mixed with the saliva secreted by the salivary glands. The tongue helps in mixing the food with saliva and swallowing it down the digestive system.

(i) Teeth: There are four main kinds of teeth in humans. These are incisors, canines, premolars and molars. The main function of teeth is to chew the food, and this process is called mastication. During mastication, food is mixed with saliva.

• The first set of teeth grows during infancy and they fall off at the age between six to eight years. These are termed milk teeth. The second set that replaces them are the permanent teeth. The permanent teeth may last throughout life or fall off during old age or due to some dental disease. Each tooth is rooted in a separate socket in the gums.

Building Concepts

What will happen if we do not clean our teeth after eating?

Explanation

Normally bacteria are present in our mouth, but they are not harmful to us. However, if we do not clean our teeth and mouth after eating, many harmful bacteria begin to live and grow in it. These bacteria break down the sugars present in the leftover food and release acids. The acids gradually damage the teeth. This is called tooth decay. Chocolates, sweets, soft drinks and other sugar products are the major culprits of tooth decay.

(ii) Tongue The tongue is a fleshy muscular organ attached at the back to the floor of the buccal cavity. It is free at the front and can be moved in all directions.

• Function of tongue: It helps in mixing the chewed food with saliva and swallowing the food. It helps us to speak. The tongue gives us a sense of taste as it has taste buds. These buds distinguish four basic tastes like sweet, salty, sour and bitter.
  
  Location of taste buds on the tongue

Salivary Glands

There are three pairs of salivary glands around our buccal cavity. A watery material called saliva is secreted by these glands. Saliva makes the food soft, slippery and also helps in its digestion. It contains an enzyme called salivary amylase (Ptyalin) which acts on starch and changes it into sugar. It is also an antiseptic as it kills germs and bacteria due to presence of an enzyme called lysozyme.

###Oesophagus (Gullet) Oesophagus connects the mouth cavity with the stomach and is also called the food pipe. It runs along the neck and the chest. No digestion takes place here. It helps only in pushing the food into stomach. The rhythmic contraction and expansion of muscles that takes place all along the alimentary canal is referred to as peristaltic movements. These movements help to push the food forward. Vomiting takes place when the food is not accepted by the stomach.

Building Concepts

Why do we feel hiccups or a choking sensation while eating food?

Explanation

Sometimes when we eat in a hurry, talk or laugh while eating, you may get hiccups or a choking sensation. This happens when food particles enter the windpipe. The windpipe carries air from the nostrils to the lungs. It runs adjacent to the food pipe. But inside the throat, air and food share a common passage. If, by chance, food particles enter the windpipe, we feel choked or get hiccups.

Stomach

The stomach is flattened J shape and thick-walled bag like organ. It is the widest part of the alimentary canal. It is located on the left side of the body. It receives food from the food pipe at one end and opens into the small intestine at the other. The inner lining of the stomach secretes mucus, hydrochloric acid and a protein digesting enzyme (pepsin).

Function of the stomach: (i) The mucus protects the lining of the stomach. (ii) The acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic so that enzymes may act on food. (iii) The protein digesting enzyme break down the proteins into simpler substances.

Building Concepts

How did we come to know so much about the working of the stomach?

Explanation

The working of the stomach was discovered by a Doctor William Beaumont. In 1822, a man named Alexis St. Martin was badly hit by a shot gun. The bullet had seriously damaged the chest wall and made a hole in his stomach. The doctor saved the patient, but he could not close the hole properly and left it bandaged. Beaumont found that the stomach was churning food. Its wall secreted a fluid which could digest the food. He also observed that the end of the stomach opens into the intestine only after the digestion of the food inside the stomach is completed.

Small Intestine

Small intestine is a long-coiled tube. It receives secretions from the liver and the pancreas. Besides, its wall also secretes digestive juices. Small intestine is longer in length (about 7.5 metres) than the large intestine (about 1.5 metres). The length of the small intestine differs in various animals depending on the food they eat. Herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Meat is easier to digest hence carnivore like tiger have a shorter small intestine.

Building Concepts

It is said that, 'small intestine is the site of the complete digestion'. Why is it so?

Explanation

In the small intestine, three digestive juices act on food i.e. from liver, from pancreas and other which is secreted by intestine itself which help in the complete digestion of carbohydrates, proteins and fats.

Associated Glands

Liver: The liver is a reddish-brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gall bladder. The bile emulsifies the fat globules and plays an important role in the digestion of fats.

Pancreas: The pancreas is a large cream coloured gland located just below the stomach. The pancreatic juice acts on carbohydrates, proteins and fats and changes them into simpler forms.

• Digestion of carbohydrates, like starch, begins in the buccal cavity. The digestion of protein starts in the stomach. The bile secreted from the liver, the pancreatic juice from the pancreas and the digestive juice from the intestinal wall complete the digestion of all components of food in the small intestine. The carbohydrates get broken into simple sugars such as glucose, fats into fatty acids and glycerol, and proteins into amino acids.
  

Absorption in the small intestine: The inner walls of the small intestine have thousands of finger-like outgrowths. These are called villi. The villi increase the surface area for absorption of the digested food. Each villus has a network of thin and small blood vessels close to its surface. The surface of the villi absorbs the digested food materials. The digested food can now pass into the blood vessels in the wall of the intestine. This process is called absorption.

Assimilation: The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as proteins required by the body. This is called assimilation. In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water and energy is released. The food that remains unabsorbed and undigested then enters into the large intestine.

Large Intestine

The large intestine is wider and shorter than small intestine. Its function is to absorb water and some salts from the undigested food material.

Rectum

It is the last part of large intestine. The remaining waste passes into the rectum and remains there as semi-solid faeces.

Anus

Posterior opening of the alimentary canal through which the faecal matter is removed from time to time is called anus and the process of removal of faecal matter through anus is called egestion.

Building Concepts

How is diarrhoea caused? What type of solution is given to the diarrhoea patient?

Explanation

Sometimes you may have experienced the need to pass watery stool frequently. This condition is known as diarrhoea. It may be caused by food poisoning, infection or indigestion. Under severe conditions it can be fatal. This is because of the excessive loss of water and salts from the body. Diarrhoea should not be neglected. Oral Rehydration Solution - ORS (plenty of water first boiled and then cooled, with a pinch of salt and sugar dissolved in it) should be given to people suffering from diarrhoea.

Some digestive enzymes and their working:

4.0Digestive system in ruminants

The cow's digestive tract consists of the mouth, oesophagus, a complex four-compartment stomach, small intestine and large intestine. The stomach is divided into four chambers - the rumen, reticulum, omasum and abomasum. Among these, the rumen is the largest. Grass-eating animals like cow, ox, buffalo and sheep quickly swallow the grass and store it in a part of the stomach called Rumen. Here the food gets partially digested and is called cud. But later the cud returns to the mouth in small lumps and the animal chews it. This process is called rumination and these animals are called ruminants.

• Abomasum is known as "true stomach" which secretes acid and enzymes. The grass is rich in cellulose, a type of carbohydrate. Many animals, including humans, cannot digest cellulose. Micro-organisms present in the stomach of the ruminants help to digest the cellulose by secreting cellulase enzyme.
• Digestion in ruminants is a good example of symbiosis. Microorganisms present in the stomach of ruminants help in digesting cellulose and in turn obtain shelter and nourishment from the animal.

Symbiotic relationship is a mutually beneficial association of two partners.

Animals like horses, rabbit etc., have a large sac-like structure called caecum in the large intestine. The cellulose of the food is digested here by the action of certain bacteria which are not present in humans.

5.0Biology Diagrams made Easy

6.0Chapter At a Glance

7.0SOME BASIC TERMS

• Swallow : Cause/allow (something especially food or drink) to pass down the throat.
• Heterotrophic: Depend on others for nutrition.
• Prey : An animal that is hunted by another animal for food.
• Mastication : To grind or crush.
• Pseudopodia : False feet.
• Peristaltic : The wave like muscular contractions of the digestive tract.
• Choking : Filling of a passage so that nothing can pass through hence, stopping air from getting into the lungs.
• Emulsify : Make into or become an emulsion.
• Coiled : Move or twist into the shape.
• Substrate : Substances on which enzyme acts.
• Utilization : Action of making practical and effective use of something.
• Glycerol : A clear thick liquid found in fats and oil.
• Enzymes: A substance which regulates the rate of chemical reactions occurring in a living organism without itself being altered.
• Incorporation: The inclusion of something as part of a whole.
• Cud : Partially digested food returned from the rumen to the mouth of ruminants for further chewing.