Chapter - 2 Nutrition in Animals

Ø Plants can prepare their own food by the process of photosynthesis but animals can not. Animals get their food from plants, either directly by eating plants or indirectly by eating animals that eat plants. Some animals eat both plants and animals.

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

Ø The components of food such as carbohydrates are complex substances. These complex substances cannot be utilized as such. So they are broken down into simpler substances. The breakdown of complex components of food into simpler substances is called digestion.

Ø The mode of taking food into the body varies in different organisms. Bees and humming-birds suck the nectar of plants, infants of human and many other animals feed on mother’s milk. Snakes like the python swallow the animals they prey upon. Some aquatic animals filter tiny food particles floating nearby and feed upon them.

Ø 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.

Ø Digestion in humans

Ø The food passes through a continuous canal which begins at the buccal cavity and ends at the anus. The canal can be divided into various compartments: (1) the buccal cavity, (2) food pipe or esophagus, (3) stomach, (4) to a small intestine, (5) large intestine ending in the rectum and (6) the anus. These parts together form the alimentary canal. The food components gradually get digested as food travels through the various compartments. The inner walls of the stomach and the small intestine, and the various glands such as salivary glands, the liver and the pancreas secrete digestive juices. The digestive juices convert complex substances of food into simpler ones. The digestive tract and the associated glands together constitute the digestive system.

Ø Food is taken into the body through the mouth. The process of taking food into the body is called ingestion. The food is chewed with the teeth and broken down mechanically into small pieces. Each tooth is rooted in a separate socket in the gums. Our teeth vary in appearance and perform different functions. Accordingly they are given different names.

Ø The salivary glands secrete saliva. The saliva breaks down the starch into sugars.

Ø 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. It mixes saliva with the food during chewing and helps in swallowing food. It also has taste buds that detect different tastes of food.

Ø The swallowed food passes into the food pipe or esophagus. The food pipe runs along the neck and the chest. Food is pushed down by movement of the wall of the food pipe.

Ø The stomach is a thick-walled bag. Its shape is like a flattened U and it is the widest part of the alimentary canal. 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 mucous, hydrochloric acid and digestive juices. The mucous protects the lining of the stomach. The acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic. The digestive juices break down the proteins into simpler substances.

Ø The small intestine is highly coiled and is about 7.5 meters long. It receives secretions from the liver and the pancreas. Besides, its wall also secretes juices.

Ø 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 juice in a sac called the gall bladder.

Ø The bile plays an important role in the digestion of fats. The pancreas is a large cream-colored gland located just below the stomach. The pancreatic juice acts on carbohydrates and proteins and changes them into simpler forms. The partly digested food now reaches the lower part of the small intestine where the intestinal juice completes the digestion of all components of the food. The carbohydrates get broken into simple sugars such as glucose, fats into fatty acids and glycerol, and proteins into amino acids.

Ø The digested food can now pass into the blood vessels in the wall of the intestine. This process is called absorption. The inner walls of the small intestine h ave thousands of finger-like outgrowths. These are called villi (singular villus). 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 absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the 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 undigested and unabsorbed then enters into the large intestine.

Ø The large intestine is wider and shorter than small intestine. It is about 1.5 meter in length. Its function is to absorb water and some salts from the undigested food material. The remaining waste passes into the rectum and remains there as semi-solid faeces. The faecal matter is removed through the anus from time-to-time. This is called egestion.

Ø In cows, buffaloes and other grass-eating animals, food is swallowed and store d in a separate part of the stomach called rumen where it is 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.  The grass is rich in cellulose, a type of carbohydrate. Many animals, including humans, cannot digest cellulose.

Ø Ruminants have a large sac-like structure between the small intestine and large intestine. The cellulose of the food is digested here by the action of certain bacteria which are not present in humans.

Ø Amoeba is a microscopic single-celled organism found in pond water. 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. It pushes out one, or more finger-like projections, called pseudopodia or false feet for movement and capture of food. Amoeba feeds on some microscopic organisms. When it senses food, it pushes out pseudopodia around the food particle and engulfs it. The food becomes trapped in a food vacuole.

Ø Digestive juices are secreted into the food vacuole. They act on the food and break it down into simpler substances. Gradually the digested food is absorbed. The absorbed substances are used for growth, maintenance and multiplication. The undigested residue of the food is expelled outside by the vacuole.