CBSE Notes Class 7 Science Chapter 2 Nutrition in Animals

All organisms, including humans, require food for growth, repair, and body functioning. Plants can prepare their own food, but animals cannot. They get ready-made food from plants or from animals that eat plants. Thus, animals show heterotrophic modes of nutrition.

1.0Steps of Nutrition

• Nutrition is a complex process involving various steps.
• Ingestion—Ingestion is the process by which organisms take in food. It varies in different animals and has different ways of being done.
• 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 - It is the process by which undigested food is removed from the body.

2.0Nutrition in Amoeba 

• Amoebas are microscopic, unicellular organisms found in pond water. They do not have a mouth or a digestive system. 
• The mode of nutrition in Amoeba is holozoic and omnivorous. 
• When Amoeba comes in contact with food particles, they send out pseudopodia, which engulf the prey by forming a food cup. This process is ingestion. 
• When the tips of encircling pseudopodia touch each other, food is encaptured into a bag called a food vacuole. The food vacuole serves as a temporary stomach, secreting digestive juices. 
• The digested food gets absorbed and diffuses into the cytoplasm and then assimilates. 
• The egestion of undigested food occurs at any point on the body's surface by the vacuole.

3.0Digestive System in Humans 

• The human digestive system consists of an alimentary canal and associated glands. 
• The process of digestion involves chewing, swallowing, digestion of food and elimination of undigested matter. 
• Alimentary canal: It is a long, muscular and coiled tube. It starts from the mouth and ends at the anus.

1. Mouth 

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

2. Buccal cavity 

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

3. Teeth 

• There are four main kinds of teeth in humans. 
• These are incisors, canines, premolars and molars.

Function of teeth 

• The main function of teeth is to chew food, and this process is called rumination. 
• During mastication, food is mixed with saliva. 

4. 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 the tongue 
• It helps mix the chewed food with saliva and swallow 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: sweet, salty, sour and bitter. 

5. Salivary glands 

• There are three pairs of salivary glands around our buccal cavity. 
• These glands secrete a watery material called saliva. Saliva makes food soft and slippery and also helps with digestion. 
• It contains an enzyme called amylase, which acts on starch and changes it into sugar called maltose. Due to the presence of an enzyme called lysozyme, it is also an antiseptic, killing germs and bacteria.

6. Oesophagus (Gullet) 

• The esophagus connects the mouth cavity with the stomach and is 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 the stomach. 
• The rhythmic contraction and expansion of muscles that occur all along the alimentary canal are referred to as peristaltic movements. 
• These movements help to push the food forward.

7. Stomach 

• The stomach is flattened in the J shape and is a 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. 
• The stomach's inner lining secretes mucus, hydrochloric acid and a protein-digesting enzyme (pepsin).

Function of the stomach 

• The mucus 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 so that enzymes may act on the food. 
• The protein-digesting enzyme breaks down the proteins into simpler substances.

8. Small Intestine 

• The small intestine is a long, coiled tube. 
• It receives secretions from the liver and the pancreas. 
• Besides, its wall also secretes digestive juices.
• The small intestine is longer (about 7.5 meters) than the large intestine (about 1.5 meters). 

Associated glands Liver 

• The liver is a reddish-brown gland on the right side of the upper abdomen. 
• It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gallbladder. 
• 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. 
• The partly digested food now reaches the lower part of the small intestine, where the intestinal juice completes the digestion of all its components. 
• 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 increases 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 body organs, which 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 undigested and unabsorbed then enters into the large intestine. 

9. Large intestine 

• The large intestine is wider and shorter than the small intestine. 
• It absorbs water and some salts from the undigested food material. 

Rectum 

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

Anus 

• The posterior opening of the alimentary canal through which the fecal matter is removed from time to time is called the anus. 
• And the process of removal of fecal matter through the anus is called egestion.

Digestive 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 cows, oxen, buffalo, and sheep quickly swallow grass and store it in a part of the stomach called the 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. 
• The grass is rich in cellulose, a type of carbohydrate. 
• Many animals, including humans, cannot digest cellulose. 
• Microorganisms present in the ruminants' stomachs help digest the cellulose by secreting cellulase enzymes.