Chapter 2 Science CBSE 7th

Nutrition: The process by which an organism takes food and utilizes it is called nutrition.
Need of Nutrition: Organisms need energy to perform various activities. The energy is supplied by the nutrients. Organisms need various raw materials for growth and repair. These raw materials are provided by nutrients.
Animal Nutrition: Plants make their food by the process of photosynthesis, but animals cannot make their food themselves. Animals get their food from plants. Some animals eat plants directly while some animals eat plant eating animals. Thus, animals get their food from plants either directly or indirectly. All organisms require food for survival and growth. Requirement of nutrients, mode of intake of food and its utilization in body are collectively known as nutrition.

Nutrition in complex animals involves following steps:

1. Ingestion: The process of taking food into the body is called ingestion. Method of ingestion, i.e. taking of food, varies from one animal to another.
2. Digestion: The process of breaking down of complex component of food into simpler substances is called digestion. The process of digestion is different in human, grass eating animals, amoeba, etc.
3. Absorption: The process of passing of digested food into blood vessels in the intestine is called the absorption.
4. Assimilation: The conversion of absorbed food in complex substances such as proteins and vitamins required by body is called assimilation. In other words, assimilation is the conversion of absorbed food (nutrients) into living tissues. Through the process of assimilation our cells are supplied with oxygen and nutrients.
5. Egestion: Removal of waste materials from the body is called egestion. The faecal matter is removed through the anus from time-to-time. Since the waste of food left after digestion is also called faeces, hence the process of egestion is also known as defecation.

Different Ways of Taking Food: The mode of taking food into the body varies in different organisms. Different organism takes food in different ways.

• Bees and humming-birds suck the nectar of plants
• Infants of human and many other animals feed upon their mother’s milk by sucking them.
• Snakes like the python swallow the animals they prey upon.
• Human beings use their hands to put food into their mouth and swallow the food after chewing.
• A snake swallows the animals they prey upon without chewing them.
• A frog captures prey with its sticky tongue.
• An earthworm uses its muscular pharynx to swallow its food.
• Spiders weave sticky web in which small insects get stuck.
• Some aquatic animals filter tiny particles floating nearby and feed upon them.
• Amoeba, a unicellular animal, engulfs tiny particles of food by using pseudopodia. Amoeba surrounds the food by pseudopodia and then makes a food vacuole to engulf the food.
• In multicellular organisms; like hydra there are numerous tentacles around their mouth. Hydra uses tentacles to surround its prey and kill them with its stinging cells. Then the food is pushed inside the body cavity.

Digestion: The process of breaking down of complex component of food into simpler substances is called digestion. The process of digestion is different in human, grass eating animals, amoeba, hydra, etc.

• Enzymes help in the breakdown of complex molecules like carbohydrates, protein, fats, etc. into simple molecules.
• Digestion in unicellular animals; like Amoeba; is intracellular. The digestive enzymes are secreted in the food vacuoles.

Digestion in Human: We take in food through the mouth, digest and utilise it. The digestive system of humans is well developed. Human digestive system consists of alimentary canal and its associated human digestive system glands.

Various organs of human digestive system in sequence are

• Mouth (Buccal Cavity)
• Oesophagus (food Pipe)
• Stomach
• Small intestine
• Large intestine
• Rectum
• Anus.

The glands which are associated with human digestive system are:

• Salivary glands- Located in mouth or Buccal Cavity
• Liver- It is the largest gland situated in the upper part of abdomen on the right side.
• Pancreas- located just below the stomach

The ducts of various glands open into the alimentary canal and pour secretion of their juices into the alimentary canal.

• Mouth: The food is ingested through the mouth. The mouth contains tongue, teeth and salivary glands. Teeth break the food into smaller particles. This process is called mastication. The chewed food is mixed with saliva. Saliva is a watery fluid secreted by the salivary glands.
• Saliva contains a type of enzyme called the salivary amylase, which converts starch into sugar.
• Teeth: Our teeth cut, tear and grind the food before we swallow it. There are four types of teeth in our mouth.
  1. Incisors: These are flat and chisel-shaped teeth. They lie in the front of the mouth. There are eight incisor teeth; four in the upper jaw and four in the lower jaw. The incisor teeth are well adapted for cutting and biting of food items.
  2. Canines: These are round shaped, sharp and pointed teeth. Canines are well adapted to hold and tear the food. There are four canine teeth found in human.
  3. Premolars: There are two premolars on each side of each jaw. Premolars help in crushing and grinding the food. There are 8 premolar teeth in an adult human.
  4. Molars: There are two molars on both sides in both the jaws. They have almost a flat surface with small projections. These teeth are meant for fine grinding of food.

• There are 12 molar teeth including the wisdom teeth in an adult human. The 4 molar teeth are also called wisdom teeth. Wisdom teeth usually grow between the ages of 18 to 21.
• The tooth is covered with a white substance called enamel. It is the hardest substance in the human body.

Milk teeth and Permanent teeth

• In human beings, the teeth grow twice. First time the teeth grow when one is a small baby (or infant). This set of teeth is called milk teeth. Thus, the first set of teeth which grow during infancy (babyhood) are called milk teeth. Milk teeth are a temporary set of teeth. The milk teeth loosen and begin to fall off at the age between 6 and 8 years. When milk teeth fall off in a child, then another set of teeth grow in their place. So, second time the teeth grow when one is a child.
• The second set of teeth is called permanent teeth. The permanent teeth grow in place of milk teeth. The permanent set of teeth remains till the old age. But when old people lose their permanent teeth, then new teeth do not grow in their place.

Tooth Decay: The white, hard outer covering of tooth is called enamel (see the below Figure). The part of tooth below the enamel is called dentine. Dentine is similar to bone. Inside the dentine is pulp cavity which contains nerves and blood vessels. lf the teeth are not cleaned regularly, then tooth decay can take place. Tooth decay is a process in which the tooth becomes rotten due to the formation of cavities (holes) inside it leading to toothache.

• If we do not clean our teeth and mouth after eating food, then many harmful bacteria begin to row and live on the teeth. These bacteria act on the sugar present in the left-over food particles sticking to the teeth to form acid. The acid thus formed eats up the enamel and dentine of the tooth gradually and ultimately makes a cavity (or hole) in the tooth [see the below Figure(a)].

• When this cavity (or hole) reaches the pulp cavity of the tooth (which contains nerves), our tooth becomes painful and we get toothache [see the below Figure(b)].

Digestive System

• Tongue: The tongue is a muscular organ. Tongue helps to mix saliva in the food. It also helps to push the food down the food-pipe or oesophagus. Taste receptors are present on tongue and give us the sense of taste.

• Foodpipe/Oesophagus: It is a tube-like structure connecting the mouth and the stomach. It is about 30 cm. long. Oesophagus has powerful muscles which gently push the food down to the stomach. The oseophagus contracts and relaxes in a rhythmic fashion to facilitate the forward movement of food. This movement happens in other parts of the alimentary canal as well and is called peristalsis. No digestion takes place in oesophagus.

• Stomach: Stomach is the thick walled bag present on the left side of the abdomen. (see human digestive system figure) It is the widest part of the alimentary canal. Oesophagus brings slightly digested food from mouth into the stomach.
• The stomach walls contains three tubular glands in it walls which secrete gastric juice. The gastric juice contains three substances: Hydrochloric acid, the enzyme pepsin and mucus. The hydrochloric creates an acidic medium which facilitates the action of the enzyme pepsin that is the digestion of protein into simple substances. The acid kills many bacteria that enter along with the food. The mucus helps to protect the stomach wall from its own secretions of hydrochloric acid. The partially digested food then goes from the stomach into the small intestine.
• Small intestine: The food leaves the stomach at certain intervals of time and enters into the small intestine.
  • The small intestine is the longest part of the digestive system. It is about 20 feet or seven meters long in an adult human. Small intestine is a highly coiled tube. It consists of three parts: duodenum, jejunum and Ileum.
  • In the duodenum, the liver and pancreas pour their secretions. Liver secretes bile juice and pancreas secretes the pancreatic juice. The bile juice contains the bile which carries out emulsification of fat. In this process, the fat is broken into tiny droplets. The pancreatic juice contains several enzymes. The enzymes of the pancreatic juice break down starch into simple sugars and proteins into amino acids.
  • Minerals and vitamins do not need to be changed because cells are able to absorb them easily. From duodenum the food goes to the lower part of the intestine. There are numerous finger-like projections on the wall of the small intestine. These projections are called villi. They have fine blood capillaries to absorb the food. After absorption; food mixes in the blood stream and is carried to all the cells of the body. The cells utilize this food to release energy.

• Large intestine: The digested food enters into large intestine after small intestine. The large intestine is wider and shorter than small intestine. It is about 1.5 metre in length. In large intestine; excess of water from the materials is absorbed. The semi solid residue is stored in the last part of the large intestine called rectum and finally throw out of the body through the anus from time to time. The throwing out of waste of digested food from rectum is called egestion. Egestion is also known as defecation.

Digestion in Grass Eating Animals:
Ruminants: None of the animal can digest cellulose which is a major component of the food eaten by herbivores. The plant eating animals digest their food in two steps. Their stomach is divided into four chamber, viz. rumen, reticulum, omasum and abomasum.

First of all, half chewed food is swallowed and it then goes from mouth to the rumen, the first chamber of the stomach. Here, it is acted upon by bacteria. These microorganisms digest the cellulose. This half digested food goes to the second muscular chamber; the reticulum. From the reticulum the food is sent back to the mouth; as cud; to be chewed again. Chewing of the cud is called rumination and such animals are called ruminating animals or ruminants. Cow, goat, buffaloes, sheep, bison, etc. are good example of ruminating animals. The re-chewed food is swallowed for the second time. After passing the first two chambers it enters the third chamber; the omasum. Here the food is further broken down into smaller pieces and finally enters the fourth chamber, the abomasum. Here, all enzymes act upon the food and the digestion is completed.

After digestion and absorption, nutrients from food are taken to the cells in all parts of the body. The cells oxidize the food to release energy.

Feeding and Digestion in Amoeba: Amoeba is a microscopic organism which consists of only a single cell. Amoeba is mostly found in pond water. The figure given below shows the structure of amoeba.Amoeba eats tiny plants and animals as food which floats in water in which it lives. The mode of nutrition in Amoeba is holozoic. The process of obtaining food by Amoeba is called phagocytosis.

Steps involved in the nutrition of Amoeba:

• Ingestion: Amoeba ingests food by forming temporary finger-like projections called pseudopodia around it. The food is engulfed with a little surrounding water to form a food vacuole (‘temporary stomach’) inside the Amoeba.
• Digestion: In Amoeba, food is digested in the food vacuole by digestive enzymes which break down the food into small and soluble molecules by chemical reactions.
• Absorption: The digested simple and soluble substances pass out of food vacuole into the surrounding environment.
• Assimilation: The absorbed food materials are used to obtain energy through respiration and make the parts of Amoeba cell which leads to the growth of Amoeba.
• Egestion: The remaining undigested material is moved to the surface of the cell and thrown out of the body of Amoeba.