Bio Ch 4 Class 6 PDF

# Food for Life You have already learnt that food is a kind of "fuel" as well as a kind of building material for all living organisms. Food contains nutrients - the raw materials our body needs to build everything from muscles and bones to the brain and the heart. We drink milk and eat different...

# Food for Life You have already learnt that food is a kind of "fuel" as well as a kind of building material for all living organisms. Food contains nutrients - the raw materials our body needs to build everything from muscles and bones to the brain and the heart. We drink milk and eat different types of food like bread, chapati, rice, butter, ghee, vegetables, fruits, meat, eggs, pulses, etc. The food provides us with nutritive substances for growth and energy. A good choice of foods gives the body what it needs to grow and do its daily tasks, while choosing the wrong foods deprives the body of some of the nutrients it needs to function properly. ## What does food provide? - Energy to do work. - Nutrients for the growth and repair of damaged cells and tissues. - Materials for keeping us healthy and disease-free. The food which provides us with all the necessary substances is known as nutritious food. A nutrient can be defined as "a constituent of food that helps one way or the other in the body's functions". # Types of Nutrients Nutrients are classified into five major groups: - **Carbohydrates** (like rice, potato and sugarcane) provide energy. - **Proteins** (like pulses, milk, egg, etc.) provide the body with building material to grow. - **Fats** (like ghee, oil and butter) provide energy and help in insulating the body. - **Minerals**, such as iron, sodium, calcium, phosphorus, etc., are needed for their specific roles in the body. - **Vitamins** are needed for the normal functioning of various body processes. Carbohydrates, fats and proteins are required in larger quantities, hence, they are called *macronutrients*. Minerals and vitamins are required in minute quantities, hence, they are called *micronutrients*. # Nutrition Nutrition: The process by which all organisms obtain or synthesise their food and convert it into simpler substances, so that it can be absorbed and utilized by the cells of the body is called nutrition. The whole process of nutrition in humans is conducted in five steps 1. Ingestion 2. Digestion 3. Absorption 4. Assimilation 5. Egestion # Digestive System in Humans The food we eat passes through the food canal or alimentary canal. It is a long, muscular and coiled tube. It starts from the *mouth* and ends at the *anus*. ## Digestive System Glands, such as liver, salivary glands and pancreas are associated with the food canal. The food canal together with these glands forms the **digestive system**. The different organs of the digestive system are as follows: ### Alimentary Canal Consists of mouth (with teeth and tongue), oesophagus, stomach, small intestine, large intestine and rectum. ### Digestive Glands Consist of salivary glands, liver and pancreas. # Digestion Digestion is a process by which the complex chemical compounds present in the food are broken down into simpler substances that are readily absorbed and utilized by the body. ## The Mouth The mouth is bordered by upper and lower lips. The lips help in closing the mouth during swallowing. Inside the mouth are present the teeth and the tongue. Food is taken through the mouth. This is called *ingestion*. The food is broken down into smaller pieces or chewed by the teeth. ## The Salivary Glands What you call the watering of the mouth is actually a secretion of saliva from the salivary glands in your mouth. The saliva is secreted by three pairs of salivary glands (one below the tongue, second at the base of the ear, and third on the inner side of the angles of the lower jaws) (Fig. 4.4). Saliva is a fluid containing water, salts and a slimy mucus. It also contains an enzyme called *amylase* which converts starch into maltose. A piece of bread or boiled rice when chewed thoroughly starts tasting sweet. The tasteless starch changes into sweet maltose (sugar). ``` Starch salivary Sugar (maltose) amylase ``` ## Functions of Saliva 1. It moistens and lubricates the mouth cavity and the tongue to make speaking and swallowing easy. 2. It cleans the mouth and destroys germs. 3. The saliva binds the food particles and makes it into a mass called the *bolus.* 4. Digestion begins in the mouth. Saliva contains an enzyme (amylase) which helps in the breakdown of starch to simple sugars (= maltose). ## The Oesophagus (Food Pipe) The oesophagus or the food pipe is a long and narrow tube which runs from the back of the throat, down through the chest to open into the stomach. The food moves down the oesophagus by *peristalsis*. No digestion occurs in the oesophagus. Peristalsis is the slow wave-like movement seen in the walls of the oesophagus due to the contraction and relaxation of its muscles. Peristalsis occurs throughout the alimentary canal. This movement enables the food to be pushed downward. ## The Stomach The stomach is an elastic bag. Its walls are highly muscular. It also contains glands called gastric glands which secrete gastric juice. The gastric juice given out by the gastric glands contains water, hydrochloric acid and an enzyme *pepsin*. The acid kills the germs (which may have entered along with the food), prevents rotting of food during its long stay in stomach, and it activates the enzyme pepsin. Pepsin converts proteins into a simpler compound called peptones. ``` Proteins Pepsin Peptones ``` The protein *casein* present in the milk is converted into curd by the help of another enzyme, the *rennin* ``` Casein Rennin Curd ``` The food remains in the stomach for about 3-4 hours or more. During this time stomach also churns the food thoroughly along with the digestive juices and changes it into a pulp-like thick paste called *chyme*. The chyme moves into the first part of the small intestine. ## The Small Intestine The small intestine is a long coiled tube of about 7 metres. The food remains in the small intestine for about 3-5 hours for digestion and absorption. The short upper 'U' shaped part of the small intestine, continuing from the stomach, is called *duodenum*. Duodenum receives a common duct that brings secretions from both the liver (called bile juice) and the pancreas (called pancreatic juice). ### The Liver Liver (Fig. 4.5) is reddish brown in colour and is the largest gland in the body. It is situated on the right side of the body, below the chest region. The liver produces a greenish yellow fluid - the bile which is stored in the gall bladder, an organ found just below the liver. The bile juice breaks down fats into tiny droplets and this process is called *emulsification*. ### The Pancreas The pancreas is a large, whitish, leaf-shaped gland situated below the stomach. It secretes pancreatic juice which is poured into the duodenum through the pancreatic duct. The pancreatic duct opens into the duodenum by an aperture common to that of the gall bladder. The pancreatic juice contains enzymes namely *amylase*, *trypsin* and *lipase* which help in the digestion of carbohydrates, proteins and fats respectively. - *Amylase* acts on the starch converting it into maltose. - *Trypsin* converts proteins and peptones into peptides. - *Lipase* converts emulsified fats into fatty acids and glycerol. ``` Starch Pancreatic → Maltose Amylase Proteins + peptones Trypsin Peptides → Emulsified Lipase, Fatty acids and glycerol fats ``` The second part of the small intestine is called *jejunum.* No digestion takes place here. The semi-digested food enters the last part of the small intestine called *ileum*. The ileum is a long, narrow, coiled tube. The inner lining of ileum contains glands called intestinal glands which produce "intestinal juice". This contains enzymes like *erepsin*, *maltase*, *sucrase*, *lactase* and *lipase*. ## Absorption of Digested Food in Small Intestine The inner lining of the small intestine contains a large number of tiny finger-like projections called *villi* (singular *villus*) (Fig. 4.6). The villi greatly increase the inner 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 amino acids and glucose to pass them into the blood system, whereas the fatty acids pass into special tubes called lymph vessels. Vitamins and mineral salts are mostly soluble in water and are directly absorbed through the walls of the intestine. The ileum of the small intestine serves functions both for the digestion and the absorption of the digested food. # The Large Intestine 1. The large intestine is about 1.5 metre long and consists of 3 regions: the *caecum*, the *colon* and the *rectum* (Fig. 4.7). 2. The large intestine does not secrete any enzyme. It *mainly absorbs* water from the undigested food. After much water is absorbed, the undigested waste matter that reaches the rectum is semi-solid. 3. The *rectum* is the last part, about 15 cm long. It stores the undigested waste matter called *faeces*. The rectum opens to the outside at the *anus*. It has a circular muscle (*sphincter*) to keep it closed. When this muscle relaxes, the anus opens to eliminate the faeces. 4. The process of eliminating the undigested food through the anus is called *egestion*. # Assimilation It is the utilization of the digested food or nutrients by the body cells. # Functions of Different Organs | Name of Organs | Functions of the Same Organs| |---|---| | Mouth | Chewing | | Salivary Glands | Saliva digests starch | | Oesophagus | Carries food to stomach | | Liver | Produces bile | | Gall Bladder | Holds bile | | Stomach | Produces gastric juice, pepsin, HCI, etc | | Pancreas | Produces digestive enzymes | | Small Intestine | Produces digestive enzymes and also absorbs digested food | | Appendix | No known function | | Large Intestine | Reabsorbs water | | Rectum | Holds undigested food until elimination | | Anal Opening | Opening through which undigested food is egested | The various parts of the digestive system and their functions are summarised in the Fig. 4.8. # Structure of a Tooth The tooth is formed of three parts - crown, neck and root. - The top portion of the tooth which can be seen is called the *crown*. The white, hard layer which surrounds the crown is called the *enamel*. Enamel is the hardest substance in the body. *Dentine* is the bone-like hard substance just below the enamel. It supports the tooth. Dentine forms bulk of the tooth and has a hollow space filled with a soft material called the *pulp*. Pulp has nerves and blood vessels. Blood (or in the villi) are transported to the rest of the body. - Glucose - (end product of carbohydrate digestion) is required to release energy for cell activities. - Amino acids - (end products of protein digestion) are used for growth, repairing worn out cells and tissues. - Fatty acids and glycerol - (end product of fat digestion) act as reserves of energy and are stored for further use.