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Biology for students of class X

Score n learn

CONTENTS Chapter 1

Life Processes


Chapter 2

Control and Coordination


Chapter 3



Chapter 4

Heredity and Evolution


Chapter 5

Our Environment


Chapter 6

Management of Natural Resources


The following 4 pages will give you a glimpse of how effectively we have used illustrations along with simple language to explain a concept. The following pages are from the part of the biology book for X grade, these pages detail “Human Respiratory system�

Respiration in animals: Simple aquatic animals exchange gases by diffusion directly through their body wall. However, complex aquatic animals have gills for the diffusion of gases. For example, in fishes, water enters through their mouths, and then moves quickly to the gills. In the gills, the O2 is taken in by the blood which transports it to all the body cells for respiration.

Cold-blooded animals such as fishes need less energy to maintain their body temperature. Therefore, their oxygen demands are less as compared to those of warm-blooded animals.

Figure 22: Respiratory system of fish

Which animal group do you think has the faster rate of breathing – terrestrial or aquatic? Aquatic animals The oxygen source, i.e., water is about 800 times denser than air. It contains less O2 as compared to air. Since the content of O2 in water is low, aquatic animals have to breathe faster to get as much oxygen as possible, resulting in a faster rate of breathing. Therefore, aquatic animals also show various adaptations for better gaseous exchange.

Sectional assessment 3 1. 2. 3. 4. 5. 6. 7. 8.

Time: 25 mins

Write the chemical reaction for respiration. What is glycolysis? Where does it occur? Write three differences between aerobic and anaerobic respiration. During heavy exercise, which type of respiration does our muscle cells undergo? Give reasons for your answer. Write the chemical reaction for the breakdown of ATP to AMP. Among a monkey and a fish, which organism has the faster rate of breathing? Give reasons for your answer. Compare the processes of photosynthesis and respiration by writing chemical reactions for each. Explain the process of anaerobic respiration in yeasts.

Human Respiratory System

The human respiratory system consists of organs that transport oxygen to the circulatory system (heart). The heart distributes it to all the body cells so that they can carry out cellular respiration. In addition to O2 supply, the respiratory system also helps in removing CO2 which is the waste product formed as a result of cellular respiration.


The intake of O2 and removal of CO2 involves two processes: 1) Inhalation: It involves bringing in air from outside the body into the lungs. When we breathe in, the size of the chest increases. When the air enters our lungs, it causes our ribs to move out and simultaneously, the diaphragm contracts and becomes flat. Thus, the size of the chest increases. 2) Exhalation: It involves the removal of CO2 from the body. When we breathe out, the size of the chest decreases. When air moves out of our lungs, the ribs move back and also the diaphragm curves upwards into the chest. Thus, the size of the chest decreases.

Dolphins and whales breathe air like human beings because the air contains 20 times more oxygen than water.

Figure 23: Inhalation and exhalation These processes involve various organs of our body. Organs involved in respiration: The organs of the respiratory system extend from the nose to the lungs. It includes nose, pharynx, trachea, bronchi, and bronchioles.

Figure 24: Human respiratory system


a) Nostrils The flow of air from the outside to the lungs starts from the nose which is divided into the left and the right nostrils. These nostrils then lead to open spaces in the nose called the nasal passages. These passages often contain hair and mucus. Function of hair and mucus present in our nasal passages. • The mucus that lines a nasal passage collects dust, bacteria, and various other foreign particles that enter with air. • The cilia embedded in the mucus membrane wave constantly and move the mucus trapped with foreign particles to the pharynx, and then to the stomach where they are destroyed by the acids present. • It also helps in moistening the air. • Below the mucus membrane, there are a large number of capillaries. The blood within these capillaries warms the entering air.

REMEMBER Pharynx is a part of the respiratory system as well as the digestive system.

Now can you guess why we should not breathe through mouth? If we take in air through our mouth, then the air is not filtered. Therefore, dust and bacteria can easily enter our respiratory tract. b) Pharynx: Air travels from the nasal passages to the pharynx which is commonly known as the throat. The pharynx is also lined with a protective mucus membrane and cilia that remove the impurities entering with the air. c) Larynx: The air from the pharynx enters the larynx or voice box. It is situated in the middle of the neck. It contains two true vocal cords which are made up of cartilage and fibres. When air passes through this area, the vocal cords vibrate and this produces sound. Human beings can control this vibration. Thus, they can make various sounds.

Figure 25: Nasal passage and trachea From the larynx, the inhaled air moves into the wind pipe or the trachea. d) Trachea: The trachea is a long narrow tube lined with ciliated mucus membrane.


The trachea branches into two tubes, the left and the right bronchi. The cilia move the mucus containing dust particles back to the pharynx and it is swallowed there. The trachea is supported by rings of cartilage. These cartilage rings prevent the air passage from collapsing. You must have seen smokers coughing frequently. Can you tell why? What are the harmful components present in tobacco? What are the harmful effects of the tar present in cigarettes?

In the Laboratory

The following experiment will help you answer the questions given above. Take an empty squeezable ketchup bottle. Remove the cap of the bottle and clean the bottle by washing it properly in running water. Place a few cotton balls in the lid of the cleaned bottle and then put the cap back on.

Fit two or more cigarettes on the tip of the bottle.

Why do we sneeze? • When more foreign particles are in the nasal passage, the cilia can remove these build ups and irritate the mucus membrane. This irritation triggers us to sneeze so as to get rid of the polluted air. • Dust and pollen are few of the many agents that can make you sneeze.

Figure 26: Experiment showing the harmful effect of cigarettes Then light the cigarettes and simultaneously, squeeze the body of the bottle slowly till the cigarette is almost finished. Repeat this process with two or more cigarettes. Then, open the cap of the bottle and remove the cotton balls. Can you guess what happens to the cotton balls? The cotton balls become black in colour. What is collected in the cotton balls? Tar, a component of cigarettes, is collected in the cotton balls.


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