Respiration in Organisms

Notes of chapter: Respiration in Organisms are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 7.

(1) Cell-

A cell is the smallest structural and functional unit of an organism which performs certain functions as nutrition, transport, excretion and reproduction.

(i) To perform all activities cell needs energy which they get from food.

(ii) The food has stored energy, which is released during respiration (breathing).

(2) Respiration-

The respiration is a process in which living organism inhale oxygen and exhale carbon dioxide.

(3) Cellular respiration-

The process of breakdown of food in the cell with the release of energy is called cellular respiration. It takes place in the cells of all organisms.

(4) Aerobic respiration-

The process in which breakdown of glucose occurs with the use of oxygen it is called aerobic respiration.

Breakdown of the food releases energy with the use of oxygen.

Glucose →carbon dioxide + water + energy

(5) Anaerobic respiration-

The process in which breakdown of glucose occurs with out using oxygen it is called anaerobic respiration.

Breakdown of the food releases energy without using of oxygen.

Glucose →carbon dioxide + alcohol + energy

(6) Anaerobes-

The organisms that can survive with out air and get energy from anaerobic respiration are called anaerobes.

Eg :- Yeast

(i) Yeast is a single – celled organism. They respire anaerobically and produce alcohol in the processes. So, they are used in making wine and beer.

(7) When human body has temporary deficiency of oxygen as in case of running, walking for long, cycling and heavy weight lifting, muscle cells can also respire anaerobically for a short time. It happens because the demand for energy is high but supply of oxygen to produce energy is limited. At that time anaerobic respiration take place to produce required energy. It also produces lactic acid.

Breakdown of glucose (in muscle) with out using oxygen releases energy and lactic acid.

Glucose (in muscle) → lactic acid + energy

(8) Cramps-

The cramps occur when lactic acid accumulates in muscles because of anaerobically respiration in muscle cells. Human muscle cells breakdown food to produce lactic acid. We get relief from the cramp after a hot bath or massage because these treatments increase blood circulation. As a result, muscle cells get supply of oxygen that breakdown the lactic acid completely in to carbon dioxide and water.

(9) Breathing –

The process of continuous inhalation of air rich in oxygen and exhalation of carbon dioxide is called breathing.

(i)The process of taking air rich in oxygen inside the body is called inhalation.

(ii) The process of giving out the air rich in carbon dioxide from the body is called exhalation.

A breath means one inhalation + one exhalation.

Breath = one inhalation + one exhalation

Activity 1

Close your nostrils and mouth tightly and note the duration to keep close your nostrils. You felt suffocating after 3-4 seconds. It is the maximum time when you can hold your breath. Now it is easy to understand that breathing is the most important for our life.

(iii) The number of times a person breathes in a minute is termed as the breathing rate.

Activity 2

Our breathing rate is different in different activities. Changes in our breathing rate under different conditions are given below:

Name of the classmate Breathing rate normal Breathing rate after a brisk walk for 10 minutes Breathing rate after running fast 100m At rest
Sita 12-18 18 24 13
Geeta 12-18 17 24 14
Lalita 12-18 17 23 13
Kavita 12-18 19 15 15
Self 12-18 17 13 13


(iv) After physical activity we feel hungry because we breathe faster and more oxygen supplies to our cells. It speeds up the breakdown of the food and more energy is released.

(v) An adult human being at rest breathes in and out 15-18 times in a minute. The breathing rate can reach to 25 times per minute during physical activities.

(vi) At the time of physical exercise we breathe fast and take deep breaths. Hence we inhale more oxygen.

(vii) Our breathe rate slows down when we feel drowsy because of deprivation of oxygen.

Activity 3

Pictures that are given below show different activities performed by a person during a day. The rate of breathing differs in all given activity.

Respiration in organisms            Respiration in organisms

i                                                          ii

Climbing Activity from NCERT Chapter Respiration in organisms              Running from NCERT Chapter Respiration in organisms

iii                                                           iv

Walking from Respiration in organisms


REading from Respiration in organisms


Pictures are arranging in the increasing rate of breathing.

  1. (ii) Resting
  2. (vi) Reading/ Sitting Work
  3. (i) Household work/Light Physical Work
  4. (v) Walking
  5. (iv) Running
  6. (iii) Climbing Stairs

(10) Human respiratory system-

Human respiratory system is a biological system which consists of different organs such as nasal cavity, trachea, lungs, chaest cavity and diaphragm and these organs are used in inhale oxygen and exhale carbon dioxide.

(i) Nasal cavity is the space for air. It is placed behind the nose and in the middle of the face.

(ii) The passage of air from the throat to the lungs is known as trachea.

(iii) The lungs are a pair (right and left lungs) of breathing organ located in chest cavity. It removes carbon dioxide from the blood and brings oxygen to the blood.

(iv) The chest cavity is the chamber of the body which is surrounded by the rib cage.

(v) The diaphragm is a large flat dome like breathing muscle that forms the floor of the chest cavity and separates the lungs from the stomach.

Human respiratory system from NCERT Chapter Respiration in organisms

(11) Breathing-

Breathing involves the movement of the diaphragm and the rib cage.

(i) Inhalation-

The upward and outward movement of rib and downward movement of diaphragm increase the space in chest cavity and air rushes into the lungs. This process is known as inhalation.


(ii) Exhalation-

The downward and inward movement of ribs and upward movement of diaphragm to its normal position reduce the size of the chest cavity and air is pushed out of the lungs. This process is known as exhalation.


Activity 4

The table given below shows the effect of breathing on the chest size of some classmates.

Name of the classmate Size of chest (cm) during inhalation 


Size of chest (cm) during exhalation


Difference in size
Ravi 35.8 32.5 3.3
Sandeep 37.0 34.0 3.0
Rahul 35.6 33.1 2.5
Adhar 35.0 31.2 3.8

Activity 5

Take a plastic wide bottle. Remove it bottom. Make a hole in the lid.  Take a y- shaped plastic tube. Fix two deflated balloons on the two ends of the tube. Put this tube in the bottle through lid as shown in figure given below. Now seal the bottle from the upper side and tie a thin rubber or plastic sheet at the open base of the bottle. Now pull the plastic sheet or rubber at the bottom balloons will inflate. It shows inhalation of the air. Now push the rubber or plastic sheet at the bottom of the bottle balloons will deflate. It shows the exhalation of the air.

Mechanism of Breathing from NCERT Chapter Respiration in organisms

                                 Model of  mechanism of breathing

The balloons in this experiment represent the lungs. The rubber sheet represents the diaphragm.

(12) When we exhale on a mirror a film of moisture appears on its surface. It happens because warm air which we exhale has water vapours that cool down when it comes in contact of comparatively cold surface of the mirror. Warm water vapours of our breath leave the air and we see it as tiny droplets. Hence, our breath condenses and we get droplets on the mirror.

Activity 6

Take a clean test tube or a plastic bottle. Make a hole in its lid and fix it on the bottle. Pour some freshly prepared lime water in it and put a straw through hole in it. The straw should dip in the lime water. Blow on the straw. The colour of the lime water will turn milky. It happens because when we blow on the straw carbon dioxide releases that formed calcium carbonate with lime water.

Effect of exhaled air on lime water

Effect of exhaled air on lime water

(13) The air which we inhaled and exhaled is a mixture of many gases. The air which we inhaled has more oxygen and less carbon dioxide. The air which we exhaled has more carbon dioxide and less oxygen. Besides these gases we inhale and exhale nitrogen and other gases. The percentage of inhalation and exhalation is approximately 78% and other gases shares only 1% or less than it.

A chart is given below that shows the percentage of oxygen and carbon dioxide in the air.

Percentages of gases in inhaled and exhaled air

(14) Breathing in Cockroach or insects-

(i) Spiracles are the small openings on the bodies of some animals as in cockroach that is used in respiration.

(ii) Tracheae are a network of air tubes in insects for gas exchange.

(iii) Air rich in oxygen passes through the spiracles into the tracheal tubes, diffuses in to the body tissue and reaches every cell of the body. Similarly, the carbon dioxide from the cell goes into the tracheal tubes and moves out through spiracles.

(15) Earthworms breathe through its skin. Its skin is moist and slimy on touching. Gases can easily pass through them.

(16) Frogs have a pair of lungs like human beings. They can also breathe through their skin which is moist and slippery.

(17) Fishes have gills that help them to breathe in water. Gills help them to use oxygen dissolve in water. Gills are projections of the skin. These are well supplied with blood vessels for exchange of gasses.

(18) Plants also respire. Each Plant takes oxygen and releases carbon dioxide. Each part of the plants takes oxygen independently.

Oxygen is taken by the roots of the plants. Roots need water to release energy. It takes oxygen and water from the soil and burn carbohydrates to release energy which is required to perform functions of root. Hence, roots take oxygen and leaves carbon dioxide in the air. Even when a plant shed off its leaves it takes oxygen through roots and releases carbon dioxide into atmosphere.

Leaves of the plants take carbon dioxide from the air and releases oxygen in the atmosphere during photosynthesis. But at the same time plants are also taking oxygen from the roots.

Roots Absorbs Air from the Soil from NCERT Chapter Respiration in organisms

(19) When we over watered a potted plant it drowns or become light green because it can not take oxygen from the air. Over wet soil do not has air pockets. So roots can not breathe as it can not take oxygen. As a result of it plant turns light green or drown.

Helping Topics

NCERT Solutions Class 7

Worksheet, Class 7

Why does an athlete breathe faster and deeper than usual after finishing the race?

Similarities and differences between aerobic and anaerobic respiration






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