Summary
Respiration is the process by which all living organisms break down food (glucose) to release energy; it includes cellular respiration inside cells and breathing, which takes in oxygen-rich air and expels air rich in carbon dioxide.
Chapter 6 explains why all living organisms respire — to get energy from food. Cellular respiration occurs in every cell, where glucose breaks down with oxygen (aerobic) to release energy, or without oxygen (anaerobic), as in yeast and muscle cells during heavy exercise. Anaerobic respiration in muscle cells produces lactic acid, causing cramps. In humans, breathing involves the diaphragm and ribs: inhalation expands the chest cavity and exhalation reduces it. Inhaled air contains 21% oxygen; exhaled air contains 16.4% oxygen and 4.4% carbon dioxide. Different animals breathe differently — insects use spiracles and tracheae, earthworms breathe through moist skin, fish use gills, and frogs use both lungs and skin. Plants exchange gases through stomata in leaves and absorb air from spaces between soil particles through roots.
Key points & formulas
- 01Cellular respiration is the breakdown of food (glucose) in the cell with release of energy; it occurs in the cells of all living organisms.
- 02Aerobic respiration uses oxygen to break down glucose into carbon dioxide and water; anaerobic respiration occurs without oxygen.
- 03Yeast respires anaerobically and produces alcohol and carbon dioxide — this is why yeast is used in wine and beer production.
- 04During heavy exercise, muscle cells respire anaerobically and produce lactic acid; accumulation of lactic acid causes muscle cramps.
- 05An average adult breathes 15–18 times per minute at rest; during heavy exercise breathing rate can rise to 25 times per minute.
- 06During inhalation, ribs move up and outwards and the diaphragm moves down, enlarging the chest cavity; during exhalation the movements reverse.
- 07Inhaled air contains 21% oxygen and 0.04% carbon dioxide; exhaled air contains 16.4% oxygen and 4.4% carbon dioxide.
- 08Respiratory organs differ across organisms: insects use spiracles and tracheae, earthworms breathe through moist skin, fish use gills, frogs use both lungs and skin, and plants use stomata in leaves and air spaces in soil for roots.
Frequently asked questions
01What is cellular respiration?
Cellular respiration is the process of breakdown of food in the cell with the release of energy. It takes place in the cells of all living organisms.
02What is the difference between aerobic and anaerobic respiration?
Aerobic respiration breaks down glucose using oxygen, producing carbon dioxide and water. Anaerobic respiration breaks down glucose without oxygen — in yeast it produces alcohol and carbon dioxide; in muscle cells it produces lactic acid.
03Why do we breathe faster during exercise?
During heavy exercise the demand for energy is high. Faster and deeper breathing supplies more oxygen to cells, which speeds up the breakdown of food and releases more energy.
04What causes muscle cramps after heavy exercise?
When oxygen supply is insufficient during heavy exercise, muscle cells respire anaerobically and produce lactic acid. The accumulation of lactic acid in the muscles causes cramps.
05How does a hot water bath relieve muscle cramps?
A hot water bath or massage improves blood circulation, increasing the supply of oxygen to muscle cells. The extra oxygen allows complete breakdown of lactic acid into carbon dioxide and water, relieving cramps.
06What is the normal breathing rate of an adult at rest?
On average, an adult human at rest breathes in and out 15–18 times per minute. During heavy exercise, the breathing rate can increase up to 25 times per minute.
07How do insects breathe?
Insects have small openings called spiracles on the sides of their body. Oxygen-rich air enters through spiracles into a network of air tubes called tracheae, which carry gases directly to every cell of the body.
08How do earthworms and frogs breathe?
Earthworms breathe through their moist, slimy skin — gases pass easily through it. Frogs have a pair of lungs like humans but can also breathe through their moist, slippery skin.
09How do fish breathe under water?
Fish breathe through gills, which are projections of the skin well supplied with blood vessels. Gills allow exchange of gases using oxygen dissolved in water.
10How do plants respire?
Leaves of plants have tiny pores called stomata through which they exchange oxygen and carbon dioxide. Roots take up air from the air spaces present between soil particles. Each part of a plant can independently take in oxygen and give out carbon dioxide.
11What happens inside the chest during inhalation and exhalation?
During inhalation, the ribs move up and outwards and the diaphragm moves down, increasing the space in the chest cavity so air rushes into the lungs. During exhalation, the ribs move down and inwards and the diaphragm moves back up, reducing the chest cavity size and pushing air out of the lungs.
12Why do we sneeze when we inhale dusty air?
Unwanted particles such as dust, smoke, and pollens are trapped by hairs in the nasal cavity. If some particles get past the hair and irritate the lining of the nasal cavity, we sneeze to expel those foreign particles from the inhaled air.
13What is the difference in composition between inhaled and exhaled air?
Inhaled air contains 21% oxygen and 0.04% carbon dioxide. Exhaled air contains 16.4% oxygen and 4.4% carbon dioxide, showing that oxygen is used and carbon dioxide is produced during cellular respiration.
14Is the NCERT PDF for Class 7 Science Chapter 6 free to download?
Yes — the NCERT PDF for Class 7 Science Chapter 6 Respiration in Organisms is free to download with no sign-up required.
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