CBSE Class 7 Biology Notes- Chapter 4 Respiration in Organisms

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A cell is the smallest functional unit of an organism. To perform the bodily functions, cells need energy. This energy comes from respiration.
The food that we eat stores energy in it. The oxygen that we breathe is transported to each cell of the body. Using this oxygen, cells break down food and release energy. This process is called cellular respiration.
When food is broken down in the presence of air, it is called aerobic respiration
When food is broken down without the presence of air, it is called anaerobic respiration. When we do some heavy exercise, the supply of oxygen to the muscles reduces and anaerobic respiration occurs.
Breathing is the process of taking in oxygen and giving out carbon dioxide.
We take in oxygen though our nose which reaches the lungs through the wind pipe. This process is called inhalation. We then give out air consisting mostly of carbon dioxide and this is called exhalation. The number of times we breathe in a minute is called breathing rate. Inhaling and exhaling once is called one breath.
The breathing rate may change according to the type of activity a person is doing. While relaxing, the breathing rate is less whereas while any brisk physical activity such as running or exercising increases the breathing rate.
When we inhale, the air passes through the nostrils, moves down the windpipe, and reaches our lungs. The lungs are in the chest cavity which is surrounded by ribs on the sides. When we inhale, the ribs move up and outwards, the diaphragm moves down which increases the space in the chest cavity and the lungs get filled with air.
When we exhale, the ribs move down and inwards, the diaphragm moves up and the chest cavity gets reduced in size which pushes out the air.

Breathing in other animals:
Most insects have small openings on the sides of their body called spiracles. They have a network of tubes called trachea for gas exchange. 

Animals like earthworms breathe through their skin.
Animals living under water, such as fishes, have small projections of the skin called gills which help them in absorbing oxygen dissolved in water.
While plants perform photosynthesis during the day and give out oxygen, during the night they perform respiration just as any other living being. They take in air, absorb oxygen and release carbon dioxide.

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