CBSE Notes Class 7 Science Chapter 6 Respiration in Organisms

All organisms are made of small microscopic units called cells. A cell is the smallest structural and functional unit of an organism. Each cell of an organism performs certain functions such as nutrition, transportation, excretion and reproduction. To perform these functions, the cell needs energy. Even when we are eating, sleeping or reading we require energy. The food has stored energy, which is released during respiration. Therefore, all living organisms respire to get energy from food.

1.0Respiration in Organisms: Introduction

• The metabolic process by which organisms obtain energy from oxidation of food (organic molecules). 
• During breathing, we breathe in air. 
• We breathe out air which is rich in carbon dioxide. 
• The air we breathe in is transported to all parts of the body and ultimately to each cell. In the cells, oxygen helps in the breakdown of food.

2.0Process of Respiration 

Cellular respiration : Cellular respiration takes place in the cells of all organisms. It is an intracellular process in which glucose is oxidized and the energy is used to make ATP (adenosine triphosphate). It consists of many separate enzymatic reactions.

3.0Types of Cellular Respiration 

Cellular respiration are of two types 

Aerobic respiration and 

Anaerobic respiration.

1. Aerobic Respiration 

• The glucose and oxygen react together in the cells to produce carbon dioxide, water and energy. 
• The reaction is called aerobic respiration because oxygen from the air is needed for it to work.
• Here is the word equation for aerobic respiration: 

Glucose + oxygen   → Carbon dioxide + Water + Energy

2. Anaerobic Respiration 

• When the food is broken down without using oxygen it is known as anaerobic respiration. 
• In this type of respiration incomplete oxidation of food takes place and in comparison to aerobic respiration, much less amount of energy is produced. (a) 
• Anaerobic respiration in yeast cells : 
• Yeast can survive in the absence of air. 
• They are called anaerobes. 
• They get energy through anaerobic respiration.
• The glucose breaks down into alcohol and carbon dioxide in the absence of oxygen .
• The reaction is called anaerobic respiration or fermentation. 
• Here is the word equation for anaerobic respiration

Glucose (without the use of oxygen)  →  Alcohol + Carbon dioxide + Energy 

Anaerobic Respiration in Muscle Cells :

• Our muscle cells can also respire anaerobically but only for a short time when there is a temporary deficiency of oxygen. 
• During heavy exercise, fast running ,cycling, walking for many hours or heavy weight lifting, the demand for energy is high but the supply of oxygen to produce the energy is limited. 
• Then anaerobic respiration takes place in the muscle cells to fulfill the demand of energy. 

Glucose in muscle cells (without the use of oxygen) →  Lactic acid + Energy

4.0Breathing

• We require oxygen for oxidation of food and breathing is essential for this purpose. 
• Breathing means taking in air rich in oxygen and giving out air rich in carbon dioxide with the help of respiratory organs.
• It is a continuous process which goes on all the time and throughout the life of an organism. Breathing includes inhalation and exhalation. 
• Inhalation :The process of taking in air rich in oxygen into the body is called inhalation. 
• Exhalation : The process of giving out of air rich in carbon dioxide is known as exhalation. 
• Breathing rate : The number of times a person breathes in a minute is termed as the breathing rate. A breath means one inhalation and one exhalation. During breathing inhalation and exhalation take place alternately.

Breathing Organs in Animals 

• Most animals use specialized organs or organ systems, such as lungs, trachea, or gills, for external respiration. 
• In all cases, exchange of gasses between the environment and an animal occurs by diffusion through a wet surface on the animal which is permeable to oxygen and carbon dioxide. 

1. Skin 

• Earthworms breathe through their skins. 
• The skin of an earthworm feels moist and slimy on touching. Gasses can easily pass through them. 
• Though frogs have a pair of lungs like human beings, they can also breathe through their skin, which is moist and slippery. 

2. Trachea 

• Cockroaches, grasshoppers and other insects have small openings on the sides of their body. 
• These openings are called spiracles. Insects have a network of air tubes called tracheae for gas exchange. 
• Oxygen rich air rushes through spiracles into the tracheal tubes, diffuses into the body tissue, and reaches every cell of the body. 
• Similarly, carbon dioxide from the cells goes into the tracheal tubes and moves out through spiracles.

3. Gills 

• Gills are projections of the skin and they help in breathing in fishes and molluscs (a type of invertebrate). 
• Gills are well supplied with blood vessels for exchange of gasses. 

4. Lungs 

• Animals such as frogs, lizards, snakes, birds, elephants, lions, cows, goats have lungs in their chest cavities like human beings. 

5.0Human Respiratory System 

• Normally we take in air through our nostrils. 
• When we inhale air, it passes through our nostrils into the nasal cavity. 
• From the nasal cavity, the air reaches our lungs through the windpipe. 
• Lungs are present in the chest cavity. This cavity is surrounded by ribs on the sides. 
• A large, muscular sheet called diaphragm forms the floor of the chest cavity. Breathing involves the movement of the diaphragm and the rib cage. 

Mechanism of Breathing 

• During inhalation, ribs move up and outwards and diaphragm moves down. 
• This movement increases space in our chest cavity and air rushes into the lungs. 
• The lungs get filled with air. 
• During exhalation, ribs move down and inwards, while diaphragm moves up to its former position. 
• This reduces the size of the chest cavity and air is pushed out of the lungs. These movements in our body can be felt easily. 

6.0Gaseous Exchange in Plants

• The process of respiration is similar in all organisms, whether plants or animals. 
• Carbon dioxide is given out and oxygen is used up, and a lot of energy is produced. 
• In plants, specific organs for breathing are absent. 
• The exchange of gasses, however, occurs by the process of diffusion. 
• Diffusion of gasses (oxygen and carbon dioxide) occurs through two sources. 
• Through stomata (singular stoma), the minute openings on the surface of leaves. 
• During the day, CO2 generated during respiration is used up for photosynthesis hence there is no CO2 release instead oxygen release in the atmosphere is the major event. 
• Rate of photosynthesis is more as compared to respiration during day time. 
• During the night, stomata are closed and photosynthesis does not occur. CO2 elimination is the major exchange activity going on because CO2 is liberated during respiration and is not used in photosynthesis. 
• Through the general surface of the roots. The roots take up oxygen from the air spaces present between the soil particles. In aquatic plants, gasses diffuse through the body surface.