Respiration in Organisms Notes

All living things need energy to live.
Respiration is the process of using oxygen to break down food to release energy, carbon dioxide and water.

Respiration in humans

In human beings, the process of respiration involves breathing, gaseous exchange and cellular respiration.

Respiratory system in Human beings:

The organ for respiration in human beings are as follows:
  • Nasal cavity
  • Pharynx
  • Trachea (windpipe)
  • Lungs (bronchi, bronchioles, alveoli)
  • Diaphragm
Lungs are present in the chest cavity, which is surrounded by ribs on the sides. The ribs form a cage-like structure called the ribcage. A large muscular sheet called diaphragm forms the base of the ribcage. The diaphragm contract and relax all the time.


Breathing is a continuous process in which humans breathe in and breathe out air from the body. It is an involuntary action controlled by the brain.
Breathing consists of two phases: inhalation, the process of taking in air and exhalation  the process of giving out air.

Mechanism of Breathing:

During inhalation, the diaphragm contract, it flattens and increases the space inside the chest cavity. This makes the air breathe in and so chest rises and the air outside rushes into lungs. During exhalation, the diaphragm relaxes, it decreases the space inside the chest cavity. This makes the air breathe out and so the chest moves inward and the expelled out.
The air that breathes in passes through the nostrils. The hair and mucus present in the nostril trap dust and germs present in the air. The air then enters a tube-like structure called the trachea (windpipe). The trachea divides at the lower end into narrow tubes called bronchi. The bronchi further divided into numerous extremely narrow tubes called bronchioles in each lung. These tubes open into tiny air sacs called alveoli. The walls of the alveoli are lined with a network of blood vessels and capillaries to facilitate the exchange of gases.

Gaseous exchange:

In respiration, the exchange of gases takes place between the alveoli of the lungs and the capillaries surrounding it.
When fresh air  enters the alveoli, oxygen from the air passes into the blood in the capillaries and binds to the red blood cells. At the same time, carbon dioxide passes out of the blood into the alveoli. This entire process is known as gaseous exchange.

Cellular respiration:

The RBCs (in blood) contain a protein celled haemoglobin which binds with the oxygen and forms a compound called oxyhaemoglobin. Oxyhaemoglobin is carried through the blood to all the cells of the body. In the cells, oxygen is released from oxyhaemoglobin.  Cells use this oxygen  to breakdown glucose (sugar) to form carbon dioxide and water and energy is released. The carbon dioxide generated diffuses into the blood. The blood carries it back to the lungs where it is expelled during exhalation.

Breathing Rate

The number of times a person breathes in a minute is called the breathing rate. At rest, a person breathes almost 14 to 16 times in a minute. During vigorous exercise or while brisk walking, the breathing rate increases.

Types of Cellular Respiration:

Note: The muscle cells in our body normally carry out aerobic respiration, but when we run fast for exercise vigorously the muscle cells work faster and require more energy. Then they respire anaerobically for a short period of time. This produces lactic acid and the person develops cramps temporarily and is forced to rest for a while.

Organs for breathing in different animals:

Different animals have different organs and mechanisms for the exchange of gases. Some of these are as follows:

Respiration in Microorganisms: 

In microorganisms such as Amoeba, the exchange of gases takes place through their most body surface.

Respiration in Earthworm

 In animals that live in the soil, such as earthworms and leeches the exchange of gases takes place through the slimy surface of the skin.

Respiration in Insects:

Respiratory system of an insect such as grasshopper and Cockroaches is known as the tracheal system. It is made up of spiracles and trachea. An insect takes air through tiny holes called spiracles which are located on the body. Spiracles open into narrow tubes called traceae, which carry the oxygen to different parts of the body. Carbon dioxide diffuses out through the trachea and exit through the spiracles.

Respiration in Birds

 Birds have lungs with air sacs.

Respiration in Aquatic organisms

Aquatic animals like fish live in water and get oxygen which is dissolved in water. Fish have a special structure called gills on either side of their mouth. Oxygen from the water diffuses into the blood in the gills and it is carried to every part of the body. Similarly, carbon dioxide in the blood of a diffuses out into the water from the gills.
Aquatic animals such as lobster, prawns, crab and shellfish also respire through gills.


Respiration in Amphibians

Amphibians like frogs, newts and salamander are both aquatic and terrestrial. Frogs have lungs, but they use their skin for the exchange of gases when in water. They use their lungs when on land.
Respiration through the lungs is known as pulmonary respiration and respiration through the skin is known as cutaneous respiration. When at rest of floating on water, the Frog takes in air through its nostril into the buccal cavity. The gaseous exchange takes place through the lining of the buccal cavity. This method of respiration is known as buccal respiration.

Respiration in Plants:

Plants also respire by taking in oxygen and giving out carbon dioxide.
  • The leaves of most plants have tiny pores called stomata on the underside. Guard cells on either side of the stomata regulate their opening and closing.
  • Roots of plants also need oxygen to respire. They take in air present in the soil. When land remains waterlogged, the roots are not able to obtain air. This damages the roots and so the plant dies.
  • The stem bark also has tiny opening called lenticels which help in gaseous exchange during night time.

Differences between Photosynthesis and Respiration:

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