Respiration is a process that provides energy required for each and every activity of the organism. It occurs in all living beings for providing energy through enzyme controlled breakdown of organic substances.

  •  In the process of respiration, oxygen is taken up for the oxidation of food in order to release energy , water and carbon dioxide . Above show the reaction of respiration :-

C6H12O +  6O2    →  6CO2  +  6H2O  +  ENERGY

In respiration, there is release of energy through oxidation of simple food molecules such as glucose. The energy released in respiration is chemical energy which gets stored in the form of adenosine triphosphate (ATP) molecules. It is the energy currency for the most of the cellular processes, such as protein synthesis, conduction of nerve impulses etc.

Oxidation of food can occur in the presence or absence of oxygen.

On the basis of oxygen utilization, respiration is of two types :

  1. Aerobic Respiration:-

In this mode of respiration , organic food is completely oxidised in presence of oxygen to carbon dioxide and water .

The organisms performing aerobic respiration are called aerobes.

Life Processes: Respiration
  1. glucose (a six carbon molecule) into a three carbon molecule called pyruvate.One molecule of glucose produces 2 molecules of pyruvate. It takes place in the cytoplasm of the cell and the process is called glycolysis.
  2. 36 or 38 molecules of ATP are produced.
  3. When the pryuvates complete oxidised in mitochondria into water , carbon dioxide and release energy . This process is known as kreb’s cycle . ( only in aerobic respiration ).
  • Anaerobic Respiration:- (absence of oxygen )

In this mode of respiration , organic food is incomplete oxidised into ethyl alcohol in the absence of oxygen .

Two molecules of ATP are released in this process .

eg:- certain bacteria and yeast.

Life Processes: Respiration

Muscle Fatigue : During heavy activities there is lack of oxygen supply and thus , anaerobic respiration takes place in muscle cells .

Life Processes: Respiration

Human Respiratory System :

Human respiratory system includes nostrils, nasal passage (cavity), pharynx, larynx, trachea, bronchi, lungs, and diaphragm.

Life Processes: Respiration


Life Processes: Respiration


1.Nostrils or internal nerves : There is a pair of small, oval apertures present at the lower end of the nose and just above the mouth. Air is inhaled by the nostrils.

2.Nasal Chambers : A pair of nasal chambers are enclosed in the nasal cavity and are present above the palate. These are separated by a nasal septum. Fine hair and mucus secreting cells are present in the lining of muscle chambers which help in filtering the air.

3.Pharynx: Pharynx is a common passage for the air we inhale and the food we eat. The pharynx leads into trachea or wind pipe through a slit like aperture called glottis. While swallowing the food, glottis gets covered by a small cartilaginous flap called epiglottis.

4`.Larynx : Larynx is a sound producing organs and contains vocal cords. These cords are made up of elastic fibers. Larynx is known as the voice box.

5.Trachea : The larynx leads into a long tube called trachea or wind pipe.These cartilaginous rings ensures that the air passage does not collapse during inspiration.

6.Bronchi and Bronchioles : The trachea bifurcates as its lower end into a pair of primary bronchi which enters the left and right lungs. As primary bronchi enters into lungs. They divide into secondary and tertiary bronchi. The walls of bronchi are also supported by cartilaginous rings. The tertiary bronchi sub-divide into smaller branches, the bronchioles which are not supported by cartilaginous rings. The bronchioles decrease in diameter and their epithelium also become thinner as they go deeper into the lungs.

7.Lungs : The lungs are soft, spongy and elastic organs which are pinkish in colour a pair of lungs lie in the thoracic cavity, one either side of the heart.

In man, the left lung, has two lobes, superior lobe and inferior lobe whereas the right lung has three lobes, superior lobe, median lobe and inferior lobe. The left lung is slightly smaller than the right and has a concavity where the heart lies within the lungs. Lung receives bronchi which divide into numerous bronchioles. The bronchioles further sub-divide and gradually lead to a number of respiratory bronchioles, which further give rise to alveolar ducts. Alveolar ducts finally terminates in balloon like structures called alveoli. The alveoli have very thin wall consisting of squamous epithelium. The alveoli of lungs provide a large surface for gaseous exchange. The gases defuse across the alveolar membrane due to difference in their partial pressures.

Life Processes: Respiration


Since the exchange due to diffusion , a respiratory surface must have following characteristics.

  • It must be thin and permeable to respiratory gases (Oand CO2).
  • It must be moist either with water or mucus.
  • It must be highly vascular and have large surface area.
  • It must be directly or indirectly in contact with source of oxygen (air or water) .

The number alveoli in human lungs has been estimated to be approximately 300 million exposing a surface area of nearly 80m2, fifty times more than the surface area of our body.

The total surface area of our body is 1.6m2. the wall of the alveoli has an extensive network of blood capillaries in contact so that the exchange of gases takes place easily. Thus, each alveolus may be called a miniature lung where exchange of gases takes places.

Hence, we see that alveoli are designed to maximise the exchange of gases. They are far more permeable and vascular than the skin. The total alveolar surface available for gas exchange, far exceeds the general body surface. So, lungs replace the skin very effectively in mammals as respiratory organs.


  1. Diaphragm is the characteristics of all mammals. It is a highly muscular dome-shaped partition elevated towards the thorax.
  2. The most important function of the diaphragm of mammals is to aid in respiration .

Mechanism of breathing

  • Breathing involves the intake of oxygen into the respiratory tract and elimination of CO2 from the body.
  • The intake of oxygen is called inhalation or inspiration. The elimination of CO2 is called exhalation or expiration.
  • Inhalation or Inspiration :
  • During inhalation the inter-costal muscles pull the ribs outwardly and at the same time, diaphragm becomes flat.
  • Due to these movements, the volume of the thoracic cavity increases and creates a low pressure area inside the cavity.
  • We know that air always moves from an area of high pressure to an area of low pressure. So, air from outer atmosphere rushes into the thoracic cavity (lungs) through respiratory tract.
  • Exhalation or Expiration :
  • During exhalation the inter-costal muscles pull the ribs inwardly and at the same time , diaphragm again becomes dome shaped .
  • Due to these movements , the volume of thorax cavity decreases . As a result a high pressure area is created inside cavity .
  • We know that air always moves from an area of high pressure to an area of low pressure . So , air from thoracic cavity ( lungs ) moves outside the body through the respiratory tract .
Life Processes: Respiration


Respiration in plants

Duringrespiration, plants obtain O2 from the atmosphere and release CO2. The exchange of gases occurs through the minute pores called stomata present on the surface of leaves .

Stomata : They are microscopic pore complexes which occur in the epidermis of leaves and other soft parts of the plants . Each stomta consists of a stomatal aperture and two surrounding gurad cell . Gurad cells are the epidermal cells that contain chloroplast the outer and lateral walls of guard cell are comparatively thinner than inner wall . Guard cells can undergo rapid turgor changes . The opening and the closing of the stomatal pore is a function of the guard cell . They swell when water flows into them , causing the stomatal pore to open . On the loss of turgidity , the guard cell becomes flaacid leading to closure of stomatal pore .

Life Processes: Respiration


Stomata help in gaseous exchange ( O2 and CO2 ) between leaves and atmosphere during repiration as well as photosynthesis . Stomata are also responsible for the loss water in the form of vapour across their surface. This phenomenon is known as transpiration . Since large amount of water can be lost through the stomata , the plants keep their stomatal pore close when they do not need CO2 for photosynthesis . During the day , CO2 is utilized by the plants for the process of photosynthesis and release oxygen whereas during the night plants utilize O2 and release CO2 in the atmosphere .

Exchange of gases in plants can occur across the surface of roots and stems as well , via root hairs and lenticels .

  •  Root Hairs : Root hairs are unicellular extensions arising from epidermal cells . Oxygen present in between the soil particles is absorbed by root hairs through diffusion . The absorbed oxygen passes from one cell to other cell of the root by diffusion . Similarly , CO2 diffuses out from root cells in the soil .
  •  Lenticels ( old roots and stems ) : Older parts of the root and stem do not have hairs . They are covered by protective layer of dead cells known as bark . Bark have tiny opening called lenticels which help in the gaseous exchange .
Life Processes: Respiration


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