Respiration in Organisms



  • Why do we respire?
  • Respiration in organisms
  • The need for respiration in organisms



Respiration: The process by which a living being utilises the food to get energy, is called respiration. Respiration is an oxidation reaction in which carbohydrate is oxidized to produce energy.

The first step is the break-down of glucose, a six-carbon molecule, into a three-carbon molecule called pyruvate. This process takes place in the cytoplasm.


This process breaks up the three-carbon pyruvate molecule to give three molecules of carbon dioxide. Breakdown of pyruvate using oxygen takes place in the mitochondria.

Respiration involves:-

Gaseous exchange (Breathing) – Intake of oxygen from the atmosphere and release of carbon dioxide

Cellular respiration – the breakdown of simple food in order to release energy inside the cell.



Aerobic respiration: This type of respiration happens in the presence of oxygen. Pyruvic acid is converted into carbon dioxide. Energy is released and water molecule is also formed at the end of this process.

Anaerobic respiration: This type of respiration happens in the absence of oxygen. Pyruvic acid is either converted into ethyl alcohol or lactic acid. Ethyl alcohol is usually formed in case of anaerobic respiration in microbes, like yeast or bacteria. Lactic acid is formed in some microbes as well as in the muscle cells.

The above diagram shows us the breakdown of glucose by various pathways.

The energy released during cellular respiration is immediately used to synthesize a molecule called ATP which is used to fuel all other activities in the cell. In these processes, ATP is broken down giving rise to a fixed amount of energy which can derive the endothermic reactions taking place in the cell

The rate of breathing in aquatic organisms is much faster than that seen in terrestrial organisms because the amount of dissolved oxygen is fairly low compared to the amount of oxygen in the air.

Aerobic respiration

Anaerobic respiration

Occurs in the presence of oxygen

Occurs in the absence of oxygen

Occurs in the Mitochondria

Occurs in the Cytoplasm

End products are water and carbon dioxide

End products are lactic acid and alcohol


More amount of energy is released

Less amount of energy is released



Nostrils: There are two nostrils which converge to form a nasal passage. The inner lining of the nostrils is lined by hair and remains wet due to mucus secretion. The mucus and the hair help in filtering the dust particles out from inhaled air. Further, air is warmed up when it enters the nasal passage.

Pharynx: It is a tube-like structure which continues after the nasal passage.

Larynx: This part comes after the pharynx. This is also called voice box.

Trachea: This is composed of rings of cartilage. Cartilaginous rings prevent the collapse of trachea in the absence of air.

Bronchi: A pair of bronchi comes out from the trachea, with one bronchus going to each lung.

Bronchioles: A bronchus divides into branches and sub-branches inside the lung.

Alveoli: These are air sacs at the end of bronchioles. The alveolus is composed of a very thin membrane and is the place where blood capillaries open. This is alveolus, where the oxygen mixes with the blood and carbon dioxide exits from the blood. The exchange of gases, in alveoli, takes place due to the pressure differential.

Mechanism of Breathing



During inhalation, the thoracic cavity expands

Thoracic cavity contracts

Ribs lift up

Ribs move downwards

Diaphragm become flat in shape

The diaphragm becomes dome shaped

Volume of lungs increases and air enters the lungs

Volume of lungs decreases and air exits from the lungs

The blood brings carbon dioxide from the rest of the body for release into the alveoli, and the oxygen in the alveolar air is taken up by blood in the alveolar blood vessels to be transported to all the cells in the body. During the breathing cycle, when air is taken in and let out, the lungs always contain a residual volume of air so that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released. When the body size of animals is large, the diffusion pressure alone cannot take care of oxygen delivery to all parts of the body. Instead, respiratory pigments take up oxygen from the air in the lungs and carry it to tissues which are deficient in oxygen before releasing it. In human beings, the respiratory pigment is haemoglobin which has a very high affinity for oxygen. This pigment is present in the red blood corpuscles. Carbon dioxide is more soluble in water than oxygen is and hence is mostly transported in the dissolved form in our blood.

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