The food digested in our body is used up in two ways :

  1. As fuel for various activities
  2. For growth and repair of body cells and tissues


RESPIRATION - Teachoo.jpg


  • When we say that food is digested and assimilated, it means that molecules are broken down into smaller parts. 
  • These molecules have to be used and energy has to be released from them.
  • Most living things need oxygen from the air to obtain energy from food
  • This oxygen reacts with glucose present in the body cells and burns them slowly to release energy . This energy is stored as ATP molecules.


Respiration is the process of releasing energy from food.


What are ATP molecules?

  • ATP is a substance called adenosine triphosphate which is present inside the cell and has a high energy content
  • It is a tri-phosphate because it contains three phosphate groups in its molecule.
  • Just as a battery provides electrical energy for different purposes such as lighting, heating, running etc., the energy stored in ATP is used by the body cells for various purposes like synthesis of proteins, contraction of muscles and other activities.


Role of Oxygen in Respiration :


Respiration involves taking in oxygen (of air) into the cells using it for releasing energy by burning glucose and then eliminating the waste products (carbon dioxide and water) from the body.


  • However, some types of Respiration can take place either in the lack of oxygen (Fermentation) or in the complete absence of oxygen (Anaerobic respiration). 

Types of respiration:


The two types of respiration or aerobic respiration and anaerobic respiration.


1.  Aerobic respiration.

  • The respiration which uses oxygen to break down food is called aerobic respiration .
  • The glucose is completely broken down into carbon dioxide and water by oxidation .
  • It produces energy in a considerable amount.
  • It is stored in the form of ATP molecules.
  • During aerobic respiration 1 molecule of glucose produces 38 energy-rich ATP molecules.
  • Mitochondria is the site of aerobic respiration in the cell.

Eg. Human beings obtain energy by aerobic respiration.


2.   Anaerobic respiration:

  • The respiration which t akes place without oxygen is called anaerobic respiration.
  • In anaerobic respiration microorganisms like Yeast break down glucose into ethanol and carbon dioxide and release energy. 
  • The breakdown of sugars by yeast to make alcohol, in the absence of air , is called fermentation

Fermentation by Yeast. - Teachoo.jpg


  • The energy produced is much less as compared to aerobic respiration. We can say that 1 molecule of glucose produces only 2 energy-rich ATP molecules in anaerobic respiration.


Human muscle cells also respire anaerobically when they do not receive sufficient oxygen

  • During vigorous physical exercise , oxygen gets used up faster in the muscle cells than can be supplied by the blood. 
  • Thus muscle cells (in humans as well as animals) obtain energy from anaerobic respiration.
  • They break down glucose in the absence of oxygen to make lactic acid. 
  • The accumulation of lactic acid causes muscle cramps.
  • Muscle cramps can be relieved with a hot bath as it increases circulation. 
  • I ncreased blood flow increases oxygen levels again after which muscle cells break down lactic acid to CO 2 and H 2 O.

Anaerobic Respiration reaction in muscles - Teachoo.jpg

  • Glucose is a six carbon atom compound. 
  • The oxidation of glucose to pyruvic acid or pyruvate is called glycolysis
  • One molecule of glucose in glycolysis produces two molecules of pyruvic acid. 
  • Pyruvic acid is a three carbon atom compound.

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The fate of pyruvate formed during respiration depends on whether oxygen is present in the cells or not. 

  • If oxygen is present in the cells then pyruvate is completely oxidized to carbon dioxide and water and a lot of energy is produced in the form of ATP.
  • If however oxygen is not present in the cells, there is absence of oxygen, pyruvate is converted to either ‘ethanol and carbon dioxide’ or ‘lactic acid’ depending on whether such processes take place in the plant cell or an animal cell.


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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo