Life Processes

05 Respiration – A Life Process



  • All living organisms eat food to produce energy for the normal functioning of the body.
  • The biochemical process of respiration involves taking in oxygen (of air) into the cells to get the energy to perform certain life processes essential for survival.
  • Oxygen is used for releasing energy by burning food, and then eliminating the waste products (carbon dioxide and water) from the body.
  • Glucose from the food that organisms consume, gets broken down into simpler substances and energy is released.
  • Since the process of respiration which release energy takes place inside the cells of the body, it is known as cellular respiration.
  • Respiration is just opposite of photosynthesis.
  • Respiration involves the exchange of gases and burning of food.
  • It is a redox reaction which can take place with or without oxygen and takes place in mitochondria of the cell and releases energy in the form of ATP.


  • Glucose is C6H12O6. It is a six carbon atom compound. It is the simple food which gets oxidised in the cells of organisms during respiration.
  • The oxidation of glucose to pyruvic acid (or pyruvate) it called glycolysis.
  • Pyruvic acid is a three carbon atom compound. It is also called pyruvate. The formula of pyruvic acid or pyruvate is
  • ATP (Adenosine triphosphate) is known as the energy currency for most of the cellular processes. It is formed from ADP (Adenosine diphosphate) and inorganic phosphate.

Types of respiration:

  • Organisms obtain energy in different ways, like humans need air to live.
  • Oxygen from the air is used to release energy because of which it is called aerobic respiration.
  • However, some species of bacteria and algae or some prokaryotes do not need oxygen to generate energy and the process by which they obtain energy is called anaerobic respiration.
  • There are two types of respiration:
  • Aerobic Respiration
  • Anaerobic Respiration

Aerobic Respiration:

  • The respiration which uses oxygen is called aerobic respiration.
  • Energy is released by the breakdown of glucose into carbon dioxide and water in the mitochondria.
  • Glucose is converted into pyruvate in the first step, similar to anaerobic respiration.
  • However, the amount of energy released is much greater than during anaerobic respiration.
  • If oxygen is present in the cells, then pyruvate is completely oxidised to carbon dioxide and water, and produces a lot of energy is produced which is immediately used to synthesize a molecule called ATP.
  • It acts as the energy currency of cellular processes like the contraction of muscles, protein synthesis, conduction of nervous impulses, etc in the human body.
  • Anaerobic Respiration:
    • The respiration which takes place without oxygen is called anaerobic respiration.
    • The first step is the conversion of glucose which is a 6 Carbon molecule into a simpler sugar called pyruvate, a 3 Carbon molecule in the cytoplasm.
    • In the absence of oxygen, pyruvate gets converted to other substances, and energy is released.
    • The process of anaerobic respiration in yeast can be depicted by the following equation.
  • Bacteria and yeast respire anaerobically i.e., in the absence of oxygen.
  • In this way, the process of fermentation takes place.
  • In yeast, pyruvate gets broken down into ethanol, and carbon dioxide and energy are released.
  • Anaerobic respiration takes place in our muscles during vigorous physical exercise when oxygen gets used up faster in the muscle cells than can be supplied by the blood.
  • And lactic acid is produced which causes muscle cramps.

Differences between Aerobic and Anaerobic Respiration

Aerobic Respiration Anaerobic Respiration
Aerobic respiration takes place in the presence of oxygen. Anaerobic respiration takes place in the absence of oxygen.
Complete breakdown of food occurs in aerobic respiration. Partial breakdown of food occurs in anaerobic respiration.
The end products in aerobic respiration are carbon dioxide and water. The end products in anaerobic respiration maybe ethanol and carbon dioxide (as in yeast plants), or lactic acid (as in animal muscles).
Aerobic respiration produces a considerable amount of energy. Much less energy is produced in anaerobic respiration.

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