All living organisms require energy for various metabolic activities. The energy is released through the process of respiration (where molecules like glucose and fatty acids are oxidised). Let us know how living organisms get energy through respiration by using oxygen.

Oxidation of food materials, especially glucose, fatty acids and proteins to CO2 and water is called Respiration. Energy is produced in this process. As this process occurs at cellular level it is called Cellular Respiration.
Cellular respiration is the process by which food is broken down by the body's cells to produce energy, in the form of ATP molecules.

Cellular respiration is carried out by every cell in both plants and animals and is essential for daily living. Energy is produced only when glucose and fatty acids are oxidised which are transported from the digestive system to all the other cells in the body which are carried by blood.

Respiratory Substrates:
Substances which oxidised in the body during respiration to produce energy are called respiratory substrates. In all organisms from microbes to man the major respiratory substrates are carbohydrates and fats.

Carbohydrates like starch and cellulose are complex molecules. Such complex molecules are not oxidised directly to produce energy. First of all complex molecules (carbohydrates) are broken down into simple molecules and then oxidised. Between these process so many reactions takes place. But energy is not produced in each and every reaction during the oxidation of glucose or fatty acids. Only in some of the reactions small amount of energy is produced. This energy is converted into chemical energy and is stored in ATP (Adenosine Tri Phosphate).

The ultimate goal of cellular respiration is to take carbohydrates, diassemble them into glucose molecules, and then use this glucose to produce energy-rich ATP molecules.
The general equations for cellular respiration is: One glucose molecule + 6 oxygen molecules produces six carbon dioxide molecules, 6 H2O and approximately 36 -38 ATP molecules.

Mitochondria - Sits of Cellular Respiration
Cellular respiration occurs in Mitochondria. Oxidation of food materials takes place in them. The energy released through oxidation is utilised by the mitochondria to synthesis ATP which are energy rich compounds. Therefore mitochondria are called as Power houses of the cell.

O2, ADP + Pi (inorganic phosphate), carbohydrates, fats are required for the respiratory activity in mitochondria.

  • Mitochondria are sac like structures present in the cytoplasm of the cells.
It has two compartments.
1. An inner compartment
2. An outer compartment
  • The substance in the inner compartment is called Matrix.
  • The inner membrane is thrown into several foldings called Cristae.
  • The space between the foldings is continuous with the outer compartment.
  • On the inner membrane large number of elementary particles are seen which have a spherical head and a stalk.
  • Outer compartment is surrounded by outer membrane.
  • Inner membrane, matrix and elementary particles have large number of enzymes which required for the respiration and energy production.
There are three main stages of cellular respiration.
1. Glycolysis
2. Krebs Cycle
3. The electron transport chain

First step in glucose oxidation is called glycolysis. (In the first stage of glycolysis energy is used to phosphorylate the 6-carbon glucose is used to molecule.) This means that a phosphate is taken from ATP (which becomes ADP) and added to the glucose molecule. This addition of phosphate makes the molecule much more energised and converted into glucose-6 phosphate. Later glucose-6 phosphate is converted to fructose-1, 6-Diphosphate. In this process another ATP is used. Fructose-1, 6-Diphosphate is finally converted into two molecules of pyruvic acid.

Result: 4 ATP and 2 NADH are formed. 2ATP molecules are used in this process. So, net gain is 2 ATP molecules.

Krebs Cycle
It takes place in mitochondrial matrix. This cycle involves series of reactions in which pyruvic acid is oxidised. This cycle discovered by Sir Hans Krebs, that's why it is called as Krebs Cycle.

When oxygen is available pyruvic acid molecules (end product of glycolysis) are converted into Acetyl Co-enzyme A with the release of CO2.

Acetyl Co-enzyme A is attached to oxalo acetic acid and formed into citric acid. When the two carbons are removed in the form of CO2 again oxalo acetic acid (4 C) is formed.

This combines with acetyl co- enzyme A and the cycle continues. So, this cycle is also called as citric acid cycle.

In this process small amount of energy is formed when two molecules of pyruvic acid are oxidised. Therefore 2 ATP molecules are gained.
Electron Transport Chain
Very little amount of energy has been produced during glycolysis and the Krebs Cycle. Most of the energy locked in the original glucose molecule will be released by the electron transport chain and oxidative phosphorylation.

NADH and FADH2 are oxidised in the elementary particles of mitochondria, where one proton and two electrons are formed from each molecule of NADH and are ejected into the inner membrane of mitochondria. These electrons are transferred from one acceptor to another which are arranged in a chain (The energy released is used for production of ATP).

These electrons and protons are finally transferred to oxygen to form water.

Know about a Scientist

Sir Hans Krebs previously been known for his excellent work on the urea science cycle in 1920's and 30's. Hans Krebs (1900 - 1981) received the Noble prize for working out the path way for oxidation of the 2 carbon acetyl group on acetyl Co A via a series of tricarboxylic intermediate. A cyclic pathway is now known as the Krebs Cycle, the Citric acid cycle and TCA Cycle.

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