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Types of Respiration

All living organisms require a continuous supply of energy for carrying out various functions of the body related to growth, development and movement. The energy requirement of the organism is fulfilled by respiration at the cellular level. Respiration is a process by which organic substances such as carbohydrates, proteins, fats (or) organic acids are degraded to release energy. It is a catabolic process.

Releasing Energy equation

All living organisms require a continuous supply of energy for carrying out various functions of the body related to growth, development and movement. The energy requirement of the organism is fulfilled by respiration at the cellular level. Respiration is a process by which organic substances such as carbohydrates, proteins, fats (or) organic acids are degraded to release energy. It is a catabolic process.

Respiration Equation

It is essentially an energy releasing process.

Seat of respiration - Mitochondria

Mitochondria are small, spherical or rod shaped organelles present in the cytoplasm of eucaryotic cells. They are the sites of cellular respiration. Respiratory


substrate is oxidized and the energy is stored in mitochondria in the form of ATP. Hence they are also called as the powerhouse of the cell.

Each mitochondrion is 0.2 to 1.0 pm in diameter and 2 to 8 |im long. It consists of two membranes, outer and inner enclosing a matrix. The outer membrane is smooth but the inner membrane is subjected to finger like infolds called cristae. The cristae possesses knob like projections called elementary particles or F or ETP (Electron Transport Particles). These are associated with ATP synthesis during oxidative phosphorylation. Mitochondria contain RNA and DNA also to carry on protein synthesis. Hence it is called semi-genetic autonomous organelle.

Adenosine Tri Phosphate(ATP)

The energy released during respiration is stored as ATP in the cells. ATP consists of a nitrogenous base Adenine, Ribose Sugar and three phosphate groups. Last two phosphate groups are attached by means of high-energy bonds. ATP is formed from ADP (Adenosine Di Phosphate) by an energy coupling mechanism called phosphorylation. ATP is hence called energy currency of the cell.

Types of Respiration

Based on the availability of oxygen, respiration is classified into two types.

Aerobic Respiration

The respiratory substrate is catabolised in the presence of oxygen with the liberation of carbon dioxide, water and a large amount of energy. This is the common type of respiration in higher organisms.

Anaerobic Respiration

Catabolism of the respiratory substrate in the absence of oxygen is termed anaerobic respiration. It is seen in lower organisms like bacteria and fungi.

Mechanism of Aerobic Respiration:

Aerobic breakdown of glucose involves 4 steps, Glycolysis, Oxidation of pyruvic acid, Krebs cycle and electron transport chain. The overall equation for aerobic respiration is written as follows.

Aerobic Respiration equation


It involves several enzyme-mediated reactions by which a glucose molecule (6 carbon compound) is split into two molecules of pyruvic acid (3 carbon compound). Three German microbiologists Embden, Meyerhof and parnas discovered this pathway and hence it is called EMP pathway. It takes place in the cytoplasm of the cell and does not require oxygen. The net gain of energy rich molecules during this process is 2ATP and 2NADH2 molecules.

Oxidation of Pyruvic Acid

Pyruvic acid formed by glycolysis undergoes oxidative decarboxylation in the presence of an enzyme pyruvic dehydrogenase. Two molecules of pyruvic acid get reduced to two molecules of Acetyl CoA (2 carbon compound). Carbon dioxide is released as a by-product and 2 NADH2 molecules are formed. This occurs in the mitochondrial matrix.

Kreb's Cycle or TCA Cycle (Tricarboxylic acid Cycle)

This cycle was elucidated by Hans Krebs (1935) for which he received a Noble Prize. This series of chemical reactions occur in the mitochondrial matrix. Acetyl CoA ( 2 carbon compound) combining with oxalo acetic acid (4 carbon compound) to form citric acid (6 carbon compound) triggers this reaction. This is followed by several enzyme catalyzed reactions. Complete breakdown of pyruvic acid takes place resulting in the production of carbon dioxide and water. Two molecules of Acetyl CoA entering Kreb's Cycle generate 6NADH2, 2ATP and 2FADH2 molecules.

Electron Transport Chain

Transfer of electrons and protons from NADH2 and FADH2 occur through the electron transport chain in the mitochondria. Molecular oxygen is the terminal acceptor of electrons and protons resulting in formation of water molecule. The energy loss during the transfer of electrons is coupled to the production of ATP. This is called oxidative phosphorylation.Three ATP molecules are formed when NADH2 donates electrons to the electron transport chain and 2 ATP molecules are formed is FADH2 is the electron donor.

Therefore in aerobic respiration, there is a stepwise breakdown of the substrate and also complete breakdown thereby ensuring higher proportion of energy being utilized in ATP synthesis.

Electron Transport Chain

Aerobic Breakdown of Glucose Total gain of ATP

Electron Transport Chain table

Thus one molecule of glucose yields energy equivalent to 3 8 ATP molecules by aerobic respiration.

Mechanism of Anaerobic Respiration

Pasteur (1860) discovered that yeast could respire in the absence of molecular oxygen. He was experimenting with yeast and explained alcoholic fermentation of glucose by the same. This process was called Anaerobic respiration. Anaerobic respiration can be defined as enzyme mediated partial breakdown of respiratory substrate without using oxygen. This results in the production of only a fraction of energy. It is commonly seen in bacteria, mould, roots of some waterlogged plants, etc. The term fermentation is applicable to anaerobic respiration since it takes place outside living cells by the extra-cellular enzymes produced by micro organisms.

Glycolysis is a common pathway for aerobic and anaerobic organisms. It involves breakdown of respiratory substrate into pyruvic acid. This is further anaerobically broken down to produce product's like ethyl alcohol, lactic acid, acetic acid, etc. depending on the organism involved. The following equation represents the anaerobic breakdown of glucose by yeast.

Anaerobic Respiration equation

The enzyme zymase in yeast brings about fermentation resulting in production of ethyl alcohol and C02 from glucose. The net gain during anaerobic respiration is 2 molecules of ATP only. Anaerobic respiration yields much less energy than aerobic respiration because

1. There is incomplete breakdown of respiratory substrate.

2. Electron transport chain is not seen as in aerobic respiration. Hence NADH2 cannot be reutilized for energy production.

Significance of Anaerobic Respiration:

1. Various types of wines can be prepared by alcoholic fermentation of sugar solution by yeast.

2. The fermentation activity of yeast finds application in baking & brewing Industry. (Refer economic importance of fungi)

3. Vinegar is obtained by fermentation activity of acetic acid bacteria.

4. It is used in clearing of hides in Tanning Industry.

5. Curing of tea and tobacco leaves is based on fermentation by bacterial species.

Fermentation is a basic principle in the field of Biotechnology with regard to manufacture of several products. On the other hand, it should be noted that fermentation by microbes could spoil food and cause food poisoning, stomach disorders, etc.

Differences between Aerobic and Anaerobic respiration

Aerobic and Anaerobic respiration

Experiment to Demonstrate Aerobic Respiration:

Some germinating seeds are taken in a conical flask. The flask is fitted with a cork. A delivery tube bent twice is fitted to the cork. The free end of the tube dips into a beaker of coloured water. A small test tube containing potassium hydroxide solution is suspended in the conical flask by means of a thread and the connections are made airtight.

Experiment to demonstrate Aerobic Respiration

It is observed that after sometime water rises in the tube. The germinating seeds respire aerobically utilizing the oxygen inside the conical flask. They liberate carbon dioxide, which is absorbed by the potassium hydroxide. This creates a partial vacuum in the flask. Therefore the level of water rises in the bent tube. This experiment proves that during aerobic respiration oxygen is taken in and carbon dioxide is liberated by living organisms.

Experiment to Demonstrate Anaerobic Respiration(fermentation)

A trough is filled with mercury. Some germinating seeds are taken and their seed coats are removed. They are taken in a small test tube which is then filled with mercury. The test tube is slowly inserted in the trough using the thumb. The experimental set up is kept undisturbed for a day. The level of mercury in the test tube comes down due to accumulation of a gas in the test tube. A few crystals of potassium hydroxide are introduced into the test tube. The gas is absorbed and the level of mercury in the test tube rises up. The seeds respired anaerobically since they could not absorb atmospheric oxygen due to presence of mercury.

Experiment to Anaerobic Respiration

Thus in anaerobic respiration, carbon dixoide is. Produced by living organism/cells.

Points to Remember

• Mitochondria is the seat of cellular respiration.

• Energy is stored in the form of ATP in cells.

• Glycolysis is also called as EMP pathway.

• Formation of ATP during electron trasnsport chain is called oxidative phosphorylation.

• Anaerobic respiration is also called fermentation.

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