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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Respiration Page 1

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  • Respiration: It is a catabolic process during which organic compounds like carbohydrates are broken down to release energy.
  • Every living cell carries out cellular respiration.
  • The energy in the cell is stored in the molecules of ATP, which are then utilised for the actual release of energy.





Ultrastructure and Functions of Mitochondrion



  • The site for respiration is 'mitochondria' present in cytoplasm.
  • Mitochondria release energy and are, thus called, "power houses of cells."
  • Each mitochondrion is an oval, rod-like structure which is bounded by two unit membranes.
  • The outer membrane is smooth and continuous, while the inner membrane is thrown into many folds called 'cristae'.
  • Space between the two membranes is called 'outer chamber'. The space between the folds of inner membrane is the 'inner chamber', which is filled with 'matrix'.
  • Many small, spherical, stalked particles come out from the inner membrane in the matrix. These are called 'F1' particles or 'oxysomes'; which are made up of base, stalk and head.
  • Oxysomes contain the enzymes necessary to synthesise ATP during electron transport.




An Overview of Cellular Respiration


Energy is released in a step wise manner during the process of respiration.

Overall process of respiration takes place in three steps, viz;
a. Splitting up of glucose molecule into pyruvic acid
b. Oxidation of pyruvic acid
Kreb's cycle
c. Oxidative phosphorylation of ADP Electron transport system

By-products of respiration are CO2 and water.





ATP (Adenosine triphosphate) is a complex compound consisting of a purine base adenine, a 5-carbon sugar, the ribose and three molecules of phosphate.

The adenine and ribose together form adenosine.

ATP is formed in both chloroplasts and mitochondria.

In chloroplasts, ATP is synthesized by the process of photophosphorylation from the physical energy of light while in mitochondria, it is formed by the process of oxidative phosphorylation of glucose.

ATP is the main source of energy for various metabolic activities that take place in a cell.




Types of Respiration


  • Depending on the presence or absence of oxygen in the process of respiration, there are two main types of respiration, namely 'aerobic'and 'anaerobic'.
  • Aerobic respiration takes place in the presence of oxygen. Thus, there is complete oxidation of glucose. It takes place in higher plants and animals. Higher amount of heat energy is released during aerobic respiration. Thus, it is supposed to be more efficient. The by-products are CO2 and water.
  • Anaerobic respiration takes place in absence of free atmospheric oxygen. There is incomplete oxidation of glucose. It takes place in primitive organisms. Comparatively less amount of heat energy is released during anaerobic respiration. The by-products are CO2, ethyl alcohol or lactic acid.
  • Reactions for aerobic and anaerobic respiration are:
    1. Aerobic:
      C6H12O6 + 6O2
      6CO2 + 6H2O + 38 ATP
    2. Anaerobic:


C6H12O6 2C2H5OH + 2CO2 + 2ATP




ethyl alcohol









  • The first step of respiration, whether it is aerobic, or anaerobic is splitting of glucose molecules into pyruvic acid. This is called 'glycolysis'.
  • It is also called 'EMP' pathway as three German physiologists - Embden, Meyerhof and Parnas - discovered it.
  • It takes place in the cytoplasm and oxygen is not involved in the process.
  • There are ten steps involved in the process, which are divided into three stages, viz. (a) Activation (b) Splitting (c) Oxidation
  • The sequence is:








Glucose 6 phosphate

Fructose 6 phosphate





















Fructose 1,6 diphosphate











Pyruvic acid

Phosphoenol pyruvic acid

2 Phospho glyceric acid

3 Phospho glyceric acid

1, 3 Diphospho glyceric acid

Glyceraldehyde 3 phosphate

  • During glycolysis, four ATP molecules are synthesised and two are used up. Thus, the net gain is two molecules of ATP.
    Glucose 3 pyruvic acid + 2ATP




TCA Cycle (Kreb's cycle)


  • The end product of glycolysis, i.e. pyruvic acid, enters the mitochondrion and complete oxidation takes place, which is described in two stages, namely,
    (a) Preparatory step and (b) Kreb's cycle.
  • Kreb's cycle is also called tricarboxylic acid (TCA) cycle or citric acid cycle.
  • During the preparatory step, formation of acetyl coenzyme A takes place as:
    Pyruvic acid + CoA + NAD
    CO2 + NADH2 + acetyl CoA
    This Acetyl CoA is a link between glycolysis and Kreb's cycle.
  • The reactions of Kreb's cycle are:
    1. Oxaloacetic acid + Acetyl CoA + H2O Citric acid + CoA
    2. Citric acid H2O + Cis Aconitic acid
    3. Cis Aconitic acid + H2O Isocitric acid
    4. Isocitric acid + NADP Oxalosuccinic acid + NADPH2
    5. Oxalosuccinic acid CO2 + a Keto glutaric acid
    6. a Keto glutaric acid + NAD + CoA CO2 + Succinyl CoA + NADH2
    7. Succinyl CoA + H2O Succinic acid + CoA
    8. Succinic acid + FAD Fumaric acid + FADH2
    9. Fumaric acid + H2O Malic acid
    10. Malic acid + NAD Oxalo acetic acid + NADH2





Electron Transport Chain


  • During respiration, there is transfer of electrons through an electron transport chain (ETC).
  • In ETC, the substrate gets oxidised and the acceptor gets reduced.
  • When the electron moves from one acceptor to the other, it loses energy.
  • Electron acceptors in the ETC are





Cytochrome b







molecular oxygen

Cytochrome a3

Cytochrome a

Cytochrome c

  • The ETC involves oxidation, i.e. removal of hydrogen and phosphorylation of ADP to ATP. This process is also called 'oxidative phosphorylation'.
  • Hydrogen is passed through a series of acceptors. The last one of these is atmospheric oxygen. This reaction is called 'terminal oxidation'.






  • Fermentation is actually a process of anaerobic respiration of microbes.
  • The by-products of this process is either 'alcohol' or 'lactic acid' depending upon the type of organism.
  • During fermentation, the first step is always 'glycolysis'.
  • When the end product of fermentation is an alcohol, then, it is called as 'alcoholic fermentation' while if the end product is lactic acid then it is called 'lactic acid fermentation'.

Alcoholic Fermentation

  • Alcoholic Fermentation: This is carried out by micro-organisms such as 'yeast', which release enzyme 'zymase'.
  • Zymase acts on sugar and converts it into ethyl alcohol.
  • Alcoholic fermentation involves three steps as:
    a. Glucose 2 pyruvic acid + 2 ATP
    b. Pyruvic acid
    Acetaldehyde + CO2
    c. Acetaldehyde
    Ethyl alcohol + CO2+ ATP

Lactic acid Fermentation

  • Lactic acid Fermentation: This is carried out by the bacteria present in milk. 'Lactose' is a milk sugar. Lactic acid fermentation involves the following steps as:
    a. Lactose
    Glucose + Galactose
    b. Glucose pyruvic acid
    c. Pyruvic acid
    lactic acid.
  • Commercially, the process of fermentation is useful in various following ways:
    1. Bakery products: In bread making for raising loaf of bread and making it spongy.
    2. Production of antibiotics like penicillin, streptomycin, etc.
    3. Production of vitamins.
    4. Production of cheese.
    5. Production of ethyl alcohol, acetic acid, glycerol, etc.




Significance of Respiration


  • Energy released during respiration is very important for the organism to perform various life activities.
  • Energy is needed for growth and repair of the body.
  • For conduction of nerve impulses, energy is essential.
  • Energy released also maintains the body temperature in warm-blooded animals.
  • In some varieties of mushrooms, fire flies and glow worms, the energy produced during respiration is transformed into light energy.




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