Life Processes

  • Question 89

    Differentiate between:
    (i) Respiration and Combustion
    (ii) Aerobic and Anaerobic respiration.





    1. Respiration being a biological process occurs in living cells.

    1. Combustion is a non-biological process and hence it is non-cellular.

    2. Respiration is the slow oxidation of organic substances (glucose) which results in stepwise release of energy.

    2. Combustion is a rapid oxidation of a substrate (like coal) and results in the release of the energy abruptly.

    3. The heat evolved during respiration is stepwise, hence it does not harm the cells and organelles.

    3. The heat evolved during combustion is abrupt hence it raises the temperature which results in fire.

    4. The heat energy evolved is either made available to the cells or is stored as chemical energy in the form of ATP.

    4. The heat energy evolved is made use of at the time of evolution and is not stored.

    5. Respiration reactions are catalysed by enzymes.

    5. Enzymes are not used in combustion.


    Aerobic respiration

    Anaerobic respiration

    1. Aerobic respiration takes place in the presence of oxygen.

    1. It takes place in the absence of oxygen.

    2. In aerobic respiration complete oxidation of glucose takes place.

    2. In anaerobic respiration the glucose molecule is incompletely broken down.

    3. End products of aerobic respiration are CO2, water and energy.


    3. End product of anaerobic respiration are ethyl alcohol (or lactic acid) CO2 and a little energy.


    4. Large amount of energy is released i.e., 38 molecules of A.T.P. per glucose molecule.

    4. Small amount of energy is released i.e., 2 A.T.P. molecules per glucose molecule.

    5. First step of aerobic respiration (glycolysis) takes place in cytoplasm while second (Kreb’s cycle) and third (Electron transport chain) steps take place in mitochondria.

    5. Complete anaerobic respiration occurs in cytoplasm.

    Question 90

    Diagrammatically represent the basic processes involved in respiration in the human body.



     The basic processes involved in respiration in the human body
    Question 91

    Briefly explain the process of aerobic cellular respiration.


    Aerobic Respiration: Break down of food and release of energy takes place in each and every cell of the body. Aerobic respiration takes place in the following two steps:

    First step: The first step does not require oxygen (anaerobic phase). The glucose is only partially broken down into an intermediate compound (pyruvic acid). In this step only a small amount of energy is released.

    Second step: This step requires oxygen and, therefore, known as aerobic (in air). The intermediate compound formed by incomplete break down of glucose undergoes complete oxidation into CO2 and H2O. A large amount of energy (38 ATP) is released in this step. The complete oxidation of glucose takes place in the Mitochondria.

    Overall reaction of aerobic respiration
    C6H12O6 +6O2 → 6CO2 + 6H2O + 2830 kJ (38 ATP).

    Question 92

    In what way air breathed in differs from the air breathed out during respiration? How much water one loses daily from the body by breathing?


    During respiration organisms use oxygen and give out CO2. Therefore, the air breathed in contains more percentage of oxygen and less percentage of carbon dioxide. The air breathed out contains more percentage of carbon dioxide and water vapour and less percentage of oxygen. A comparison of composition of air breathed in and breathed out is given in the table below:
    Composition of Respired Air


    Air breathed in (Approx)

    Air breathed out (Approx)







    Carbon dioxide



    Water vapour



    One loses approximately 400 cm3 of water daily from the body by breathing.

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