What do you understand by chemosynthesis? Differentiate between photorespiration and respiration as well as aerobic respiration and anaerobic respiration.


There are some organisms that are able to manufacture their food in the absence of light.  They get chemical energy for this purpose by oxidizing different types of substances present in their medium.  The reaction is exergonic (energy releasing).  The process of manufacture of food from inorganic raw materials by using chemical energy is known as chemosynthesis.

Winogradsky (1887) discovered a group of organisms that can assimilate carbon dioxide in the dark.  He discovered another group of organisms which grew in the presence of carbon dioxide and NH3 or nitrite.  Both the group of organisms were colorless and although they grew in the dark and in the environment free of organic matter, they were autotrophic.  The energy required for the reduction of CO2 was obtained from simultaneous oxidation of simple inorganic compound such as H2S, NH3 or nitrite.  These groups were bacteria which released energy from chemical reaction mostly in the form of ATP.  This energy is much less than required for the reduction of one molecule of carbon dioxide and only a part of energy released becomes available in the form of energy-rich phosphate group (ATP).  Such an  energy must, therefore, be pooled before being used.

Chemosynthetic bacteria are colourless sulphur bacteria, nitrifiers, iron bacteria, hydrogen bacteria, carbon bacteria and methane oxidizers.


  1. Colourless sulphur bacteria: These bacteria are classified into the following three groups:

a.)  Sulphur oxidizing bacteria:  These include Thiobacillus thiooxidans, Thiobacillus thioparus, etc.  These bacteria are able to grow with sulphur thiosulphates or thiocyanates.  They are obligate aerobes, obligate autotrophs and are acid tolerant.  The exergonic reactions carried out by these bacteria are as follows:


2S+3O2+2H2O →2H2S04+energy

Na2S2O3+H2O+2O2 →Na2SO4+H2SO4+energy

b).        Sulphide oxidizing bacteria:  Many Thiobacilli oxidize sulphides to sulphur.  These bacteria are entirely dependent for their development on the oxidation of sulphides (e.g., Beggiatoa).  The exergonic reactions are as follows:


2Na2S+O2+H2O →2S+4NaOH+energy

2H2S+O2 →2S+2H2O+energy

c).        Sulphate reducing bacteria:  These obligate anaerobes use sulphates instead of oxygen to oxidize their food (e.g., Desulphovibrio).


C6H12O6+3H2SO4 →3H2S+6CO2+6H2O+energy


  1. Nitrifiers: These bacteria oxidize NH3 into NO2 and NO3.  They belong to two groups:  a) Nitrosomonas, Nitrosocystis, Nitrospira which oxidize NH3 to NO2.

2NH3+3O2 →2HNO2+2H2O+energy


b).        Nitrobacter which oxidizes NO2 to NO3.

2KNO2+O2 →2KNO3+energy

  1. Hydrogen bacteria: These bacteria oxidize H2 gas to form water (e.g. Hydrogenomonas)


2H2+O2 → 2H2O+energy


  1. Carbon bacteria: These bacteria oxidize carbon monoxide to carbon dioxide (e.g., Bacillius oligocarbophilus).

O2+2CO →2CO2+energy

  1. Iron bacteria: A number of bacteria have the property of converting ferrous salt to ferric salt.  They are termed as iron bacteria.  The reaction is exergonic.  Thiobacillus ferro-oxidans and Ferrobacillus ferro-oxidans are known to be autotrophic.  The other iron bacteria like Leptothrix ochracea, Spirophyllum ferrugineum are not completely autotrophic.

4FeCO3+6H2O+O2 → 4Fe(OH)2+4CO3+energy

  1. Methane oxidizer: These bacteria oxidize methane (e.g., Methanomonas).


CH4+2O2 →CO2+2H2O+energy




Photorespiration Respiration
1.  It occurs only inside photosynthetic cells.

2.  Photorespiration takes place only in the presence of light.

3.  In photorespiration, uptake of oxygen and evolution of carbon dioxide are light dependent.

4.  It takes place in C3 plants and negligible in C4 plants.

5.  Photorespiration increases with the availability of oxygen.

6.  It occurs in the chloroplast and may require the help of peroxisomes and mitochondria.

7.  The substrate for photorespiration is RuBP and its breakdown product is called, glycolate.

8.  The substrate is formed at the time of utilization.

9.  Toxic hydrogen peroxide may be formed during oxidation of the substrate.

10.  The end products of photorespiration are carbon dioxide and phosphoglyceric acid.

11.  Both the end products, i.e., carbon dioxide and phosphoglyceric acid are re-utilized in photosynthesis.

12.  Photorespiration requires three cell organelles -chloroplasts, peroxisomes and mitochondria.

13.  It is wasteful method and does not produce energy.

14.  Photorespiration rises rapidly with the rise in temperature.

15.  Photorespiration is not essential.

1.  It is found in all living cells.

2.  Respiration continues both in dark and light.

3.  Exchange of gases is independent of light.

4.  It takes place in all organisms except the anaerobic ones.

5.  It is not influenced by the change in oxygen concentration between 10 to 25%.  Beyond it, there is a decline in the rate of respiration.

6.  It occurs in cytoplasm and mitochondria.

7.  The substrate is commonly glucose though the others like(fat, protein, organic acid) can also be used.

8.  The substrate is already present in the cell.

9.  Hydrogen peroxide is not produced.

10.  The end products of respiration are carbon dioxide and water.

11.  The end products are not re-cycled in respiration.

12.  Respiration requires only mitochondria besides cytoplasm.

13.  It produces energy for cellular maintenance, synthesis and work.

14.  The rise with temperature is normal as for other biochemical reactions.

15.  Normal or dark respiration is essential for the survival of organisms.




Aerobic respiration Anaerobic respiration
1.  It occurs in all living cells 1.  It occurs only in some bacteria, fungi, germinating seeds, fleshy fruits, etc.
2.  It requires oxygen. 2.  It does not require oxygen.
3.  The end products of aerobic respiration are carbon dioxide and water. 3.  The end products are carbon dioxide and alcohol.
4.  Complete oxidation of one molecule of glucose by aerobic oxidation produces a net of 38 ATP molecules. 4.  Only 2 molecules of ATP are gained in this process.
5.  Reactions of glycolysis occur in the cytoplasm and those of Krebs cycle in mitochondria. 5.  All reactions occur in cytoplasm.  Mitochondria is not needed in this process.