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Mechanism of Photosynthesis


       The process of photosynthesis can be divided into two main phases:


  1. Light reaction (or light dependent reaction)
  2. Dark reaction (or light independent reaction)


Light reaction

       It occurs in the grana present in chloroplast. During light reaction, chlorophyll absorbs light energy in the form of photons. Actually, light is emitted in the form of ‘packets of energy’ called photons. The light energy absorbed by chlorophyll is used to do following reactions:

1. Formation of ATP

       ATP is the main energy source of a cell. Its full form is Adenosine Tri Phosphate. A molecule of ATP is formed by combining inorganic phosphate with ADP (Adenosine DiPhosphate) in the presence of energy.

ADP    +    phosphate    +    energy    Formation of ATP    ATP


This ATP is used as a source of energy during dark reaction.

2. Photolysis of water

       The process of decomposition of water into hydrogen and oxygen by light energy absorbed by chlorophyll is called photolysis. The oxygen produced by the photolysis of water is released into the air as a byproduct of photosynthesis.


H2O    Photolysis of water    2H+    +    1/2O2    +    2e-


It should be noted that oxygen released during photosynthesis comes from water.

3. Formation of NADPH (reduced Nicotinamide Adenine Dinucleotide Phosphate)

       The hydrogen ions (H+) released during second reaction reduces the NADP (Nicotinamide Adenine Dinucleotide Phosphate) into NADPH in the presence of energy.


NADP    +    Hydrogen    +    Energy Formation of NADPH    NADPH


NADPH is also used during dark reaction.

Dark reactions (light independent reactions)

       In dark reactions, the ATP molecule and NADPH produced during light reaction, are used in the matrix (or stroma) of chloroplast to make carbohydrates from carbon dioxide. In this process, carbon dioxide enters into a cycle of reactions in the presence of a substance called RuDP (Ribulose DiPhosphate) and enzymes. At the end of the cycle, carbohydrate is synthesized and ribulose diphosphate is regenerated.


RuDP + CO2 + NADPH +ATPFormation of NADPH RuDP + Carbohydrate + NADP

                                                                  + ADP + phosphate

This cycle of reactions is called Calvin-Benson cycle because it was discovered by Melvin Calvin and Andy Benson.




Test Your Understanding and Answer These Questions:




  1. What is a photon?

  2. Explain the mechanism of photosynthesis.

  3. What are the two phases of photosynthesis?

  4. Give differences between light reactions and dark reactions.

  5. Explain Calvin-Benson’s cycle.

  6. From which raw material O2 is liberated during photosynthesis?

  7. How water reaches the leaves from roots of a plant?



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