Question

The site of light reaction is
A) Granum
B) Stroma
C) Unit membrane
D) Stroma lamellae

Answer 1

Hint:Photosynthesis has two phases- a light phase and a dark phase. A light phase requires light and hence is also called photochemical reaction but reactions of dark phase require no light and are purely chemical reactions.

Complete answer:
>The chloroplast is the green colored plastid that acts as the site for photosynthesis. It contains a proteinaceous matrix or fluid called stroma and the membrane system called lamellae or thylakoid. At some places the thylakoids get aggregated to form the stacks of discs called grana.
>There is a clear division of labor within the chloroplast. The membrane system is responsible for trapping solar energy and synthesis of ATP and NADPH while stroma has enzymes for reduction of carbon dioxide into carbohydrates.
>So light reaction occurs in the granum while dark reaction occurs in the stroma of the chloroplast.
>During light reaction, ATP and NADPH2 are formed that are called assimilatory power.
  It involves two major steps:
1) Photolysis of water- It involves breaking up of water into hydrogen and oxygen in the chloroplast. A small complex of protein called oxygen evolving complex is attached to photosystem II which oxidizes water into oxygen, hydrogen ion and electrons.
2) Production of assimilatory power- The electrons released in the above reaction are picked up by the reaction centre of PS II. On receiving a photon of light energy P680 expels an electron. It is the primary reaction of photosynthesis that involves conversion of light energy into chemical energy.

Hence the correct answer is option ‘A’ Granum.

Note:The dark reaction involves using the assimilatory power for the fixation and reduction of carbon dioxide. There are two pathways for it- Calvin cycle and dicarboxylic acid cycle pathways.