March 4, 2014

10th Biology New Syllabus - Mechanism of Photosynthesis

Mechanism of Photosynthesis : There are two major phases are found in Photosynthesis. They are as follows: 1. Light reaction 2. Dark reaction

1) Light Reaction (Photo chemical Phase)
In this reaction light plays the key role. A series of chemical reactions occurs in a very quick succession initiated by light and therefore the phase is technically called the photochemical phase. The light reaction takes place in chlorophyll containing thalakoids called Grana of Chloroplasts . The light reaction occurs in several steps.

Step-I : The chlorophyll on exposure to light energy becomes activated by absorbing
photons. (Photon is the smallest energy of light)

Step-II: The energy is used in splitting the water molecule into its two component ions H2O H+ + OH
The reaction is known as photolysis, which means splitting by light (photo means light, lysis means breaking)

Step-III: The highly reactive ions of water undergoes quick change in two different
directions of OH- and H+ ions as described below OH- ions through a series of steps produce water (H20) and Oxygen (O2). The Water may be used inside the plant but O2 is released into the atmosphere. H+ ions undergo series of changes in dark reaction. ATP and NADPH are formed at the end of the light reaction these are called as assimilatory powers.

2. Dark reaction (Bio synthetic phase)
This reaction in the phase do not require light energy and occur simultaneously with
the light reaction. (time gap between the two being less than even one thousands of a second)

The term dark reaction does not mean that they occur when it is dark at night. It only means that the reactions are not depend on light. H+ Ions produced in photolysis are immediately picked up by special compound NADP to form NADPH. In the dark phase the hydrogen of the NADPH is used to combine it with Co2 by utilizing ATP energy and to ultimately produce glucose (C6H12O6). This synthesis occurs in a number of steps
using certain special intermediate compounds (mainly RUBP Ribulose bi phosphate) and enzymes. Finally the glucose to convert to starch. During photosynthesis several events occur in the chloroplast some of them are :
1. Conversion of light energy to chemical energy
2. Splitting of water molecule
3. Reduction of carbondioxide to carbohydrates

Light is required to initiate several events while several may continue even in the absence of it. That would mean, once light energy has been captured it can help reactions to continue even in the dark. Light dependent events or reactons are called light reactions and it has been found to take place in grana, while the rest are called dark reactions and they occur in the stroma.

Plants are capable of working under a range of situations, from very lighted hot dry conditions to wet, humid dim light conditions and requirement of light and other factors varies from one plant to another.

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