Page 7 - LN-PHOTOSYNTHESIS IN PLANTS

from 0.4μm to 0.7μm (400-700 nm).
Assimilatory Power
The chemicals (ATP and NADPH) synthesised in photosynthesis, termed as
assimilatory power. They are used in the process to reduce CO 2 into carbohydrates.

Topic 2 Mechanism of Photosynthesis

It was observed that the rate of photosynthesis is directly proportional to the
intensity of light, i.e., rate increases with the increase in intensity of light till the plant
achieved the saturation point.
The process ofphotosynthesis takes place in following two steps
(i) Light reaction or the photochemical phase.
(ii) Dark reaction or the biosynthetic phase.
Light Reaction (The Photochemical Phase)
Light reaction includes the following steps, i. e., absorption of light, splitting of water,
release of oxygen and finally the formation of high energy chemical intermediates,
i.e., ATP and NADPH. ‘
During the course of light reaction, light is trapped by photosynthetic pigments
present in the quantasomes of grana thylakoids.
These photosynthetic pigments are organised into two discrete photochemical Light
Harvesting Complexes (LHCs) known as Photosystem-I (PS-I) and Photosystem-II
(PS-II).
Photosystems
The light harvesting complexes or photosystems are made up of hundreds of
pigment molecules bounded by proteins. Each photosystem has a photocentre or
reaction centre, where actual reaction takes place.
This reaction centre contains a special chlorophyll- molecule is fed by hundred other
pigment molecules forms the light harvesting system called antennae. These
antennae molecule absorb light of different wavelength, but shorter than reaction
centre in order to make photosynthesis more efficient.
These photosystems are, named according to the sequence of their discoveries not
in the sequence in which function during the light reaction.

The reaction centre is different
in both the photosystem as given below
(i) In PS-I, the reaction centre or chlorophyll-a has peak of absorption at 700 nm,