Photosynthesis - Notes | Class 11 | Part 2: Photosynthesis- Site and Pigments

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13. PHOTOSYNTHESIS

PHOTOSYNTHESIS: SITE & PIGMENTS


Photosynthesis occurs in green leaves & other green parts.Chloroplasts present in the walls of mesophyll cells of leaves. It helps to get optimum quantity of incident light.

Chloroplast contains a membranous system. It consists of grana, stroma lamellae and matrix stroma.

Each granum is a group of membrane-bound sacs called thylakoids (lamellae). They contain leaf pigments.

The membrane system traps light energy and synthesise ATP and NADPH. It is called light reactions.

In stroma, enzymatic reactions synthesize sugar, which in turn forms starch. It is called dark reactions (carbon reactions). It does not mean that they occur in darkness or that they are not light dependent.


PIGMENTS INVOLVED IN PHOTOSYNTHESIS


Pigments are substances that have ability to absorb light at specific wavelengths.

Chromatography shows the following leaf pigments:
  • Chlorophyll a (bright or blue green in chromatogram)
  • Chlorophyll b (yellow green)
  • Xanthophylls (yellow)
  • Carotenoids (yellow to yellow-orange)
Chlorophyll b, Xanthophylls and Carotenoids are Accessory pigments.

Functions of accessory pigments:
  • They absorb light at different wavelength and transfer the energy to chlorophyll a.
  • They protect chlorophyll a from photo-oxidation.
The absorption spectrum & action spectrum coincide closely showing that photosynthesis is maximum at the blue & red regions of the spectrum.

The graphs also show that chlorophyll a is the chief pigment associated with photosynthesis.


Graph showing absorption spectrum of chlorophyll a, b & carotenoids 


Graph showing action spectrum of photosynthesis


Graph showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a 

Photosystems 

Pigments are organised into two Photosystems called Photosystem I (PSI) & Photosystem II (PSII). These are named in the sequence of their discovery.

Each photosystem has a chlorophyll a and accessory pigments bound by proteins.

All pigments (except one molecule of chlorophyll a) form a light harvesting complex (LHC or antennae).

Single chlorophyll a acts as reaction centre.


In PS I, the reaction centre absorbs light at 700 nm, and so called P700.

In PS II, the reaction centre absorbs light at 680 nm, and so called P680.
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