ACTION SPECTRUM

abhishek.m

Accessory pigments extend the range of light

absorption

In addition to chlorophyll , thylakoid membrane contain secondary light absorbing

pigments or accessory pigments, called as CAROTENOIDS

These pigments may be YELLOW,RED OR PURPLE

Beta-carotene is a

red-orange while Lutein is a yellow carotenoid.

These carotenoids absorbs light at wavelengths not absorbed by chlorophylls, and thus are supplementary light receptors

ABSORPTION v/s ACTION Spectra

ABSORPTION SPECTRUM:

A Plot showing absorption of light of different wavelength of a pigment is

called as its absorption spectrum

Both chlorophyll a & b show maximum absorption in the violet and orange region of

visible spectrum.

V

Absorption spectrum of chlorophyll ‘a’ & ‘b’

V

Absorption of chlorophyll a shows peak at 680nm and that of chlorophyll b at 450nm.Carotenoids show absorption peak at 478nm and Xanthophylls at 440 and 490 nm.

ACTION SPECTRUM

The plot showing relative effectiveness of different wavelengths of light in

photosynthesis is called as Action spectrum.

Absorption spectrum is given by all pigments, but action spectrum is given only by

chlorophyll ‘a’.

G

A:ABSORPTION SPECTRUMB:ACTION SPECTRUM

There is difference b/w the absorption and action spectrum of chlorophyll a.

The peaks of its action spectrum have more height and valleys are much narrower as compared to its absorption spectrum.

From the former diagram it is rather clear that the action spectrum of chlorophyll a is much more efficient than its absorption spectrum.This is an indication of the fact that chlorophyll a performs more photosynthesis than it actually absorbs light.

An action spectrum is the rate of physiological activity plotted against the wavelength of light.

It shows which wavelength is more effectively used in a specific chemical reaction{in this case photosynthesis.}

EXPERIMENTAL METHODS TO

DETERMINE THE ACTION SPECTRUM

ENGELMANN’S EXPERIMENT:

T .W . Engelmann , in 1882 conducted a classic experiment that determined the wavelength

of light that is most effective in photosynthesis.

He performed the experiment on filamentous alga.

He illuminated the alga with light from a prism.

His aim was to determine which algal cells carried out photosynthesis most actively.

To achieve his objective he introduced on the microscopic slide bacteria known to migrate towards regions of high O2.

after a period of illumination, the distribution of bacteria showed highest O2 levels in regions illuminated by violet and red light.

THE DISTRIBUTION OF BACTERIA IN DIFFERENT REGIONS OF THE VISIBLE SPECTRUM.

MODERN EXPERIMENTS

Similar experiments were conducted by scientists in the

twentieth century.

But here modern technologies were used .

Like the use of an O2 electrode for measurement of oxygen

production.

An action spectrum was obtained as a result.

An action spectrum describes the relative rate of photosynthesis for illumination with a constant number of photons of different wavelength.

It is useful because, by comparison with an absorption spectrum, it suggests which pigments can channel energy into photosynthesis.

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