Mechanism of Photosynthesis

LIGHT REACTION OR HILL REACTION

It occurs in lamellar part of chloroplast an antasomes are units of light reaction. It includes following main processes.

(1) Light and its absorption :

White light, as it reaches us from the sun, is composed of different wavelengths.

When passed through a glass prism it resolves into seven colours-violet, indigo, blue, green, yellow, orange and red.

This band of seven colours constitutes the visible spectrum.

Visible light consists of radiations having a wavelength between 390-760 nm.

Part of spectrum used in photosynthesis has a wavelength between 400 -700 nm and is called PAR i.e., photosynthetically active radiation.

In the range of visible spectrum violet bas the shortest wavelength (390-430 nm) and red the longest (660-760nm).

Wavelength longer than 760 nm (infra red radiation) and shorter than 390 nm (ultraviolet radiation) also exist but these are invisible.

Visible spectrum obtained when a ray of white light is passed through a prism

Only a small portion of visible spectrum reaction the earth is utilized in photosynthesis.

Chlorophyll - the green pigment of leaves, absorbs mainly the blue 470 -500 nm) and red (660 -760 nm) wavelengths.

It reflects the green wavelengths (500-580 nm) and that is why the leaves appear green in colour.

        Graph (a) showing absorption spectra of chlorophylls a, b and carotenoids.

Graph (b) showing action spectrum of photosynthesis. Graph (c) showing action spectrum of
photosynthesis superimposed on absorption spectrum of chlorophyll a

Absorption and Action Spectra

Different plant pigments absorb only certain wave lengths of light and these wavelengths are not absorbed at the same rate.

A curve obtained by plotting the amount of absorption of different wavelengths of light by a particular pigment is called absorption spectrum.

An action spectrum is a curve showing the effectiveness of different wavelengths of light in stimulating the process being investigated.

The effectiveness of different wavelengths of light on photosynthesis is measured and plotted by quantum yield or amount of action (expressed through CO2 reduction, O2 evolution etc.) Action spectrum of photosynthetic pigments was studied by Engleman on Cladophora and Spirogyra for the first time.

(2) Excitation of chlorophyll by light :

Each molecule of chlorophyll is composed of many atoms of hydrogen, oxygen, carbon, nitrogen, etc. Negatively charged electrons of different energies revolve in specific orbits around positively charged nucleus of the atom.

Each electron possesses a definite amount of energy depending upon its distance from the nucleus.

This state of electron is known as ground state -a condition of maximum stability.

If a sufficiently energetic photon of light strikes one of the electrons, the electron acquires the energy of photon (quantum) and jumps to another orbit of higher energy.

This is said to be the excited state of the electron.

It is an unstable state and the electron stays in this state for only a very short time (10–5 – 10–9 second).

As the excited electron returns back to its ground state, the trapped energy may be emitted as heat, light or as fluorescence.

In green plants, however, the energy emitted by the excited chlorophyll molecule is trapped and stored in the form of chemical energy (ATP) rather than being lost simply as light or heat.

If the excited electron returns to its ground state in one step, the energy stored in it would be released in one large dose and may not be fully converted into chemical energy.

Therefore, the excited electron, as it returns to its ground state, is passed on to a number of acceptors so that the maximum amount of released energy is converted into chemical energy.

Concept Builder

Photoluminescence :

It is the property of reradiation of absorbed energy. The reradiated energy is usually or longer wavelength. It of two types.

(i) Fluorescence : It is immediate reradiation of fight energy. It stops immediately after the withdrawal of source of illumination. All types of photosynthetic pigments are fluorescent. Most of the fluorescence produced by green parts are due to chlorophyll-a.

Isolated chlorophyll 'a' when dissolved in organic solvents and exposed to light emits red colour immediately. It is called fluorescence; and this quality is called kutusky effect. It is, infact, a manifestation of loss of light energy absorbed in excess. Actually, when the absorbed energy is not utilised for a purpose (phosphorylation), it is emitted out as radiations of longer wavelength. In green plants, of course, this is utilised to synthesise chemical energy (ATP).

(ii) Phosphorescence: The delayed re-radiation of absorbed energy is called phosphorescence.