Photosynthetic Units / Pigment Systems

Photosynthetic Units

These are group of pigment molecules which takes part in change of light energy into chemical energy.

Each photosynthetic unit has a reaction center (photocentre) of specific chlorophyll­ a molecule like P₇₀₀ and P₆₈₀, which absorb light energy of long wavelength.

These can release electron upon absorption of energy.

Reaction center is surrounded by a number of light harvesting pigment molecules.

These are those photosynthetic pigments which absorb photons of different wavelength.

Harvesting molecules occur in the form of specific complexes called LHC I and LHC II.

Harvesting molecules are of two types :

I. Antenna molecule : Those occurs on outer side of photosynthetic unit. They absorb photons of different wavelength depending upon their absorbance maxima. They get excited and energy is transferred to core molecule through resonance transfer.

II. Core molecules: They are pigment molecules which are present around the reaction center. They transfer the energy to reaction center, so the reaction center gets excited.

(3) Evolution of oxygen :

Equations of photosynthesis proposed before 1930s gave the impression that the source of oxygen was carbon dioxide.

However, experiments of C.B. Van Niel performed during 1930s showed that oxygen gas liberated during photosynthesis is released as a result of photolysis of water.

While comparing photosynthetic activity of sulphur bacteria and higher plants, Van Niel found that sulphur bacteria assimilate carbon dioxide using hydrogen sulphide (H₂S) as a hydrogen source, whereas in higher plants the source of hydrogen is water. 6CO₂ +12H₂S C₆H₁₂O₆ + 6H₂O + 12S ... (1)

6CO₂ +12H₂O C₆H₁₂O₆ + 6H₂O + 6O₂ ... (2)

In both the above processes carbon dioxide is used but no oxygen is evolved by sulphur bacteria, while on the other hand, it is evolved in higher plants.

It confirms that source of oxygen is water and not carbon dioxide as was believed earlier.

On this basis Van Niel gave following generalized equation for photosynthesis in higher plants and bacteria:

CO₂ + 2H₂A (CH₂O) + H₂O + 2A

In this presentation, H₂A represents a hydrogen donor; in green plants this is water (H₂O) and in sulphur bacteria it is hydrogen sulphide (H₂S).

The action of light might be to produce a reducing entity [H] and an oxidising entity [O] from water.

The views of Van Niel were further confirmed by Ruben, Hassid and Kamen (1941) using radioactive isotope of oxygen (O¹⁸).

When they used water (containing radioactive oxygen, H₂O¹⁸), (O¹⁸) was evolved.

It thus confirms Van Niel's observation that the source of oxygen evolved in photosynthesis is water.

Equation (2) provides the most adequate explanation of photosynthetic process, since it shows that oxygen evolved during photosynthesis comes from water.

(4) Red Drop:

Robert Emerson measured the efficiency of photosynthesis of Chlorella cells in monochromatic light (i.e. only one wavelength of light at a time).

They observed that the quantum yield of photosynthesis decreased very sharply towards 680nm and longer wavelengths.

Since this decrease in quantum yield is observed at the far-red region of the spectrum, hence it is called red drop.

Quantum requirement: It is the number of light quanta needed for production of one O₂ molecule or reduction of one CO₂ molecule. It is 8.

Quantum yield: It is the number of oxygen molecules evolved per light quanta absorbed. Two light quanta are required to move one electron. Such four electrons are involved in the evolution of one molecule of oxygen. Eight quanta are, therefore required for evolution of one oxygen molecule. Thus quantum yields or number of oxygen molecules evolved per light quanta is 1/8.

Enhancement Effect

Emerson found that when light of shorter wavelengths was used superimposed with the light of long wavelengths, the rate of photosynthesis was greater (25% more) than could be expected from adding the rates found when either light was given alone.

Thus both light beams had a synergistic effect i.e. one helped the other.

It leads to the conclusion that two group of pigments, one absorbing in lower range and other absorbing in higher range of wavelengths are involved in photosynthesis and are called pigment systems.

(5) Two pigment systems :

It is a group of pigment molecules having chlorophyll a, chlorophyll b, carotenoids etc.

One molecule of chlorophyll a functions as a reaction centre or trapping centre and others act as light harvesting complex (LHC) or antenna molecules.

These antenna molecules collect light energy and transfer this energy (by inductive resonance process) to reaction centre where primary photochemical act occurs (quantum conversion) i.e. absorption of a photon and release of an electron.