PHOTOSYNTHESIS

Photosynthesis is a mechanism in which complex organic substances are synthesised from the inorganic substances or in other words it is a oxidation – reduction process in which water molecules are oxidised to form oxygen and carbon dioxide molecules are reduced to form sugars (carbohydrates). This reduction of CO₂ into sugars requires an energy source like ATP and NADPH (assimilatory powers). This reduction process takes place in dark conditions while production of assimilatory powers takes place in light. So there are 2 phases that occurs during photosynthesis.

Light Dependent Phase: This phase requires light so it light dependent and it is also best known as HILL Reaction after the name of its discoverer.

Light Independent Phase: This phase does not require light so it is called as DARK Reaction or BLACKMAN’s Reaction (after the name of its discoverer).

MECHANISM OF ABSORPTION OF LIGHT:

When a visible photon of light with a wavelength between 380nm and 780nm strikes the chlorophyll molecule it releases an electron from chlorophyll to an outer molecular orbital. This state of chlorophyll molecule is called as excited or an activated state. Usually the normal state of an atom or molecule is called as Ground state or Singlet state (S₀) in which the electrons are present in the even number and paired state. The excited chlorophyll molecule shows four states:

Singlet state first (S₁)

Singlet state second (S₂)

Triplet state first (T₁₎

Triplet state second (T₂)

So S₁ state would be the one when a red light photon strikes a chlorophyll molecule which becomes photo excited and an electron will be released from its ground molecular orbital to the outer molecular orbital. This state is unstable state as it has half life period of only 10^-9 s and it produces two molecular orbitals each having one electron.

S₂ state  will be produced in the same way as the S₀; the only difference is that this time blue light photon will photo excite the chlorophyll molecule and release electron into outer MO. The electron is raised more higher than the S₁ state because blue light photon possesses more energy than red light photons. This state also produces 2 MO’s and is unstable with a half life period of 10_-9 s.

Since S ₁ and S ₂states both are unstable they are converted into S₀ state by releasing energy through some processes like heat, phosphorescence and fluorescence. The S₁ state is converted in to S₀ state by releasing energy in the form of heat. This is called as fluorescence.

But all of the energy is not lost as fluorescence; still some of energy is left which is being utilised to drive photosynthetic reactions. The S₂ state is first converted into S₁ state by releasing small amount of energy and when S₁ state loses energy it is converted into 2 interconvertible states called as T₁ and T₂ states. T₁ state is then converted in to S₀ state by loosing small amount of energy. This phenomenon is known as phosphorescence.

ABSORPTION SPECTRUM

It can be defined as the amount of the light of different wavelengths absorbed by the pigment. if the light of different wavelengths is passed through the extracted chlorophyll the absorption of each wavelength can be measured by spectrophotometer. And when this absorption is plotted the resulting plot we get is called as absorption spectra. Different pigments absorb different wavelength of light. For instance, the absorption spectra of chl a is in blue and red region. Other lights like yellow green and orange are absorbed only slightly. That’s why chlorophyll is not green because it absorbs green but actually it reflects and transmits green. The exact position of the peaks in the spectra depends upon the solvent used for the extraction of pigments. Chl a shows max absorption at 662nm in red region and 430nm in blue region. While Chl b shows max absorption peak at 644 nm in red region and 455nm in blue region.

Figure shows absorption spectra of chl a and chl b : vertical axis has absorption and horizontal axis is wavelength in nm. 

In next post we will be doing action spectra and mechanism of photosynthesis!!!!!!