Light Absorption
Photosynthesis in plants is initiated by the absorption of light in the visible range (wavelength § 400 to 700 nm) by pigment 0 molecules, mainly chlorophylls and arytenoids. Xot all wavelengths of light in the visible range are effective in photosynthesis.
Absorption spectrum (see Box I) of chlorophyll the blue and red parts of spectrum are effective in photosynthesis. Light moves in discrete particle called as photons. The energy content of a photon is called a quantum. The energy content of
photon varies with its wavelength; the energy of photon is higher when its wavelength is shorter and vice versa. When a chlorophyll molccule in its lowest energy or ground state absorbs a photon (represented by hi), the energy of the photon is transferred to one of the electrons in the outermost orbit of chlorophyll molccule.
This electron, having higher energy level, is elevated to a higher orbit equivalent to its energy level. The chlorophyll molecule is then said to be in a higher energy, or excited state as shown below:
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Absorption of blue light excites the chlorophyll molecule to a higher energy state (called as the second excited singlet state) than the absorption of red light because, as stated above? – The energy of a photon is higher when its wavelength is shorter.
In the second excited singlet state, the chlorophyll molecule is extremely unstable having the half-life of ions. It very rapidly gives up some of its energy to its surroundings as heat and comes to the first excited singlet state.
The chlorophyll molecule can directly enters into the first excited singlet state by the absorption of a photon of red light. The first excited singlet state is much more stable than the second excited singlet state; its half life is ions. The excited chlorophyll in this state has the inherent tendency of returning back to the ground state after dissipating the excess energy in different ways.
1. The excited chlorophyll can return to its ground state by directly converting its excitation energy into heat with no emission of a photon (called as radiation- less transition). The heat is lost to the environment.
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2. The excited chlorophyll can return to its ground state by releasing the excitation energy in the form of light. This emission of light is called fluoresccnec. The wavelength of fluorescent light is slightly longer (and, therefore, has lower energy) than the wavelength of absorbed light which is required for attaining the first excited singlet state.
This is because part of the excitation energy is usually lost as heat before the fluorescent photon is emitted. Therefore, chlorophyll fluoresces always ill the red region of the spectrum irrespective of whether blue or red light is absorbed. Chlorophyll fluoresces during energy transition from first excited singlet state to ground state.
3. By releasing part of the excitation energy in the form of heat,-the chlorophyll molecule can attain an excitation state of lower energy, called as the triplet state. Triplet state can not be reached directly from ground state by excitation. In the triplet state, spin of the excited electron is reversed.
The transition from singlet state to triplet state is, therefore, called as’ inter system crossing. As the probability of spin reversal is low, the triplet state docsr occur frequently. By emitting excitation energy in the form of light (called phosphoresccnce), or by heat loss, the chlorophyll molecule can return from the tripl state to the ground state.
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4. Chlorophyll in the first excited singlet state can return to the ground state by transferring the excitation energy to a neighbouring molecule which requires lesser energy for excitatio this energy transfer is very important during photosynthesis.
5. The fifth process is photochemistry in which the energy of excited chlorophyll is utilized for chemical reactions. The chlorophyll molecule transfers the excited electron from the fr excited singlet state to an electron accepter.
The electron accepter is reduced leaving behind positively charged chlorophyll radical. This step is also known as charge separation which constitute the most important step in photosynthesis.