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Question : Describe the photoexcitation of chlorophyll-a in photosynthesis. |
Answer» Solution :(1) Chlorophyll-a is capable of converting the physical energy of light into chemical energy. It also acts as reaction centre of photosynthesis. Therefore chlorophyll-a is considered as an essential pigment.  (2) Initially, the chlorophyll-a lies at the ground level. But when it absorbs photons (solar energy) it gets activated and becomes energy RICH. This state of chlorophyll is called the excited state. (3) In excited state, the chlorophyll-a expels an electron. The expelled electron is energy rich as it possesses extra amount of energy. (4) With the loss of electron `(e^(-))` the chlorophyll-a becomes positively charged. This state of chlorophyll-a is called ionized state. The molecule of chlorophyll-a cannot remain in ionized state for more than 10 seconds. (5) The energy rich expelled electron is TRANSFERRED through various acceptors and donors. (6) During the transfer, the electron emits energy which is utilized for the synthesis of ATP. This shows that light energy PRESENT in the electron is converted into chemical energy in the form of ATP. (7) There are TWO forms of chlorophyll-a, viz. `P_(700)` and `P_(680)` which form the reaction centres of PS I and PS II respectively for the photochemical reaction. |
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