Photophosphorylation the Light Reaction - Assignment Example

The light-harvesting molecules otherwise called antennae molecules absorb light energy and transmit it to the reaction center. Once they absorb light, the molecules are in an excited state which is generally unstable. Therefore the excited molecules (electrons) rapidly decay to the ground state with simultaneous transfer of excitation energy to neighboring molecules. This process is called exciton transfer. This transfer continues to the third and fourth or subsequent neighbor until a special pair of chlorophyll a molecule at the reaction center is excited.

Upon being excited the electron is promoted to higher energy orbital and then it passes to an electron acceptor leaving the photochemical center with an electron ‘hole’. This electron is then replaced by another electron from a neighboring electron donor molecule. Experiments set up to investigate photosynthesis in plants in the 1950s by Louis Duysens with bacterium Rhodospirillum found that there was a temporary decrease in absorption of light at the wavelength of 870 nm. This temporary decrease in light absorption was referred to as ‘bleaching’.

Similar studies also demonstrated similar ‘bleaching’ at wavelengths of 680 and 700 nm. Therefore, P680 and P700 stand for bleaching which occurs in plant chloroplasts at the wavelength 680 and 700 nm respectively. The P680 and P700 represent the maximum wavelength of maximum bleaching (needed to excite) in photosystem I and photosystem II respectively.A chloroplast is a double membrane organelle present in the photosynthetic organisms. The organelle contains the photosynthetic pigment, chlorophyll which absorbs light in the visible spectra and converts the light energy into chemical via two distinct phases, the light phase and dark phase with the final step of assimilation of CO2 into the cell as a hexose sugar.

Photosynthesis is a process that takes place in the chloroplast of photosynthetic organisms where light is absorbed.