Structural Biochemistry/Light Reaction


Light Reaction converts light to ATP and NADPH, an electron carrier. This reaction occurs in the thylakoid, and it requires visible light for the chlorophyll (a type of molecule known as pigment that absorbs light) to absorb all visible light except for green. Chlorophyll are organized in the thylakoid membrane (in the photosystems).

Then the electron gains energy from light and jumps up to a higher electron shell; this excited state is unstable and temporary, and so it jumps back to the original position. As it returns to its ground state, it gives off energy in the form of heat, and this heat is then absorbed by a neighboring chlorophyll, which passes on the heat to another neighboring chlorophyll and so on until the heat reaches the reaction center chlorophyll. At this point, the reaction center chlorophyll's electron gets excited and passed on to the primary electron acceptor, and the reaction center chlorophyll results with a missing electron that is regained from H2O (electron donor). Before this stage, H2O is converted to 2e- + 2H + 1/2 O2 and this is how it is able to donate an electron. The primary electron acceptor now has an extra electron, which is donated to the electron transport chain that has H+ pumping. The ATP synthase pumps the H+ back in to the stroma and converts ADP + P → ATP. All of this occurs in Photosystem II. Then, the final electron acceptor is the reaction center chlorophyll of PhotoSystem I. PS I receives an electron from PS II, and the primary electron acceptor of PS I donates the electron to another electron transport chain. This electron transport chain has no H+ pumping and no ATP; instead, it generates NADPH from NADP+. Thus, the purpose of the light reaction is to convert light to chemical energy in the form of ATP and NADPH [1]

Photosystem II and I

There are three steps in converting Light energy to ATP and NADPH

  1. To capture the light energy from the sun
  2. Undergo Photosystem II and I to convert ATP and NADPH
  3. The converted ATP and NADPH is used to reduce CO2 to sugar

The photosystems undergo in the internal membrane called Thylakoid membrane. Thylakoid membrane is membrane full of collections of pigments and proteins. There are four types of pigments they are: chlorophylls, carotenes, xanthophylls and phycobilins. The light reaction uses the light power to create ATP and NADPH2 to provide reducing energy to undergo the chemical reaction to reduce the CO2 to sugar. [2]