Biology: The Light Dependent Reaction

Essay by mjj328High School, 10th gradeA+, April 2004

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In the Light Dependent phase of photosynthesis, light strikes Photosystem II, causing the electron to become excited. The excited electron bounces up to an electron acceptor, and is then passed down the electron transport chain. As the electron goes down the ETC, the electron

loses energy. The energy released is used to produce ATP. At the same time, light is hitting Photosystem I, which makes it's electron's excited. The electrons are passed to another electron acceptor and another ETC. The electrons leaving Photosystem I are replaced by those from Photosystem II. The electrons from Photosystem II must then be replaced, so water is split (photolysis). Oxygen is released to the atmosphere. At the end of the second ETC, the electron is picked up by NADP+. The hydrogen + ions that resulted from photolysis are also picked up by the NADP+, making NADPH.

The light dependent reaction takes place in the thylakoid membrane.


Light Energy







Oxygen- byproduct or used in cell

ATP- goes to light independent reaction

NADPH- goes to light independent reaction