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Light Dependent Reaction

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Photosystem I and II

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Electron Transport Chain

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Photosystem II

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Light Dependent Reactions

All living things need energy, and energy comes from food. Animals eat plants and/or other animals, but you'll never see a tree grazing on grass or chowing down on chicken. Before a plant can become food for an animal, it has to first produce its own food. Plants are the original solar power generators. They take in water and nutrients from the soil and carbon dioxide and water from the air and, powered by pure solar power, produce glucose and oxygen through the process of photosynthesis. Photosynthesis is broken down into two processes, the light dependent reactions and the Calvin cycle.

Light dependent reactions, as the name suggests, require sunlight. They work to turn solar energy into chemical energy in the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide hydrate (NADH). There are two photosystems of light dependent reactions, both of which take place in the thylakoid membrane of the chloroplasts.Light Dependent Reaction

Photosystem II

Light dependent reactions start in Photosystem II. Sunlight is used to provide energy that is used to split water molecules. The result of this split is two electrons, two hydrogen atoms, and one oxygen atom. Chlorophyll a absorbs a photon and takes the electron that has been released to a higher energy level. This unstable electron needs to get on the electron transport chain so that it can be used in the next stage, Photosystem I.

Photosystem I

The electron goes from Photosystem II to an intermediary stage, cytochrome b6f, then to Photosystem I. Energy is lost during this transfer, but is replaced during Photosystem I. This occurs through the absorption of another proton. In the Photosystem I reaction center, the high energy electron is used to reduce NADH to NADPH.

There are several things that are occurring at the same time in both photosystems I and II. When the water molecule is broken down, a proton is given off along with the electron. A proton is added to the lumen, and a proton is reduced from the stroma.


There are two different types of photophosphorylation, cyclic and non-cyclic.

In cyclic photophosphorylation, energy from both photosystems I and II is used. During this process, cytochrome b6f uses the energy to form ATP and to stop the production of NADPH. Because of this, a balance is maintained that allows the light dependent reactions to continue.

In Non-cyclic photophosphorylation the energy that comes from Photosystem II is used to move hydrogen ions from the lumen to the stroma. The energy from the hydrogen ion allows ATP synthase to add a third phosphate group to an adenosine diphosphate (ADP) molecule and form ATP. The process is called chemiosmosis.

At the beginning, we stated that the light dependent reactions were responsible for creating chemical energy from sunlight. The result of the light dependent reactions is:

2H2O + 2NADP+ + 3ADP + 3Pi → O2+ 2NADPH + 3ATP


We all need energy, so we all have to eat. This is true for the most basic of single cell organisms, for humans, and for every living thing in between. While we consumers eat plants, and sometimes each other, the plants themselves have to produce their food. Light dependent reactions are a critical first step in the solar powered process photosynthesis, and none of this could occur without sunlight.

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