Simple Explanation of Light Reactions in Photosynthesis

For years now I have asked my mom to "help" me study for the many science tests I have had. If you were one of my close friends you would, unfortunately for them, know that when I study for science tests, I rant on and on about the specific subject at hand. I remember about a week ago I was doing my usual rant in preparation for a test the next day, this time I was telling my mother all about the light reactions of photosynthesis. Now, not to undermine my mother's intelligence what so ever, but, being special education teacher, she does not know a lot about biology. So, like she does always, she simply sat attentively and listened, occasionally asking questions. I feel, much of the time, I explain things without really considering whether she understands what I am talking about, so this is a simple explanation of light reactions in photosynthesis.

Diagram from: http://bioap.wikispaces.com/file/view/fig10.17.jpg/189210409/661x576/fig10.17.jpg




Refer to diagram.

Step 1: Energy in the form of light waves from the sun are absorbed by the chlorophyll

Step 2: This absorbed energy excites the electrons which are built up in the reaction centers of the photosystems (I and II)

Step 3: The the reaction centers built up so much energy that the electrons "jump" through the electron transport system. (follow arrows)

Step 4: To replenish photosystem II's electrons it oxidizes water to yield protons and electrons which are also used in other steps of the light reaction process.

Step 5: In the meantime, as the electrons are traveling through the electron transport system, protons (H+), in a way, "follow" them and get stuck in the thylakoid space.

Step 6: Once of electrons travel to photosystem I they receive an energy boost which changes them into NADP+.

Step 7: The protons and electrons from the oxidation of water in step 4 are combined to form NADPH, a molecule that will later be used in the Calvin cycle.

Step 8: The build up of protons (positive charge) in the thylakoid space, and the presence of electrons (negative charge) on the outside of the thylakoid space creates potential energy (i.e. a battery).

Step 9: This potential energy is used by ATP Synthase (enzyme that makes ATP) to make ATP from ADP and phosphate. This ATP is also used later in the Calvin cycle.

Step 10: Done! You now have the two major products of the light reactions of photosynthesis, NADPH and ATP.