You Are Now Going to Study the Light Reaction of Photosynthesis

Virtual Cell Photosynthesis Goals, Orientations, and Help Menu texts. Ganesh Padmanabhan 3/17/2004

You are now going to study the light reaction of photosynthesis. Photosynthesis takes place in chloroplasts. You first need to find a chloroplast and click on it.

Take some time to study this animation. You will next perform some small experiments to learn the key concepts found in this process.

When you are finished studying, click on the Lab Guy icon, and select 'I have watched the animation'. You will be able to return to the animation or view the diagram later if you wish.

DIAGRAM CAPTION: In this module you will study the process of photosynthesis in the chloroplast. Photosynthesis is a complex series of reactions that can be divided into a part that relies on the presence of light (the light reactions) and another part that is a series of chemical reactions that generate sugar as a final product (the dark reactions or Calvin Cycle). The light reactions involve two photosystems and an electron transport chain which are imbedded in the thylakoid membranes of the chloroplast. Water molecules are split to release electrons, H+ and oxygen. The electrons are donated to chlorophyll and then energy from photons of light is absorbed by chlorophyll and other pigments in Photosystem 2. The absorbed energy is transferred to electrons which are passed by a carrier, plastoquinone, to the cytochrome b6-f complex. Cytochrome b6-f pumps H+ ions across the thylakoid memberane, generating a H+ gradient. The gradient between the inside of the thylakoids and the stroma space of the chloroplast is necessary to produce ATP by the ATP Synthase complex. The electrons are passed through plastocyanin to Photosystem 1, where more light energy is absorbed to boost the electron energy level a second time. Electrons are passed on to ferridoxin and finally donated to NADP+ to produce NADPH. The ATP and NADPH produced in the light reactions are used to drive the dark reactions in which CO2 is converted into sugar.

YOUR GOAL:

Put Photosystem 1,Photosystem 2,NADP reductase and Cytb6-f into their proper slots so that a photosynthetic system is established and can work well. Step 1. Select proper slots for these components from the drop down buttons.

Step 2. Use the "Set Components" button to put these components into their slots.

Step 3. Use the "Run" button to run the experiment to test your selections. When the experiment is completed, a message will pop up to tell you if your selections are right.

In the photosynthesis light reaction, there is a light-drived flow of electrons, from water to NADPH.

The chlorophyll molecules in Photosystem 2 lose an electron, which is derived from water, and this electron is moved through a series of carriers to the chlorophyll molecule in Photosystem 1. Finally, two such electrons can reduce NADP+ to NADPH.

In this level the system lacks several things it needs to work properly: water, NADP, light P680 and light P700. You must add one of these components at each step. The first time you will have a guide to follow. One NADPH molecule will be produced at the end of the process. Then you should produce two more NADPH molecules without the guide. Through this exercise, you should learn how the photosynthetic light reaction works.

First go to Photosynthesis Level Two, then work through activities one and two.

The chlorophyll molecule in photosystem 2 is now positively charged. It needs an electron to return it to its original energy state. Which substrate that you add to the system can provide electrons to fill the positive hole?

Well Done! The electrons come from breaking apart water molecules.Now,the electron void in chlorophyll molecule is filled. Next question is: What makes one of the electrons in the chlorophyll molecule gain enough energy to break away.Select one substrate from the drop down button.

Well done! The chlorophyll molecule in photosystem 2 loses a high-energy electron when it is struck by light energy P680.The energy in the high-energy electron came from light energy P680.Two electrons are needed for NADP reduction, so repeat the same action again. ----

Well done! Two electrons are now moved through a series of carriers to the chlorophyll molecules in photosystme 1. How the electrons are transferred from chlorophyll molecules in photosystem 1 to NADP reductase? Select one substrate and add to the system so that the chlorophyll molecule in photosystem 1 loses an electron. Hint: it is a similar process with what happened in photosystem 2.

The chlorophyll molecule in photosystem 1 loses an electron when it is struck by light energy P700 and the excited electron is transported to NADP reductase through Fd. Repeat the action that you did last time and transfer another electron to NADP reductase also.

Well done! There are two electrons in the NADP reductase now. They need to be transported to the terminal electron acceptor, but who is the terminal electron acceptor? Select one from the drop down button.

Well done! The terminal electron acceptor is NADP.The excited electrons are used to reduce NADP to NADPH , which is used to convert CO2 to carbohydrate. Congratulations! You have finished your first task. If you want to try again, reset the system. Otherwise select task two and continue.

Now,you should know how the electrons is transfered from water to NADP molecule and reduce NADP to NADPH. If you are not familiar with this process, please go to task 1.In this section, the system is lack of light energy P680, light energy P700, water, and NADP. You need to add one of them at each step so that two NADP molecules are reduced to two NADPH molecules at the end of the process