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
----