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Light Dependent Reactions

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Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The light dependent reaction is the first step of photosynthesis which converts light energy into ATP. The light dependent reaction takes place inside the chloroplasts along the thylakoid membrane.

Photosynthesis is a 2 step process. The first step of that process is the harvesting of the energy from the sun, and that step is called "the light dependent reactions," because it depends on light. So what it does is it happened on thylakoid membrane and it creates the ATP and NADPH that is required for the last half of photosynthesis fthe Calvin's cycle to occur. And it does it through a process known as chemiosmosis. Let's take a closer look at this, I always want to help kids remember that a plant is made out of a thousands of millions of individual cells. Every cell has dozens to hundreds of chloroplast and each one of these chloroplasts is carrying out photosynthesis. If we take a closer look at the chloroplast we'll see these stacks of membrane that are in stacks called granum and each individual disk is called a thylakoid and it's in the thylakoid membrane that you find the imbedded molecules that make up the light dependent processes mechanisms. So here we see the light dependent reactions or sometimes it's just short to light reactions and what's going on is the harvesting of light is being used to generate the ATP energy molecule and the high energy electron carrier called NADPH that the Calvin cycle requires.

Where does it get the building materials for this? Well you get adenosine diphosphate and phosphate ions that have been used up by the Calvin cycle as well as NADP positive empty electron carriers that also have been used up by the Calvin cycle. Ultimately oxygen gas is kicked out as a waste product of the light dependent reactions. The oxygen there came from water. Now that we've gone over the basics of it, let's dive into the details. Now you may see a diagram like this in your text book, don't get too worried about it, I'm not going to go through and then demand that you memorize "oh that's plastic well I know" instead I'm going to focus on the overall process so that you can then go back to your class and go "oh I get this." Alright so this depends on 2 major collections of light harvesting pigments or chlorophyll molecules.

And they're called photosystem 2 and photosystem 1; it really sucks for you guys that this one was names first and then this one second. Sorry, because generally when we describe what's going on during photosynthesis we talk about number 2 first and then number one, oh well. So what basically happens is that energy in the form of light is absorbed by special pigments like I mentioned before called chlorophyll they pass that energy on to one of the central ones called the reaction center and that reaction center molecule can actually lose a couple of electrons and those electrons carry the energy away. Just like a base ball, if I fling it all of a sudden it's got energy. So it flings away these high energy electrons and you can use the energy of those high energy electrons to do things, just like you use high energy electrons to make a fan move, your iPod to work or your TV to work. Now if you keep removing electrons from molecules those electrons are being used to form the bonds of those molecules and so the molecules will fall apart. So we need to get replacements and that's the job of this end of the photosystem where you have a special enzyme that can grab electrons from water.

Which makes the water fall apart into hydrogen ions and oxygen gas, now those hydrogen ions got positively charged why? Because we took away their electrons. Now here we see something called generically the electron transport system or electron carrier system and what happens you have a bunch of molecules kind of acting like wires and that they allow the passage of electrons. And this thing right here in the middle that acts as a pump and what it does is it grabs hydrogen ions from outside of the thylakoid and shoves them inside the thylakoid. Now we've already started making some hydrogen ions inside of here so we're forcing even more in here and those hydrogen ions will be repelled by this process actually because there's already a bunch of hydrogen ions and they're all positively charged. So that's why it requires energy to force them in just like you're trying to shove kids into a small closet or wherever. The pressure starts to build up, don't put children in a closet.

Eventually those electrons have lost their energy why? It's been used to do something, so we have this low energy electron. Luckily photosystem 1 also at the same time has been absorbing light and given its energy from that light to some high energy electrons. They go to this molecule here which gives those high energy electrons to the electron carrier NADP positive. If you give it 2 electrons it becomes negatively charged and it grabs a hydrogen ion from again outside and it becomes NADPH that's one of the two things required by the Calvin cycle for it to operate in do carbon fixation. So the NADPH goes off to the enzymes that are floating around the stroma to do the Calvin cycle.

The last thing that we need to generate for the Calvin cycle is ATP, and that's where all these hydrogen ions that we've been building up inside the thylakoid membrane come into play. Now this high concentration of hydrogen ions gives us the ability to do some work. This difference of hydrogen ions on one side to the other side is called a chemiosmotic gradient, it's more than just a concentration gradient because all of these hydrogen ions that we've been adding to this side and removing from that side they've got a charge and so we've got a positive region inside of here comparatively the outside is negative so they're attracted to the other side as well as repelling each other out. And this thing right here is a molecule or a collection of molecules actually and collectively called ATP synthase it's a specialized channel that as the hydrogen ions go zipping down the their gradient kind of like water through a dam.

They force the turbine of the dam to turn to generate electricity, well as the hydrogen ions go through this channel they force the ATP synthase thing here to spin literally it spins and as it spins it changes shape, grabs an ADP and phosphate and rams them together to make ATP and then that ATP floats away to the Calvin cycle and there you have it. The light reactions, the absorption of light creates this chemiosmotic gradient so that you can create ATP and the absorption of light creates high energy electrons that hop a ride to the Calvin's cycle on NADPH.