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Photosynthesis

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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.

Photosynthesis is a process found in plant cells which converts light energy into chemical energy in the form of sugars that the plant can store and use at any time. Photosynthesis takes place in the chloroplasts, an organelle only found in plant cells, and consists of two parts: the light dependent reaction, which converts light energy into ATP, and the Calvin cycle, which converts ATP into glucose.

Photosynthesis is one of the major processes on this planet it's the way that all plants and algae are able to add energy to our eco-system so it's very important for you to understand. In general photosynthesis is the using of the light energy in order to make sugars and oxygen gas. Now sometimes kids think that plants do photosynthesis to give them the food, they don't plants are greedy they're doing it to make food themselves. Also if you've ever been in the force you very rarely been worried about being eaten by the tree. So how is it that a tree or a bush makes more tree or more bush, they do it by adding in more building materials from the air. Unlike you where you can just say, give me my berry and you can toss more building materials into your stomach and then use whatever it is that you ate whether it's a rabbit or a tomato or a banana you use those materials to build your body they have to use air and so they do it through this process here.

Let's go through the basic chemical equation of photosynthesis and then we'll start getting into some of the details. The basic equation is 6 carbon dioxide molecules plus 6 water molecules plus the energy of light and remember to make any chemical reaction occur often you need to add energy especially when you're building a more complicated molecule. You use this light energy to force these atoms to rearrange themselves to form glucose C6H12O6 plus as a waste product 6 oxygen molecules. So let's look a little bit closer at all of this and discuss the chloroplast which is the organelle of photosynthesis now I want to make sure that we remember that this is something that's happening in a plant cell. So here we see a plant cell and remember every leaf and plant is made up of thousands cells or a natural plant is probably out of millions if not billions of cells and each one of these cells will have several chloroplasts in them.

If we take a closer look at a chloroplast we'll see the inner and outer membrane and then we'll see these stacks of discs each stack is called a granum and individual disks are called thylakoids. Floating around them, these stacks are floating within this liquidy medium called the stroma and we've got to remember these 2 main areas the stroma and the thylakoid because those are where the 2 major processes of photosynthesis occur. The 2 basic parts of photosynthesis are the light dependent reactions and the light independent reactions. Now those are the proper names, but a lot of times you'll notice that those are long and scientists like everybody else, are humans and we're lazy and so we like to shorten things.

A lot of times people call the light dependent reactions simply the light reactions because they're the ones that depend on light, they take in the light. And Dr. Calvin is the person who figured out a lot of the steps of the light independent reactions the ones that don't require light. So very often you'll just call these the light reactions in the Calvin cycle. So basically what happens is that the light reactions on the thylakoid membrane you've got special molecules that can absorb light. Using that energy they strip off electrons and put them onto a high energy electron carrier called NADPH they also use the energy of those electrons to make a molecule called ADP an energy molecule that's going to be needed later on in the Calvin cycle. Where do they get some of these electrons that they're putting on to NADPH? Ultimately they take these electrons from water molecules, so they rip apart the water taking the electrons that were holding the hydrogen's onto the oxygen and release the oxygen as oxygen gas. Now the ATP and NADPH are sent off to the Calvin cycle that is happening in the stroma. In the stroma you've got all these enzymes floating around, they grab carbon dioxide then using the energy of ATP and the high energy electrons of NADPH and hydrogens that are being carried on the NADPH. They build glucose or other sugars ultimately you recycle the building materials that you're using and you send the now used up NADPH which become NADP positive and you send the used up ATP it's now adenosine diphosphate and unattached phosphate ion and those go back to be recharged by the light dependent reactions to begin the cycle again.

And one last thing it is you may notice this NADPH if you've done aerobic respiration the break down of glucose in cells, you've seen a very similar molecule called NADH I've often had my students get confused between NADH which is used in respiration and NADPH which is used in photosynthesis. Very similar and they wonder if there was only some easy way to remember that plants during photosynthesis use NADPH you may want to think about that for a while before you van figure out a way to remember.