Erika Tan explains the process of photosynthesis at the molecular level.
*** If there are any pictures used in this video, they are NOT MINE and I will not take credit for them. ***
Before we take a look at photosynthesis as the process itself, you should get used to the structure of the chloroplast first. Here’s a diagram of the chloroplast and all of its labeled parts. So you can see that there’s a double membrane; this one is the inner membrane and that one’s the outer membrane. The intermembrane space is the area between the two membranes. Then there are these sacs inside of the chloroplast. They’re called thylakoids, and one column of these thylakoids makes up a singular granum. All of these grana are surrounded by a fluid called the stroma, which is kind of like the matrix of the mitochondrion if you’ve seen my cellular respiration video. And of course, there’s chlorophyll and other photosynthetic pigments that give the plant its color in the thylakoid membranes.
Here’s an action spectrum of the pigment called chlorophyll a, and we can see that it mostly absorbs the red and blue-ish violet colors, but not the green. That’s because it’s reflecting the green waves, so that’s why we see that many plants are, in fact, green.
There are two steps of photosynthesis: the light reactions, also called photophosphorylation, and the Calvin cycle. Let’s talk about the two types of photophosphorylation now: noncyclic and cyclic. During noncyclic photophosphorylation, water is split in order to donate electrons and protons to the electron transport chain. When two water molecules are split, it gives off O2 as a byproduct since oxygen is a diatomic molecule, meaning it’s naturally found consisting of two atoms.
Alright, so we know that water is split, but what happens next? Well, there are two photosystems involved in the process that harvest light and power the reactions. Photosystem II starts it off by absorbing light wavelengths of 680 nanometers (nm), exciting the electrons in the complex. Then, a primary acceptor accepts these electrons, powering the electron transport chain that happens between photosystems II and I. By the way, even though photosystem II functions first, they were named as they are by the order of discovery, so photosystem I was found first although it functions second. Anyways, during the electron transport chain, a proton gradient is created in the thylakoid membrane so that H+ protons are pumped from the stroma into the interior of the thylakoid. As the proton supply in the thylakoid interior builds up, the H+ will diffuse down the gradient through an ATP synthase complex and power the making of ATP. Remember, this is called photophosphorylation because the phosphorylation of ADP to ATP is powered by light. You can also describe this process using the word “chemiosmosis” because the protons are diffusing down a gradient in order to power a reaction.
So now, more light energy is absorbed by photosystem I, which absorbs wavelengths of 700 nm. Another acceptor accepts the excited electrons, and a molecule called NADP+ picks up the two H+ protons from the water molecule that was split before, turning into NADPH. We say that NADP+ has been reduced to NADPH, since it’s gaining electrons and is therefore getting more negative, reducing its charge.
Okay, so the light reactions make NADPH and ATP. Let’s move on to the Calvin cycle. But let me just get one thing straight with you before we start: you do NOT need to know all of the specific steps for the AP exam, thankfully. I’m just going to give you the important information that you do need to know. So first of all, CO2 enters the stroma of the chloroplast where the cycle actually takes place. This first step is called carbon fixation, which is catalyzed by an enzyme called rubisco. A molecule of CO2 is attached to a five-carbon sugar called RuBP, or ribulose biphosphate. Now the sugar is a six-carbon molecule, which immediately breaks down into two three-carbon molecules. Through reduction and a series of chemical reactions powered by ATP and NADPH, a molecule called G3P or PGAL is formed. This is the three-carbon sugar produced from the Calvin cycle, and it is NOT glucose. Glucose is a product that can be FORMED from G3P molecules. It’s important to notice that NADPH and ATP power the reduction of PGA into G3P, resulting in NADP+ and ADP. We can see that both the light reactions and the Calvin cycle therefore depend on each other because the products of one process powers the other. So if one shut down, the other would, too.
Finally, other G3P molecules regenerate RuBP, allowing the cycle to continue again.