Photosynthesis: Crash Course Biology

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Photosynthesis! It is not some kind of abstract scientific thing. You would be dead without plants and their magical- nay, SCIENTIFIC ability to convert sunlight, carbon dioxide and water into glucose and pure, delicious oxygen. This happens exclusively through photosynthesis, a process that was developed 450 million years ago and actually rather sucks. It’s complicated, inefficient and confusing. But you are committed to having a better, deeper understanding of our world! Or, more probably, you’d like to do well on your test…so let’s delve. There are two sorts of reactions in Photosynthesis…light dependent reactions, and light independent reactions, and you’ve probably already figured out the difference between those two, so that’s nice.

The light independent reactions are called the “calvin cycle” no…no…no…no…YES! THAT Calvin Cycle. Photosynthesis is basically respiration in reverse, and we’ve already covered respiration, so maybe you should just go watch that video backwards. Or you can keep watching this one. Either way. I’ve already talked about what photosynthesis needs in order to work: water, carbon dioxide and sunlight. So, how do they get those things? First, water. Let’s assume that we’re talking about a vascular plant here, that’s the kind of plant that has pipe-like tissues that conduct water, minerals and other materials to different parts of the plant. These are like trees and grasses and flowering plants. In this case the roots of the plants absorb water and bring it to the leaves through tissues called xylem. Carbon dioxide gets in and oxygen gets out through tiny pores in the leaves called stomata. It’s actually surprisingly important that plants keep oxygen levels low inside of their leaves for reasons that we will get into later. And finally, individual photons from the Sun are absorbed in the plant by a pigment called chlorophyll.

Alright, you remember plant cells? If not, you can go watch the video where we spend the whole time talking about plant cells. One thing that plant cells have that animal cells don’t… plastids. And what is the most important plastid? The chloroplast! Which is not, as it is sometimes portrayed, just a big fat sac of chlorophyl. It’s got complicated internal structure. Now, the chlorophyll is stashed in membranous sacs called thylakoids. The thykaloids are stacked into grana. Inside of the thykaloid is the lumen, and outside the thykaloid (but still inside the chloroplast) is the stroma. The thylakoid membranes are phospholipid bilayers, which, if you remember means they’re really good at maintaining concentration gradients of ions, proteins and other things. This means keeping the concentration higher on one side than the other of the membrane.

You’re going to need to know all of these things, I’m sorry. Now that we’ve taken that little tour of the Chloroplast, it’s time to get down to the actual chemistry. First thing that happens: A photon created by the fusion reactions of our sun is about to end its 93 million mile journey by slapping into a molecule of cholorophyll. This kicks off stage one, the light-dependent reactions proving that, yes, nearly all life on our planet is fusion-powered. When Chlorophyll gets hit by that photon, an electron absorbs that energy and gets excited. This is the technical term for electrons gaining energy and not having anywhere to put it and when it’s done by a photon it’s called photoexcitation, but let’s just imagine, for the moment anyway, that every photon is whatever dreamy young man 12 year old girls are currently obsessed with, and electrons are 12 year old girls.

The trick now, and the entire trick of photosynthesis, is to convert the energy of those 12 year old- I mean, electrons, into something that the plant can use. We are literally going to be spending the entire rest of the video talking about that. I hope that that’s ok with you. This first Chlorophyll is not on its own here, it’s part of an insanely complicated complex of proteins, lipids, and other molecules called Photosystem II that contains at least 99 different chemicals including over 30 individual chlorophyll molecules. This is the first of four protein complexes that plants need for the light dependent reactions. And if you think it’s complicated that we call the first complex photosystem II instead of Photosystem I, then you’re welcome to call it by its full name, plastoquinone oxidoreductase. Oh, no? You don’t want to call it that? Right then, photosystem II, or, if you want to be brief, PSII.

PSII and indeed all of the protein complexes in the light-dependent reactions, straddle the membrane of the thylakoids in the chloroplasts. That excited electron is now going to go on a journey designed to extract all of its new energy and convert that energy into useful stuff. This is called the electron transport chain, in which energized electrons lose their energy in a series of reactions that capture the energy necessary to keep life living. PSII’s Chlorophyll now has this electron that is so excited that, when a special protein designed specifically for stealing electrons shows up, the electron actually leaps off of the chlorophyll molecule onto the protein, which we call a mobile electron carrier because it’s… …a mobile electron carrier. The Chlorophyll then freaks out like a mother who has just had her 12 year old daughter abducted by a teen idol and is like “WHAT DO I DO TO FIX THIS PROBLEM!” and then it, in cooperation with the rest of PSII does something so amazing and important that I can barely believe that it keeps happening every day.

It splits that ultra-stable molecule, H2O, stealing one of its electrons, to replenish the one it lost. The byproducts of this water splitting? Hydrogen ions, which are just single protons, and oxygen. Sweet, sweet oxygen. This reaction, my friends, is the reason that we can breathe. Brief interjection: Next time someone says that they don’t like it when there are chemicals in their food, please remind them that all life is made of chemicals and would they please stop pretending that the word chemical is somehow a synonym for carcinogen! Because, I mean, think about how chlorophyll feels when you say that! It spends all of it’s time and energy creating the air we breathe and then we’re like “EW! CHEMICALS ARE SO GROSS!” Now, remember, all energized electrons from PSII have been picked up by electron carriers and are now being transported onto our second protein complex the Cytochrome Complex! This little guy does two things.

..one, it serves as an intermediary between PSII and PS I and, two, uses a bit of the energy from the electron to pump another proton into the thylakoid. So the thylakoid’s starting to fill up with protons. We’ve created some by splitting water, and we moved one in using the Cytochrome complex. But why are we doing this? Well…basically, what we’re doing, is charging the Thylakoid like a battery. By pumping the thylakoid full of protons, we’re creating a concentration gradient. The protons then naturally want to get the heck away from each other, and so they push their way through an enzyme straddling the thylakoid membrane called ATP Synthase, and that enzyme uses that energy to pack an inorganic phosphate onto ADP, making ATP: the big daddy of cellular energy. All this moving along the electron transport chain requires energy, and as you might expect electrons are entering lower and lower energy states as we move along.

This makes sense when you think about it. It’s been a long while since those photons zapped us, and we’ve been pumping hydrogen ions to create ATP and splitting water and jumping onto different molecules and I’m tired just talking about it. Luckily, as 450 million years of evolution would have it, our electron is now about to be re-energized upon delivery to Photosystem I! So, PS I is a similar mix of proteins and chlorophyll molecules that we saw in PSII, but with some different products. After a couple of photons re-excite a couple of electrons, the electrons pop off, and hitch a ride onto another electron carrier. This time, all of that energy will be used to help make NADPH, which, like ATP, exists solely to carry energy around. Here, yet another enzyme helps combine two electrons and one hydrogen ion with a little something called NADP+.

As you may recall from our recent talk about respiration, there are these sort of distant cousins of B vitamins that are crucial to energy conversion. And in photosynthesis, it’s NADP+, and when it takes on those 2 electrons and one hydrogen ion, it becomes NADPH. So, what we’re left with now, after the light dependent reactions is chemical energy in the form of ATPs and NADPHs. And also of course, we should not forget the most useful useless byproduct in the history of useless byproducts…oxygen. If anyone needs a potty break, now would be a good time…or if you want to go re-watch that rather long and complicated bit about light dependent reactions, go ahead and do that…it’s not simple, and it’s not going to get any simpler from here. Because now we’re moving along to the Calvin Cycle! The Calvin Cycle is sometimes called the dark reactions, which is kind of a misnomer, because they generally don’t occur in the dark.

They occur in the day along with the rest of the reactions, but they don’t require energy from photons. So it’s more proper to say light-independent. Or, if you’re feeling non-descriptive…just say Stage 2. Stage 2 is all about using the energy from those ATPs and NADPHs that we created in Stage 1 to produce something actually useful for the plant. The Calvin Cycle begins in the stroma, the empty space in the chloroplast, if you remember correctly. And this phase is called carbon fixation because…yeah, we’re about to fix a CO2 molecule onto our starting point, Ribulose Bisphosphate or RuBP, which is always around in the chloroplast because, not only is it the starting point of the Calvin Cycle, it’s also the end-point… which is why it’s a cycle. CO2 is fixed to RuBP with the help of an enzyme called ribulose 1,5 bisphosphate carboxylase oxidase, which we generally shorten to RuBisCo. I’m in the chair again! Excellent! This time for a Biolo-graphy of RuBisCo. Once upon a time, a one-celled organism was like “Man, I need more carbon so I can make more little me’s so I can take over the whole world.

” Luckily for that little organism, there was a lot of CO2 in the atmosphere, and so it evolved an enzyme that could suck up that CO2 and convert inorganic carbon into organic carbon. This enzyme was called RuBisCo, and it wasn’t particularly good at its job, but it was a heck of a lot better than just hoping to run into some chemically formed organic carbon, so the organism just made a ton of it to make up for how bad it was. Not only did the little plant stick with it, it took over the entire planet, rapidly becoming the dominant form of life. Slowly, through other reactions, known as the light dependent reactions, plants increased the amount of oxygen in the atmosphere. RuBisCo, having been designed in a world with tiny amounts of oxygen in the atmosphere, started getting confused.

As often as half the time RuBisCo started slicing Ribulose Bisphosphate with Oxygen instead of CO2, creating a toxic byproduct that plants then had to deal with in creative and specialized ways. This byproduct, called phosphogycolate, is believed to tinker with some enzyme functions, including some involved in the Calvin cycle, so plants have to make other enzymes that break it down into an amino acid (glycine), and some compounds that are actually useful to the Calvin cycle. But plants had already sort of gone all-in on the RuBisCo strategy and, to this day, they have to produce huge amounts of it (scientists estimate that at any given time there are about 40 billion tons of RuBisCo on the planet) and plants just deal with that toxic byproduct. Another example, my friends, of unintelligent design. Back to the cycle! So Ribulose Bisphosphate gets a CO2 slammed onto it and then immediately the whole thing gets crazy unstable. The only way to regain stability is for this new six-carbon chain to break apart creating two molecules of 3-Phosphoglycerate, and these are the first stable products of the calvin cycle.

For reasons that will become clear in a moment, we’re actually going to do this to three molecules of RuBP. Now we enter the second phase, Reduction. Here, we need some energy. So some ATP slams a phosphate group onto the 3-Phosphoglycerate, and then NADPH pops some electrons on and, voila, we have two molecules of Glyceraldehyde 3-Phosphate, or G3P, this is a high-energy, 3-carbon compound that plants can convert into pretty much any carbohydrate. Like glucose for short term energy storage, cellulose for structure, starch for long-term storage. And because of this, G3P is considered the ultimate product of photosynthesis. However, unfortunately, this is not the end. We need 5 G3Ps to regenerate the 3 RuBPs that we started with. We also need 9 molecules of ATP and 6 molecules of NADPH, so with all of these chemical reactions, all of this chemical energy, we can convert 3 RuBPs into 6 G3Ps but only one of those G3Ps gets to leave the cycle, the other G3Ps, of course, being needed to regenerate the original 3 Ribulose Bisphosphates.

That regeneration is the last phase of the Calvin Cycle. And that is how plants turn sunlight, water, and carbon dioxide into every living thing you’ve ever talked to, played with, climbed on, loved, hated, or eaten. Not bad, plants. I hope you understand. If you don’t, not only do we have some selected references below that you can check out, but of course, you can go re-watch anything that you didn’t get and hopefully, upon review, it will make a little bit more sense. Thank you for watching. If you have questions, please leave them down in the comments below..

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