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Last sync:2023-11-09 12:30
Hank explains the extremely complex series of reactions whereby plants feed themselves on sunlight, carbon dioxide and water, and also create some by products we're pretty fond of as well.

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Table of Contents:
1) Water 1:16
2) Carbon Dioxide 1:32
3) Sunlight/Photons 1:43
4) Chloroplasts 1:57
5) Light Reaction/Light-Dependent 2:42
a. Photosystem II 3:33
b. Cytochrome Complex 5:54
c. ATP Synthase 6:16
d. Photosystem I 7:06
6) Dark Reactions/Light-Independent 7:55
a. Phase 1 - Carbon Fixation 8:50
b. Phase 2 - Reduction 11:31
c. Phase 3 - Regeneration 12:02

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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.

(Intro Music Plays)

There are two sorts of reactions in photosynthesis. The light-dependent reactions, and the 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 could 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?

 1) Water (1:16)

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 the water and bring it to the leaves through tissues called xylem.

 2) Carbon dioxide (1:32)

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.

 3) Sunlight/Photons (1:43)

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.

 4) Chloroplasts (1:57)

One thing that plants 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 sack of chlorophyll. It's got a complicated, internal structure.

Now the chlorophyll is stashed in membranous sacks called thylakoids, and the thylakoids are stacked into grana. Inside the thylakoid is the lumen and outside of the thylakoid, but still inside of the chloroplast, is the stroma.

The thylakoid membranes are phospholipid bilayers, which, if you remember, means that they are really good at maintaining concentration gradients of ions and 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 these things. I'm sorry.

Now that we've taken our little tour of the Chloroplast, it's time to get down to the actual chemistry. 

 5) Light Reaction/Light-Independent (2:42)

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 chlorophyll. 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 twelve-year-old girls are currently obsessed with and electrons are twelve-year-old girls.

The trick now, and the entire trick of photosynthesis is to convert the energy of those twelve-year, I mean electrons, into something that the plant can use. We are literally going be spending the entire rest of the video talking about that I hope that that's ok with you. 

 a. Photosystem II (3:33)

Now first, chlorophyll is not on its own here. It's part of an insanely complicated complex of proteins and 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, which is 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.

Now, that excited electron is 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.

So 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 twelve-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 Photosystem II, 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 by-products 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 its time and energy creating the air we breathe and we're like "Ewww chemicals are so grosssss."

 b. Cytochrome Complex (5:54)

Now, remember, all energized electrons from PSII have been picked up by electron carriers and are now being transported onto the second protein complex the Cytochrome Complex! This little guy does two, it serves as an intermediary between PSII and PS I and, two, uses a little bit of the energy from the electron to pump another proton into the thylakoid. 

 c. ATP Synthase (6:16)

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. 

 d. Photosystem I (7:06)

Luckily, as 450 million years of evolution would have it, our electron is now about to get re-energized upon delivery to Photosystem I. So, PSI 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 out 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 two electrons and one hydrogen ion, it becomes NADPH. 

 6) Dark Reactions/Light-Independent (7:55)

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. 

 a) Phase 1 - Carbon Fixation (8:50)

The Calvin Cycle begins in the stroma, or the empty space inside of 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. 

(9:13) CO2 is fixed to RuBP with the help of an enzyme called ribulose-1,5-bisphosphate carboxylase oxidase which we generally shorten to RuBisCo.

(9:28) I'm in the chair again! Excellent! This time for a Bio-lography 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.

(10:06) 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 of the time, RuBisCo started slicing ribulose bisphosphate with oxygen, instead of CO2, creating a toxic by-product that plants had to deal with in creative and specialized ways.

(10:36) This by-product, called phosphoglycolate, 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 million tonnes of RuBisCo and plants just deal with that toxic by-product. Another example, my friends, of unintelligent design. Back to the cycle!

(11:11)  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 6-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 this to three molecules of RuBP

  b) Phase 2 - Reduction (11:31)

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 VIOLA we have two molecules of glyceraldehyde-3-phosphate, or G3P. This is a high-energy, three-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. Because of this, G3P is considered the ultimate product of photosynthesis. 

 c) Phase 3 - Regeneration (12:02)

However, unfortunately, this is not the end. We need five G3Ps to regenerate the 3 RuBPS that we started with. We also need nine molecules of ATP and six molecules of NADPH. So with all these chemical reactions, all of this chemical energy, we can convert three RuBPS into six G3Ps but only one of those G3Ps gets to leave the cycle. The other G3Ps, of course, being needed to regenerate the original three ribulose bisphosphonates.

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. 

 Conclusion (12:44)

Now 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, will make a little bit more sense. Thank you for watching. If you have questions, please leave them down in the comments below.