This video is part of an epic collab at hashtag Team Trees, but more on that later. Hey Crazies. Most of you probably get the basic idea of photosynthesis. Plants pull water up from their roots, inhale some carbon dioxide from the air, and use energy from the Sun to power a chemical reaction. It’s basically how they eat. But, like a lot of things in biochemistry, it’s a little more complicated than that. Photosynthesis is wild. Actually, plants are not the only thing that can photosynthesize. Yeah yeah, I know. Organisms like algae and cyanobacteria can do it too. Photosynthesis has been around a lot longer than plants have. It’s just easier to talk about plants because they’re more familiar to people. Hashtag Team Trees! Alright, some of you may not know this, but I’m married to a biologist. She helped me a lot with this video, so if I get anything wrong, it’s her fault. What was that? It’s my fault. It’s always my fault. That’s what I thought! Anyway, something I’ve learned from our conversations is that, in biology, there’s always a deeper level where we can ask more questions. Think of them as various levels of abstraction. At the top, you’ll find our original explanation for photosynthesis. Plants use water, carbon dioxide, and sunlight to make food. Down at the bottom is the full atomic level, which is usually unnecessary. And often overwhelming to the point of uselessness. As a physicist, I like to see how deep I can go before that happens, so let’s do this. So far, photosynthesis is like one big mystery box. The water, carbon dioxide, and sunlight go in and then glucose and oxygen come out. That glucose is a sugar and sugar equals food. But this visual is pretty misleading. It gives you the impression that, if you have a bunch of water and carbon dioxide, and you shine some light on it, then the molecules will just rearrange themselves, which is false. If that were true, you could get the same reaction to happen by just shining a light on this carbonated water. The truth is none of those molecules can absorb enough light to react like that. They need help. A lot of help. There’s a lot going on in this box. Let’s go deeper. In reality, the water and the carbon dioxide aren’t even used at the same stage. Stage 1 is where water and light come together to make oxygen and some energy. The oxygen is released, some of which makes it out to the atmosphere. The energy is passed on to stage 2, where it’s used to transform carbon dioxide into something called a sugar precursor. It isn’t actually turned into the sugar glucose until stage 3. Technically speaking, that sugar precursor can become all sorts of sugars, not just glucose. Are you going to be on my case the whole video?! If you keep messing up, yes. Ugh! He’s right, as usual, but this process is going to be complicated enough. I’ve got to draw the line somewhere. So, for the rest of this video, stage 3 makes glucose. Just go with it. We’re going to focus most of our attention on stage 1. Because that’s where light goes in and oxygen comes out. "Photo-" means light, so we should focus on the light reactions. To understand how the heck that works, you
know where we have to go. Deeper! The process that happens in stage 1 is called the z-scheme because some of it’s parts kind of look like the letter Z. There are 5 separate parts, each taking in some particles and spitting out others. I warned you this was going to get involved, but it’s worth it, I promise. Part A splits some water into an oxygen and
a bunch of ions. That’s 4 electrons and 4 protons. Usually, those protons are labeled as ionized Hydrogen or H plus. But I’m a physicist, not a chemist. I’m just going to call it what it is: a proton. As I said before, the oxygen is simply released, but most of those ions are going to get used in the rest of stage 1. Part B is where the magic happens. Light comes in and gives up its energy to an electron, knocking it loose. One of the electrons from part A quickly falls in to fill the gap left behind. The loose electron is free to move on to part C. How the heck does that thing work? Chlorophyll. Deeper! Each plant cell is filled with little green packets called chloroplasts. Chlorophyll being inside Chloroplasts? That’s not a coincidence. Biology is real big on prefixes and suffixes. Inside each chloroplast, there are even smaller pancake things. This is where stage 1 is actually happening. On the surface, you’ll find a lump of of various pigment molecules. Many of which are chlorophyll, which give plants their distinctive green color. Each of those pigments can absorb a different color of light. As you can imagine, the more light a plant
can absorb, the better off it is, so the more variety of pigments the better. A pair of special chlorophyll hang out in the central opening. That’s where our electron is attached. Light comes in, the appropriate pigment absorbs it, and that energy gets passed around until it gets to that middle chlorophyll. The electron is knocked loose and another falls in to fill the gap. The mechanism is actually kinda cool. We call it a photosystem and there are two photosystems in stage 1 of photosynthesis. So what happens with that loose electron? It’s got some excess energy we can use. That electron hops from molecule to molecule in part C, making biological energy along the way. Well, it’s not like it’s loose energy floating around. That’s not a thing. Energy is just a property. But, in biology, the word almost always refers to ATP. A biologically useful molecule that stores energy. This molecule is constructed from stuff already floating around nearby. The energy from the electron is given up to combine those parts into ATP. That’s all part C does. It makes ATP. Part D is another photosystem. Light comes in, an electron is knocked loose, and the electron from part C falls in to take its place. Part E is another assembly machine. But, instead of assembling ATP, this one assembles something called NADPH. Biologist and their acronyms. Just think of it like a little bus that carries
electrons around. Seriously, they call it an electron bus. It just helps the cell move the electrons over to stage 2. That’s it. Nothing crazy. This is what stage 1 looks like overall. For simplicity, we’re only showing the molecules that get moved around. Unfortunately, for stage 2 to actually work properly, we need 3 of these to be running simultaneously. That’s the only way to give stage 2 all the energy and ions it needs to work. And that’s where the carbon dioxide comes in. Remember the carbon dioxide? Plants can’t grow without carbon. Stage 2 happens in the main space of those green packets inside the cell. Three carbon dioxide molecules come in along with a bunch of energy and charge from stage 1. Through a process called the Calvin cycle, those molecules are turned into CGP. I mean, G3P. That’s the sugar precursor we talked about earlier, which will be made into glucose in stage 3. Wait a minute! It takes two of those G3P molecules to make a glucose. That means the cell needs six stage ones and two stage twos just to make one glucose. That’s forking ridiculous! So how do photosynthesis? Well, plants and other photosynthsizing organisms use water, carbon dioxide, and sunlight to literally make their food. This process is nowhere near simple though. A whole bunch of complex chemical processes are used. Some molecules need to be added to the system, but others continuously cycle around. All just to make sugar molecules that allow them to grow and continue to exist. So, were you prepared for the complexity of that process? Let us know in the comments. Thanks for liking and sharing this video. Don’t forget to subscribe if you’d like to keep up with us. And until next time, remember, it’s OK to be a little crazy. This video is part of an epic collab at hashtag Team Trees. So, crazies, let’s talk. Global warming, climate change, whatever you want to call it. It’s the greatest challenge humans have ever faced and it’s our fault it’s even happening. Speaking of challenges, back in May 2019, Reddit challenged Mr. Beast to plant 20 million trees as a celebration for hitting 20 million subscribers. And, to literally nobody’s surprise, he was crazy enough to attempt it. Unfortunately, that’s a lot of trees and he can’t possibly do this by himself. So he’s partnered with the Arbor Day Foundation and recruited hundreds of creators across multiple genres to make it happen. Myself included. Photosynthesis is still the best control we have for carbon dioxide levels in the atmosphere, so the more trees the better. For every dollar we donate to the Arbor Day Foundation by January 1st, 2020; they’ll plant a single tree in a forest that needs them. One dollar equals one tree. Ten dollars equals ten trees. A hundred dollars equals a hundred trees! Of course, that means 20 million trees is going to take 20 million dollars. Hence needing to recruit all of us to spread the word. Just so you know I’m putting my money where my mouth is, I’ll be donating $100 to the cause. If you can donate, there’s a link in the doobly-doo. If you can’t donate, that’s fine too. Just share this video and others like it to spread the word. Let’s do this! Hashtag Team Trees! This was a special video today, so no featured comment. I’ll get to those next time when I get back to my normal routine. Thanks for watching.
To answer his question, yes, I was prepared for the complexity of the process. I was not prepared for the inanity of the presentation.