Previous: What Do These Creepy Plant Mouths Do? (Plant Tissues): Crash Course Botany #4
Next: What Biologists Do: Crash Course Biology #3



View count:145,198
Last sync:2024-06-25 06:45


Citation formatting is not guaranteed to be accurate.
MLA Full: "Photosynthesis and Cellular Respiration: Crash Course Botany #5." YouTube, uploaded by CrashCourse, 22 June 2023,
MLA Inline: (CrashCourse, 2023)
APA Full: CrashCourse. (2023, June 22). Photosynthesis and Cellular Respiration: Crash Course Botany #5 [Video]. YouTube.
APA Inline: (CrashCourse, 2023)
Chicago Full: CrashCourse, "Photosynthesis and Cellular Respiration: Crash Course Botany #5.", June 22, 2023, YouTube, 13:00,
Plants and trees may seem pretty passive, but behind the scenes, their cells are working hard to put on a magic show. In this episode of Crash Course Botany, we’ll explore how the processes of photosynthesis and cellular respiration work, why they’re so critical for all life on Earth, and how they’re helping us to forge a greener path to the future.

Plants' Magic Show 00:00
Photosynthesis 1:27
The Light-Dependent Reactions 3:02
The Light-Independent Reactions 4:18
Cellular Respiration 5:32
Biofuels 8:50
Review & Credits 11:43

Special thanks to Hannah Bodenhausen for additional post-production support on this episode!


Crash Course is on Patreon! You can support us directly by signing up at

Thanks to the following patrons for their generous monthly contributions that help keep Crash Course free for everyone forever:
David Fanska, Andrew Woods, Tawny Whaley, Sean Saunders, Katie, DL Singfield, Ken Davidian, Stephen Akuffo, Toni Miles, Steve Segreto, Kyle & Katherine Callahan, Laurel Stevens, Burt Humburg, Aziz Y, Perry Joyce, Scott Harrison, Mark & Susan Billian, Alan Bridgeman, Rachel Creager, Breanna Bosso, Matt Curls, Jennifer Killen, Jon Allen, Sarah & Nathan Catchings, team dorsey, Trevin Beattie, Eric Koslow, Indija-ka Siriwardena, Jason Rostoker, Siobhán, Ken Penttinen, Nathan Taylor, Les Aker, William McGraw, ClareG, Rizwan Kassim, Constance Urist, Alex Hackman, Pineapples of Solidarity, Katie Dean, Stephen McCandless, Wai Jack Sin, Ian Dundore, Mark, Caleb Weeks.

Want to find Crash Course elsewhere on the internet?
Instagram -
Facebook -
Twitter -

CC Kids:
Plants… grow.

Okay, yeah, you probably knew that. In fact, you also probably know that plants grow because they make their own food from a combination of water, sunlight, and gases from the air.

To those of us who have to grocery-shop for food, photosynthesis seems almost like a magic trick. [magic noises] But there’s a lot of work going on behind the scenes. Inside many, many cells, inside every single leaf, of every single plant on Earth is a cranking, shunting, thundering workshop that never stops. These living workshops are consumed with photosynthesis and cellular respiration— processes dedicated to making, breaking, and reshaping molecules with the singular purpose of keeping a plant alive.

Though, we also have these processes to thank for our beautiful green planet, and all the creatures who call it home. And plants may be structured perfectly for these intricate tasks, but they can’t do it all on their own. This magic show includes audience participation —that’s us.

Hi! I’m Alexis, and this is Crash Course Botany. [THEME MUSIC] When you consider the amount of living stuff on this planet, plants really do come out on top. They make up about 80% of the world’s biomass, or the total weight of living organisms.

But where does all that heft actually come from? You might initially think that soil plays a big part. After all, most plants grow in soil, and it’s similar to the solid, heavy stuff that we use to make building materials.

But the botanical world actually does something much more remarkable. The building blocks for all that biomass come predominantly from the air. In this biological magic act, plants pull microscopic carbon molecules from the atmosphere, which become the major materials for building plant bodies, and then —alakazam!— we have a plant kingdom to rule the world.

Okay, it’s not that simple — I’ll break it down for you bit by bit. You’ll recognize this first trick as the process of photosynthesis. The word comes from Greek and basically means “put together with light,” which is a fairly accurate description of what’s going on.

Nearly all of Earth’s energy comes from the Sun. So in photosynthesis, plant cells combine a zap of that energy with carbon dioxide pulled from the air and water. The end products of this chemical reaction are the sugar glucose that plants use as food, and the byproduct, or unintended extra product, oxygen.

There are two stages to the photosynthesis process: the “photo” part, and the “synthesis” part. The “photo” part is more accurately known as the light-dependent reactions because, as the name suggests, they can only happen with the help of light energy. Here, tiny organelles called chloroplasts inside the cells of leaves take center stage.

They contain the green pigment chlorophyll, which is able to capture the light energy that falls on it, and then harness that energy to do work in the plant. So you can think of chloroplasts a bit like the original solar panels —taking energy from the Sun and converting it to a usable form. With the help of light energy, water molecules are split apart, generating spare oxygen in the process.

And to be clear: the goal of photosynthesis is to help plants thrive, full stop. It’s really not to make oxygen for us humans to breathe. But on the other hand, we help plants accomplish photosynthesis, too.

Carbon dioxide is released into the air in a few different ways, including as an unintentional byproduct for us when we exhale. That CO2 is essential to the “photo” part of photosynthesis. So we’re not just volunteers ready to “pick a card, any card.” We’re full-fledged magician’s assistants.

Does this mean I get a cape? Okay, so with the energy trapped, water split, and some CO2 absorbed, it’s time for the “synthesis” phase of photosynthesis, more commonly known as the light-independent reactions, which don’t need any input from the Sun. At this point, the energy that’s already been captured is used, along with the split water, to convert carbon dioxide into glucose, completing the reaction.

Can we get some applause, please? [Poof] Plants don’t usually disappear in a puff of smoke at the end. Although this final stage doesn’t need light, it does need the products of the light-dependent reactions, which don’t last for very long inside plant cells. So photosynthesis really only happens during the day.

In any case, the whole point of photosynthesis is to make food for the plant in the form of glucose, and those small sugar molecules have  several different potential  fates inside the plant. Some of them are used as the building blocks for much larger cellulose molecules, which surround every cell in the plant and are the main construction material for roots, stems, and leaves. And some are converted to other types of molecules, like sucrose and starch, that can save energy for later.

The rest become fuel for the other major function of the cellular workshop: cellular respiration. By the way, cellular respiration isn’t unique to plants. Every living thing does it, including you and me.

It’s happening all the time – day and night, and, in plants’ case, often at the same time as photosynthesis. Cellular respiration is the process organisms use to transfer the energy they get from food into the usable energy we  need to do everything we do, like hiking up a mountain, taking a test, or plotting elaborate plant-related fan-fiction. Oh, is that just me?

The only difference between cellular respiration in animals like us and in plants is that we need to eat food to get our glucose fuel, whereas plants can make their own. And because plants can do that, they give us things to eat— from plants themselves, to the animals that eat them. If it weren’t for photosynthesis converting energy from the Sun to a form usable — AKA edible — to us, there’d be no food chains at all.

And we wouldn’t be able to power our own cellular respiration process. So, for plants and people alike, cellular respiration happens in organelles called the mitochondria, often referred to (affectionately, or not) as the powerhouses of the cell. What that old cliché actually means is that mitochondria acquire the energy needed to power cells’ chemical reactions.

They do this by breaking down raw fuel to make special molecules known as ATP. ATP are like rechargeable batteries — you can fill them with energy again and again, which can be used to power life-sustaining reactions anywhere in the plant. And actually, in us, too.

And in every organism on Earth. There are two main ways that plants can generate these energy-storing molecules. The first, aerobic respiration, is the most common, and the “aerobic” part of its name tells us that it involves oxygen.

This kind of respiration is a reversal of the photosynthesis reaction: glucose and oxygen combine to release energy, with water and carbon dioxide as waste products, instead of the other way around. That released energy can be used to make stuff happen in the plant on a microscopic level. But that work translates into things we can observe like flowers growing, fruit ripening, or leaves arching toward the Sun.

And it’s efficient. Through aerobic respiration, each molecule of glucose can make over thirty molecules of ATP. That’s like the energy you get from a nice, big breakfast —a warm bowl of oatmeal, some avocado toast, and half a grapefruit.

Mmm. The other way that plants respire is by burning their glucose fuel without oxygen around, in a process known as anaerobic respiration. This is much less efficient than the aerobic version, producing only two molecules of ATP for each glucose molecule used.

That’s more like a running-out-the-door- granola-bar kind of breakfast. But it can be crucial for plants that don’t have much oxygen available. Roots growing in water-logged soil can be heroes by using anaerobic respiration.

Aside from providing us with oxygen to breathe, and the sugar molecules in everything we eat, the living workshops inside plant cells have yet another important application to us humans. They produce biomass that can be used as biofuel. For example, around the world, more than 180 million tons of tomatoes are produced every year.

In my ideal world, these would all go towards making delicious salads, pasta sauces, and ketchup. But in reality, not every tomato makes the grade. Some are too wonky for our supermarket shelves, and many are rejected as waste from various sauce-making processes.

Enter environmental engineer Dr. Venkataramana Gadhamshetty and his team in South Dakota, USA. In 2016, they and their collaborators figured out how to give these poor, discarded tomatoes a new lease on life, by using them to generate electricity!

It works a bit like the lemon or potato batteries you might have made at school. An electrochemical device called a fuel cell helps to break down the tomato waste and extract electrons, making an electric current. It turns out that tomato pulp is an ideal fuel for this because it has loads of high-energy sugars —sugars that were originally  created by photosynthesis!

So not only are plants perfect for photosynthesis, but some plants, like tomatoes, corn, and soybeans, seem to be perfect for generating electricity, too. And we’re already using the energy stored in biomass today —every time we burn wood in an open fire, or charcoal on a barbecue. That energy came from the Sun, was made usable by photosynthesis, and then, rather than eating it, we converted it into electricity to power our lives.

In the U. S., up to 10% of the gas we put in our cars contains a biofuel called ethanol, which comes from corn, and other fuels from plants are increasingly being used for heating and generating electricity. Biofuels are burned in the same way as fossil fuels, and they do release carbon dioxide into the air.

The difference is that the carbon dioxide biofuels release was already in the air until very recently, when a living plant worked its photosynthesis magic to turn the CO2 into glucose. By contrast, when we burn traditional fossil fuels, we’re releasing carbon that’s been locked up for millions of years. So the magic of burning biofuels is that there’s practically no overall increase in the amount of carbon dioxide in the atmosphere – which is good news for anyone looking to slow down the pace of climate change.

While there are pros and cons to any energy source, it’s helpful to have a variety of options to work with, and biofuels are just one of them, thanks to the power of photosynthesis. So, the next time you come across a plant, whether it’s a mighty oak tree or a tiny daisy, spare a thought for the relentless chemical industry that’s going on behind the scenes. Photosynthesis and cellular respiration may seem as simple as some slight-of-hand, but those amazing tricks are the result of super-efficient processes that plants are doing all the time.

And we get to be a part of that. We may rely on plants’ power of photosynthesis, but we get to contribute carbon dioxide when we exhale stinky garlic breath, and plants make it something beautiful. Next time, we’ll uproot the great tree of life to discover how plants evolved into the incredible organisms we know today.

Hey, before we go, let’s branch out! What organisms make up the next largest percentage of Earth’s biomass, after plants? Look closely to find the answer in the comments, ‘cause they’re pretty tiny!

Thanks for watching this episode of Crash Course Botany which was filmed at the Damir Ferizović Studio and made in partnership with PBS Digital Studios and Nature. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.