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Cells need energy to power the chemical reactions that keep their microscopic cities running, and most of that energy comes from a chemical called ATP. In this episode of Crash Course Biology, we’ll learn how our cells use energy, what an enzyme’s role is in chemical reactions, and what makes a reaction exergonic or endergonic.

Cellular Cities 00:00
What Is Energy? 1:18
The Laws of Thermodynamics 2:22
ATP 4:07
Chemical Reactions 6:42
Enzymes 9:13
Metabolic Pathways 11:04
Review & Credits 12:09

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CC Kids:
Cells are like microscopic cities.

They hustle and bustle with work to be done, and things that need to get  from point A to point B. And cities of all kinds need energy to run a whole range of important processes.

We use energy to drive our cars, heat our homes, and even charge our cell phones. Though mine is always just clinging to life at five percent. Like cities, cells use energy to carry out a variety of functions — all of the bodily functions, in fact.

And all the functions of all  living organisms on Earth. But before we can understand how and why cells harness energy, we need to take a look at what exactly energy is. Sure, it’s the stuff that comes out of our walls to power our homes and let John Green Bot write his next book “The Anthropocene Rebooted.” But what is it, really?

And what does something as seemingly abstract as energy have to do with biology? Hi! I'm Dr.

Sammy, your friendly neighborhood entomologist, and this is Crash Course Biolo— [cell phone rings] Who is this calling me right  in the middle of a shoot? Sorry guys, I gotta take  this, it’s the theme music. [THEME MUSIC] Now, when I say the word “energy” you might picture a lot of things. Power plants, lightning, batteries.

Those are physical representations of the distinctly “non-physical” thing that is energy. Energy is not made of matter, but it does affect matter. We scientists define energy as the ability to cause change, specifically in the form of work.

And the scientific definition of “work” is to move an object over a given distance. So when I say that you’re over there werking it on the dance floor, I mean that scientifically too. Most of the usable energy on Earth comes from the Sun.

But most animals like us can’t use it right away. That’s where plants come in. Through photosynthesis, plants combine energy from the Sun with carbon dioxide and water to make sugars like glucose, which plants use as food.

It’s sort of like cooking with an oven. You don’t eat the heat from your oven, just like plants don’t eat sunlight, but they use it to transform ingredients into food they can eat. And when an animal eats that plant, some of the energy is transferred to the animal. [In a Hippie voice] So like truthishly, energy is like this whirling, swirling vibe that connects us all, bro. [Laughs] I’m kidding, — but it is kind of like that.

Because the laws of thermodynamics explain how the energy in the universe transforms and moves from one thing to another, operating as one big, connected system. So how does energy work? Well, the first law of thermodynamics is that energy can’t be created or destroyed.

Instead, it’s transformed  from one form to another,  or transferred from one thing to another. Like this stretched rubber band knocking over this plastic campfire. It worked!

Did it break? Hah, so things exchange both energy and matter with our surroundings, so when we use our energy — whether it’s to digest a burger, write a love note, or nearly break a prop — it’s not available for us to use anymore. But it’s not gone.

It's just been transferred to something else or transformed into something that we can’t use anymore to do our body’s work. So, when I eat that burger, I receive energy that the  cow got from eating grass, and that the grass got from the Sun. But I still can’t use that energy right away.

Because it’s chemical energy, which is a form of stored, or potential energy. You can think of the chemical  energy inside plants, or inside us, like a spring that’s coiled up, ready to sproing! Is that what a spring does?

A spring sproings? I feel like that’s right. Let’s fact-check it though.

Anyway, the point is, it has the potential to release energy, but only when it’s activated in the right way. When cells need that stored potential energy, chemical reactions have to unlock it from the sugar molecules and get the work of life done. And to do that, we — and all organisms on Earth — have to produce a special molecule called adenosine triphosphate, or ATP.

ATP is like a rechargeable battery — we can fill it with energy over and over again. And ATP is especially good at its job because of its unique chemical makeup. You see, it has three connected phosphate groups huddled together, each with negative charges.

Now, if you’ve ever tried to push the negative ends of magnets together, you’ll know that like-charges repel. Well, that same struggle is happening inside each ATP molecule. Those three negatively charged phosphate groups are all pushing against each other trying to get away.

This constant repelling makes ATP’s phosphate bonds pretty unstable, but it’s also why there’s so much potential energy in the molecule. Those bonds in ATP are easy to break, and when they do, that  potential energy gets released. One of those phosphate groups pops off like a rubber band where its energy can be transferred to other molecules in the cell.

Like, when you introduce our good friend H2O to ATP, ATP gets B-R-O-K-E, or, uh, broke. Water breaks ATP into two pieces, separating the phosphate bonds in a chemical reaction known as hydrolysis. When combined with other key reactions, this process releases potential energy to do whatever work is needed in the cell.

And there’s a lot of work to be done! Cells use chemical reactions to make all sorts of stuff they need, like hormones to talk to other cells, and even to construct their  own internal organelles. They also break down food and move chemicals across membranes.

So it’s not just the daily muscle movement that we’re aware of; it’s countless microscopic movements that are each essential to keep your body functioning. If a cell ever stops doing its chemical reactions, it’s officially dead. That’s actually the definition of dead that we use in science: the point at which an organism can’t continue those chemical reactions that create order.

And when a cell dies, that’s it – no save points, no phoenix down. It’s game over man; game over! And, by the way, your cells  are dying all the time.

In fact, you’ll lose about the equivalent of your body weight in dead cells every year. Ahh, Isn’t science fun!? But don’t worry, your cells  get replaced all the time.

It’s fine. You’ll be okay. OK, the second law of thermodynamics is that disorder, or what we call entropy, in the universe is always increasing.

It’s sort of like how your room always gets messy again, no matter how often you clean it – except in this case the dirty clothes are atoms and your room is the entire universe. After all, there are infinitely more ways for things to be disorganized than there are for them to be perfectly ordered. Now, chemical reactions  sometimes increase the entropy, or the number of ways the molecules can be arranged in the system.

This happens, for example, when bonds break so molecules are now attracted or repelled from other molecules in new ways. And in order for life to be ordered  at all, we need that chaos. You see, every chemical reaction is a balance between the changes in entropy and the changes in energy that happen during that reaction.

To find out why, let’s head  over to the Thought Bubble… It’s time for a Crash Course camping trip! In our campfire, wood and oxygen are our reactants, or the raw materials for our reaction. Once the fire gets going, the wood and oxygen undergo a chemical reaction to form new products, or the results of the reaction.

In this case: CO2, water, and other compounds. The energy contained in the bonds of the original reactant is more than the energy contained in the products. In other words, energy that  was stored in the orderly bonds  between molecules in the wood has been released.

Larger molecules are broken  into smaller molecules,  ripe for new combinations  — and entropy is increased. That makes this an exergonic, or energy-releasing reaction. Energy-releasing reactions  are considered spontaneous, but that doesn’t mean they’ll start on their own. “Spontaneous” just means that once exergonic reactions start, they don’t need outside energy to keep them going.

On the other hand, there are reactions that are not spontaneous; they require work. Take, for example, the  photosynthesis that occurred  in the tree before it became a part of our fire. In this endergonic reaction, energy is essentially absorbed rather than released.

The tree took molecules of carbon dioxide and water bouncing around in the air and soil and whipped them into shape: a very specific, woody shape. But whether it’s a tree converting sunlight into energy, or you dropping that tree’s hard-grown wood into a fire pit, every reaction needs activation energy —something to set it in motion initially, like the spark of a match. Thanks, Thought Bubble!

So, while all exergonic reactions do happen spontaneously, sometimes the required activation energy to get them started is such a lift that the actual reactions happen very slowly. Too slow, in fact, to keep our bodies alive and functioning if they happened on their own time. Thankfully, nature has a trick to speed up these reactions– proteins called enzymes.

Enzymes stick to reactants because they have a complementary shape, like puzzle pieces. Then, they catalyze reactions, speeding them up by doing things like pushing two reactants super close together, or putting stress on a bond that needs to break. By getting the chemicals  closer to their reactive form, the enzyme lowers the energy it takes to activate the reaction, making it happen much faster.

And the cool thing about these enzymes is they can be used over and over. The reaction that occurs doesn’t permanently  alter the enzyme, so it  just keeps going and going. Enzymes power reactions in more life functions than you can count, both in and outside of the human body.

One cool example: newborns need to eat a lot, so there is a special enzyme in breast milk that helps babies absorb fat more quickly. Or sometimes, enzymes help reactions team up in what’s called coupling. See, when two reactions love each other very much and they want to spend their lives together… wait, not that kind of couple.

When enzymes facilitate coupling, it allows the energy generated from an exergonic reaction to power an endergonic reaction. Sort of like a stationary bike that couples the kinetic energy of pedaling with electrical energy to charge your phone. Most living things on Earth rely on enzyme-facilitated coupling to break down the carbohydrate glucose and use it as a major source of energy.

And that’s just to name one example of many. Usually, we need more than one reaction to occur to build a complex molecule or to break it down. When a series of enzymes helps lower the activation costs of a series of chemical reactions in this way, we call it a metabolic pathway.

A metabolic pathway either breaks down or builds some specific biological molecule that the living thing needs for a particular function. For example, these pathways help assemble complex molecules, like nucleotides, which are the building blocks of our DNA. And the more we understand about how these processes come together, the better we’ll get at  fighting a number of diseases.

Cancer and diabetes, for example, are often treated in part with enzyme inhibitors — which work to slow or block enzymes that aren’t working as they should. And, amazingly, all living things have strikingly similar metabolic pathways — so when we learn about ourselves, we also learn about other species. And likewise, when we study other organisms, we learn more about ourselves [in a Hippie voice] and the energy we all share in this big, connected universe, man.

Just like energy powers work all throughout a city, energy in our cells powers cellular work, which keeps our bodies running in so many beautiful ways. Whether you’re playing  video games, doing homework,  or mourning your billions  and billions of dead cells, your body is using energy. Energy that’s involved in chemical reactions happening within you every moment of every day, with a little help from enzymes.

In our next episode, we’re going to take what we learned about ATP and metabolic pathways a few steps further to talk  about cellular respiration. I’ll see you then! Peace!

This series was produced in collaboration with HHMI BioInteractive. If you’re an educator, visit for classroom resources and professional development related to the topics covered in this course. [In a Hippie voice] So like, thanks for watching this episode of Crash Course Biology which was filmed at our studio in Indianapolis, Indiana and was made with the help of all, like, these nice people. So like, if you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon, bruh.