It’s 1665, and scientist Robert Hooke has just used his newly-invented light microscope to look at a thin slice of cork up close.

He’s stunned to see that the tree bark is made up of thousands of tiny compartments, which he names for the little rooms that monks live in, called “cells”. He feverishly writes in his book, Micrographia, “They were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this.” Which is 1665-speak for “Woah, did I just discover cells?

I think I just discovered cells.” And from that point on, scientists  have never stopped studying them. Cells are a fundamental unit of all life on Earth that help us understand everything from the teeniest microbes to the blubberiest whales. But botanists won’t let you forget that the first cell ever discovered was from a plant.

And hundreds of years later, we’ve learned more about these things — and about ourselves — than Robert Hooke could have ever imagined. Hi! I’m Alexis, and this is Crash Course Botany. [THEME MUSIC] While you might think that plants and animals don’t have much in common, it turns out that our cells actually have a lot of similarities.

First off, both plant and animal cells are surrounded by a barrier called a cell membrane that allows the cell to decide what kind of molecules it wants to let in or out. And both types of cells manufacture lots of proteins using ribosomes, which are the little granules spread throughout the cell, like the sprinkles in a funfetti cake. Those proteins have tons of different  responsibilities in both animals and plants, like helping with immunity from  disease and transporting nutrients.

Plants and animals are also both eukaryotic organisms, meaning our cells contain organelles. And if you’re thinking “Hey, that just sounds like cute little organs,” you’re right. Organelles are functional units of the cell, just like organs are the functional units of our bodies.

Like hearts and brains for us and stems and leaves for plants, organelles each have a unique job to do. On the other hand, prokaryotic  organisms like bacteria lack organelles, so their cellular contents are a little more willy-nilly, like that junk drawer where you keep the chip clips and sauce packets. One key organelle that plant and animal cells share is the nucleus, the home of all our DNA.

The nucleus uses the information stored in the DNA to tell the other parts of the cell what to do. It’s like the coach of a sportball team. And we can’t forget mitochondria, the kidney-bean-shaped sites of cellular respiration in both plant and animal cells.

And when we say “respiration” here, we don’t mean the way humans breathe using our lungs. Cellular respiration is the process by which chemical energy stored in sugars is converted to energy molecules that fuel life’s essential processes. So even though animals and plants  get food in different ways, they all still have to perform respiration  to convert their food to usable energy. [action noises] But there are some pretty major differences between plant and animal cells.

For one thing, plant cells contain chloroplasts, which are organelles that convert carbon dioxide gas from the air into sugars using energy from the Sun— AKA, photosynthesis. You won’t find chloroplasts in animals; they’re more of a plant thing.  You wouldn’t understand, Blobfish. And vacuoles are also more of a plant thing— they’re fluid-filled organelles that maintain pressure in the plant cell to keep the plant from wilting.

While some animal cells have small ones, as much as ninety percent of a plant cell’s volume can be taken up by the vacuole. Vacuoles help cells grow, store proteins and sugars, and often contain colored pigments, so we have them to thank for the pretty pink petals on our roses. Plant cells are also surrounded by  a thick cell wall made of cellulose, which is a carbon-containing molecule  that is super abundant here on Earth.

It’s hard to comprehend just how much cellulose is on this planet — it’s wrapped around every single cell of every single blade of grass, tree trunk, and superbloom. It’s also stronger than steel and very economically important. Cotton and paper, for example, are made from cellulose.

The cell wall also has profound implications  for how plants build their bodies. In animals, when new cells are made from existing ones, the parent cell and resulting cell sort of pinch off from each other and go their separate ways. Maybe it’ll stay close, maybe it’ll move across the country for college.

An animal cell has to learn to make it on its own. But plant cells don’t have that choice because of their cell walls. Their parent cells are a little more… controlling.

Instead of pinching off and saying goodbye, plant cells are forever locked in next to their parent cells. They do build a new, permanent, cellulose wall to separate parent cells from their offspring, though. So at least they’re able to establish some boundaries.

All this wall-building among plant cells doesn’t  mean the parent and offspring will never speak again. They’re still connected through tiny channels in their cell walls called plasmodesmata that allow them to communicate. In fact, the cytoplasm, or cell juice, is completely interconnected with all of the cells  in a plant thanks to the plasmodesmata.

They’re like windows that plant cells can communicate through, telling each other how to respond to stimuli, or asking for the WiFi password. So, those were a bunch of things that our cells don’t have in common with plant cells. But there’s another thing we do have in common.

And that’s hormones. That’s right: plant cells are raging with them, just like an angsty pre-teen. Hormones are chemical signals that regulate growth and metabolism and make you want to say stuff like, “Ugh, Mom, get out of my room.” We’ll explore five major types of plant hormones, but beyond that, botanical researchers are still routinely discovering new ones.

Let’s start with ethylene— the only plant hormone that’s a gas. It’s one that humans have been using since way before we knew we were using it. Like, in ancient Egypt, fig harvesters cut open under-ripe figs.

In ancient China, farmers kept under-ripe pears near burning incense. And in East Africa and Samoa, people ripened bananas by burying them near a fire. Three different techniques; all ethylene.

But it wasn’t until the 1890s that Russian plant physiologist Dimitry Neljubov pinpointed ethylene as that ripening agent. He wanted to figure out why trees that grew next to gas street lamps would lose their leaves and get all distorted. So, in his lab, he replicated and studied this effect in pea seedlings.

And it turned out, the ethylene released from burning gas lamps could keep plant cells from expanding,  which is a huge part of plant growth. It took a few more decades for scientists to figure out that ethylene doesn’t only come from burning fuel, but can also be produced as a hormone by plants themselves. That’s what triggered those ancient figs  to ripen when they were sliced open.

And if you keep your green tomato  next to a bunch of bananas, ethylene will deliver you a ripe tomato very soon. OK, so those are the effects of just one plant hormone. But just like your hormones affect your mood, your armpit hair, and so much more, plant hormones affect a whole range  of things throughout plants’ lives.

Like, let’s go back to the beginning — or, okay, technically before the beginning — with seeds. The hormone abscisic acid helps seeds stay dormant, or asleep, until they land in a good spot to start growing into plants. Next up, Gibberellins.

These hormones act like a seed’s alarm clock, telling it to start growing and helping cells glow up accordingly. Once we have a growing plant on our hands, Cytokinins stimulate cell division, which allows the plant to build new  organs like leaves, stems, and roots. Aw, they grow up so fast. [Wrestling Announcer Alexis] And now for the main event, we have auxin— [Alexis sings the John Cena theme] not you, auxin.

It’s involved with nearly every aspect of the plant’s life cycle, from its birth…to its death! [Host Alexis] When a plant’s an embryo, auxin helps decide which half of it will become the root and which will be the shoot. It tells plant cells when to grow bigger so that the plant itself can grow bigger, and it helps determine what shape that plant’ll be. Auxin helps form vascular tissue that carries  water and nutrients throughout the plant.

And it’s responsible for how it reacts to things like light, gravity, and touch. Basically, without auxin, a plant would be knocked out…of life! Now, hormones don’t live in their own tidy boxes and do their own separate jobs.

There is frequent cross-talk among all of these hormones and the genes that regulate them. Cytokinin and auxin are often fighting with each other, and sometimes gibberellin and auxin  team up to stimulate fruit development. Through every stage of a plant’s life cycle, hormones are chatty and messy.

And botanists have their own messy ideas about plants. Like, there’s a debate that’s been simmering among plant people for decades about the philosophical significance of the cell. Some botanists are on team cell theory; they believe that cells are the building blocks of life and that organisms are the sum of millions of individual specialized cells that work together to coordinate the activities within the organism.

Team organismal theory, on the other hand, believes that the whole organism is what matters most, and it’s merely subdivided into cells. In the words of German botanist Anton de Bary, “It is the plant that forms cells,  not the cell that forms plants.” As for me? I’m gonna sit this one out and stay botanically neutral — I guess that makes me Edelweiss, the national flower of Switzerland.

Who knew that the humble plant cell is a  place of such drama and hormonal turbulence? And that it’s triggered an existential debate  among botanists about what it means to be a plant? What we do know is that the makeup and function  of cells are essential to how plants, well, exist.

They’re involved with everything from the  beginning to the end of a plant’s life and are the microscopic parts that make big changes occur. Rest assured, cellular harmony awaits us in the next episode, where we’ll find out how these cells come together to form the tissue systems that make a plant function. Hey, before we go, let’s branch out!

Besides bananas, what other fruits emit a ton of ethylene? Find the answers in the comments! [Wrestling Announcer Alexis] Thanks for watching this episode of Crash Course Botany which was filmed at the Damir Ferizović Studio and was 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.