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The cell is the basic unit of life, and our understanding of it has advanced as science, and the tools available to scientists, has advanced. In this episode of Crash Course Biology, we’ll take a look at the difference between prokaryotic and eukaryotic cells, take a guided tour of the eukaryotic cell, and learn why most cells are small. We’ll explore the eukaryotic cell’s surprising beginnings through an endosymbiosis that occurred about 1.5 billion years ago.

Introduction to the Cell 00:00
Classical Cell Theory 2:12
Parts of a Cell 5:37
Endosymbiosis 9:11
Modern Cell Theory 10:41
Review & Credits 12:23

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CC Kids:
Back when Aristotle was around, people didn’t know much about the origins of life.

A lot of folks, including the famous Greek philosopher himself, thought that nonliving matter could, just, create life. They called it "spontaneous generation." And they used it to explain all sorts of things.

Like, if someone was storing grain in a silo and happened to find mice that hadn’t been there the day before, well, they assumed the mice arose spontaneously from the nonliving bricks, mud, and some bits of grain. Aristotle even suggested that semen had a unique property that allowed it to "enliven menstrual blood," and that’s how babies were made. And sure, it’s closer to the truth than the "Unified Stork Theory" that your parents came up with – but still definitely wrong.

These days, we know that life doesn’t arise spontaneously from inorganic material, it’s actually made of tiny, individual building blocks called cells. But it took a lot of science to get there, science that relied on all of the work that came before it. Hi!

I'm Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. Now ready your mind for one of the most complex theme songs ever constructed. [THEME MUSIC] We actually knew about the  cell a few hundred years  before spontaneous generation had been debunked, thanks to a physicist named Robert Hooke.

In 1665, around the same time that Isaac Newton was thinking about gravitational forces, Hooke was focused on science at a smaller scale - a microscopic scale actually. Having made some tweaks to an existing microscope, Hooke discovered something astonishing. Looking at a slice of cork  under his improved scope, he was amazed by the tiny pores he saw.

To him, they looked like the little rooms in monasteries that monks lived in, which are called "cells." And, well, the name stuck. If Aristotle had a microscope, we probably wouldn’t have gotten hung up on spontaneous generation for so long. But, that’s how scientific advancement works, it’s all about having the right tools for the right task and being able to iterate, or “build on to”, the work of past scientists.

In the mid-1800s, Hooke’s sketches of the  microscopic world led a  couple of other scientists, physiologist Dr. Theodor Schwann and botanist Matthias Schleiden, to propose that all organisms are made from cells,  and that the cell is the  basic building block of life. Then, in 1855, the work continued when Dr.

Rudolf Virchow added his own proposal: all cells come from pre-existing cells that have multiplied. These three ideas became the cornerstones of what we now call classical cell theory. It wasn’t until the 1900s that we figured out the key differences between the two major cell types prokaryotic and eukaryotic, thanks in part to more advanced microscopes.

You’re probably more familiar with the eukaryotic variety because those are the cells that make up most of the living things you see every day like bees, trees, and people. And unless you’re using a microscope,  you’re unlikely to see any  prokaryotes like E. coli – a type of bacteria that can cause some nasty infections. Although we humans do owe a lot to prokaryotes, since they form itty-bitty communities in our guts and on our skin, helping us digest our dinner and even ward off infection.

The best evidence so far  suggests that single-celled  prokaryotes were among the first forms of life, and that our eukaryotic cells evolved from them about 2.7 billion years ago. So they’re like your great, great, great, great, well, you get it, you’re related. There are a few important differences between the two types of cells.

Our eukaryotic cells have a defined nucleus, usually near the center of the cell. Prokaryotic cells don’t. In fact, their name means “pre-nucleus.” The nucleus is where the genetic material is stored in a eukaryote, packed up neatly within the nuclear membrane, a double-layered shell that surrounds the nucleus.

Most prokaryotes have their single, circular piece of DNA just kinda hanging out in the main compartment of the cell with everything else, in a water-based jelly called the cytoplasm. So, prokaryotic cells kind of look like they packed their suitcase in a hurry, while eukaryotes made sure to bubble-wrap their cellular accessories and tuck them away in special compartments. This compartmentalization lets eukaryotes develop more complex, coordinated cellular processes than prokaryotes.

Like, take plants for example. Not only do the eukaryotic plant cells run some complex processes, but they also do some extra work to support the unique structure of plant life. For starters, much like the buffalo wings I had for lunch, plants are boneless.

To make up for that, each cell membrane is braced by a surrounding cell wall that helps plants maintain their structure. This thick barrier consists  of structural molecules,  including carbohydrates and proteins. Plants also have a large  central vacuole, or cavity, that stores a lot of water and chemicals the plant needs.

The central vacuole also provides additional structural support alongside the cell wall. And then there’s the chloroplast, which converts sunlight into an energy the plant can use. Both a plant’s vacuole and its chloroplast are membrane-enclosed structures called organelles.

In the same way our heart pumps blood and our lungs exchange gases, these little mini-organs inside a cell each serve their own unique function. In a way, a cell is like a city, with each organelle performing its own civic duty  to keep the city-cell  working in a coordinated way. Let’s say goodbye to the plant cells for now and go on a Thought Bubble tour of the eukaryotic animal cell.

Welcome to Cellular City. Here at the entrance to the city, we find a barrier of biomolecules called lipids surrounding the cell. The barrier is the cell membrane.

It’s studded with proteins, some of which open doors allowing us access. We have a very efficient transportation department here in Cellular City. The highways are made up of the protein filaments of the cytoskeleton.

They help move cargo, and let the cell maintain—or change—its shape as needed. They’re quite dynamic, too. See how they assemble at one end while disassembling on the other.

As we approach the center of the cell you’ll catch a glimpse of City Hall, the nucleus. Here, important genetic messages get sent out in the form of the nucleic acid RNA. Ah, please, no flash photography.

Once the instructions reach the ribosomes over in the cytoplasm, they turn the instructions from City Hall into a protein that helps make action happen in the body. Now if you look to your right, you’ll see that sometimes the nucleus dumps the RNA right into these buildings of the endomembrane system, where they enter the endoplasmic reticulum. We just call it the ER around here, and it has two sections: the rough and the smooth.

The rough ER, as you can see, is dotted with ribosomes of its own, so it makes proteins, same as the cytoplasm. But here in the ER, the cell can make more complex proteins with modifications. The Smooth ER is the lipid manufacturing plant, where new pieces of the cell membrane are made, along with those famous message-carrying lipids called hormones – don’t worry, they’ll be around at the end of the tour for autographs.

These cellular products are shipped out of the ER in a vesicle – a fluid-filled structure that buds from the smooth ER’s lipid membrane carrying cargo to the rest of the cell. Those vesicles need to make a quick pit stop at the Golgi apparatus. Part manufacturing plant, part protein sorting facility, the Golgi is another member of the endomembrane system which packages proteins into  vesicles, like this one.

And finally, we end our tour at the mitochondria, the city’s power plants, where energy for all of life’s cellular processes is produced by breaking down the right molecules at the right times. All this while making versatile chemicals that can be used as building blocks in other areas of the city. And that ends our tour for today.

Buh-bye now, Buh-bye. Buh-bye now, Buh-bye. Thanks, Thought Bubble!

We gotta give that tour guide a raise, they were great! Anyway, how amazing that so many complicated things are happening inside a single, microscopic cell. You might have noticed that our tour of the animal cell didn’t include a stopover in the chloroplast.

That’s because the chloroplast  is unique to plants. While animals don’t have chloroplasts, plants do have mitochondria – double the power production for our leafy friends. I’m not jealous… okay, I’m a little jealous.

So while we non-plants  might have chloroplast envy, both of these organelles are really special – and really strange – when you consider their origin. One day, 1.5 billion years or so ago, we think a small bacterium found its way inside of a larger bacterium, whether on purpose or by accident who can say, but this new living arrangement actually worked well for both bacteria. These days we call this arrangement endosymbiosis.

Both mitochondria and chloroplasts might have arose from this kind of symbiotic relationship that worked so well that it became permanent, and it allowed them to co-evolve into the eukaryotic cells we know today. In other words, we think  that’s what led to the cells that allow bugs, bananas, and bears to exist. Scientists had long believed that this could be the case, but it wasn’t until the 1960s that the idea of endosymbiosis as the origin for eukaryotic cells really took off, thanks to evolutionary biologist and zoologist Dr.

Lynn Margulis. Margulis, drawing on the work of many scientists before her, hypothesized that both mitochondria and chloroplasts were descended from the remnants of prokaryotic cells that had merged in an endosymbiotic relationship. But what she had that earlier scientists didn't have were the right tools thanks to advancements in microscope technology, which helped her present experimental evidence to back up the hypothesis.

Further proof that science  doesn’t happen in a void; it builds and grows and changes as different people pick it up across generations. People who are in the right place, at the right time, with the right tools. And speaking of continuing the work that came before you, remember classical cell theory?

Well, we call it "classical" for a reason. Today, we have modern cell theory. Modern cell theory builds on its classical counterpart in the same way Margulis built on the work of her predecessors, by adding three more central ideas: energy flows within cells, similar species have similar cells, and cells divide and pass along their genetic information to new cells.

And, just like classical  cell theory wasn’t possible  without the right advancements in science, neither was modern cell theory, which also relied on advancements in microscopy. We’ve come a long way since the old-school microscopes of Robert Hooke’s day. Of course, the reason it takes such powerful microscopes to observe cells, is because the vast majority of cells are really, really small.

As cell size increases, the volume inside the cell increases faster than the surface area along the membrane, and the surface-to-volume ratio puts a limit on cell size. If a cell got too big, there wouldn’t be enough membrane to support all of its processes. That’s why we’re not made up of just a couple really big cells.

Some cells dodge the size limit by changing their shape, taking the form of rods, or spiky balls that increase their surface-to-volume ratio. Like Caulerpa taxifolia, a single-celled organism that can grow larger than a human arm. It’s plant-shaped, with these cool fronds that increase surface area.

There’s also more than one nucleus to support this giant water-dwelling thing. So even though most cells are really small, nature is always full of surprises. From Hooke to Margulis, to, well, us, the history of the cell is a long chain of scientific iteration and experimentation.

As far back as the world of  ancient Greek philosophy, folks were asking one of the most important questions in science. It’s a simple question, only four words long,  but it’s one that can help  us understand cell theory, invent new microscopes, and figure out new ways of thinking: “why did that happen?” By asking that simple question, we’ve been able to discover not only the existence of the cell and its major types. But we’ve also learned about the busy, bustling internal structures of the organelles, and hypothesized how cells adapted over billions of years to become the shapes of life we see today.

Next time, we’ll check out the cattywampus membrane that holds them together. I’ll see you then. 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. 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 these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.