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  • This is an animal.

  • This is also an animal.

  • Animal. Animal. Animal carcass. Animal. Animal. Animal carcass again. Animal.

  • The thing that all of these other things have in common is that they're made out of the

  • same basic building block: the animal cell.

  • Animals are made up of your run-of-the-mill eukaryotic cells. These are called eukaryotic because

  • they have a "true kernel," in the Greek. A "good nucleus".

  • And that contains the DNA and calls the shots for the rest of the cell

  • also containing a bunch of organelles.

  • A bunch of different kinds of organelles and they all have very specific functions.

  • And all this is surrounded by the cell membrane.

  • Of course, plants have eukaryotic cells too, but theirs are set up a little bit differently,

  • of course they have organelles that allow them to make their own food which is super

  • nice.  We don't have those.

  • And also their cell membrane is actually a cell wall that's made of cellulose. It's rigid,

  • which is why plants can't dance.

  • If you want to know all about plant cells, we did a whole video on it and you can click

  • on it here if it's online yet. It might not be.

  • Though a lot of the stuff in this video is going to apply to all eukaryotic cells, which

  • includes plants, fungi and protists.  

  • Now, rigid cells walls are cool and all, but one of the reasons animals have been so successful

  • is that their flexible membrane, in addition to allowing them the ability to dance, gives

  • animals the flexibility to create a bunch of different cell types and organs types and

  • tissue types that could never be possible in a plant. The cell walls that protect plants

  • and give them structure prevent them from evolving complicated nerve structures and

  • muscle cells, that allow animals to be such a powerful force for eating plants.

  • Animals can move around, find shelter and food, find things to mate with

  • all that good stuff.  In fact, the ability to move oneself around using specialized muscle

  • tissue has been 100% trademarked by kingdom Animalia.

  • >>OFF CAMERA: Ah! What about protozoans?

  • Excellent point! What about protozoans?

  • They don't have specialized muscle tissue.  They move around with cillia and flagella

  • and that kind of thing.

  • So, way back in 1665, British scientist Robert Hooke discovered cells with his kinda crude,

  • beta version microscope. He called them "cells" because hey looked like bare, spartan monks'

  • bedrooms with not much going on inside.

  • Hooke was a smart guy and everything, but he could not have been more wrong about what

  • was going on inside of a cell.  There is a whole lot going on inside of a eukaryotic

  • cell. It's more like a city than a monk's cell.  In fact, let's go with that

  • a cell is like a city.

  • It has defined geographical limits, a ruling government, power plants, roads, waste treatment

  • plants, a police force, industry...all the things a booming metropolis needs to run smoothly.

  •  But this city does not have one of those hippie governments where everybody votes on

  • stuff and talks things out at town hall meetings and crap like that.  Nope.  Think fascist

  • Italy circa 1938.  Think Kim Jong Il's-

  • I mean, think Kim Jong-Un's North Korea, and you might be getting a closer idea of how

  • eukaryotic cells do their business.  

  • Let's start out with city limits.

  • So, as you approach the city of Eukaryopolis there's a chance that you will notice something

  • that a traditional city never has, which is either cilia or flagella.  Some eukaryotic cells

  • have either one or the other of these structures--cilia being a bunch of little tiny arms that wiggle

  • around and flagella being one long whip-like tail.  Some cells have neither. Sperm cells,

  • for instance, have flagella, and our lungs and throat cells have cilia that push mucus

  • up and out of our lungs.  Cilia and flagella are made of long protein fibers called microtubules,

  • and they both have the same basic structure: 9 pairs of microtubules forming a ring around

  • 2 central microtubules. This is often called the 9+2 structure. Anyway, just so you know--when

  • you're approaching city, watch out for the cilia and flagella!

  • If you make it past the cilia, you'll encounter what's called a cell membrane, which is

  • kind of squishy, not rigid, plant cell wall, which totally encloses the city and all its

  • contents.  It's also in charge of monitoring what comes in and out of the cell--kinda like

  • the fascist border police. The cell membrane has selective permeability, meaning that it

  • can choose what molecules come in and out of the cells, for the most part.  

  • And I did an entire video on this, which you can check out right here.

  • Now the landscape of Eukaryopolis, it's important to note, is kind of wet and squishy. It's

  • a bit of a swampland.

  • Each eukaryotic cell is filled with a solution of water and nutrients called cytoplasm.  And

  • inside this cytoplasm is a sort of scaffolding called the cytoskeleton, it's basically just

  • a bunch of protein strands that reinforce the cell.  Centrosomes are a special part

  • of this reinforcement; they assemble long microtubules out of proteins that act like

  • steel girders that hold all the city's buildings together.

  • The cytoplasm provides the infrastructure necessary for all the organelles to do all

  • of their awesome, amazing business, with the notable exception of the nucleus, which has

  • its own special cytoplasm called "nucleoplasm" which is a more luxurious, premium environment

  • befitting the cell's Beloved Leader. But we'll get to that in a minute.  

  • First, let's talk about the cell's highway system, the endoplasmic reticulum, or just

  • ER, are organelles that create a network of membranes that carry stuff around the cell.

  • These membranes are phospholipid bilayers. The same as in the cell membrane.

  • There are two types of ER: there's the rough and the smooth. They are fairly similar, but

  • slightly different shapes and slightly different functions. The rough ER looks bumpy because

  • it has ribosomes attached to it, and the smooth ER doesn't, so it's a smooth network of

  • tubes.

  • Smooth ER acts as a kind of factory-warehouse in the cell city. It contains enzymes that

  • help with the creation of important lipids, which you'll recall from our talk about

  • biological molecules -- i.e. phosopholipids and steroids that turn out to be sex hormones.

  • Other enzymes in the smooth ER specialize in detoxifying substances, like the noxious

  • stuff derived from drugs and alcohol, which they do by adding a carboxyl group to them,

  • making them soluble in water.

  • Finally, the smooth ER also stores ions in solutions that the cell may need later on,

  • especially sodium ions, which are used for energy in muscle cells.  

  • So the smooth ER helps make lipids, while the rough ER helps in the synthesis and packaging of proteins.

  • And the proteins are created by another typer of organelle called the ribosome. Ribosomes

  • can float freely throughout the cytoplasm or be attached to the nuclear envelope, which

  • is where they're spat out from, and their job is to assemble amino acids into polypeptides.

  • As the ribosome builds an amino acid chain, the chain is pushed into the ER. When the

  • protein chain is complete, the ER pinches it off and sends it to the Golgi apparatus.

  • In the city that is a cell, the Golgi is the post office, processing proteins and packaging

  • them up before sending them wherever they need to go. Calling it an apparatus makes

  • it sound like a bit of complicated machinery, which it kind of is, because it's made up

  • of these stacks of membranous layers that are sometimes called Golgi bodies. The Golgi

  • bodies can cut up large proteins into smaller hormones and can combine proteins with carbohydrates

  • to make various molecules, like, for instance, snot.  

  • The bodies package these little goodies into sacs called vesicles, which have phosopholipid

  • walls just like the main cell membrane, then ships them out, either to other parts of the

  • cell or outside the cell wall. We learn more about how vesicles do this in the next episode

  • of Crash Course.

  • The Golgi bodies also put the finishing touches on the lysosomes. Lysosomes are basically

  • the waste treatment plants and recycling centers of the city. These organelles are basically

  • sacks full of enzymes that break down cellular waste and debris from outside of the cell

  • and turn it into simple compounds, which are transferred into the cytoplasm as new cell-building materials.

  • Now, finally, let us talk about the nucleus, the Beloved Leader.  The nucleus is a highly

  • specialized organelle that lives in its own double-membraned, high-security compound with

  • its buddy the nucleolus.  And within the cell, the nucleus is in charge in a major

  • way.  Because it stores the cell's DNA, it has all the information the cell needs to do its job.

  • So the nucleus makes the laws for the city

  • and orders the other organelles around, telling them how and when to grow, what to metabolize,

  • what proteins to synthesize, how and when to divide. The nucleus does all this by using

  • the information blueprinted in its DNA to build proteins that will facilitate a specific

  • job getting done.  For instance, on January 1st, 2012, lets say a liver cell needs to

  • help break down an entire bottle of champagne. The nucleus in that liver cell would start

  • telling the cell to make alcohol dehydrogenase, which is the enzyme that makes alcohol not-alcohol

  • anymore. This protein synthesis business is complicated, so lucky for you, we will have

  • or may already have an entire video about how it happens.

  • The nucleus holds its precious DNA, along with some proteins, in a weblike substance

  • called chromatin. When it comes time for the cell to split, the chromatin gathers into

  • rod-shaped chromosomes, each of which holds DNA molecules. Different species of animals

  • have different numbers of chromosomes. We humans have 46. Fruit flies have 8. Hedgehogs,

  • which are adorable, are less complex than humans and have 90

  • Now the nucleolus, which lives inside the nucleus, is the only organelle that's not

  • enveloped by its own membrane--it's just a gooey splotch of stuff within the nucleus.

  • Its main job is creating ribosomal RNA, or rRNA, which it then combines with some proteins

  • to form the basic units of ribosomes. Once these units are done, the nucleolus spits

  • them out of the nuclear envelope, where they are fully assembled into ribosomes. The nucleus

  • then sends orders in the form of messenger RNA, or mRNA, to those ribosomes, which are

  • the henchmen that carry out the orders in the rest of the cell.

  • How exactly the ribosomes do this is immensely complex and awesome, so awesome, in fact,

  • that we're going to give it the full Crash Course treatment in an entire episode.

  • And now for what is, totally objectively speaking of course, the coolest part of an animal cell:

  • its power plants!  The mitochondria are these smooth, oblong organelles where the amazing

  • and super-important process of respiration takes place. This is where energy is derived

  • from carbohydrates, fats and other fuels and is converted into adenosine triphosphate or

  • ATP, which is like the main currency that drives life in Eukaryopolis. You can learn

  • more about ATP and respiration in an episode that we did on that.

  • Now of course, some cells, like muscle cells or neuron cells need a lot more power than

  • the average cell in the body, so those cells have a lot more mitochondria per cell.  

  • But maybe the coolest thing about mitochondria is that long ago animal cells didn't have

  • them, but they existed as their own sort of bacterial cell.

  • One day, one of these things ended up inside of an animal cell, probably because the animal

  • cell was trying to eat it, but instead of eating it, it realized that this thing was

  • really super smart and good at turning food into energy and it just kept it. It stayed around.

  • And to this day they sort of act like their own, separate organisms, like they do their

  • own thing within the cell, they replicate themselves, and they even contain a small

  • amount of DNA.

  • What may be even more awesome -- if that's possible -- is that mitochondria are in the

  • egg cell when an egg gets fertilized, and those mitochondria have DNA. But because mitochondria

  • replicate themselves in a separate fashion, it doesn't get mixed with the DNA of the father,

  • it's just the mother's mitochondrial DNA. That means that your and my mitochondrial

  • DNA is exactly the same as the mitochondrial DNA of our mothers. And because this special

  • DNA is isolated in this way, scientists can actually track back and back and back and

  • back to a single "Mitochondrial Eve" who lived about 200,000 years ago in Africa.  

  • All of that complication and mystery and beauty in one of the cells of your body. It's complicated,

  • yes. But worth understanding.

  • Review time! Another somewhat complicated episode of Crash Course Biology. If you want

  • to go back and watch any of the stuff we talked about to reinforce it in your brain or if

  • you didn't quite get it, just click on the links and it'll take you back in time to when

  • I was talking about that mere minutes ago.

  • Thank you for watching. If you have questions for us please ask below in the comments, or

  • on Twitter, or on Facebook. And we will do our best to make things more clear for you.

  • We'll see you next time.

This is an animal.

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