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 havein 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 eukaryoticcells. these are called eukaryotic because they have a "true kernel," in the greek.a "good nucleus". and that contains the dna and calls the shotsfor the rest of the cell also containing a bunch of organelles.
a bunch of different kinds of organelles andthey 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 allowthem to make their own food which is super nice. â we don't have those. and also their cell membrane is actually acell 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 notbe.
though a lot of the stuff in this video isgoing to apply to all eukaryotic cells, which includes plants, fungi and protists. â now, rigid cells walls are cool and all, butone of the reasons animals have been so successful is that their flexible membrane, in additionto allowing them the ability to dance, gives animals the flexibility to create a bunchof different cell types and organs types and tissue types that could never be possiblein a plant. the cell walls that protect plants and give them structure prevent them fromevolving complicated nerve structures and muscle cells, that allow animals to be sucha powerful force for eating plants. animals can move around, find shelter andfood, find things to mate with
all that good stuff. â in fact, the abilityto move oneself around using specialized muscle tissue has been 100% trademarked by kingdomanimalia. >>off camera: ah! what about protozoans? excellent point! what about protozoans? they don't have specialized muscle tissue.â they move around with cillia and flagella and that kind of thing. so, way back in 1665, british scientist roberthooke 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, buthe could not have been more wrong about what was going on inside of a cell. â there isa whole lot going on inside of a eukaryotic cell. it's more like a city than a monk'scell. â in fact, let's go with that a cell is like a city. it has defined geographical limits, a rulinggovernment, power plants, roads, waste treatment plants, a police force, industry...all thethings a booming metropolis needs to run smoothly. â but this city does not have one of thosehippie governments where everybody votes on stuff and talks things out at town hall meetingsand crap like that. â nope. â think fascist italy circa 1938. â think kim jong il's-
i mean, think kim jong-un's north korea, andyou might be getting a closer idea of how eukaryotic cells do their business.â let's start out with city limits. so, as you approach the city of eukaryopolisthere's a chance that you will notice something that a traditional city never has, which is eithercilia or flagella. â some eukaryotic cells have either one or the other of these structures--ciliabeing a bunch of little tiny arms that wiggle around and flagella being one long whip-liketail. â some cells have neither. sperm cells, for instance, have flagella, and our lungsand throat cells have cilia that push mucus up and out of our lungs. â cilia and flagellaare 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 calledthe 9+2 structure. anyway, just so you know--when you're approaching city, watch out for thecilia and flagella! if you make it past the cilia, you'll encounterwhat's called a cell membrane, which is kind of squishy, not rigid, plant cell wall,which totally encloses the city and all its contents. â it's also in charge of monitoringwhat comes in and out of the cell--kinda like the fascist border police. the cell membranehas selective permeability, meaning that it can choose what molecules come in and outof the cells, for the most part. â and i did an entire video on this, which youcan check out right here.
now the landscape of eukaryopolis, it's importantto note, is kind of wet and squishy. it's a bit of a swampland. each eukaryotic cell is filled with a solutionof water and nutrients called cytoplasm. â and inside this cytoplasm is a sort of scaffoldingcalled the cytoskeleton, it's basically just a bunch of protein strands that reinforcethe cell. â centrosomes are a special part of this reinforcement; they assemble longmicrotubules out of proteins that act like steel girders that hold all the city's buildingstogether. the cytoplasm provides the infrastructurenecessary for all the organelles to do all of their awesome, amazing business, with thenotable 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. butwe'll get to that in a minute. â first, let's talk about the cell's highwaysystem, the endoplasmic reticulum, or just er, are organelles that create a network ofmembranes 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 roughand the smooth. they are fairly similar, but slightly different shapes and slightly differentfunctions. the rough er looks bumpy because it has ribosomes attached to it, and the smoother doesn't, so it's a smooth network of tubes.
smooth er acts as a kind of factory-warehousein 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. phosopholipidsand steroids that turn out to be sex hormones. other enzymes in the smooth er specializein detoxifying substances, like the noxious stuff derived from drugs and alcohol, whichthey do by adding a carboxyl group to them, making them soluble in water. finally, the smooth er also stores ions insolutions that the cell may need later on, especially sodium ions, which are used forenergy in muscle cells. â so the smooth er helps make lipids, whilethe rough er helps in the synthesis and packaging of proteins.
and the proteins are created by another typerof organelle called the ribosome. ribosomes can float freely throughout the cytoplasmor be attached to the nuclear envelope, which is where they're spat out from, and theirjob 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 pinchesit off and sends it to the golgi apparatus. in the city that is a cell, the golgi is thepost office, processing proteins and packaging them up before sending them wherever theyneed 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 thatare sometimes called golgi bodies. the golgi
bodies can cut up large proteins into smallerhormones and can combine proteins with carbohydrates to make various molecules, like, for instance,snot. â the bodies package these little goodies intosacs called vesicles, which have phosopholipid walls just like the main cell membrane, thenships them out, either to other parts of the cell or outside the cell wall. we learn moreabout how vesicles do this in the next episode of crash course. the golgi bodies also put the finishing toucheson the lysosomes. lysosomes are basically the waste treatment plants and recycling centersof the city. these organelles are basically sacks full of enzymes that break down cellularwaste and debris from outside of the cell
and turn it into simple compounds, which aretransferred into the cytoplasm as new cell-building materials. now, finally, let us talk about the nucleus,the beloved leader. â the nucleus is a highly specialized organelle that lives in its owndouble-membraned, high-security compound with its buddy the nucleolus. â and within thecell, the nucleus is in charge in a major way. â because it stores the cell's dna, ithas 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, tellingthem how and when to grow, what to metabolize, what proteins to synthesize, how and whento divide. the nucleus does all this by using the information blueprinted in its dna tobuild proteins that will facilitate a specific
job getting done. â for instance, on january1st, 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 iscomplicated, so lucky for you, we will have or may already have an entire video abouthow it happens. the nucleus holds its precious dna, alongwith some proteins, in a weblike substance called chromatin. when it comes time for thecell to split, the chromatin gathers into rod-shaped chromosomes, each of which holdsdna molecules. different species of animals have different numbers of chromosomes. wehumans have 46. fruit flies have 8. hedgehogs,
which are adorable, are less complex thanhumans and have 90 now the nucleolus, which lives inside thenucleus, is the only organelle that's not enveloped by its own membrane--it's justa gooey splotch of stuff within the nucleus. its main job is creating ribosomal rna, orrrna, which it then combines with some proteins to form the basic units of ribosomes. oncethese units are done, the nucleolus spits them out of the nuclear envelope, where theyare fully assembled into ribosomes. the nucleus then sends orders in the form of messengerrna, or mrna, to those ribosomes, which are the henchmen that carry out the orders inthe rest of the cell. how exactly the ribosomes do this is immenselycomplex and awesome, so awesome, in fact,
that we're going to give it the full crashcourse treatment in an entire episode. and now for what is, totally objectively speakingof course, the coolest part of an animal cell: its power plants! â the mitochondria are thesesmooth, oblong organelles where the amazing and super-important process of respirationtakes place. this is where energy is derived from carbohydrates, fats and other fuels andis converted into adenosine triphosphate or atp, which is like the main currency thatdrives life in eukaryopolis. you can learn more about atp and respiration in an episodethat we did on that. now of course, some cells, like muscle cellsor neuron cells need a lot more power than the average cell in the body, so those cellshave a lot more mitochondria per cell. â
but maybe the coolest thing about mitochondriais that long ago animal cells didn't have them, but they existed as their own sort ofbacterial cell. one day, one of these things ended up insideof an animal cell, probably because the animal cell was trying to eat it, but instead ofeating it, it realized that this thing was really super smart and good at turning foodinto energy and it just kept it. it stayed around. and to this day they sort of act like theirown, separate organisms, like they do their own thing within the cell, they replicatethemselves, and they even contain a small amount of dna. what may be even more awesome -- if that'spossible -- is that mitochondria are in the
egg cell when an egg gets fertilized, andthose 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 mitochondrialdna of our mothers. and because this special dna is isolated in this way, scientists canactually track back and back and back and back to a single "mitochondrial eve" who livedabout 200,000 years ago in africa. â all of that complication and mystery and beautyin one of the cells of your body. it's complicated, yes. but worth understanding. review time! another somewhat complicatedepisode of crash course biology. if you want
to go back and watch any of the stuff we talkedabout to reinforce it in your brain or if you didn't quite get it, just click on thelinks 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 questionsfor us please ask below in the comments, or on twitter, or on facebook. and we will doour best to make things more clear for you. we'll see you next time.
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