Eukaryopolis - The City of Animal Cells: Crash Course Biology #4

Summary

This script delves into the intricate world of animal cells, highlighting their eukaryotic nature with a nucleus governing cellular activities. It compares cells to a bustling city, complete with transportation via endoplasmic reticulum, protein processing in the Golgi apparatus, and waste management by lysosomes. The video also explores the cell's power plants, mitochondria, and their fascinating evolutionary history, ending with a discussion on mitochondrial DNA and its link to 'Mitochondrial Eve.'

Takeaways

• 🐾 Animals are composed of eukaryotic cells, which have a 'true kernel' or nucleus containing DNA and various organelles with specific functions.
• 🌿 Unlike animal cells, plant cells have a rigid cell wall made of cellulose, which provides structure but limits their mobility and complexity.
• 💃 The flexible cell membrane of animals allows for a variety of cell types, organs, and tissues, contributing to their adaptability and success.
• 🚀 The ability to move using specialized muscle tissue is unique to the kingdom Animalia.
• 🔬 Robert Hooke first discovered cells in 1665, but the complexity within eukaryotic cells was much more than he initially realized.
• 🏙️ A eukaryotic cell can be likened to a city with its own infrastructure, including a cell membrane that acts as border police, and cytoplasm that provides a 'wet and squishy' environment.
• 🛤️ The endoplasmic reticulum (ER) serves as the cell's highway system, with rough ER involved in protein synthesis and smooth ER in lipid creation and detoxification.
• 🧬 Ribosomes are the protein factories of the cell, assembling amino acids into polypeptides, which are then processed by the Golgi apparatus.
• 📦 The Golgi apparatus functions as the cell's post office, modifying, packaging, and dispatching proteins to their destinations.
• 🗑️ Lysosomes are the waste management and recycling centers of the cell, breaking down waste into reusable components.
• 🌟 The nucleus is the 'Beloved Leader' of the cell, containing DNA and controlling cell functions through protein synthesis.
• 🔋 Mitochondria are the power plants of the cell, converting nutrients into ATP, the energy currency, and retaining their own DNA, hinting at an ancient symbiotic relationship with early cells.

Q & A

What is the basic building block of animals?

The basic building block of animals is the animal cell.

Why are animal cells called eukaryotic?

Animal cells are called eukaryotic because they contain a 'true kernel' or nucleus, which in Greek means 'good nucleus', that houses the DNA and directs the cell's activities.

What is the main function of the cell membrane in animal cells?

The main function of the cell membrane in animal cells is to enclose the cell and regulate what substances can enter or exit the cell, providing selective permeability.

How do plant cells differ structurally from animal cells?

Plant cells differ from animal cells in that they have a rigid cell wall made of cellulose instead of a flexible cell membrane, and they contain organelles that allow them to produce their own food.

What is the significance of the flexible cell membrane in animal cells?

The flexible cell membrane in animal cells allows for the creation of various cell types, organs, and tissues, and enables animals to move, find shelter, food, and mates, which has contributed to their evolutionary success.

What are cilia and flagella, and how do they function in cells?

Cilia and flagella are hair-like or whip-like structures in some eukaryotic cells that aid in movement. Cilia are multiple small arms that wiggle, while flagella is a single long tail. They are made of microtubules and help in moving substances or the cell itself.

What is the role of the endoplasmic reticulum (ER) in a cell?

The endoplasmic reticulum (ER) serves as a network of membranes that facilitate the transport of materials within the cell. It has two types: rough ER, which is involved in protein synthesis, and smooth ER, which is involved in lipid synthesis and detoxification.

What is the function of the Golgi apparatus in a cell?

The Golgi apparatus functions as the 'post office' of the cell, processing and packaging proteins and other molecules before sending them to their destinations within or outside the cell.

What is the primary role of the nucleus in a eukaryotic cell?

The nucleus serves as the control center of the cell, storing DNA and directing cellular activities such as growth, metabolism, and reproduction by regulating gene expression.

What are mitochondria, and why are they important in cells?

Mitochondria are the 'power plants' of the cell, where respiration occurs, and energy from carbohydrates, fats, and other fuels is converted into ATP, which is the primary energy currency of the cell.

How do mitochondria relate to the concept of 'Mitochondrial Eve'?

Mitochondria contain their own DNA, which is inherited solely from the mother and does not mix with paternal DNA. This allows scientists to trace human lineage back to a single female ancestor known as 'Mitochondrial Eve' who lived around 200,000 years ago in Africa.

Outlines

00:00

🐾 Animal Cells: The Basic Building Blocks

This paragraph introduces the concept of animal cells as the fundamental units of animals, highlighting their eukaryotic nature with a 'true kernel' or nucleus that contains DNA and controls cellular functions. It explains the presence of various organelles within the cell, each with a specific role, and contrasts the flexible cell membrane of animals with the rigid cell wall of plants, which restricts their mobility and complexity. The paragraph also touches on the historical discovery of cells by Robert Hooke and the metaphorical comparison of a eukaryotic cell to a bustling city, governed by the nucleus and equipped with various organelles for different functions.

05:01

🌿 The Infrastructure of Eukaryotic Cells

This section delves into the structural and functional aspects of eukaryotic cells, beginning with the cell's protective and selective cell membrane. It describes the cytoplasm, a nutrient-rich fluid within the cell, and the cytoskeleton that provides structural support. Centrosomes and microtubules are highlighted as key components of this support system. The endoplasmic reticulum (ER) is introduced as the cell's transportation network, with two types—rough and smooth ER—each serving different roles in protein synthesis and lipid production. Ribosomes, the protein synthesis factories, are also discussed, along with the Golgi apparatus, which processes and packages proteins for transport. Lysosomes, the cell's waste management system, and the nucleus, the control center containing DNA, are also covered, emphasizing the nucleus's role in cell regulation and the nucleolus's function in ribosome production.

10:02

🔋 Mitochondria: The Powerhouses of the Cell

The final paragraph focuses on mitochondria, the energy-producing organelles within eukaryotic cells. It explains the process of cellular respiration, where energy is derived from nutrients and converted into ATP, the cell's primary energy currency. The paragraph also explores the evolutionary origin of mitochondria as independent organisms that became incorporated into animal cells, retaining their own DNA and replication mechanisms. The unique inheritance pattern of mitochondrial DNA is discussed, which is passed only from mother to offspring, allowing scientists to trace human lineage back to a common 'Mitochondrial Eve.' The summary concludes by emphasizing the complexity and beauty of cellular processes and invites viewers to engage with the content through comments and social media.

Keywords

💡Eukaryotic cells

Eukaryotic cells are a type of cell that contains a nucleus and membrane-bound organelles. They are fundamental to the structure and function of all organisms in the kingdoms of animals, plants, and fungi. In the video's theme, eukaryotic cells are compared to a city, highlighting their complexity and the various roles of their components. An example from the script is the comparison of the nucleus to the 'Beloved Leader' of the city, emphasizing its central role in controlling the cell's activities.

💡Cell membrane

The cell membrane is a selectively permeable barrier that encloses the cell, regulating the passage of substances in and out. It is crucial for maintaining the cell's internal environment and is described in the script as 'kind of squishy', contrasting it with the rigid cell wall of plants. The script also mentions the cell membrane's role in the cell's ability to move and adapt, which is a key feature of animal cells.

💡Cytoskeleton

The cytoskeleton is a network of protein fibers that provide structural support within the cell, similar to a scaffold. It is mentioned in the script as a part of the cell's infrastructure that helps maintain its shape and assists in various cellular processes. The centrosome, which is part of the cytoskeleton, is highlighted for its role in assembling microtubules, which act like 'steel girders' in the cell.

💡Endoplasmic reticulum (ER)

The endoplasmic reticulum is an organelle that forms a network of membranes involved in the synthesis and transport of proteins and lipids. The script distinguishes between rough ER, which is studded with ribosomes, and smooth ER, which lacks ribosomes and has different functions. The ER is likened to a 'highway system' within the cell, transporting materials and playing a role in detoxification and lipid synthesis.

💡Ribosomes

Ribosomes are the cellular structures responsible for protein synthesis, assembling amino acids into polypeptide chains. They are found floating in the cytoplasm or attached to the ER. The script describes ribosomes as 'henchmen' that carry out the nucleus's orders for protein synthesis, emphasizing their role in executing the genetic instructions from the DNA.

💡Golgi apparatus

The Golgi apparatus is an organelle that modifies, sorts, and packages proteins and lipids for secretion or use within the cell. In the script, it is likened to the 'post office' of the cell, processing and dispatching proteins to their correct destinations. The Golgi apparatus is also involved in the formation of lysosomes and the modification of proteins with carbohydrates.

💡Lysosomes

Lysosomes are organelles that contain enzymes for breaking down waste materials and cellular debris. They are described in the script as the 'waste treatment plants and recycling centers' of the cell, converting waste into simple compounds that can be reused by the cell. This process is crucial for maintaining cellular health and function.

💡Nucleus

The nucleus is the control center of the cell, containing the cell's DNA and directing its activities. It is depicted in the script as the 'Beloved Leader' of the cell, responsible for issuing commands for growth, metabolism, and protein synthesis. The nucleus's role in the cell is analogous to a government in a city, with the nucleolus inside it being responsible for creating ribosomal RNA.

💡Mitochondria

Mitochondria are the organelles responsible for producing energy in the form of ATP through cellular respiration. They are referred to in the script as the 'power plants' of the cell. The script also delves into the endosymbiotic theory, suggesting that mitochondria were once independent organisms that became incorporated into eukaryotic cells, and highlights the unique inheritance of mitochondrial DNA.

💡Cilia and flagella

Cilia and flagella are hair-like structures that extend from the cell surface and are involved in cell movement. The script describes them as 'a bunch of little tiny arms' for cilia and 'one long whip-like tail' for flagella. They are made of microtubules and are important for the movement of substances over the cell surface, such as mucus in the lungs, and for the motility of sperm cells.

💡Chromatin

Chromatin is a complex of DNA and protein found in the nucleus, which condenses into chromosomes during cell division. The script mentions chromatin as the substance within which the DNA is stored and how it forms the rod-shaped chromosomes that carry genetic information. The number of chromosomes varies among species, with humans having 46, fruit flies having 8, and hedgehogs having 90.

Highlights

Animals are composed of eukaryotic cells, characterized by a 'true kernel' or nucleus containing DNA and various organelles.

Plant cells differ from animal cells with their rigid cell walls made of cellulose and the ability to produce their own food.

Animal cells' flexible membranes allow for diverse cell types, organs, and tissues, unlike the rigid cell walls of plants.

The movement of animals is facilitated by specialized muscle tissue, a feature unique to the kingdom Animalia.

Protozoans, unlike other animals, move using cilia and flagella instead of specialized muscle tissue.

Robert Hooke's discovery of cells in 1665 and his initial misunderstanding of their complex internal structure.

Eukaryotic cells are likened to a bustling city with defined limits, a government, and various functional units.

Cilia and flagella are eukaryotic cell structures that aid in movement, composed of microtubules in a 9+2 arrangement.

The cell membrane's selective permeability acts as a border police, controlling the ingress and egress of molecules.

Cytoplasm is the wet and squishy environment within eukaryotic cells, containing the cytoskeleton for cell reinforcement.

Centrosomes are part of the cytoskeleton and play a role in the assembly of microtubules, like steel girders in a city.

The endoplasmic reticulum (ER) serves as the cell's highway system, with rough and smooth ER having distinct functions.

Ribosomes are the protein synthesis factories of the cell, assembling amino acids into polypeptides.

The Golgi apparatus functions as the cell's post office, processing and packaging proteins for distribution.

Lysosomes are the waste treatment plants and recycling centers of the cell, breaking down waste into reusable materials.

The nucleus is the 'Beloved Leader' of the cell, storing DNA and controlling cellular activities and protein synthesis.

The nucleolus, within the nucleus, is responsible for creating ribosomal RNA and assembling ribosomes.

Mitochondria are the power plants of the cell, converting food into energy in the form of ATP.

Mitochondria contain their own DNA and are thought to have originated from a symbiotic relationship with ancient bacteria.

Mitochondrial DNA is inherited only from the mother and can be traced back to 'Mitochondrial Eve', providing insights into human evolution.

Transcripts

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.