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

Summary

このビデオ脚本では、動物細胞の複雑な構造と機能について説明しています。動物は真核細胞で構成されており、核にDNAが含まれていることで知られる。細胞膜、細胞質、内質網、ゴルギ体、リボソーム、線粒体など、さまざまな細胞器が特定の機能を担っています。特に核は、細胞の中心的存在であり、DNAを保持し、他の細胞器に命令を出す「指導者」的な役割を果たします。また、線粒体は細胞のエネルギー工場であり、独自のDNAを持ち、母系から受け継がれるミトコンドリアDNAについても触れています。

Takeaways

• 🐾 動物は真核細胞からできています。真核細胞とは、核を持っている細胞です。
• 🌿 植物も真核細胞を持っていますが、動物とは異なり、細胞壁を形成するセルロースからできています。
• 💃 動物細胞は柔軟性があり、様々な細胞タイプ、器官タイプ、組織タイプを形成できます。
• 🏃 動物は移動能力を持っており、食べ物を探す、避難所を見つける、配偶者を見つけることができます。
• 🔬 1665年にロバート・フックは原始的な顕微鏡を用いて細胞を発見しましたが、細胞内は非常に複雑です。
• 🏙️ 真核細胞は都市のような複雑な構造を持っており、様々な機能を持つ器官と組織があります。
• 🛃 細胞膜は細胞の境界線であり、選別透過性を持っているため、細胞内外の物質の移動を管理します。
• 🚛 エンドプラスミックレチクルは細胞内の輸送網であり、物質を運ぶ役割を果たします。
• 🏢 ゴルギ体は細胞の郵便局であり、タンパク質を加工し、パッケージングして適切な場所に送ります。
• 🔋 ミトコンドリアは細胞のパワープラントであり、呼吸作用を通じてエネルギーを生成します。
• 🧬 ミトコンドリアは独自のDNAを持ち、母系から受け継がれるため、科学者たちはこれを利用して人類の祖先を追跡することができます。

Q & A

質問1: 動物細胞の特徴は何ですか？

動物細胞は真核細胞であり、核にDNAが含まれていて、他の細胞小器官を持っています。柔軟な細胞膜を持ち、筋肉細胞や神経細胞などのさまざまな細胞タイプを作り出すことができます。

質問2: 動物細胞と植物細胞の違いは何ですか？

動物細胞には柔軟な細胞膜がありますが、植物細胞にはセルロースでできた硬い細胞壁があります。また、植物細胞は自分で食物を作るための葉緑体を持っており、これが大きな違いです。

質問3: 1665年にロバート・フックが細胞を発見した際、彼はどのようにそれを説明しましたか？

ロバート・フックは初期の顕微鏡で細胞を発見し、それを修道士の質素な部屋に例えましたが、実際には細胞内には多くの活動が行われており、フックの理解は間違っていました。

質問4: 真核細胞の細胞膜の役割は何ですか？

真核細胞の細胞膜は、細胞を囲む柔軟な膜で、選択的透過性を持ち、どの分子が細胞に入るか、または出るかを制御します。

質問5: 動物細胞において、筋肉や神経のような複雑な構造が進化できる理由は何ですか？

動物細胞の柔軟な細胞膜は、植物細胞のような硬い細胞壁がないため、異なる種類の細胞や組織を形成し、筋肉や神経のような複雑な構造を進化させることができるからです。

質問6: ゴルジ体の役割は何ですか？

ゴルジ体は細胞内の「郵便局」として機能し、タンパク質を処理し、梱包して必要な場所に送ります。また、ゴルジ体はタンパク質を他の分子と結びつけて、特定の機能を持つ分子を作り出します。

質問7: ミトコンドリアの主な役割は何ですか？

ミトコンドリアは細胞の「発電所」であり、呼吸過程で炭水化物や脂肪などの燃料からエネルギーを生成し、アデノシン三リン酸（ATP）として細胞に提供します。

質問8: リソソームの役割は何ですか？

リソソームは細胞内の「廃棄処理施設」として機能し、細胞内の老廃物や外部から取り込んだデブリを分解し、新しい細胞構造の材料に再利用します。

質問9: ミトコンドリアが動物細胞にどのように取り込まれたのかについての理論は何ですか？

ミトコンドリアはもともと独立した細菌細胞で、動物細胞に取り込まれた後、食べられずにエネルギーを生成する能力を評価されて共生するようになったと考えられています。

質問10: 核の役割は何ですか？

核は細胞の「指導者」として機能し、DNAを保持し、細胞がどのように成長し、何を代謝し、どのタンパク質を合成するかなど、細胞全体の活動を指示します。

Outlines

00:00

🐾 動物細胞の基礎

本段落では、動物が同じ基本的な構成要素である動物細胞からできていると説明しています。真核細胞と呼ばれるこれらの細胞には、DNAを保持する核と、特定の機能を持つ様々な器官が含まれています。植物細胞も真核細胞ですが、構造的に微妙に異なります。例えば、植物細胞には自己栄養のための器官や、セルの柔軟性を制限するセル壁があります。一方で動物は柔軟な細胞膜を持ち、多様な細胞タイプ、器官タイプ、組織タイプを生み出すことができます。また、1665年に英国の科学者であるロバート・フックがセルを発見し、単純な空間であると誤解しましたが、真核細胞内は非常に複雑で、都市のような機能を有しています。

05:01

🏭 真核細胞の機能と構造

この段落では、真核細胞の様々な機能と構造について詳しく説明しています。細胞膜は細胞の境界を定義し、細胞内外の物質の移動を管理します。細胞質は細胞の内部を満たし、細胞スケルトンと呼ばれる蛋白線で強化されています。細胞内の輸送システムである内質網は、細胞内の物質を運ぶ役割を果たします。内質網には、タンパク質の合成とパッケージングを行うROUGH ERと、脂質の合成や解毒を行うSMOOTH ERがあります。核糖体はタンパク質を形成し、ゴルギ体はタンパク質を加工してパッケージングします。リボソームは細胞の廃棄物や外来物を分解し、セルから出る単純化合物を作成します。細胞の中心的な役割を果たす核は、DNAを保持し、細胞の活動を指令します。

10:02

🧬 ミトコンドリアと遺伝子の謎

最後の段落では、細胞のエネルギー源であるミトコンドリアの重要性と、その起源について説明しています。ミトコンドリアは、食物からエネルギーを作り出し、ATPに変換する重要な役割を果たしています。さらに、ミトコンドリアは独自のDNAを持ち、そのDNAは母系から受け継がれます。この特性を利用して、科学者は人類の共同の母親である「ミトコンドリア・イヴ」を追跡することができます。この段落では、細胞の複雑さと美しさを強調し、生物学の理解の重要性を示しています。

Keywords

💡真核細胞

真核細胞とは、細胞核を有する細胞の総称です。このビデオでは、動物が真核細胞からできていると説明されており、DNAを保持し、他の細胞の機能を指揮する重要な役割を果たします。真核細胞は、特定の機能を持つ様々な器官と組織を作るために柔軟な細胞膜を持ち、植物とは異なる構造を持っています。

💡細胞膜

細胞膜は、細胞の外側に位置し、細胞内外の物質の交換を管理する役割を持ちます。ビデオでは、動物細胞の細胞膜が柔軟であり、植物細胞の細胞壁とは異なると強調されています。この柔軟性は、動物が複雑な神経構造や筋細胞を持つことができる要因の一つです。

💡細胞器

細胞器は、細胞内の特定の機能を担う小さな構造体です。ビデオでは、細胞核、線粒体、内質網、ゴルギ体、リボソームなど、さまざまな細胞器が紹介されており、それぞれが細胞の運行に不可欠な役割を果たしています。

💡細胞核

細胞核は、真核細胞の中核的な役割を果たし、DNAを保持しています。ビデオでは、細胞核が細胞の「最高指導者」であると表現されており、他の細胞器に蛋白質の合成や細胞分裂などの命令を出すことで、細胞の活動を管理しています。

💡線粒体

線粒体は、細胞のエネルギー工場として知られており、ビデオではその重要な役割が強調されています。細胞内の糖質や脂肪からATP（アデノシントリリン酸）に変換し、細胞の活動に必要なエネルギーを供給します。また、線粒体は独自のDNAを持ち、母系から子に引き継がれるため、科学者たちが人類の祖先を追跡することができると説明されています。

💡内質網

内質網は、細胞内の膜構造で、タンクや工場の役割を果たします。ビデオでは、内質網がタンクとして脂質の合成や解毒作用を行う滑らかな内質網（滑らかなER）と、タンクとパッケージングセンターとしての粗面内質網（粗面ER）があると説明されています。

💡ゴルギ体

ゴルギ体は、細胞内の「郵便局」として機能し、タンクから送られたタンクの最終加工とパッケージングを行います。ビデオでは、ゴルギ体がタンクから受け取ったタンクを加工し、他の細胞部へ配送したり、細胞外に送り出したりする役割を果たしていると説明されています。

💡リボソーム

リボソームは、タンパク質を合成する場所であり、ビデオではその重要な役割が強調されています。リボソームはmRNAの指示に基づいてアミノ酸を組み立て、タンパク質の合成を促進します。

💡細胞骨架

細胞骨架は、細胞内の繊維状構造で、細胞を支え、形を保つ役割を果たします。ビデオでは、細胞骨架が細胞膜から伸びていて、細胞の形を維持し、細胞内の物質移動を助ける役割を果たしていると説明されています。

Highlights

动物由真核细胞构成，这些细胞具有真正的细胞核。

动物细胞包含多种具有特定功能的细胞器。

与植物细胞不同，动物细胞没有细胞壁，这使得它们能够发展出复杂的肌肉和神经结构。

动物细胞的灵活性允许它们形成多种细胞类型和器官。

动物能够通过专门的肌肉组织移动，这是动物界特有的。

罗伯特·胡克在1665年首次发现了细胞。

真核细胞内部活动丰富，类似于一个繁忙的城市。

细胞膜具有选择性通透性，控制物质的进出。

细胞质是细胞内充满水和营养物质的溶液，为细胞器提供必要的基础设施。

内质网是细胞内的运输系统，负责在细胞内运输物质。

核糖体是蛋白质合成的场所，它们可以自由浮动或附着在内质网上。

高尔基体是细胞的邮局，负责处理和包装蛋白质。

溶酶体是细胞的废物处理和回收中心，含有能够分解细胞废物的酶。

细胞核是细胞的指挥中心，包含DNA并控制细胞的所有活动。

线粒体是细胞的能量工厂，负责将食物转化为能量。

线粒体拥有自己的DNA，并且只通过母系遗传。

线粒体的DNA可以帮助科学家追踪到大约20万年前的“线粒体夏娃”。

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.