How We Study Cells
A-Level Biology Topic 1 16:26 English narration · English + 中文 subtitles burned in
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You are built from about thirty trillion cells, yet you have never seen a single one.
你由大约三十万亿个细胞构成，却从未见过其中任何一个。
Cells are simply too small for the eye.
细胞实在太小，肉眼看不见。
A grain of sand is about a millimetre across.
一粒沙子大约一毫米宽。
A typical cell is a hundred times smaller than that.
一个典型的细胞比它小一百倍。
And a virus is a hundred times smaller again — a mere hundred nanometres.
而一个病毒又要再小一百倍—— 仅有一百纳米左右。
To explore this hidden world, biology reaches for its most important tool: the microscope.
为了探索这个隐藏的世界，生物学拿起了它最重要的工具：显微镜。
In this lesson we learn how we study cells.
在这节课里，我们学习如何研究细胞。
We will measure them with magnification and resolution, tour the eukaryotic cell and its organelles, compare plant and animal cells, meet the smaller, simpler prokaryotes, and finally look at viruses — strange particles that sit right at the edge of life.
我们会用放大倍数和分辨率来测量它们， 游览真核细胞及其细胞器，比较植物细胞和动物细胞，认识更小更简单的原核生物， 最后看看病毒——那种恰好处在生命边缘的奇特颗粒。
Let's begin.
让我们开始吧。
The first tool you meet is the light microscope.
你最先接触的工具是光学显微镜。
Light from a lamp travels up through a condenser, then through a thin specimen on the slide.
灯发出的光向上穿过聚光镜，再穿过载玻片上薄薄的标本。
Two glass lenses enlarge the view: the objective lens near the slide, and the eyepiece lens near your eye.
两片玻璃透镜放大图像：靠近载玻片的是物镜，靠近眼睛的是目镜。
That is how a thin slice of tissue becomes a picture you can study.
就这样，薄薄一片组织变成了你能研究的画面。
The material you look at is called the specimen.
你观察的材料叫做标本。
Keep it thin, or light cannot pass through.
一定要切得薄，否则光就穿不过去。
To look at living material, you make a temporary preparation, also called a wet mount.
要观察活的材料，你做的是临时装片，也叫水装片。
Put a small, thin piece of material on a glass slide.
把一小块薄薄的材料放在载玻片上。
Add a drop of stain — a coloured liquid that makes parts easier to see.
加一滴染色剂——有颜色的液体，让各部分更容易看清。
Then lower a thin cover slip on top, at an angle, so it flattens the specimen and keeps air bubbles out.
然后把薄薄的盖玻片斜着放下来， 压平标本，并把气泡挤出去。
That simple three-step routine is how you get a clear view of real cells under the light microscope.
就是这三步简单操作，让你在光学显微镜下看清真实的细胞。
When you draw cells from a slide or a photograph, examiners want three simple habits.
当你根据装片或照片画细胞时，考官要的是三个简单习惯。
Use a sharp pencil and clear single lines — no shading and no sketchy double lines.
用尖细的铅笔和清晰的单线——不要涂阴影，也不要潦草的双线。
Draw only what you can really see, with the parts in the correct relative sizes.
只画你真正能看见的东西，并且各部分的相对大小要正确。
And label each part with a straight line that does not cross another label line.
给每个部分做标注时，用直线引出，标注线不要互相交叉。
Those three rules keep a biological drawing accurate, neat, and worth full marks.
这三条规则让生物图画得准确、整洁，也更容易拿满分。
First, how big is what we see?
首先，我们看到的东西有多大？
Magnification tells you how many times larger the image is than the real object.
放大倍数告诉你图像比真实物体大多少倍。
It is simply the image size divided by the actual size — and because it is a ratio, it has no unit.
它就是图像大小除以实际大小——因为是一个比值，所以没有单位。
There is one golden rule: the top and bottom must be in the same unit.
有一条黄金法则： 分子和分母必须用同一个单位。
Cells are tiny, so we drop down the ladder — a millimetre is a thousand micrometres, a micrometre is a thousand nanometres.
细胞很小，所以我们沿着阶梯往下走——一毫米是一千微米， 一微米是一千纳米。
Here is a worked example: a chloroplast measures twenty-five millimetres on an image magnified five thousand times.
看一个例子：一个叶绿体在放大五千倍的图像上量得二十五毫米。
Divide, and its real size is just five micrometres.
相除，它的真实大小只有五微米。
You can rearrange the same equation to find any one value from the other two.
你可以把同一个方程变形，用其中两个量求出第三个。
Actual size equals image size divided by magnification.
实际大小等于图像大小除以放大倍数。
Image size equals actual size times magnification.
图像大小等于实际大小乘以放大倍数。
Always convert to the same unit first, then divide or multiply.
一定先换算成同一个单位，再除或再乘。
Remember the ladder in full: one millimetre is a thousand micrometres, one micrometre is a thousand nanometres, so one millimetre is one million nanometres.
把整条阶梯记牢：一毫米是一千微米， 一微米是一千纳米，所以一毫米是一百万纳米。
The equation works for drawings, photomicrographs from a light microscope, and electron micrographs.
这个方程适用于绘图、 光学显微镜拍的显微照片，以及电子显微照片。
So how do you measure a real cell under the microscope?
那么，怎样在显微镜下测量一个真实的细胞呢？
You use an eyepiece graticule — a tiny ruler etched inside the eyepiece.
你用目镜测微尺——刻在目镜里的一把小尺子。
And here is the catch that makes this a two-step job instead of one.
而这里有个关键之处，它使这件事变成两步而不是一步。
The graticule sits in the EYEPIECE, above the objective lens, so it is never magnified along with the specimen.
目镜测微尺装在目镜里，位于物镜的上方，所以它从来不会跟着标本一起被放大。
Its divisions stay exactly the same size to your eye, while the specimen underneath them grows or shrinks every time you change objective.
它的分度在你眼里始终是同样大小，而它下面的标本， 每换一次物镜就会变大或变小。
So one graticule division stands for a different real distance at every magnification, and it has no fixed value at all until you go and find it.
所以同一个分度，在不同的放大倍数下代表着不同的真实距离， 在你去把它测出来之前，它根本没有固定的值。
That is what calibrating means.
这就是"校准"的含义。
Count how many graticule divisions fit a known length on a stage micrometer — a slide with an accurate scale — and you have the value for that objective, and only that one.
数一数在载物台测微尺——一张带精确刻度的载玻片——上， 一段已知长度里能放下多少个目镜分度，你就得到了那个值， 而且只对那一个物镜有效。
Change objective and you must calibrate again.
换了物镜，就必须重新校准。
Look carefully at the two scales lined up.
仔细看这两把对齐的尺子。
The eyepiece graticule often runs from zero to one hundred.
目镜测微尺常常从零标到一百。
The stage micrometer is a special slide with an accurate scale — often one millimetre split into one hundred parts, so each part is ten micrometres.
载物台测微尺是一张带精确刻度的特殊载玻片—— 往往把一毫米分成一百份，所以每一份是十微米。
Line the zeros up, count how many graticule divisions match a known length on the micrometer, and divide.
把零点对齐，数一数一段已知长度对应多少个目镜分度， 再相除。
Once calibrated at that objective, you can remove the micrometer, put your specimen in, and measure real cell lengths with the graticule alone.
在这个物镜下校准之后，你可以取下载物台测微尺，换上标本， 只用目镜测微尺就能量出真实的细胞长度。
But magnification is only half the story.
但放大倍数只是故事的一半。
Making an image bigger does not always show more detail.
把图像放大，并不总能显示更多细节。
Past a point, you just get a bigger blur.
过了某个点， 你只会得到更大的模糊。
The real limit is resolution — the smallest distance between two points that still lets you see them as two, not one.
真正的极限是分辨率——能让你把两个点看成两个、而不是一个的最小距离。
A light microscope has poor resolution, because light waves are relatively long, so two close points smear into a single blob.
光学显微镜分辨率较差，因为光波相对较长，于是两个靠近的点会糊成一团。
An electron microscope uses electrons, whose wavelength is far shorter, so it resolves those same two points cleanly.
电子显微镜用电子，它的波长短得多，于是能把同样的两个点清晰地分开。
That is why examiners want you to keep the two words apart.
这就是为什么考官要你把这两个词分清楚。
Electron microscopes come in two kinds, and examiners love both names.
电子显微镜有两种，考官两个名字都爱考。
A scanning electron microscope, or SEM, bounces electrons off the surface and builds a three-dimensional view of the outside.
扫描电子显微镜，简称扫描电镜， 让电子从表面弹回，构建外面的三维图像。
A transmission electron microscope, or TEM, sends electrons through a very thin slice, so you see fine detail inside the cell.
透射电子显微镜，简称透射电镜， 让电子穿过极薄的切片，于是你能看见细胞内的精细结构。
Both beat a light microscope on resolution, because electrons have a much shorter wavelength than light.
两者在分辨率上都胜过光学显微镜， 因为电子的波长比光短得多。
That is why tiny structures only appear clearly on electron micrographs.
所以很小的结构只有在电子显微照片上才显得清楚。
Now step inside a eukaryotic cell — the kind that builds plants and animals.
现在走进一个真核细胞——构成植物和动物的那种细胞。
Around the outside is the cell surface membrane, controlling what goes in and out.
最外面是细胞膜，控制什么进、什么出。
The defining feature is the nucleus, a large compartment that holds the DNA and runs the cell.
它的决定性特征是细胞核，一个存放DNA、指挥整个细胞的大隔间。
Floating in the jelly-like cytoplasm are the organelles — tiny structures, each doing one job.
漂浮在果冻般的细胞质里的是各种细胞器——微小的结构，各司其职。
The sausage-shaped mitochondria release energy.
腊肠形的线粒体释放能量。
The rough endoplasmic reticulum, dotted with ribosomes, builds proteins.
粗面内质网上点缀着核糖体，制造蛋白质。
And the Golgi body packs them into vesicles for secretion.
而高尔基体把它们打包。
A cell is a busy, organised factory.
细胞是一座繁忙而有序的工厂。
Here is a generalised animal cell, the map examiners expect you to recognise.
这是一个概括的动物细胞，是考官希望你认得出的那张地图。
Spot the large nucleus near the centre, the scattered oval mitochondria, the folded sheets of endoplasmic reticulum, the stacked Golgi body, and the tiny dots that are ribosomes.
注意靠近中央的大细胞核、散落的卵圆形线粒体、折叠的内质网片层、 成叠的高尔基体，以及那些小点点——核糖体。
In a photomicrograph or electron micrograph you identify organelles by shape, size and position.
在显微照片或电子显微照片里， 你靠形状、大小和位置辨认细胞器。
In a drawing, show their outlines and label them clearly.
画图时，画出轮廓并清楚标注。
Learn this layout so you can name each part under pressure in the exam.
把这张布局记牢，考试时就能在压力下叫出每个部分的名字。
For the exam, always pair an organelle with its job.
考试时，永远把一个细胞器和它的功能配成对。
The nucleus holds the DNA and controls the cell.
细胞核存放DNA并控制细胞。
The mitochondria are the site of respiration, releasing energy as ATP — the cell's usable fuel.
线粒体是呼吸作用的场所，把能量以ATP的形式释放出来——那是细胞可用的燃料。
The ribosomes join amino acids together to build proteins.
核糖体把氨基酸连接起来，构建蛋白质。
And the Golgi body modifies and packs those proteins into little sacs called vesicles, ready to be sent out of the cell.
而高尔基体修饰这些蛋白质，把它们装进叫做囊泡的小袋子里， 准备送出细胞。
Structure and function, every time.
每一次，都要写结构加功能。
Zoom in on the nucleus.
再把细胞核放大看。
It is surrounded by a nuclear envelope — a double membrane with holes — and inside it sits a denser spot called the nucleolus, which makes ribosomes.
它被核膜包围——一层有孔的双层膜——里面有一个更致密的点，叫核仁， 负责制造核糖体。
Now the endoplasmic reticulum comes in two forms.
内质网则有两种形态。
Rough ER has ribosomes stuck to its surface, so it makes and transports proteins, for example antibodies.
粗面内质网表面粘着核糖体，所以制造并运输蛋白质， 例如抗体。
Smooth ER has no ribosomes; it makes lipids instead.
滑面内质网没有核糖体；它制造脂质。
Name the surface feature, then name the product — that is how you earn both structure and function marks.
先说出表面特征，再说出产物—— 这就是同时拿到结构和功能分的写法。
Three more organelles often appear in mark schemes.
还有三种细胞器常常出现在评分标准里。
Lysosomes are small sacs of enzymes that break down old organelles and waste.
溶酶体是装满酶的小囊，分解衰老的细胞器和废物。
Cilia are tiny hairs on the cell surface that beat to move fluid, or to move the cell itself.
纤毛是细胞表面的细小毛发，通过摆动来推动液体，或推动细胞本身。
Microvilli are tiny folds of the cell surface membrane that increase surface area for absorption — think of gut lining cells.
微绒毛是细胞膜的微小折叠，增大吸收面积——想想肠壁细胞。
Centrioles and microtubules are small protein tubes that help move chromosomes and form the cell's internal skeleton.
中心粒和微管是由蛋白质构成的细管，帮助移动染色体，并形成细胞的内部骨架。
Ribosomes in the cytoplasm are the larger eighty-S type; the smaller seventy-S type lives inside mitochondria and chloroplasts.
细胞质里的核糖体是较大的八十艾斯型；较小的七十艾斯型住在线粒体和叶绿体里。
Cells burn ATP from respiration for every energy-hungry job: building proteins, moving materials, and dividing.
细胞用呼吸作用产生的ATP，去做每一个耗能的工作：制造蛋白质、运输物质，以及分裂。
Two organelles deserve a closer look, because their shape is the answer to a question examiners ask every year.
有两个细胞器值得看得更仔细，因为它们的形状正是考官每年都会问的那个问题的答案。
Look inside a mitochondrion.
往线粒体里面看。
It has two membranes, and the inner one is deeply folded into shelves called cristae.
它有两层膜，内层深深地折叠成一层层的嵴。
Those folds are not decoration: they pack an enormous membrane area into a tiny volume, and it is on that membrane that aerobic respiration happens.
这些褶皱不是装饰：它们把巨大的膜面积塞进极小的体积里， 而有氧呼吸正是在那层膜上进行的。
So the mark scheme wants the pair — folded inner membrane, therefore a large surface area for respiration.
所以评分标准要的是这一对——内膜折叠，因此为呼吸作用提供了巨大的表面积。
A chloroplast plays the same trick with a different job.
叶绿体玩的是同一个把戏，只是任务不同。
Inside its envelope, membranes are stacked into piles called grana, giving a huge area to hold the pigments that trap light for photosynthesis.
在它的双层膜里面，膜被堆叠成一摞摞的基粒， 提供巨大的面积来容纳捕捉光的色素，用于光合作用。
And here is the detail that ties this lesson together: both of these organelles keep their own small circular DNA and their own seventy-S ribosomes — the smaller kind you met in bacteria, not the eighty-S kind out in the cytoplasm.
还有一个把这节课串起来的细节： 这两个细胞器都保有自己的小型环状DNA和自己的七十S核糖体—— 就是你在细菌里见过的那种较小的核糖体，而不是细胞质里的八十S。
That is why the seventy-S ribosome appears twice in your notes, and it is a favourite way to test whether you really read the table.
这就是为什么七十S核糖体在你的笔记里出现了两次， 也是考试最爱用来检验你有没有真的读过那张表的地方。
Plant and animal cells are both eukaryotic, but they are not identical.
植物细胞和动物细胞都是真核的，但它们并不相同。
A plant cell carries three extra features you must remember: a tough cell wall made of cellulose, green chloroplasts where photosynthesis happens, and one large permanent vacuole full of watery sap that keeps the cell firm.
植物细胞多带三个你必须记住的特征： 一层由纤维素构成的坚固细胞壁，进行光合作用的绿色叶绿体， 以及一个装满水样细胞液、让细胞保持坚挺的大而永久的液泡。
What do they share?
它们共有什么？
Plenty — a cell surface membrane, cytoplasm, a nucleus, mitochondria, ribosomes, and endoplasmic reticulum.
很多——细胞膜、细胞质、细胞核、线粒体、核糖体和内质网。
The neat reversal: animal cells have centrioles, which most plant cells lack.
一个巧妙的反转：动物细胞有中心粒，而大多数植物细胞没有。
Now the plant cell diagram.
现在看植物细胞图。
The strong outer layer is the cell wall of cellulose — it supports and protects the cell and stops it bursting.
最外面坚固的一层是纤维素细胞壁——它支撑并保护细胞，防止细胞胀破。
Green chloroplasts, with stacked membranes and their own small circular DNA, are the site of photosynthesis.
绿色的叶绿体带有成叠的膜，还有自己小小的环状DNA，是光合作用的场所。
The large permanent vacuole is a big sac of watery fluid, bounded by a membrane called the tonoplast; it stores water and keeps the cell firm.
大而永久的液泡是装满水样液体的大囊，外面有一层叫做液泡膜的膜；它储存水分，让细胞保持坚挺。
Tiny channels through the walls, called plasmodesmata, link the cytoplasm of neighbouring plant cells.
穿过细胞壁的细小通道叫做胞间连丝，把相邻植物细胞的细胞质连在一起。
Plant cells tend to have a fixed, regular shape; animal cells are rounder and more flexible.
植物细胞往往形状固定而规则；动物细胞则更圆、更柔软。
Put the two side by side and the exam table almost writes itself.
把两者并排放在一起，考题表格几乎自己就写出来了。
Cell wall: present in plant cells, made of cellulose; absent in animal cells.
细胞壁：植物细胞有，由纤维素构成； 动物细胞没有。
Chloroplasts: plant only.
叶绿体：只有植物有。
Large permanent vacuole: plant only — animal cells may have small temporary ones.
大而永久的液泡：只有植物有——动物细胞可能只有小的临时液泡。
Centrioles: present in animal cells, absent in most plant cells.
中心粒：动物细胞有，大多数植物细胞没有。
Shape: plant cells are fixed and regular; animal cells are rounder and flexible.
形状：植物细胞固定而规则；动物细胞更圆、更柔软。
Both share a membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER and a Golgi body.
两者共有细胞膜、细胞质、细胞核、线粒体、核糖体、内质网和高尔基体。
Not all cells are so elaborate.
并非所有细胞都这么精巧。
A prokaryotic cell, like a bacterium, is much smaller and far simpler — just one to five micrometres across.
一个原核细胞，比如细菌，要小得多、简单得多——只有一到五微米宽。
Its wall is made of peptidoglycan, not cellulose.
它的壁由肽聚糖构成，而不是纤维素。
The biggest difference is this: there is no nucleus.
最大的区别在于：没有细胞核。
Its DNA is a single circular loop, floating free in the cytoplasm, sometimes with tiny extra rings called plasmids.
它的DNA是单一的环状回路， 自由漂浮在细胞质里，有时还带着叫做质粒的微小额外环。
Its ribosomes are smaller too — the seventy-S type.
它的核糖体也更小——是70S型。
And crucially, a prokaryote has no membrane-bound organelles at all: no nucleus, no mitochondria, no chloroplasts.
而且关键是，原核细胞完全没有膜包被的细胞器：没有细胞核，没有线粒体，没有叶绿体。
A labelled prokaryote shows extra surface features you should name.
一张带标注的原核细胞图，还会多出一些你应该叫得出的表面结构。
Outside the wall there may be a capsule.
细胞壁外面可能有荚膜。
Short hair-like pili help the cell stick to surfaces or exchange DNA.
短短的毛发状菌毛帮助细胞粘在表面，或交换DNA。
A long whip-like flagellum beats to move the bacterium.
一条长长的鞭状鞭毛摆动，推动细菌移动。
Inside, the free circular DNA sits in a region called the nucleoid — not a true nucleus, because there is no nuclear envelope.
在内部，游离的环状DNA位于叫做拟核的区域——那不是真正的细胞核，因为没有核膜。
Small plasmids float nearby.
小小的质粒漂在附近。
Ribosomes are scattered through the cytoplasm.
核糖体散布在整个细胞质里。
Prokaryotes are unicellular: each organism is a single cell.
原核生物是单细胞的：每个生物体就是一个细胞。
Now the comparison table that wins easy marks.
现在是那张能轻松拿分的比较表。
Size: prokaryotes about one to five micrometres; eukaryotes about ten to one hundred micrometres.
大小：原核大约一到五微米；真核大约十到一百微米。
DNA: circular and free in the cytoplasm in prokaryotes; linear and inside a nucleus in eukaryotes.
DNA：原核是环状、游离在细胞质里；真核是线状、包在细胞核里。
Nucleus: none versus present.
细胞核：没有对有。
Double-membrane organelles: none in prokaryotes; mitochondria, and chloroplasts in plants, in eukaryotes.
双层膜细胞器：原核没有；真核有线粒体，植物还有叶绿体。
Ribosomes: seventy-S versus eighty-S in the cytoplasm — though eukaryotes also keep seventy-S ribosomes inside mitochondria and chloroplasts.
核糖体：原核是七十艾斯， 真核细胞质里是八十艾斯——不过真核在线粒体和叶绿体里也保留七十艾斯核糖体。
Cell wall: peptidoglycan in prokaryotes; cellulose in plants, or none in animals.
细胞壁：原核是肽聚糖；植物是纤维素，动物没有。
Finally, viruses — which stretch the very idea of a cell, because they are not cells at all.
最后，病毒——它们挑战了细胞这个概念本身，因为它们根本不是细胞。
A virus is astonishingly simple.
病毒简单得惊人。
At its core sits its genetic material, either DNA or RNA, but never both.
它的核心是遗传物质，要么是DNA，要么是RNA，但绝不会两者都有。
Wrapped around that is a protein coat called the capsid.
包在外面的是一层叫做衣壳的蛋白质外壳。
Some viruses add an outer envelope of lipids, studded with spikes.
有些病毒还加上一层由脂质构成、镶着刺突的外包膜。
And that is all.
就这些了。
A virus has no cytoplasm, no ribosomes, and cannot make its own proteins or release energy.
病毒没有细胞质，没有核糖体， 不能制造自己的蛋白质，也不能释放能量。
It can only copy itself inside a living host cell — which is why we say it sits at the very edge of life.
它只能在活的宿主细胞内复制自己—— 这就是为什么我们说它处在生命的最边缘。
The generalised diagram shows the three possible parts.
这张概括图显示了可能的三个部分。
First, a coiled nucleic acid strand — the genetic material, either DNA or RNA.
第一，盘绕的核酸链——遗传物质，要么是DNA，要么是RNA。
Second, the protein capsid wrapped tightly around that core.
第二，紧紧包住核心的蛋白质衣壳。
Third, in some viruses only, a lipid envelope of phospholipids studded with glycoprotein spikes.
第三，仅在有些病毒中才有的脂质包膜，由磷脂构成， 上面镶着糖蛋白刺突。
All viruses are non-cellular: they are not built from cells.
所有病毒都是非细胞的：它们不是由细胞构成的。
They have no cytoplasm, no organelles and no ribosomes, so they cannot respire or synthesise proteins on their own.
它们没有细胞质、没有细胞器、没有核糖体，所以自己不能呼吸，也不能合成蛋白质。
Here is a real virus, magnified hugely under a transmission electron microscope.
这是一个真实的病毒，在透射电子显微镜下被放大了很多倍。
The rounded head is the protein capsid wrapped around the nucleic acid core.
圆圆的头部是包住核酸核心的蛋白质衣壳。
The tail injects that nucleic acid into a bacterium — this kind is called a bacteriophage.
尾部把那份核酸注入细菌—— 这种病毒叫做噬菌体。
Note the scale bar: the whole particle is about one hundred nanometres, far smaller than any cell.
注意比例尺：整个颗粒大约一百纳米，比任何细胞都小得多。
Viruses sit at the edge of what we call living, because they can only copy themselves inside a living host cell.
病毒处在我们所说的生命边缘，因为它们只能在活的宿主细胞内复制自己。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, for magnification, convert every length to the same unit before you divide.
第一，算放大倍数时，相除之前先把每个长度换成同一个单位。
Second, never confuse resolution with magnification — resolution is the limit on detail, and electrons beat light because their wavelength is shorter.
第二，绝不要把分辨率和放大倍数搞混——分辨率是细节的极限，电子胜过光，是因为它的波长更短。
Third, describe each organelle as a structure plus a function, joined together.
第三，把每个细胞器描述成结构加功能，合在一起写。
Fourth, nail the prokaryote differences: no nucleus, smaller seventy-S ribosomes, no membrane-bound organelles, and circular DNA.
第四，牢记原核细胞的区别：没有细胞核， 更小的70S核糖体，没有膜包被的细胞器，以及环状DNA。