Cell Membranes
A-Level Biology Topic 4 14:03 English narration · English + 中文 subtitles burned in
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Every cell must keep the right things in and the wrong things out.
每个细胞都必须把该留的留住、把不该进的挡在外面。
That job falls to a single, astonishingly thin layer — the cell surface membrane, just two molecules thick.
这项工作落在一层薄得惊人的结构上—— 细胞膜，只有两个分子那么厚。
It is not a solid wall.
它不是一堵实心的墙。
It is partially permeable: small, non-polar molecules like oxygen slip straight through, while large or charged particles are turned away.
它是半透的：像氧气这样又小又非极性的分子 能直接穿过，而大分子或带电粒子则被挡回。
Understanding that gatekeeper is the key to understanding the cell.
理解这个守门人，就是理解细胞的关键。
In this lesson we build the membrane from its parts using the fluid mosaic model, watch cells send chemical messages to one another, then work through every way a substance can cross: diffusion, facilitated diffusion, osmosis, active transport, and bulk transport.
在这节课里，我们用流动镶嵌模型把膜一部分一部分搭起来，看细胞之间如何传递化学信息， 然后逐一走过物质跨膜的每一种方式：扩散、易化扩散、渗透、主动运输和大量运输。
Finally we ask why size matters, and what happens to a cell placed in the wrong solution.
最后我们要问：为什么大小很重要，以及把细胞放进不合适的溶液会发生什么。
Let's begin.
让我们开始吧。
We describe the membrane with the fluid mosaic model, and it begins with one molecule that cannot make up its mind.
我们用流动镶嵌模型来描述细胞膜，而它的起点是一种"拿不定主意"的分子。
The foundation is a phospholipid bilayer: each phospholipid has a hydrophilic, water-loving head and two hydrophobic, water-fearing tails.
基础是磷脂双层：每个磷脂都有一个亲水的头和两条疏水的尾。
Drop those into water and nothing has to arrange them — the heads turn outward towards the water on both sides, the tails hide from it in the middle, and a sheet two molecules thick assembles itself.
把它们扔进水里，不需要任何东西去摆布它们—— 头自动转向两侧的水，尾巴躲进中间，一层两个分子厚的膜就这样自己组装起来。
That is worth holding on to: nothing builds a membrane, it forms because it is the only arrangement that keeps every part of the molecule happy.
这一点值得记住：没有谁去"建造"细胞膜，它之所以形成， 是因为这是唯一能让分子每一部分都各得其所的排列方式。
Dotted through this layer are proteins, and two extras complete the picture — cholesterol, and carbohydrate chains on the outer face.
这一层里散布着蛋白质，还有两样补全了这幅图——胆固醇， 以及外表面上的糖链。
What each of them is FOR is the next beat.
它们各自是干什么用的，下一段就讲。
Look at the model more carefully.
再仔细看这个模型。
It is fluid because the phospholipids are not locked in place — they slide past one another, so the membrane can bend and reseal.
它叫流动，是因为磷脂并不固定，它们彼此滑过，所以膜可以弯曲并重新封闭。
It is a mosaic because proteins sit dotted through the sheet like tiles in a picture.
它叫镶嵌，是因为蛋白质像马赛克瓷砖一样散布在这张薄片里。
Channel proteins form water-filled pores for ions; carrier proteins bind one kind of molecule and ferry it across.
通道蛋白形成充水的孔供离子通过； 载体蛋白结合某一种分子并把它运送过去。
Cholesterol sits between the tails: it steadies fluidity and adds strength.
胆固醇夹在尾巴之间：它稳住流动性并增加强度。
Glycoproteins and glycolipids carry carbohydrate chains on the outer face for cell recognition; some act as antigens that the immune system can read.
糖蛋白和糖脂在外表面带着糖链，用于细胞识别；有些充当抗原，可被免疫系统读出。
Together these molecules give the membrane its stability, its fluidity, its partial permeability, plus its transport, signalling, and recognition jobs.
这些分子合在一起，赋予膜稳定性、流动性、部分通透性，以及运输、信号传递和识别的工作。
Cells also talk to each other, and the membrane is where the conversation lands.
细胞之间也会交谈，而细胞膜正是对话落地的地方。
One cell secretes a signalling chemical called a ligand — a hormone, for example.
一个细胞分泌一种叫做配体的信号化学物质—— 比如激素。
It travels, often in the blood, to a target cell.
它常常随血液运送到靶细胞。
There it binds to a receptor on the target cell's membrane, and here is the beautiful part: the receptor's shape matches that one ligand and no other.
在那里它与靶细胞膜上的受体结合，而美妙之处在于： 受体的形状只与那一种配体相配，别无其他。
So only cells carrying the right receptor respond.
所以只有带着对应受体的细胞才会响应。
Binding then triggers a specific response inside that cell.
结合随后在那个细胞内部触发一个特定的反应。
Here are the three stages drawn out.
这里画出了三个阶段。
First a signalling cell secretes the ligand.
第一，信号细胞分泌配体。
Second the ligand is carried — often in the blood — until it reaches a target cell.
第二，配体被运送，常常在血液中，直到到达靶细胞。
Third it binds a receptor whose shape fits that ligand only, like a key in one lock.
第三，它与一个形状只适合该配体的受体结合，就像一把钥匙只开一把锁。
Binding then sets off a cascade inside the target cell.
结合随后在靶细胞内启动一连串反应。
Cells without that receptor ignore the message completely.
没有那种受体的细胞会完全忽略这条信息。
So the membrane is not only a barrier: it is a mailbox that only opens for the right letter.
所以膜不只是屏障：它是一个只给对的信打开的信箱。
Exam answers should name ligand, target cell, and receptor, and say the shapes match.
答题时应写出配体、靶细胞和受体，并说明形状相互匹配。
Now, how do things cross?
那么，物质是怎么跨膜的？
The simplest way needs no help and no energy.
最简单的方式既不需要帮助，也不需要能量。
In simple diffusion, particles move from a high concentration to a low concentration — down the concentration gradient — passing straight through the bilayer.
在简单扩散中， 粒子从高浓度移向低浓度——沿着浓度梯度——直接穿过双层。
Only small or non-polar molecules manage it, like oxygen and carbon dioxide.
只有又小或非极性的分子能做到， 比如氧气和二氧化碳。
But charged ions and large polar molecules such as glucose cannot cross that oily middle.
但带电的离子和像葡萄糖这样的大极性分子，无法穿过那层油性的中间。
They use facilitated diffusion, slipping through a channel protein or riding a carrier protein — still down the gradient, still no energy needed.
它们用易化扩散，从通道蛋白里溜过去，或者搭上载体蛋白——依然是顺着梯度，依然不需要能量。
Both are passive.
两者都是被动的。
Water gets its own name for the same process: osmosis.
水在同一个过程里有它自己的名字：渗透。
It is the diffusion of water across a partially permeable membrane.
它是水通过半透膜的扩散。
But we describe it with water potential, written psi.
但我们用水势来描述它， 记作 psi。
Pure water has the highest water potential, and adding any solute lowers it.
纯水的水势最高，加入任何溶质都会把它降低。
Water always moves from a higher water potential to a lower one — so it flows towards the side with more solute, because the solute itself is too big to cross.
水总是从较高的水势流向较低的水势—— 所以它流向溶质更多的那一侧，因为溶质本身太大，过不去。
Get the direction right and osmosis questions become easy marks.
方向搞对了，渗透题就是轻松得分。
Picture a container split by a partially permeable membrane.
想象一个容器被半透膜隔开。
On one side there are few solutes and a high water potential; on the other, many solutes and a lower water potential.
一侧溶质很少、水势高；另一侧溶质很多、水势较低。
Water crosses towards the lower side.
水朝较低的一侧穿过。
The solute particles are too large to pass, so only water moves.
溶质粒子太大过不去，所以只有水在移动。
That is why we never say water moves from dilute to concentrated as if solutes were the story — water potential is the story.
这就是为什么我们不要说水从稀溶液移向浓溶液，好像溶质才是主角，水势才是主角。
Pure water has the highest water potential; every solute you dissolve lowers it.
纯水的水势最高；你溶解的每一种溶质都会把它降低。
Osmosis is always passive: no energy is spent.
渗透始终是被动的：不消耗能量。
You can see osmosis in a classic school set-up: a potato osmometer.
你可以在一个经典的学校装置里看到渗透：马铃薯渗透计。
A well of concentrated sugar solution sits in the potato tissue, with a tube standing above it.
浓缩糖溶液的小井开在马铃薯组织里， 上面竖着一根管子。
Water from the potato cells — and from any surrounding water of higher water potential — moves into the sugar well by osmosis.
来自马铃薯细胞，以及周围水势更高的水，的水，通过渗透进入糖井。
The liquid level in the tube rises.
管中液面上升。
The potato tissue is living, so its own water potential matters too.
马铃薯组织是活的，所以它自己的水势也很重要。
To estimate the water potential of plant tissue in the lab, place pieces in sucrose solutions of different strengths.
要在实验室估计植物组织的水势，把组织块放进不同浓度的蔗糖溶液里。
The solution that causes no change in mass or length has about the same water potential as the tissue.
使质量或长度几乎不变的那份溶液，水势大约与组织相同。
Sometimes a cell needs to move a substance the other way — from a low concentration to a high one.
有时候细胞需要把物质往相反的方向搬——从低浓度搬到高浓度。
Diffusion will never do that, so the cell must pay.
扩散绝不会这么做， 所以细胞必须付出代价。
This is active transport.
这就是主动运输。
A carrier protein binds the substance, changes shape, and pushes it across against the concentration gradient.
载体蛋白结合物质，改变形状，逆着浓度梯度把它推过去。
That shape change costs energy, supplied by ATP splitting into ADP and phosphate.
这个形状改变要消耗能量，由 ATP 分解成 ADP 和磷酸来提供。
So the test is simple: if it goes against the gradient, it needs ATP.
所以判断很简单： 只要是逆着梯度，就需要 ATP。
Three panels side by side make the contrast clear.
三幅并排的图把对比说清楚。
Simple diffusion: a small non-polar molecule slips straight through the bilayer, high concentration to low, no protein, no energy.
简单扩散：又小又非极性的分子直接穿过双层，从高浓度到低浓度， 不要蛋白质，不要能量。
Facilitated diffusion: an ion or a large polar molecule such as glucose uses a channel or a carrier, still down the gradient, still passive.
易化扩散：离子或像葡萄糖这样的大极性分子，借助通道或载体， 仍然顺着梯度，仍然是被动的。
Active transport: a carrier pumps the substance against the gradient and burns ATP to change shape.
主动运输：载体逆着梯度泵送物质，并消耗三磷酸腺苷来改变形状。
If the exam shows an arrow going uphill and a little ATP symbol, you know which process it is.
如果考题画了一支往上爬的箭头和一点点能量符号，你就知道是哪一种过程。
If the arrow goes downhill with no energy cost, it is one of the two passive routes.
如果箭头往下走且没有能量代价，那就是两条被动路径之一。
For anything too large for a protein — a whole bacterium, or a package of hormone — the membrane moves it in bulk, and this also costs ATP.
对于任何大到蛋白质无法运送的东西——一整个细菌，或一包激素——细胞膜会把它整批搬运， 这同样需要 ATP。
In endocytosis, the membrane folds inwards around the material and pinches off a vesicle, carrying it into the cell.
在胞吞作用中，膜向内折叠，包住物质，掐断形成一个囊泡，把它带进细胞。
Watch a white blood cell do exactly that as it engulfs a bacterium.
看一个白细胞正是这样吞下一个细菌。
Exocytosis is the reverse: a vesicle travels to the membrane, fuses with it, and releases its contents outside.
胞吐作用则相反：囊泡移动到膜上，与膜融合， 把内容物释放到外面。
The diagram freezes both directions.
这张图把两个方向都定格下来。
In endocytosis the membrane sinks inward around the cargo, then pinches closed so a vesicle floats free inside the cytoplasm.
在胞吞作用中，膜向内陷包住货物，然后掐断封闭， 于是一个囊泡在细胞质里自由漂浮。
That is how cells take in large particles, droplets of fluid, or whole microbes.
细胞就是这样吞进大颗粒、液滴或整只微生物。
In exocytosis a vesicle made earlier — often by the Golgi body — docks with the surface membrane, the two bilayers fuse, and the contents spill outside.
在胞吐作用中，先前做好的囊泡，常常来自高尔基体，停靠在表面膜上，两层双分子层融合， 内容物倾泻到外面。
Both need ATP because the membrane must actively reshape.
两者都需要能量，因为膜必须主动改变形状。
Secreted hormones, digestive enzymes, and waste packages all leave this way.
分泌的激素、消化酶和废物包裹都这样离开。
Bulk transport is active, even though no single carrier protein is pumping one molecule at a time.
大量运输是主动的， 尽管并没有单个载体蛋白一次只泵一个分子。
Let's sort all six.
我们把六个过程分类。
The passive processes need no ATP and go down the concentration gradient: simple diffusion straight through the bilayer, facilitated diffusion through a channel or carrier, and osmosis, which is water moving from high to low water potential.
被动过程不需要 ATP，顺着浓度梯度进行：简单扩散直接穿过双层， 易化扩散通过通道或载体，以及渗透——水从高水势流向低水势。
The active processes need ATP and can go against the gradient: active transport pumped by a carrier, endocytosis bringing material in by vesicle, and exocytosis releasing it out.
主动过程需要 ATP，可以逆着梯度： 主动运输由载体泵送，胞吞作用用囊泡把物质带进来，胞吐作用把它释放出去。
If an exam asks you to classify a process, this is the split it wants.
如果考题让你给某个过程分类，它要的就是这个划分。
Why can't a cell just grow bigger?
细胞为什么不能一直长大？
Because everything enters and leaves across its surface.
因为一切进出都要经过它的表面。
For a cube of side L, the surface area is six L squared, the volume is L cubed, and the ratio is simply six over L.
对一个边长为 L 的正方体， 表面积是六 L 平方，体积是 L 三次方，而比值就是六除以 L。
As L grows, the volume outruns the surface, so the ratio falls.
随着 L 变大， 体积增长快过表面积，于是比值下降。
Compare a cube of side four with one of side ten: the small one gives ninety-six to sixty-four, about one point five to one; the large one gives six hundred to a thousand, only zero point six to one.
把边长四的正方体和边长十的比一比： 小的是九十六比六十四，约一点五比一；大的是六百比一千，只有零点六比一。
The bigger cell exchanges more slowly for its size — which is exactly why large organisms need lungs and gills.
大的细胞按它的大小来说交换得更慢——这正是大型生物需要肺和鳃的原因。
The cubes of side one, two and three show the pattern at a glance: ratios of six to one, three to one, and two to one.
边长为一、二和三的立方体一眼就看出规律：比值是六比一、三比一和二比一。
As the cube grows, volume outruns surface, and the ratio falls.
立方体变大时，体积增长快过表面积，比值下降。
You can prove the idea with agar blocks of different sizes soaked in dye or acid: the smallest block, with the largest surface area to volume ratio, changes colour all the way through first.
你可以用不同大小的琼脂块浸在染料或酸里来证明： 最小的块表面积与体积之比最大，整块变色也最快。
Diffusion across non-living materials is also studied with dialysis tubing — sometimes called Visking tubing — a partially permeable bag that lets water and small solutes through but holds back large ones.
非生命材料上的扩散也可用透析袋研究，有时叫维斯金管，这是一种半透袋， 让水和小溶质通过，却挡住大分子。
Small cells and thin flat shapes exchange quickly; large cells cannot rely on diffusion alone.
小细胞和薄而扁的形状交换快；大细胞不能单靠扩散。
Finally, put a cell into a solution and watch what the wall does, because the wall is the whole story.
最后，把细胞放进溶液，看看细胞壁在做什么，因为细胞壁才是关键。
A plant cell in a solution of higher water potential takes water in and swells — and as the contents press outward, the wall pushes back.
植物细胞在水势较高的溶液中吸水膨胀——当内容物向外挤压时，细胞壁会往回顶。
That rising back-pressure opposes any more water entering, so the cell stops swelling before it can burst.
这个不断上升的反压力会阻止更多的水进来， 于是细胞在胀破之前就停止了膨胀。
The pressure is turgor, and it is why a plant stands upright with no skeleton at all.
这个压力就是膨压， 它正是植物没有骨骼却能挺立的原因。
An animal cell has no wall, so nothing ever pushes back: water keeps arriving until the membrane gives way.
动物细胞没有细胞壁， 所以没有任何东西往回顶：水会一直进来，直到细胞膜撑不住为止。
Same gradient, same direction of flow, opposite ending — and the only difference is the wall.
同样的梯度，同样的水流方向，结局却相反——而唯一的差别就是那道壁。
The grid next holds all four outcomes together.
接下来的表格会把四种结果放在一起看。
Take the effect on plant cells and the effect on animal cells separately.
把对植物细胞的影响和对动物细胞的影响分开看。
In a solution of lower water potential a plant cell's contents shrink and the membrane pulls away from the wall — that is plasmolysis.
在水势更低的溶液里，植物细胞的内容物收缩，细胞膜脱离细胞壁——这就是质壁分离。
An animal cell has no wall, so the same conditions burst it or shrivel it instead.
动物细胞没有细胞壁，所以同样的条件下它会胀破或者皱缩。
Hold the four outcomes together.
把四种结果放在一起记。
A plant cell in distilled water — a solution of higher water potential — takes water in and becomes turgid; the wall stops it bursting.
植物细胞在蒸馏水，水势更高的溶液，里吸水并变得膨胀坚挺； 细胞壁阻止它胀破。
In a solution of lower water potential it loses water and becomes plasmolysed: the membrane peels away from the wall.
在水势较低的溶液里它失水并发生质壁分离：膜从壁上剥离。
An animal cell has no wall.
动物细胞没有细胞壁。
In higher water potential it swells and may lyse; in a red blood cell that bursting is haemolysis.
在较高水势中它膨胀，甚至可能裂解；在红细胞里这种胀破叫做溶血。
In lower water potential water leaves and the cell shrinks or crenates.
在较低水势中水离开，细胞皱缩或出现锯齿状皱缩。
Always link the outcome to the water potential gradient, not just to the word concentrated.
永远把结果和水势梯度联系起来，不要只说浓缩。
One last exam habit for water potential.
关于水势，还有一个考试习惯。
Pure water is set at zero — the highest value.
纯水定为零，最高值。
Every real solution is negative, and more solute makes the number more negative, so the water potential is lower.
每一种真实溶液都是负的， 溶质越多数值越负，水势也就越低。
Water always moves from high — less negative — to low — more negative.
水总是从高，负得较少，流向低，负得较多。
Say that phrase in full when you write about osmosis.
写渗透时把这句话说完整。
Name the cell type and the outcome: plant cells become turgid or plasmolysed; animal cells lyse or crenate.
点明细胞类型和结果：植物细胞变得膨胀坚挺或发生质壁分离； 动物细胞裂解或皱缩。
Link each outcome to which side had the higher water potential.
把每一种结果与哪一侧水势更高联系起来。
That language scores the marks that vague talk of concentration misses.
这样的用语能拿到空泛地说浓度所拿不到的分数。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, when you describe the membrane, name all the parts of the fluid mosaic — bilayer, proteins, cholesterol, glycoproteins.
第一，描述细胞膜时，把流动镶嵌的各个部分都写全—— 双层、蛋白质、胆固醇、糖蛋白。
Second, sort every process into passive or active, and say whether it needs ATP.
第二，把每个过程都归入被动或主动，并说明它是否需要 ATP。
Third, always talk about osmosis using water potential, moving from high to low — never just say concentration.
第三，讲渗透时永远用水势，从高到低——不要只说浓度。
Fourth, remember that a bigger cell has a smaller surface area to volume ratio, so it exchanges more slowly.
第四，记住细胞越大，表面积与体积之比越小，所以交换越慢。