Cell Structure and Function
AP Biology Topic 2 8:05 English narration · English + 中文 subtitles burned in
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Transcript
Look at a leaf under a microscope.
在显微镜下看一片树叶。
You see rows of green boxes, and each box is packed with tiny green discs.
你会看到一排排绿色的小盒子，每个盒子里都塞满了细小的绿色圆盘。
Now look at a thin slice of onion. Neat brick walls in perfect lines, with a dark dot inside each one.
再看洋葱的一薄片：整齐的砖墙排成一行行，每一格里都有一个深色的点。
And now your own blood: soft red discs with a dent in the middle.
再看你自己的血液：柔软的红色圆盘，中间有一个凹陷。
Three very different shapes, and every one of them is a cell.
三种完全不同的形状， 而它们每一个都是细胞。
Unit Two, in six steps.
第二单元，分六步。
First, the organelles and the endomembrane system.
第一，细胞器和内膜系统。
Second, why cells stay small: surface area to volume.
第二，细胞为什么这么小：表面积与体积之比。
Third, the fluid mosaic membrane.
第三，流动镶嵌模型的膜。
Fourth, what can cross it.
第四，什么能穿过它。
Fifth, passive and active transport.
第五，被动运输和主动运输。
And sixth, tonicity, water potential, and where organelles came from.
第六，张力、水势，以及细胞器是从哪里来的。
A typical animal cell.
一个典型的动物细胞。
The cell membrane is the outer skin; it decides what goes in and out.
细胞膜是最外面的皮肤，它决定什么进来、什么出去。
Inside is the cytoplasm, the watery jelly where most reactions happen.
里面是细胞质，那层水样的胶质，大多数反应都在这里进行。
These tiny dots are ribosomes, which build proteins.
这些小点是核糖体，负责合成蛋白质。
The big round one is the nucleus; it stores the DNA.
这个又大又圆的是细胞核，它储存DNA。
And these are the mitochondria, where respiration makes ATP. The little sacs are lysosomes, full of enzymes that break down worn-out parts.
这些是线粒体，呼吸作用在这里制造ATP。
A plant cell has three extras.
植物细胞多了三样东西。
Outside the membrane is a cell wall of cellulose; it is strong, so the cell cannot burst.
细胞膜的外面是纤维素做的细胞壁，它很结实，所以细胞不会胀破。
In the middle sits a large vacuole full of sap, which keeps the cell firm.
中间是一个装满细胞液的大液泡，它让细胞保持坚挺。
And the green ovals are chloroplasts, for photosynthesis.
这些绿色的椭圆是叶绿体，用于光合作用。
Note the order: the wall is outside the membrane.
注意顺序：细胞壁在细胞膜的外面。
Now watch one protein travel.
现在看一个蛋白质的旅程。
It starts in the nucleus, where the gene is copied.
它从细胞核开始，基因在这里被转录。
The message goes to the rough endoplasmic reticulum, rough because ribosomes sit on it, and the protein is built there.
信息传到粗面内质网，之所以叫粗面，是因为上面附着核糖体，蛋白质就在那里合成。
A bubble takes it to the Golgi, which trims and packs it.
一个小泡把它送到高尔基体，在那里被修剪和包装。
The Golgi buds off a vesicle.
高尔基体再出芽形成一个囊泡。
The vesicle reaches the cell membrane and pushes the protein out.
囊泡到达细胞膜，把蛋白质推到细胞外面。
Make it, modify it, package it, and ship it out of the cell.
合成，修饰，包装，然后送出细胞。
Why do cells stay small?
细胞为什么这么小？
Look at three cubes.
看这三个立方体。
The little one has six units of surface for every unit of volume: six to one.
最小的那个，每一份体积对应六份表面积：六比一。
Double the side and the ratio falls to three to one.
边长加倍，比值就降到三比一。
Triple it and it is only two to one.
边长变成三倍，就只剩二比一了。
Volume grows faster than surface, so a big cell cannot feed its own inside.
体积增长得比表面积快，所以大细胞养不活自己的内部。
Now the exam calculation.
现在做考试计算。
A cube-shaped cell is two micrometres on each side.
一个立方体形状的细胞，边长两微米。
First the surface area: a cube has six faces, so six times two squared is twenty-four.
先求表面积：立方体有六个面，六乘以二的平方，等于二十四。
Next the volume: two cubed is eight.
再求体积：二的三次方等于八。
So the ratio is twenty-four over eight, which is three.
所以比值是二十四除以八，等于三。
Now double the side to four micrometres.
现在把边长加倍到四微米。
Surface area is ninety-six; volume is sixty-four.
表面积变成九十六，体积变成六十四。
The ratio is now one point five.
比值现在是一点五。
Doubling the size halved the ratio, and that is why cells stay small.
尺寸加倍，比值就减半，这正是细胞保持很小的原因。
Now the membrane itself: a phospholipid bilayer.
现在看膜本身：磷脂双分子层。
The water-loving heads face the water; the water-hating tails hide inside.
亲水的头部朝向水，疏水的尾部藏在里面。
Floating in it are proteins.
膜里漂浮着蛋白质。
A channel protein makes a pore.
通道蛋白形成一个孔。
A carrier protein changes shape to move a passenger.
载体蛋白改变形状来运送乘客。
Cholesterol steadies the fluidity.
胆固醇稳定膜的流动性。
And sugar chains outside make glycoproteins, the cell's name tags.
外面的糖链构成糖蛋白，也就是细胞的名牌。
Everything drifts, so we call it the fluid mosaic model.
一切都在漂动，所以我们叫它流动镶嵌模型。
So what actually gets through?
那么到底什么能通过？
The oily middle decides.
由中间那层油性的部分决定。
Small non-polar molecules, like oxygen and carbon dioxide, slip straight through, and water crosses in small amounts.
小的非极性分子，比如氧气和二氧化碳， 可以直接穿过，水也能少量通过。
But anything large, or anything charged, is blocked.
但任何大分子，或者任何带电的粒子，都会被挡住。
Glucose cannot pass, and neither can a sodium ion.
葡萄糖过不去，钠离子也过不去。
That is what selectively permeable means.
这就是选择透过性的意思。
Three ways across, side by side.
三种跨膜方式，并排比较。
On the left, simple diffusion: down the gradient, straight through the lipid, no energy.
左边是简单扩散：顺着浓度梯度，直接穿过脂质，不需要能量。
In the middle, facilitated diffusion: still down the gradient and still free, but through a channel or a carrier protein, because the particle is too big or too charged.
中间是易化扩散：同样顺着梯度，同样不耗能，但要通过通道蛋白或载体蛋白， 因为这些粒子太大或者带电。
On the right, active transport: a pump pushes particles against the gradient, and that costs ATP.
右边是主动运输：泵把粒子逆着梯度推过去，这要消耗ATP。
Water has its own name for this.
水有它自己的名字。
Osmosis is the diffusion of water across a partially permeable membrane.
渗透是水通过半透膜的扩散。
Here water can cross, but the solute cannot.
这里水可以通过，但溶质不能。
There is more solute on the right, so that side has less free water.
右边的溶质更多，所以右边的自由水更少。
Water crosses to the right, and the level climbs.
水就向右边移动，右边的液面升高。
Only water is moving.
只有水在移动。
Now put real cells into solutions. That comparison has a name: tonicity.
现在把真实的细胞放进不同的溶液。
In a hypotonic solution the outside has a higher water potential, so water enters.
左边外界的水势更高，所以水进入细胞。
In a hypertonic one it is lower, so water leaves.
右边水势更低，所以水流出。
In an isotonic solution there is no net change, and keeping that balance is osmoregulation.
先看植物细胞。 水进去让它变得坚挺，紧压着细胞壁。
Take the plant cell. Water in makes it turgid, pressing on its wall. Water out makes it plasmolysed: the contents pull away.
水出来就发生质壁分离：内容物从细胞壁上收缩离开。
Now the animal cell, with no wall.
再看动物细胞，它没有细胞壁。
Water in makes it swell, and it may burst.
水进去让它膨胀，可能会胀破。
Water out makes it shrink, so your blood needs an isotonic solution.
水出来让它皱缩，所以你的血液需要等渗溶液。
Now the second AP formula.
现在是第二个AP公式。
Water potential is a pressure term plus a solute term.
水势等于压力势加上溶质势。
Our solution: zero point one molar sugar, open beaker, twenty-five degrees.
我们的溶液：零点一摩尔的糖溶液，敞口烧杯，二十五摄氏度。
Start with the pressure term: the beaker is open, so it is zero.
先看压力势：烧杯是敞开的，所以它等于零。
Then the solute term: multiply the ionisation constant, the concentration, the gas constant and the temperature, and make it negative.
再看溶质势：把电离常数、浓度、气体常数和温度相乘， 然后取负号。
That gives minus two point four eight bar.
结果大约是负二点四八巴。
Now compare.
现在来比较。
The plant cell in it sits at minus one point zero bar.
里面的植物细胞是负一点零巴。
Water moves to the lower, more negative potential.
水会向更低、更负的水势移动。
So water leaves the cell, and the cell loses its turgor and shrinks.
所以水离开细胞，细胞失去膨压并皱缩。
These are real red onion cells, left in a strong salt solution.
这些是真实的红洋葱细胞，泡在浓盐水里。
Look at the edge of each cell.
看每个细胞的边缘。
The purple contents have pulled away from the wall, leaving a clear gap.
紫色的内容物已经从细胞壁上收缩离开，留下一条明显的空隙。
That is plasmolysis, and it is exactly what the calculation predicted.
这就是质壁分离，和我们刚才算出来的结果完全一致。
Some cargo is far too big for any pore.
有些货物太大了，任何蛋白通道都过不去。
For that, the cell uses the membrane itself.
这时细胞就用膜本身。
In endocytosis the membrane folds inward around the material, then pinches off a vesicle inside the cell.
内吞作用中，细胞膜向内凹陷包住物质，然后掐断形成一个囊泡进入细胞。
In exocytosis a vesicle travels out, fuses with the membrane, and releases its contents outside.
外排作用中，囊泡向外移动，与细胞膜融合，把内容物释放到外面。
Both cost energy.
两者都要消耗能量。
One last idea — compartmentalization inside the cell, and where it came from.
最后一个概念。 膜不只是包裹细胞。
Membranes do more than wrap the cell. Inside, they build separate compartments — rooms — so reactions that would fight each other can run at the same time.
在细胞内部，膜隔出一个个独立的小房间， 让本来会互相干扰的反应可以同时进行。
But where did the rooms with two membranes come from?
那么这些有两层膜的小房间是从哪里来的？
That is the endosymbiotic theory. Long ago, a large cell swallowed a bacterium, and it was not digested.
很久以前，一个大细胞吞下了一个细菌，而这一次它没有被消化。
It stayed inside and became the mitochondrion.
它留在里面，变成了线粒体。
Three pieces of evidence support this: its own circular DNA, its own ribosomes, and two membranes.
有三条证据支持这一点：它自己的环状DNA、它自己的核糖体，以及两层膜。
The chloroplast has the same story.
叶绿体也是同样的故事。
Four marks students lose.
四个学生常失的分。
One: water moves to the lower water potential, and lower means more negative.
第一：水向水势更低的一侧移动，更低意味着更负。
Two: facilitated diffusion is still passive; it uses a protein, but no ATP.
第二：易化扩散仍然是被动的，它要用到蛋白质，但不消耗ATP。
Three: when something is blocked, say why: small non-polar molecules cross the oily core, but large or charged ones need a protein.
第三：说某样东西被挡住时，一定要说清原因：小的非极性分子能穿过油性核心， 而大分子或带电粒子需要蛋白质。
Four: as a cell grows, the ratio falls, because volume grows faster than surface.
第四：细胞长大时，比值会下降，因为体积增长得比表面积快。
Get those right, and Unit Two is yours.
把这些做对，第二单元就是你的了。