Movement Into and Out of Cells
IGCSE Biology Topic 3 7:44 English narration · English + 中文 subtitles burned in
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A cell is surrounded by a membrane, and it must keep trading with the world outside.
细胞被细胞膜包围，它必须不断和外界交换物质。
Oxygen has to get in.
氧气必须进来。
Carbon dioxide has to get out.
二氧化碳必须出去。
Water moves both ways.
水两个方向都在移动。
Minerals have to be pulled in from the soil.
矿物质必须从土壤中被吸进来。
There are three ways this happens, and the exam wants you to tell them apart.
这有三种方式， 考试要求你能区分它们。
Two of them are free.
其中两种是免费的。
One of them costs the cell energy.
有一种要消耗细胞的能量。
Welcome to movement into and out of cells.
欢迎来到物质进出细胞。
Diffusion, osmosis, and active transport.
扩散、渗透，还有主动运输。
Let's begin.
让我们开始吧。
Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated.
扩散是粒子从浓度较高的区域向浓度较低的区域的净移动。
We say they move down a concentration gradient.
我们说它们沿着浓度梯度移动。
Notice the word net.
注意"净"这个字。
Particles move in both directions all the time, but the overall flow is downhill.
粒子一直在两个方向上移动， 但总体的流动是向下的。
Nothing pushes them — they spread out because of their own random movement, and the energy comes from the kinetic energy those particles already have.
没有东西推它们——它们因为自身的随机运动而散开， 能量来自这些粒子本来就有的动能。
So diffusion is free.
所以扩散是免费的。
It needs no energy from the cell.
它不需要细胞提供能量。
Why diffusion matters: many gases and dissolved solutes move this way in living things. Oxygen diffuses into a cell for respiration, and carbon dioxide diffuses out.
在生物体内，氧气扩散进细胞用于呼吸作用，二氧化碳扩散出去。
Four things change how fast diffusion happens, and each one is worth a mark.
有四样东西会改变扩散的快慢，每一个都值分。
A larger surface area is faster, because more particles can cross at once.
表面积越大越快， 因为同时能通过的粒子更多。
A higher temperature is faster, because the particles have more kinetic energy and move more quickly.
温度越高越快，因为粒子有更多动能，移动得更快。
A steeper concentration gradient is faster — that means a bigger difference between the two sides.
浓度梯度越陡越快——就是说两边的差别越大。
And a shorter distance is faster, which means a thinner barrier to cross.
距离越短越快， 也就是要穿过的屏障越薄。
Bigger, hotter, steeper, thinner: all four make diffusion faster.
更大、更热、更陡、更薄：这四样都让扩散更快。
Now watch water.
现在看水。
Water is an excellent solvent — substances dissolve in it to make a solution — and that is what makes digestion, excretion and transport possible at all: food must dissolve before it can be used, wastes leave dissolved, and everything travels round the body dissolved in blood or sap.
水是极好的溶剂——物质溶解在水里形成溶液—— 而这正是消化、排泄和运输之所以可能的根本原因： 食物必须先溶解才能被利用，废物以溶解的形式排出， 一切物质都溶解在血液或树液中，在体内运送。
Here a membrane separates two solutions, and only the small water molecules can get through it.
这里一层膜隔开两种溶液，只有小的水分子能通过它。
The larger solute particles are stuck.
较大的溶质颗粒过不去。
Water crosses both ways, but more of it crosses toward the concentrated side.
水两个方向都在穿过，但更多的水流向浓的那一边。
Osmosis is a special kind of diffusion, and you must be exact about it.
渗透是一种特殊的扩散，你必须说得准确。
Osmosis is the net movement of water molecules, and only water, across a partially permeable membrane.
渗透是水分子的净移动，只有水， 穿过半透膜。
A partially permeable membrane has holes small enough to let water molecules through while holding the bigger solute particles back.
半透膜上的孔小到只让水分子通过，而把较大的溶质颗粒挡住。
Remember which solution is which.
记住哪种溶液是哪种。
A concentrated solution has a lot of solute and little water.
浓溶液有很多溶质、很少水。
A dilute solution has little solute and a lot of water.
稀溶液有很少溶质、很多水。
For the supplement paper, say it with water potential: water moves from a higher water potential, which is the dilute side, to a lower water potential, which is the concentrated side.
补充部分要用水势来说：水从水势较高的一边，也就是稀的一边， 移动到水势较低的一边，也就是浓的一边。
Put a plant cell in pure water or a dilute solution, and water moves in.
把植物细胞放进纯水或稀溶液里，水就移动进去。
The cell swells and pushes out against its cell wall, and the cell becomes turgid.
细胞胀大，向外顶住细胞壁， 细胞变得膨胀。
That outward push is called turgor pressure, and it is what holds a plant up straight.
这个向外的压力叫膨压，正是它让植物挺立。
Now put the cell in a concentrated solution instead.
现在把细胞放进浓溶液里。
Water moves out, the cell loses its firmness, and the cell goes flaccid.
水移动出去，细胞失去硬挺，细胞变得松软。
If even more water leaves, the membrane pulls away from the cell wall — this is plasmolysis, and the cell is plasmolysed.
如果流失的水更多，细胞膜就从细胞壁上拉开——这就是质壁分离，细胞发生了质壁分离。
This is the classic experiment.
这是经典实验。
You can use dialysis tubing — an artificial partially permeable membrane — or cut cylinders from a potato, measure their mass, soak each one in a different solution, then measure the mass again.
你从马铃薯切出圆柱，测量它们的质量，把每一个泡在不同的溶液里， 然后再测一次质量。
There are three possible results.
有三种可能的结果。
In a dilute solution, water enters, so the cylinder gains mass and gets longer.
在稀溶液中，水进入，所以圆柱增加质量、变长。
In a concentrated solution, water leaves, so it loses mass and gets shorter.
在浓溶液中，水流出，所以它失去质量、变短。
And if the solution matches the cell contents, nothing changes.
如果溶液和细胞内容物一样浓，就没有变化。
Now the marks.
现在说得分点。
Always calculate the percentage change, not just the change in grams.
永远要算百分比变化，不能只算克数的变化。
Take the change in mass, divide it by the starting mass, and multiply by one hundred.
用质量的变化，除以起始质量，再乘一百。
A cylinder going from five point zero grams to five point six grams gained nought point six grams.
一个圆柱从五点零克变成五点六克， 增加了零点六克。
Nought point six divided by five point zero, times one hundred, is a gain of twelve percent.
零点六除以五点零，乘一百，就是增加百分之十二。
Percentages let you compare cylinders that started at different masses fairly.
百分比让你能公平地比较起始质量不同的圆柱。
The last one is different.
最后一种不一样。
Watch this particle being moved the wrong way — from where it is scarce to where it is already crowded.
看这个颗粒被朝错误的方向搬运——从它稀少的地方， 搬到它已经很拥挤的地方。
That cannot happen on its own.
这不可能自己发生。
Active transport is the movement of particles through a cell membrane from a lower concentration to a higher concentration — that is, against the concentration gradient.
主动运输是粒子通过细胞膜，从浓度较低处移动到浓度较高处——也就是逆着浓度梯度。
Because it is uphill, it cannot be free.
因为是往上走，它不可能免费。
Active transport needs energy from respiration.
主动运输需要来自呼吸作用的能量。
It also needs a protein carrier: a protein sitting in the membrane picks the particle up on one side and carries it across to the other.
它还需要载体蛋白：一个位于细胞膜上的蛋白质在一边把颗粒接住，把它运到另一边。
This is how root hair cells take in mineral ions from the soil, even when those ions are already more concentrated inside the root than outside it.
根毛细胞就是这样从土壤中吸收矿物离子的，即使这些离子在根内部已经比外面更浓。
Now compare the three processes, side by side.
把三者放在一起比较。
Diffusion moves any particle, down the gradient, with no energy from respiration.
扩散移动任何粒子，沿着梯度向下，不需要呼吸作用的能量。
Osmosis moves water only, across a partially permeable membrane, also with no energy from respiration.
渗透只移动水，穿过半透膜，同样不需要呼吸作用的能量。
Active transport is the only one that moves particles against the gradient, and the only one that uses energy from respiration.
主动运输是唯一逆着梯度移动粒子的，也是唯一使用呼吸作用能量的。
If you can fill in this table, you can answer most of this topic.
如果你能填出这张表，这个专题的大部分题目你都能答。
Three marks to lock in, and all three are about wording rather than understanding — you can know this topic perfectly and still lose them.
要锁住三个分，而这三个都关乎措辞，而不是理解—— 你完全可以把这个专题学透，却仍然把它们丢掉。
First: always write the word net when you define diffusion or osmosis.
第一：定义扩散或渗透时，永远要写"净"这个字。
Particles move both ways the whole time, so an answer without "net" describes something that is not actually true.
粒子无时无刻不在两个方向上移动，所以不写"净"的答案， 描述的其实是一件并不成立的事。
Second, name what is moving: osmosis moves water only, so writing "particles move by osmosis" throws the mark away even though you meant the right thing.
第二，说清楚移动的是什么：渗透只移动水， 所以写"粒子通过渗透移动"就把分丢了，哪怕你想的是对的。
Third, name where the energy comes from: only active transport uses energy from respiration — and it is the phrase "from respiration" that is being marked, not the bare word energy.
第三，说清楚能量从哪来：只有主动运输使用呼吸作用的能量—— 而被判分的正是"来自呼吸作用"这几个字，不是光秃秃的"能量"。
Say what moves, which way it moves, and what pays for it.
说清楚：什么在动、往哪个方向动、由什么来买单。
Five leftover facts.
还剩五件事实。
First, water is a very good solvent: food must dissolve for digestion, wastes are carried away dissolved for excretion, and substances travel dissolved in blood and sap.
第一，水是很好的溶剂：食物必须溶解才能消化，废物溶解后被带走排泄， 物质溶解在血液和树液里运输。
Second, the energy for diffusion is the kinetic energy of the random movement of molecules and ions — so diffusion needs no extra energy.
第二，扩散的能量来自分子和离子随机运动的动能——所以扩散不需要额外能量。
Third, you can investigate osmosis with dialysis tubing, an artificial partially permeable membrane, as well as with potato cylinders.
第三，可以用透析管（人造半透膜）研究渗透，也可以用土豆柱。
Fourth, always work out the percentage change in mass, not just the change in grams, so cylinders that started at different masses compare fairly.
第四，一定要算质量的百分比变化，不要只算克数，这样才能公平比较起始质量不同的柱。
Fifth, protein carriers in the membrane pick up molecules or ions and carry them across, which is how root hairs take mineral ions from the soil.
第五，膜上的载体蛋白把分子或离子带过去，根毛就是这样从土壤吸收矿物离子的。