Transport in Plants
IGCSE Biology Topic 8 5:54 English narration · English + 中文 subtitles burned in
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A tall tree lifts water a hundred metres into the air, every day, with no pump and no moving parts.
一棵高大的树每天把水提升一百米高，没有泵，也没有活动部件。
It does it using nothing but evaporation from its leaves and the fact that water molecules stick to one another.
它靠的只是叶片的蒸发，以及水分子彼此吸引这个事实。
Two separate pipelines run through every plant, and the exam wants you to keep them apart.
每一株植物体内都有两条独立的管道，考试要求你把它们分清楚。
Welcome to transport in plants.
欢迎来到植物的运输。
Xylem and phloem, water uptake, transpiration, and translocation.
木质部和韧皮部、水分吸收、蒸腾作用，还有转运。
Let's begin.
让我们开始吧。
Two transport tissues run through the roots, the stems and the leaves, and you must never mix them up.
有两种运输组织贯穿根、茎和叶，你绝不能把它们弄混。
Xylem carries water and mineral ions upwards from the roots, and it also gives the plant support.
木质部把水和矿物离子从根向上运输，它还为植物提供支撑。
Phloem carries sucrose and amino acids to wherever they are needed.
韧皮部把蔗糖和氨基酸运送到需要它们的地方。
Notice the differences.
注意它们的区别。
Xylem goes one way only, upwards; phloem can go either way.
木质部只有一个方向，向上；韧皮部两个方向都可以。
Xylem moves water; phloem moves food.
木质部运水；韧皮部运食物。
In a root, a stem or a leaf, the two lie side by side in vascular bundles.
在根、茎或叶里，两者并排位于维管束中。
For the supplement paper, learn three things about how xylem is built — and link each one to the job it does.
补充部分要记住木质部结构的三点——并且把每一点和它的功能联系起来。
First, the vessels have thick walls strengthened with lignin, and that lignin is also what supports the plant.
第一，导管有被木质素加固的厚壁，而木质素也正是支撑植物的东西。
Second, the cells are dead and empty, with no cell contents in the way, so water flows through freely.
第二，细胞是死的、空的，里面没有细胞内容物挡路，所以水能自由流过。
Third, the cells are joined end to end with no walls across the tube, making one long continuous pipe from root to leaf.
第三，细胞首尾相连，管子中间没有横壁，形成一条从根到叶的连续长管。
Dead, hollow, lignified, continuous.
死的、中空的、木质化的、连续的。
Every one of those is a mark.
每一个都是得分点。
Water and mineral ions get in through root hair cells.
水和矿物离子通过根毛细胞进入。
There are a huge number of root hairs, and together they give a very large surface area, which speeds up uptake.
根毛数量巨大， 它们一起提供了非常大的表面积，加快了吸收。
Now learn this pathway, in this exact order: root hair cell, then root cortex cells, then xylem, then the mesophyll cells in the leaf.
现在按这个准确的顺序记住这条路径：根毛细胞，然后根的皮层细胞， 然后木质部，然后叶片里的叶肉细胞。
You can show that pathway for yourself.
你可以自己演示这条路径。
Stand a plant, or a white flower, in water containing a coloured stain.
把一株植物，或者一朵白花，放在含有有色染色剂的水里。
The stain is carried up the xylem and colours the veins, so you can see exactly where the water went.
染色剂会沿着木质部被运上去，把叶脉染上颜色，你就能清楚看到水去了哪里。
Watch the whole stream at once — in at the roots, up the stem, and out through the leaves.
一次看完整条水流——从根进入，沿茎向上，再从叶片出去。
Transpiration is the loss of water vapour from the leaves, and it happens in two steps.
蒸腾作用是叶片失去水蒸气，它分两步发生。
Both steps earn marks, so write both.
两步都得分，所以两步都要写。
Step one: water evaporates from the wet surfaces of the mesophyll cells into the air spaces inside the leaf.
第一步：水从叶肉细胞的湿表面蒸发到叶片内部的气腔里。
Step two: that water vapour then diffuses out through the stomata.
第二步：这些水蒸气再通过气孔扩散出去。
Evaporation first, then diffusion — two different processes, in that order.
先蒸发，再扩散——两个不同的过程，按这个顺序。
Three things change the rate, and each answer needs a reason, not just a direction.
有三样东西改变速率，每个答案都需要一个原因，不能只说方向。
A higher temperature is faster: hotter, so water evaporates faster.
越热越快，因为水蒸发得更快。
Windier is faster, because the wind carries the water vapour away from the leaf.
风越大越快，因为风把水蒸气从叶片带走。
And for the supplement paper, humidity: drier air is faster, because a bigger difference in water vapour between the inside of the leaf and the outside makes water diffuse out more quickly.
补充部分还要说：空气越干越快，因为叶片内外水蒸气的差别越大， 水就扩散得越快。
Hot, windy and dry is the worst combination for a plant.
又热、又有风、又干燥，是对植物最糟糕的组合。
A potometer measures how fast a shoot takes up water.
蒸腾计测量枝条吸水的快慢。
Here is the standard question.
这是标准的题目。
In a potometer the air bubble moves sixty millimetres in five minutes.
在蒸腾计中，气泡在五分钟内移动了六十毫米。
What is the rate?
速率是多少？
Rate is distance divided by time, so sixty divided by five is twelve millimetres per minute.
速率等于距离除以时间，所以六十除以五等于每分钟十二毫米。
Always give a rate its full unit — millimetres per minute, not just twelve.
永远要给速率写完整的单位——毫米每分钟，不能只写十二。
Now the second half: switch a fan on, and what happens?
现在是第二半：打开一台风扇，会发生什么？
A fan raises the wind speed, which carries water vapour away from the leaf, so transpiration speeds up and the bubble moves further in the same five minutes.
风扇提高了风速，把水蒸气从叶片带走，所以蒸腾加快， 气泡在同样的五分钟内移动得更远。
So what actually lifts the water?
那么究竟是什么把水提上去的？
As water vapour leaves the leaf, it pulls more water up behind it, and that is called the transpiration pull.
当水蒸气离开叶片时， 它把后面的水往上拉，这就叫蒸腾拉力。
It works because water molecules stick to one another by forces of attraction, so they are drawn up the xylem together as one long column of water — not as separate drops.
它之所以有效，是因为水分子之间靠吸引力彼此粘连， 所以它们作为一条长长的水柱一起被拉上木质部——而不是一滴一滴分开的。
Notice there is no pump anywhere: the leaf pulls, it does not push.
注意任何地方都没有泵：叶片是在拉，不是在推。
And if a plant loses water faster than it can take it up, its cells go soft and the plant wilts — leaves and stem droop.
如果植物失水比吸水快，它的细胞就会变软，植物就萎蔫——叶子和茎都下垂。
That happens on a hot, dry, windy day, which is exactly the combination we just met.
这发生在又热、又干、又有风的日子，正是我们刚说过的那个组合。
Last, for the supplement paper: translocation.
最后是补充部分：转运。
Translocation is the movement of sucrose and amino acids in the phloem, from sources to sinks.
转运是蔗糖和氨基酸在韧皮部中的移动，从源到库。
Sources release them — the leaves are the obvious source, because they make sugar by photosynthesis.
源释放它们——叶片是最明显的源，因为它们通过光合作用制造糖。
Sinks use or store them — a growing root, or a storage organ.
库使用或储存它们——比如正在生长的根，或者储存器官。
And here is the part students miss: the direction can change.
这里是学生容易漏掉的部分：方向是可以改变的。
In summer a potato is a sink, storing sugar sent down from the leaves.
夏天，马铃薯是库，储存从叶片送下来的糖。
In spring that same potato becomes a source, sending its stored sugar up to the new shoots.
春天，同一个马铃薯变成源，把储存的糖送上去给新芽。
Three marks to lock in.
锁住三个分。
First: xylem is water upwards in dead lignified tubes, plus support; phloem is sucrose and amino acids in living cells, either way.
第一：木质部是水向上，走在死的、木质化的管子里，还提供支撑； 韧皮部是蔗糖和氨基酸，走在活细胞里，两个方向都行。
Second: transpiration is evaporation then diffusion — evaporation from the mesophyll, diffusion out of the stomata.
第二：蒸腾作用是先蒸发再扩散——从叶肉蒸发，从气孔扩散出去。
Third: the leaf pulls the water up, it never pushes, and the molecules travel as one column.
第三：是叶片把水拉上去的，它从不推，而且水分子作为一条水柱一起移动。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。