Transport in Plants
A-Level Biology Topic 7 8:36 English narration · English + 中文 subtitles burned in
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A giant redwood lifts water more than a hundred metres into the air, every single day — and it has no heart, no muscle, no pump of any kind.
一棵巨杉每天都把水提升到一百多米的高空——而它没有心脏、没有肌肉，也没有任何泵。
So what does the lifting?
那么是什么在提水呢？
The answer is evaporation.
答案是蒸发。
Water escaping from the leaves at the top pulls the whole column up behind it, through narrow pipes running the length of the tree.
水从顶端的叶子里逃逸出去，把整根水柱在后面拉上来， 经过贯穿整棵树的细管。
In this lesson we follow that water from the soil to the sky, and then follow the sugar back down.
在这节课里，我们跟着这些水从土壤走到天空，再跟着糖走回下面。
Plants move two things in two tissues.
植物用两种组织运输两样东西。
Xylem carries water and mineral ions upwards; phloem carries dissolved sugars wherever they are needed.
木质部把水和矿物离子往上运；韧皮部把溶解的糖运到任何需要的地方。
We will look at how each tissue is built, how water crosses the root, what drives it up the stem, how a plant survives where water is scarce, and finally how sugar is pushed from a source to a sink.
我们会看看每种组织是怎么构成的、水如何穿过根、是什么把它推上茎、 植物在缺水的地方如何生存，最后看糖是如何从源被推到库的。
Let's begin.
让我们开始吧。
Start with the two tissues.
先看这两种组织。
Xylem carries water and dissolved mineral ions, and it only ever goes one way — upwards, from the roots.
木质部运输水和溶解的矿物离子，而且只朝一个方向走——从根往上。
Phloem carries assimilates, mainly sucrose, and it can go either direction, to wherever the plant needs them.
韧皮部运输同化物，主要是蔗糖，它可以朝任一方向走，去到植物需要的地方。
Phloem carries assimilates — sucrose mainly, and amino acids.
而它们长在哪里，对考试很重要。
Now, where they sit matters for the exam. Take a transverse section, a cut straight across a dicotyledonous plant. In a stem, xylem and phloem sit together in vascular bundles arranged in a ring near the outside, with the xylem on the inner side of each bundle.
在茎里，木质部和韧皮部一起组成维管束， 排成一圈靠近外缘，每束的木质部在内侧。
In a root, the xylem forms a star right in the centre, with phloem between the arms.
在根里，木质部在正中央形成一个星形， 韧皮部夹在星芒之间。
Before we look inside the tissues, two practical points, because the exam puts a real section in front of you.
在深入组织内部之前，先讲两个实用要点，因为考试会把一张真实的切片放在你面前。
When a question asks for a plan diagram, it wants the outlines of the tissues and nothing else — you draw where the xylem is and where the phloem is, as blocks, and you do not draw a single cell.
当题目要你画组织图时，它要的是各组织的轮廓，别的都不要—— 你画出木质部在哪里、韧皮部在哪里，画成一块一块的，一个细胞也不要画。
Drawing individual cells on a plan diagram loses marks, even when the drawing is beautiful.
在组织图上画出单个细胞是要扣分的，哪怕画得很漂亮。
Then, looking down the microscope, how do you tell them apart?
接着，在显微镜下你怎么区分它们？
Xylem vessels are the large, round cells with obviously thick walls, and because lignin takes up the stain they usually come out bright red.
木质部导管是那些又大又圆、细胞壁明显很厚的细胞， 而且因为木质素会着色，它们通常呈现鲜红色。
Phloem cells are smaller with thin walls, and in a stem they sit on the outer side of the bundle, just beyond the xylem.
韧皮部细胞比较小、壁很薄，在茎里它们位于维管束靠外的一侧，就在木质部的外面。
And do not forget the leaf.
还有，别忘了叶。
We have placed the tissues in the stem and in the root, but they are in the leaf too — xylem and phloem run side by side along every vein.
我们把这两种组织在茎和根里定了位，但它们在叶里也有—— 木质部和韧皮部并排走在每一条叶脉里。
Now their structure — and in biology, structure always fits the job.
再看它们的结构——在生物学里，结构总是与功能相配。
A xylem vessel is made of dead, empty cells stacked end to end, with the end walls dissolved away, so it forms one long open pipe.
导管由死的、空的细胞首尾相接而成， 端壁已经溶解消失，所以它形成一根长长的开放管道。
Nothing blocks the flow.
没有任何东西挡住水流。
Its walls are thickened with lignin, which waterproofs them and gives the plant support.
它的壁被木质素加厚，既防水又给植物提供支撑。
Phloem is different: sieve tubes are living cells, and their end walls remain as sieve plates full of holes for the sap to pass through.
韧皮部则不同：筛管是活细胞， 它们的端壁保留下来，成为布满小孔的筛板，供汁液通过。
To make room, the sieve tube loses most of its contents and has no nucleus — so beside each one sits a companion cell, which keeps its nucleus and packs in mitochondria, doing the living work and loading the sugar.
为了腾出空间， 筛管失去了大部分内容物，也没有细胞核——所以每根筛管旁边都有一个伴胞， 它保留细胞核、装满线粒体，替筛管完成生命活动，并把糖装载进去。
Water enters a root hair cell by osmosis, because the root hair has a lower water potential than the soil water.
水通过渗透进入根毛细胞，因为根毛的水势低于土壤水。
From there it crosses the root by two pathways.
从那里，它经由两条途径穿过根。
The apoplast route goes through the cell walls and the spaces between cells, never entering the cytoplasm — and it is fast.
质外体途径走细胞壁和细胞之间的空隙，从不进入细胞质——而且很快。
The symplast route goes through the cytoplasm, passing from cell to cell through the plasmodesmata.
共质体途径走细胞质，通过胞间连丝在细胞之间传递。
But at a ring of cells called the endodermis, the apoplast is blocked by the Casparian strip, a waterproof band of suberin.
但在一圈叫做内皮层的细胞处， 质外体被凯氏带挡住了，那是一条由木栓质构成的防水带。
That forces every molecule through a cell membrane — which is exactly how the plant controls what gets into its xylem.
这迫使每一个分子都必须穿过细胞膜—— 而这正是植物控制什么能进入木质部的方式。
So what pulls the water up?
那么是什么把水拉上去的？
Transpiration.
蒸腾作用。
Water evaporates from the wet cell surfaces inside the leaf, and the vapour diffuses out through the stomata into the atmosphere.
水从叶子内部潮湿的细胞表面蒸发， 水蒸气再通过气孔扩散出去。
That loss at the top is the engine.
顶端的这种损失就是发动机。
It works because water molecules hydrogen-bond to each other, so they stick together — we call that cohesion, and it lets the entire column behave like a rope.
它之所以奏效， 是因为水分子之间以氢键相互吸引，所以它们黏在一起——我们把这叫做内聚力， 它让整根水柱表现得像一根绳子。
As water leaves the top, the whole rope is pulled up under tension.
当水从顶端离开时，整根绳子在张力下被拉上来。
Water also sticks to the cellulose of the walls — adhesion — which helps hold the column in place.
水也黏附在细胞壁的纤维素上——这叫附着力——它帮助把水柱固定住。
Together this is the cohesion-tension theory.
合起来，这就是内聚力-张力学说。
A quick calculation the exam loves.
一道考试很爱考的小计算。
A plant cell has a water potential of minus eight hundred kilopascals, and it is placed in a solution of minus four hundred.
一个植物细胞的水势是负八百千帕，把它放进负四百的溶液里。
Compare the two values.
比较这两个数值。
Water always moves down a water potential gradient — from the less negative value to the more negative one.
水总是沿着水势梯度往下走——从负得较少的一方，走向负得较多的一方。
Minus four hundred is less negative, so it is the higher one, which means water moves into the cell.
负四百负得更少，所以它更高，这意味着水会进入细胞。
The cell swells, the contents press on the wall, and the cell becomes turgid.
细胞膨胀，内容物压向细胞壁， 细胞变得膨胀坚挺。
Two traps: minus four hundred is greater than minus eight hundred, and pure water is the maximum at zero, so water potential is never positive.
两个陷阱：负四百大于负八百；以及纯水的最大值是零，所以水势永远不会是正的。
Four factors change the rate of transpiration.
有四个因素会改变蒸腾速率。
More light opens the stomata, so the rate goes up.
光照增强会打开气孔，速率上升。
Higher temperature speeds evaporation, so the rate goes up.
温度升高会加快蒸发，速率上升。
Higher humidity makes the water potential gradient smaller, so the rate goes down.
湿度升高会让水势梯度变小，速率下降。
And air movement sweeps the vapour away, keeping the gradient steep, so the rate goes up.
而空气流动把水蒸气吹走，让梯度保持陡峭，速率上升。
You measure it with a potometer — but be careful how you describe it: the moving bubble tells you the rate of water uptake, which is close to, but not the same as, the rate of water loss.
你用测水计来测量它——但描述时要小心：移动的气泡告诉你的是吸水速率， 它接近但并不等于失水速率。
A xerophyte is a plant adapted to live where water is scarce, and its leaves are built to lose as little as possible.
旱生植物是适应在缺水环境中生存的植物，它的叶子被造得尽可能少失水。
A thick waxy cuticle cuts evaporation straight through the leaf surface.
厚厚的蜡质角质层减少了直接透过叶表面的蒸发。
Stomata sunk in pits trap a pocket of humid air right outside the pore.
凹陷在小坑里的气孔， 在孔口外侧困住一小袋潮湿空气。
Hairs on the surface do the same, holding moist air next to the stoma.
表面的绒毛也起同样的作用，把潮湿空气留在气孔旁。
And some leaves roll up entirely, enclosing the humid air inside.
而有些叶子会整个卷起来，把潮湿空气包在里面。
Notice the common theme: every adaptation keeps the air just outside the stoma humid, which makes the water potential gradient smaller, so less water is lost.
注意共同的主线： 每一项适应都让气孔外侧的空气保持潮湿，从而让水势梯度变小，于是失水更少。
Now the sugar.
现在说糖。
Assimilates travel from a source — where they are made, like a photosynthesising leaf — to a sink, where they are used or stored, like a growing root.
同化物从源——它们被制造的地方，比如正在光合作用的叶子——运到库， 也就是被使用或储存的地方，比如正在生长的根。
At the source, companion cells load sucrose into the sieve tube, and they have to push it against its concentration gradient.
在源端，伴胞把蔗糖装载进筛管， 而且必须逆着浓度梯度把它推进去。
They do it in two steps: proton pumps push hydrogen ions out of the companion cell using ATP, and then cotransporter proteins let those ions back in, dragging sucrose along with them.
它们分两步做：质子泵用ATP把氢离子泵出伴胞， 然后协同运输蛋白让这些离子回来，顺带把蔗糖一起拖进来。
That is active transport.
这就是主动运输。
Loading all that sucrose lowers the water potential inside the sieve tube, so water follows by osmosis, and the pressure there rises.
装载这么多蔗糖会降低筛管内的水势，于是水通过渗透跟进来，那里的压力就升高了。
That pressure is what moves the sap.
正是这个压力推动了汁液。
At the source, sucrose is loaded, water follows, and the hydrostatic pressure is high.
在源端，蔗糖被装载，水跟进来，静水压很高。
At the sink, sucrose is removed, water leaves, and the pressure is low.
在库端，蔗糖被取走，水离开，压力很低。
So between source and sink there is a pressure difference, and sap simply flows down it, from high pressure to low.
于是在源与库之间存在一个压力差， 汁液就顺着它从高压流向低压。
This pressure-driven bulk movement is called mass flow.
这种由压力驱动的整体移动叫做集流。
Notice it needs no pumping along the tube — just a difference in pressure at the two ends.
注意，它并不需要沿着管子一路泵送——只需要两端有压力差。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, always name the tissue and the direction: xylem, water upwards, dead and lignified; phloem, assimilates either way, living.
第一，永远写清组织和方向：木质部，水向上，死的、木质化的； 韧皮部，同化物双向，活的。
Second, explain water movement as a pull, not a push — transpiration pulls a column held together by cohesion.
第二，把水的移动解释成"拉"而不是"推"—— 蒸腾拉动一根由内聚力维系的水柱。
Third, watch the signs in water potential: minus four hundred is higher than minus eight hundred, and zero is the maximum.
第三，注意水势的正负号：负四百高于负八百，而零是最大值。
Fourth, for translocation, say that mass flow needs a pressure gradient between source and sink.
第四，讲运输时要说：集流需要源与库之间的压力差。