Photosynthesis
A-Level Biology Topic 13 8:58 English narration · English + 中文 subtitles burned in
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Transcript
Here is something strange about a tree. Almost none of it came out of the ground.
关于一棵树，有件奇怪的事：它几乎没有一点是从土里长出来的。
The wood, the leaves, the fruit — nearly all of that mass was pulled out of the air, as carbon dioxide, and assembled using nothing but sunlight.
木头、叶子、果实——这些质量几乎全都是从空气中以二氧化碳的形式取来的， 而且只靠阳光就组装起来了。
Every meal you have ever eaten traces back to that.
你吃过的每一顿饭都能追溯到这里。
And the oxygen you are breathing right now is simply what was left over.
而你此刻正在呼吸的氧气，不过是剩下的副产品。
In this lesson we take that process apart, stage by stage.
这节课我们把这个过程一个阶段一个阶段地拆开。
Photosynthesis happens inside the chloroplast, and the shape of that organelle tells you the process has two halves.
光合作用发生在叶绿体内部，而这个细胞器的形状就告诉你这个过程分成两半。
Inside are stacks of flat sacs called thylakoids.
里面是一摞摞扁平的囊，叫类囊体。
Their membranes hold the light-trapping pigments, and the first stage happens there.
它们的膜上带着捕光的色素，第一阶段就发生在那里。
Around them is a fluid called the stroma, and the second stage happens there. Learn the naming.
围绕着它们的是一种液体，叫基质，第二阶段就发生在那里。
One stack is a granum, several stacks are grana, and they are joined to each other by sheets called lamellae.
记住名称：一摞叫基粒，多摞叫基粒（复数），它们之间由叫做片层的薄片连接。
So the structure is split in two, because the process is split in two.
所以结构分成两部分，因为过程本身就分成两部分。
The first stage is the light-dependent stage, and it happens in the thylakoids.
第一阶段是光反应阶段，发生在类囊体里。
It uses light energy to make two things: ATP, and reduced NADP.
它用光能制造两样东西：ATP 和还原型 NADP。
The second stage is the light-independent stage, better known as the Calvin cycle, and it happens in the stroma.
第二阶段是暗反应阶段，更常被叫做卡尔文循环，发生在基质里。
It spends that ATP and that reduced NADP to build carbon dioxide into sugars.
它花掉那些 ATP 和还原型 NADP，把二氧化碳组装成糖。
Notice how the first hands over to the second, and the second sends the empty carriers back — ADP and NADP — to be recharged. Look at what goes in and what comes out.
注意第一阶段怎样把东西交给第二阶段，而第二阶段又把空的载体——ADP 和 NADP——送回去充电。
Water and light go into the first stage, and oxygen comes out. Carbon dioxide goes into the second, and sugars come out.
看看什么进去、什么出来：水和光进入第一阶段，氧出来； 二氧化碳进入第二阶段，糖出来。
A pigment is simply a coloured substance that absorbs light, and the thylakoids hold several pigments so that between them they can trap as much light as possible.
色素就是能吸收光的有色物质，而类囊体里有好几种色素， 好让它们合起来尽可能多地捕获光。
Chlorophyll a and chlorophyll b absorb mainly red and blue light.
叶绿素 a 和叶绿素 b 主要吸收红光和蓝光。
Carotene and xanthophyll absorb other colours and pass the energy on. We study them with two graphs.
胡萝卜素和叶黄素吸收其他颜色的光，再把能量传递过去。
An absorption spectrum shows how much light the pigments absorb at each wavelength.
我们用两张图来研究它们。 吸收光谱显示色素在每个波长吸收了多少光。
An action spectrum shows how fast photosynthesis actually goes at each wavelength.
作用光谱显示在每个波长下光合作用实际有多快。
And here is the point. The two curves match closely — both high in blue and red, both low in green.
关键就在这里：两条曲线非常吻合——都在蓝光和红光处高，都在绿光处低。
That match is the evidence that these pigments really are what drives photosynthesis.
这种吻合就是证据，说明这些色素确实是驱动光合作用的东西。
It is also why leaves look green: green is the light they use least, so it is the light they reflect back at you.
这也解释了叶子为什么看起来是绿的：绿光是它们用得最少的，所以被反射回你的眼睛。
To see the pigments separately, we use chromatography.
为了分别看到这些色素，我们用色谱法。
A spot of leaf extract is placed on the paper, and a solvent creeps up the paper carrying the pigments with it.
把一点叶片提取液点在纸上， 溶剂沿着纸往上爬，把色素一起带上去。
Because each pigment is held by the paper a little differently, they travel different distances, and the single green spot separates into a line of coloured spots.
因为每种色素被纸吸附的程度略有不同， 它们走的距离也不同，那一个绿色的点就分离成一列有颜色的斑点。
Carotene travels furthest, then chlorophyll a, then chlorophyll b, then xanthophyll.
胡萝卜素走得最远，然后是叶绿素 a，再是叶绿素 b，最后是叶黄素。
To identify one, we calculate its Rf value: the distance the spot moved, divided by the distance the solvent moved.
要辨认某一种，我们计算它的比移值：斑点移动的距离除以溶剂移动的距离。
Two things to be careful about. Measure to the centre of the spot, and always measure from the origin line.
有两点要小心：量到斑点的中心，而且一律从原点线开始量。
In the thylakoids, light is used to make ATP, and because light is doing the phosphorylating we call it photophosphorylation.
在类囊体里，光被用来制造 ATP，而因为是光在做磷酸化，我们把它叫做光合磷酸化。
It comes in two forms, and the examiner wants you to tell them apart.
它有两种形式，考官希望你能分辨清楚。
In cyclic photophosphorylation, only photosystem I is used.
在循环光合磷酸化中，只用到光系统 I。
Light boosts the electrons of chlorophyll to a higher energy — that is photoactivation — and the only product is ATP.
光把叶绿素的电子提升到更高的能量——这叫光激活——唯一的产物是 ATP。
In non-cyclic photophosphorylation, both photosystem one and photosystem two are used, and something extra happens. The oxygen-evolving complex carries out photolysis: it splits water using light.
在非循环光合磷酸化中，光系统 I 和光系统 II 都要用到，而且还多发生一件事： 放氧复合体进行光解，用光把水劈开。
That is where the oxygen comes from, and non-cyclic makes both ATP and reduced NADP.
氧就是从这里来的， 而非循环途径同时产生 ATP 和还原型 NADP。
So if a question mentions oxygen or reduced NADP, it must be the non-cyclic route.
所以如果题目里提到氧或还原型 NADP，那一定是非循环途径。
In both forms, the energy is captured in the same way, and you have met this mechanism before.
两种形式捕获能量的方式是一样的，而这个机制你以前见过。
Energetic electrons pass along an electron transport chain in the thylakoid membrane, releasing energy at each step.
带能量的电子沿着类囊体膜上的一串载体传递，每一步都释放能量。
That energy is used to pump protons across the membrane, so protons build up inside the thylakoid.
这些能量被用来把质子泵过膜，于是质子在类囊体内部积累起来。
Then they flow back out into the stroma through a channel called ATP synthase, and that flow provides the energy to make ATP.
然后它们通过一个叫 ATP 合酶的通道流回基质，这股流动提供了制造 ATP 的能量。
If that sounds familiar, it should.
如果这听起来很熟悉，那就对了。
It is exactly the chemiosmosis you met in respiration — a different membrane, but the same trick.
这正是你在呼吸作用里见过的化学渗透—— 换了一层膜，但是同一个把戏。
Now the second stage, in the stroma, in three steps.
现在是第二阶段，在基质中，分三步。
Fixation.
第一步，固定。
The enzyme rubisco joins carbon dioxide onto a five-carbon molecule called RuBP, and the result immediately splits into two molecules of a three-carbon compound called GP.
核酮糖羧化酶把二氧化碳接到一个叫 RuBP 的五碳分子上， 产物立刻分裂成两个叫 GP 的三碳化合物。
Reduction.
第二步，还原。
GP is reduced to TP, and this is where the ATP and the reduced NADP from the first stage get spent.
GP 被还原成 TP，第一阶段产生的 ATP 和还原型 NADP 就在这里被花掉。
Regeneration.
第三步，再生。
Most of that TP is used to rebuild RuBP, using more ATP, so the cycle can keep turning.
大部分 TP 被用来重建 RuBP，还要再消耗 ATP，好让循环继续转动。
And some TP leaves the cycle — that is the actual product.
还有一部分 TP 离开循环——那才是真正的产物。
It goes on to make carbohydrates, lipids and amino acids.
它接着去制造碳水化合物、脂质和氨基酸。
GP can be used to make some amino acids too.
GP 也可以用来制造一些氨基酸。
A limiting factor is whichever thing is in shortest supply, holding the rate back.
限制因素就是供应最不足、拖住速率的那个因素。
There are three main ones: light intensity, carbon dioxide concentration, and temperature.
主要有三个： 光照强度、二氧化碳浓度和温度。
Raise the light intensity and the rate climbs — while it is climbing, light is the limiting factor.
提高光照强度，速率就上升—— 在上升的这一段，光就是限制因素。
But then the curve levels off.
但接着曲线就变平了。
Nothing you do to the light helps any more, which tells you something else has become limiting instead.
你再怎么调光都没用，这告诉你已经换成别的因素在限制了。
Give the plant more carbon dioxide, or warm it up, and the whole curve lifts to a higher plateau, proving that was the factor holding it back.
给植物更多二氧化碳，或者把温度升高，整条曲线就抬到更高的平台上， 证明刚才拖住它的正是那个因素。
Temperature behaves a little differently. It helps only up to an optimum; push it too high and the enzymes denature and the rate collapses.
温度的表现有点不同： 它只在到达最适温度之前有帮助；再往上，酶就变性，速率就崩掉。
And how do you measure that rate in a lab? Two ways.
那在实验室里怎么测这个速率？
For the light-dependent stage, use a redox indicator such as DCPIP or methylene blue with a suspension of chloroplasts: the dye loses its colour as the chloroplasts work, and you can time that under different light intensities or different wavelengths.
有两种做法。 测光反应，用氧化还原指示剂， 比如 DCPIP 或亚甲基蓝，配上叶绿体悬浮液：叶绿体工作时染料会褪色， 你可以在不同光照强度或不同波长下计时。
For the whole process, use a whole aquatic plant such as pondweed and count the bubbles of oxygen it gives off.
测整个过程，就用一整株水生植物， 比如金鱼藻，数它放出的氧气气泡。
Here is the question that comes up again and again.
这是反复出现的题目。
A plant is photosynthesising steadily, and then the carbon dioxide is suddenly taken away.
一株植物正在稳定地进行光合作用，然后二氧化碳突然被撤走。
What happens to GP and to RuBP? The method never changes.
GP 和 RuBP 会怎样？
Find the reaction that stops, then ask of each substance two things: is it still being made, and is it still being used.
方法永远不变：找出停下来的那个反应， 然后对每种物质问两件事：它还在被产生吗？ 它还在被消耗吗？
With no carbon dioxide, fixation stops.
没有二氧化碳，固定就停了。
So GP is no longer being made — but it is still being used up, reduced to TP.
所以 GP 不再被产生——但它仍在被消耗，被还原成 TP。
Therefore GP falls.
因此 GP 下降。
RuBP, meanwhile, is still being regenerated from TP, but it is no longer being consumed by fixation.
与此同时，RuBP 仍在由 TP 再生，但不再被固定消耗。 因此 RuBP 上升。
Therefore RuBP rises. Now take the light away instead, and you get the mirror image. With no ATP and no reduced NADP, GP cannot be reduced, so GP rises and RuBP falls.
现在改成撤走光，你会得到镜像的结果：没有 ATP 和还原型 NADP，GP 无法被还原， 于是 GP 上升，RuBP 下降。
Four things the examiner keeps asking for.
考官反复要的四点。
First, always say which part of the chloroplast you mean — thylakoid for the light-dependent stage, stroma for the Calvin cycle.
第一，永远要说清你指的是叶绿体的哪一部分—— 光反应在类囊体，卡尔文循环在基质。
Second, be precise about where the oxygen comes from.
第二，氧从哪里来要说准： 它来自水的光解，不是来自二氧化碳。
It comes from the photolysis of water, not from carbon dioxide.
第三，当速率曲线变平时，要说是另一个因素成了限制因素，并说出一个可能的因素。
Third, when a rate graph levels off, say that another factor has become limiting, and name a candidate.
第四，对卡尔文循环的任何变化，都用"产生与消耗"来分析—— 这种物质还在被产生吗？
Fourth, for any change to the Calvin cycle, use made and used — is the substance still being made, and is it still being used.
还在被消耗吗？
So there it is.
就是这样了。
Stacks of membrane that catch the light, a fluid where the carbon is assembled, and two stages that pay each other.
一摞摞捕光的膜，一片组装碳的液体，还有两个互相供应的阶段。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。