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Narrator: Listen to part of a lecture in a biology class.

好的。在我们上堂课结束之前，我们开始讨论了微量元素，在生物体里发现的小量微量元素，对生物体起到了重要的生物的、营养的功能。

Professor: OK.Just before the end of the last class, we started talking about trace metals, metals found in living organisms in very small quantities that serve an important biological, important nutritive function in those organisms.

其中一个有营养功能的微量元素就是锌。

And one trace metal that serves a nutritive function is zinc.

锌促进了很多人体内部的过程，但是我们将会着重于其中一个，一个适用于很多生物体的，不单单适用于人类的过程。

Zinc assists in a number of processes in humans, but we are going to focus on just one, one that applies to a number of organisms, not just humans.

锌在碳循环中起到了主要的作用，对各种各样含碳分子进行转化，比如二氧化碳，转化成其他生物体可以使用的含碳分子。

See, zinc plays a major role in carbon cycling, the conversion of various kinds of molecules with carbon, Iike carbon dioxide, into other kinds of molecules with carbon that organisms can use.

我们的身体，我们的细胞在分解糖分的时候会产生二氧化碳。

Our bodies, our cells produce carbon dioxide when they break down sugars.

我们需要将二氧化碳排出体外，所以二氧化碳被转化成了碳酸，而血液可以将碳酸带到肺部。

We need to get the CO2 out of our bodies, so the CO2 is converted into carbonic acid, which the blood is able to carry to the lungs.

一旦碳酸到达了肺部，它又被转化成了二氧化碳，这样我们就可以把它呼出体外。

Once the carbonic acid reaches the lungs, it's converted back into carbon dioxide so that we can breathe it out.

这整个转化的过程都依赖于一种特定的酶。

Now, this whole conversion process relies on a particular enzyme.

Uh, who remembers what an enzyme is? Bob?

学生：这是一种蛋白，一种特定的蛋白，它可以加速化学反应。

Student: Uh, it's a protein, a specific kind of protein, one that speeds up chemical reactions.

教授：非常正确。不同的酶帮助不同的化学反应。

Professor: Exactly. Different enzymes assist in different chemical reactions.

加速二氧化碳转化的酶当中含有锌。

Now, the one that speeds up the conversion of carbon dioxide has zinc in it.

所以，这个锌酶对于将二氧化碳通过肺排除体外非常关键。

So this zinc enzyme is critical for getting CO2 out of our bodies through the lungs.

并且，这个对于肺部也极其重要。

And it's also extremely important for plants.

鲍勃 ，你可以告诉我们为什么嘛？

学生：为了制造食物，为了光合作用？

Student: For making food, for photosynthesis?

植物同样也将二氧化碳转化为各种形式的含碳分子，并且使用的转化过程依赖着和人类体内相同的酶。

Plants also convert carbon dioxide into different forms of carbon-containing molecules and the conversion process used relies on the very same enzyme that works in humans.

好的。但是锌在特定环境中是非常稀有的。

OK. But zinc is scarce in certain environments.

并且它在河流湖泊表面附近的水域及海洋较浅部分的水域里尤其稀少，这个就让我们想知道在那里的植物是怎么存活的。

And it's particularly scarce in waters near the surface of rivers and lakes and shallower parts of oceans, which might make us wonder how plants could live there at all.

事实上，那里有很多存活的海洋植物，那些在水域表面生长繁衍的植物。

In fact, there are a lot of marine plants that survive, that grow and reproduce in surface waters.

In particular, there are a lot of diatoms.

硅藻类是细小的、光合的生物体，并且他们是其他海洋生物的主要食物来源。

Diatoms are microscopic, photosynthetic organisms and they are a major source of food for other organisms in the ocean.

硅藻有很多不同种类，并且他们在碳循环中起到非常重要的作用，因为它们帮主海洋深处的其他生物获得碳。

There are a number of different types of diatoms, and, well, diatoms play a very important role in the carbon cycling process, because they help make carbon available to other organisms in deeper parts of the ocean.

当这些硅藻被吃掉的时候，它们在光合作用中使用的碳就被转移到海洋的其他部分，比如被一条鱼吃掉，吸收了二氧化碳，然后游到海洋的其他部分，或者当硅藻死掉的时候，掉落到海底。

The carbon that these diatoms use in photosynthesis is transferred to other parts of the ocean when the diatoms are eaten, say, by a fish that absorbs the carbon and then swims to another part of the ocean, or when diatoms die and fall to the ocean floor.

那么，如果锌那么稀少，硅藻又是怎么存活下来的呢？

So how did diatoms survive if zinc is so scarce?

最近有研究人员发现，一种特定的硅藻会制造出不同的酶，这种酶起到同一种作用。

Well, recently researchers discovered that a specific type of diatom makes a different enzyme that serves the same purpose.

取而代之的是，这种新的酶包含了另外一种微量元素，镉。

Instead this new enzyme incorporates another trace metal, cadmium.

学生：是的，我认为镉是有毒的。

Student: Yeah.I thought cadmium was toxic.

嗯，事实上，我们过去认为它对所有生物体都有毒，它不具有任何生物功能。

Uh, actually, we used to think that it was toxic to all biological life, that it didn't serve any biological purpose.

但是，新的研究表示，镉事实上可以替代锌，当生物体的环境中没有足够锌的时候，生物体可以用它来替代锌。

But new study suggests that cadmium can actually substitute for zinc, that organisms can use it instead of zinc when there isn't enough zinc in their environment.

因为很多原因，这个以镉为基础的酶的发现是非常重要的。

Now, the discovery of this cadmium—based enzyme is really important for a number of reasons.

事实上，它是我们发现的第一个使用镉的酶。

It's actually the first enzyme we have discovered that uses cadmium.

所以，其他一些非典型的微量元素可能在化学过程中被使用，海洋生物可能在必须的微量元素非常稀有的情况下，使用其他微量元素来产生酶。

So it's possible that other not so typical trace metals may be used in chemical processes, that marine organisms might make enzymes from other trace metals when the essential one is scarce.

可能会有其他一些酶使用镉来进行碳循环。

And there may be other types of diatoms that use cadmium to cycle carbon.

但是，我们有其他一些事情需要思考。

But there's something else to think about.

什么呢我们大气中最常见的温室气体，全球变暖的主要元凶？

What is one of the most common greenhouse gases in our atmosphere, one of the major culprits in global warming.

如果这些酶正在将表面的二氧化碳拿走，将他们转化，并将它们运到海洋底部，可能关于那整个过程及循环，我们忽视了很多东西。

Now, if all these diatoms are taking carbon dioxide from the surface, converting it and transporting it to the bottom of the ocean, well, maybe there's more to that whole process, that cycle, something that we've overlooked.

所以，进一步的研究也可能告诉我们这个变暖周期的更多内容。

So further research might tell us more about these warming cycles too.

What does the professor imply about the conversion of carbon dioxide molecules in plants?

According to the professor, why is it surprising that many marine plants are able to survive near the surface of oceans?

According to the professor, what important function do diatoms serve?

What point does the professor make when she talks about cadmium being poisonous to humans?

The professor states that the discovery of an enzyme containing cadmium is important. What are two reasons that this discovery is important?

Narrator 
Listen to part of a lecture in a biology class 
Professor 
OK. Just before the end of the last class, we started talking about trace metals, metals found in living organisms in very small quantities that serve an important biological, important nutritive function in those organisms. And one trace metal that serves a nutritive function is zinc． 
Zinc assists in a number of processes in humans, but we are going to focus on just one, one that applies to a number of organisms, not just humans. See, zinc plays a major role in carbon cycling, the conversion of various kinds of molecules with carbon, like carbon dioxide, into other kinds of molecules with carbon that organisms can use. 
So, take respiration. Our bodies, our cells produce carbon dioxide when they break down sugars. We need to get the CO2 out of our bodies, so the CO2 is converted into carbonic acid, which the blood is able to carry to the lungs. Once the carbonic acid reaches the lungs, it’s converted back into carbon dioxide so that we can breathe it out． 
Now, this whole conversion process relies on a particular enzyme. Uh, who remembers what an enzyme is? Bob? 
Student 
Uh, it’s a protein, a specific kind of protein, one that speeds up chemical reactions. 
Professor 
Exactly．Different enzymes assist in different chemical reactions．Now, the one that speeds up the conversion of carbon dioxide has zinc in it．So this zinc enzyme is critical for getting CO2 out of our bodies through the lungs．And it’s also extremely important for plants．Bob, can you tell us why? 
Student 
For making food, for photosynthesis? 
Professor 
Exactly. For photosynthesis. Plants also convert carbon dioxide into different forms of carbon-containing molecules and the conversion process used relies on the very same enzyme that works in humans．So zinc is also important for plants． 
OK. But zinc is scarce in certain environments．And it’s particularly scarce in waters near the surface of rivers and lakes and shallower parts of oceans, which might make us wonder how plants could live there at all．In fact, there are a lot of marine plants that survive, that grow and reproduce in surface waters. In particular, there are a lot of diatoms． 
Diatoms are microscopic, photosynthetic organisms and they are a major source of food for other organisms in the ocean. There are a number of different types of diatoms, and, well, diatoms play a very important role in the carbon cycling process, because they help make carbon available to other organisms in deeper parts of the ocean．The carbon that these diatoms use in photosynthesis is transferred to other parts of the ocean when the diatoms are eaten, say, by a fish that absorbs the carbon and then swims to another part of the ocean, or when diatoms die and fall to the ocean floor. 
So how did diatoms survive if zinc is so scarce? Well, recently researchers discovered that a specific type of diatom makes a different enzyme that serves the same purpose．But this enzyme doesn’t contain zinc．Instead this new enzyme incorporates another trace metal, cadmium．Kelly, you’ve got a question? 
Student 
Yeah. I thought cadmium was toxic. Didn’t you say that? 
Professor 
It is poisonous to humans．Uh. actually, we used to think that it was toxic to all biological life, that it didn’t serve any biological purpose．But new study suggests that cadmium can actually substitute for zinc, that organisms can use it instead of zinc when there isn’t enough zinc in their environment． 
Now, the discovery of this cadmium—based enzyme is really important for a number of reasons．It’s actually the first enzyme we have discovered that uses cadmium．So it’s possible that other not so typical trace metals may be used in chemical processes, that marine organisms might make enzymes from other trace metals when the essential one is scarce．And there may be other types of diatoms that use cadmium to cycle carbon． 
But there’s something else to think about．What is one of the most common greenhouse gases in our atmosphere, one of the major culprits in global warming. Carbon dioxide, right? 
Now, if all these diatoms are taking carbon dioxide from the surface, converting it and transporting it to the bottom of the ocean, well, maybe there’s more to that whole process, that cycle, something that we’ve overlooked. So further research might tell us more about these warming cycles too．