Enzymes
IGCSE Biology Topic 5 5:54 English narration · English + 中文 subtitles burned in
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Transcript
Every second, thousands of chemical reactions are running inside you.
每一秒钟，你体内都在进行成千上万个化学反应。
Left to themselves, almost all of them would be far too slow to keep you alive.
如果放任不管， 它们几乎全都会慢到无法维持你的生命。
What speeds them up is a set of remarkable protein molecules — enzymes.
让它们加速的， 是一类了不起的蛋白质分子——酶。
They work by shape, they can be used again and again, and they are fussy: each one does exactly one job.
它们靠形状工作，可以反复使用， 而且很挑剔：每一种酶只做一件事。
Welcome to enzymes.
欢迎来到酶。
What a catalyst is, the lock and key idea, and how temperature and pH change enzyme activity.
什么是催化剂、锁和钥匙的概念，以及温度和pH如何改变酶的活性。
Let's begin.
让我们开始吧。
Start with the definition, because it is worth marks on its own.
先从定义开始，因为它本身就值分。
A catalyst is a substance that speeds up the rate of a chemical reaction but is not used up or changed by that reaction.
催化剂是一种能加快化学反应速率， 但在反应中不被消耗、也不被改变的物质。
Because it is not used up, the same catalyst can work again and again.
因为不被消耗，同一个催化剂可以反复工作。
Enzymes are biological catalysts — catalysts made by living cells.
酶是生物催化剂——由活细胞制造的催化剂。
And every enzyme is a protein.
而且每一种酶都是蛋白质。
Enzymes take part in all the reactions of metabolism, so they control almost every chemical reaction in a living thing.
酶参与新陈代谢的所有反应，所以它们控制着生物体内几乎每一个化学反应。
Without enzymes, the reactions that keep you alive would be far too slow.
没有酶，维持你生命的那些反应会慢得多。
Watch one enzyme at work.
看一个酶工作。
A molecule drops into a pocket on the enzyme, the reaction happens, the pieces leave — and the enzyme is left exactly as it was, ready for the next one.
一个分子落进酶上的一个凹槽，反应发生，产物离开—— 而酶还是原来的样子，准备好接待下一个。
Now the words the examiner wants.
现在说考官想要的词。
Each enzyme has a special pocket called the active site.
每一种酶都有一个特殊的凹槽，叫做活性位点。
The molecule it works on is called the substrate.
它作用的分子叫做底物。
The shape of the active site is complementary to the shape of the substrate — that is the key word, complementary.
活性位点的形状与底物的形状互补—— 这就是关键词，互补。
They fit together like a key in a lock.
它们像钥匙和锁一样吻合。
The substrate slots into the active site, and for the supplement paper that gives an enzyme-substrate complex.
底物嵌进活性位点， 补充部分把这叫做酶底物复合物。
The reaction then takes place, and the substrate is changed into products.
然后反应发生，底物被转变成产物。
The products leave, and the active site is free to be used again.
产物离开，活性位点又空出来，可以再次使用。
That shape idea explains something important: each enzyme is specific.
形状这个概念解释了一件重要的事：每一种酶都是专一的。
It works on only one kind of substrate.
它只作用于一种底物。
Why?
为什么？
Because only that substrate has the right shape to fit the active site.
因为只有那种底物有合适的形状能嵌进活性位点。
A substrate of a different shape simply will not go in — just as the wrong key will not open a lock.
形状不同的底物根本进不去—— 就像错的钥匙打不开锁。
So if a question asks why an enzyme only works on one substance, do not say it "chooses" — say the shapes are complementary, and no other molecule fits.
所以如果题目问为什么一种酶只作用于一种物质， 不要说它会"选择"——要说形状是互补的，别的分子都不吻合。
Temperature changes enzyme activity, and the graph has a very particular shape.
温度改变酶的活性，而且这个图形有一个非常特别的形状。
As the temperature rises from cold, activity speeds up.
当温度从低处上升， 活性加快。
The enzyme and substrate molecules gain kinetic energy, so they move faster and meet more often — for the supplement paper, there are more useful collisions each second.
酶和底物分子获得动能，所以它们移动得更快、相遇得更频繁—— 补充部分会说，每秒有更多有效碰撞。
Activity is highest at the optimum temperature, about thirty-seven degrees Celsius in the human body.
活性在最适温度时最高， 人体大约是三十七摄氏度。
But above the optimum, activity falls fast, and it falls much more steeply than it rose.
但超过最适温度后，活性迅速下降， 而且下降得比上升时陡得多。
The heat changes the shape of the active site, so the substrate no longer fits.
热改变了活性位点的形状，所以底物不再吻合。
The enzyme denatures — it is denatured, and that is permanent.
酶变性了，而且是永久的。
It cannot recover.
它无法恢复。
Here is the calculation students get wrong.
这是学生常做错的计算。
A reaction finishes in fifty seconds at twenty degrees, and in twenty seconds at thirty-seven degrees.
一个反应在二十度时五十秒完成，在三十七度时二十秒完成。
Which is faster, and by how much?
哪个更快，快多少？
A shorter time means a faster rate, so first turn each time into a rate.
时间越短表示速率越快，所以先把每个时间换算成速率。
Use rate equals one thousand divided by the time.
用速率等于一千除以时间。
At twenty degrees, one thousand divided by fifty is twenty units.
在二十度，一千除以五十等于二十个单位。
At thirty-seven degrees, one thousand divided by twenty is fifty units.
在三十七度，一千除以二十等于五十个单位。
Fifty divided by twenty is two point five, so thirty-seven degrees is two point five times faster.
五十除以二十等于二点五， 所以三十七度快二点五倍。
Never say the rate "went up by thirty seconds" — a time is not a rate.
绝不要说速率"增加了三十秒"——时间不是速率。
Turn the time into a rate first, and remember: the bigger rate always goes with the smaller time.
先把时间换成速率，并记住：较大的速率总是对应较短的时间。
pH does something similar, but the curve is a different shape.
pH的作用类似，但曲线的形状不同。
Each enzyme works best at one particular pH — its optimum.
每一种酶在某一个特定的pH下工作得最好—— 也就是它的最适pH。
Move away from that optimum in either direction, too high or too low, and the shape of the active site changes, the substrate stops fitting, and the enzyme is denatured.
从最适值向任一方向偏离，太高或太低， 活性位点的形状都会改变，底物不再吻合，酶就变性了。
So the pH curve is a hill with the peak at the optimum and both sides falling away.
所以pH曲线是一座小山，峰在最适值，两边都往下降。
Different enzymes have different optimum values: the protease in your stomach works in strong acid, while most enzymes in your cells prefer a pH near neutral.
不同的酶有不同的最适值：你胃里的蛋白酶在强酸中工作， 而细胞里的大多数酶喜欢接近中性的pH。
One last warning, and it costs marks every year.
最后一个警告，它每年都让学生失分。
Students write that the enzyme was killed.
学生会写酶被"杀死"了。
That answer gets nothing, because enzymes are not alive.
这个答案得不到分，因为酶不是活的。
An enzyme is a protein molecule — it was never alive, so it cannot be killed.
酶是一个蛋白质分子—— 它从来就不是活的，所以不能被杀死。
Here is what to write instead: the shape of the active site has changed, so the substrate no longer fits, and the enzyme is denatured.
你应该这样写： 活性位点的形状改变了，所以底物不再吻合，酶变性了。
Shape, fit, denatured.
形状、吻合、变性。
Those are the three ideas the mark scheme is looking for.
这就是评分标准要找的三个要点。
Three marks to lock in.
锁住三个分。
First: an enzyme is a protein and a catalyst — it speeds up a reaction and is not used up.
第一：酶既是蛋白质又是催化剂——它加快反应，而且不被消耗。
Second: use the word complementary — the active site is complementary to the substrate, and each enzyme is specific to one substrate.
第二：要用"互补"这个词——活性位点与底物互补，每一种酶对一种底物专一。
Third: never write killed.
第三：绝不要写"杀死"。
Too hot or the wrong pH changes the shape of the active site, so the enzyme is denatured, permanently.
太热或pH不对会改变活性位点的形状，所以酶永久性地变性了。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。