Chemistry of Life
AP Biology Topic 1 9:24 English narration · English + 中文 subtitles burned in
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Transcript
Drop an ice cube into a glass and it floats.
把一块冰放进玻璃杯，它会浮起来。
That sounds ordinary, but it is strange.
这听起来平平无奇，其实很奇怪。
Almost every other solid sinks in its own liquid, because freezing packs the particles closer.
几乎所有其他固体都会沉在自己的液体里，因为凝固会让粒子挤得更近。
Water does the opposite.
水恰恰相反。
And there is more.
还不止如此。
Water resists heating.
水很难被加热升温。
Water climbs up a tree against gravity.
水能逆着重力爬上大树。
Water dissolves an enormous range of substances.
水能溶解种类极多的物质。
Four odd behaviours, and there is one reason behind all of it. The shape of a single water molecule, and the weak bond it makes with its neighbours.
四种反常的行为，而它们背后只有一个原因：一个水分子的形状， 以及它和邻居之间形成的那种弱的键。
Biology is chemistry that happens to be alive.
生物学就是恰好活着的化学。
Let's begin.
让我们开始吧。
A water molecule is one oxygen joined to two hydrogens, and it is bent, not straight.
一个水分子是一个氧连着两个氢，而且它是弯的，不是直的。
Now the important part. Oxygen pulls the shared electrons harder than hydrogen does. So the electrons spend more time near the oxygen end.
现在讲关键的部分： 氧比氢更强地把共用电子往自己这边拉，所以电子待在靠近氧那一端的时间更多。
That leaves the oxygen slightly negative and each hydrogen slightly positive.
这就使氧带上轻微的负电，每个氢带上轻微的正电。
A molecule with separated partial charges like this is called polar.
像这样带有分离的部分电荷的分子，叫做极性分子。
And opposite charges attract, so the slightly positive hydrogen of one molecule is drawn to the slightly negative oxygen of another.
而异性电荷相吸， 所以一个分子上带轻微正电的氢，会被另一个分子上带轻微负电的氧吸引。
That attraction is a hydrogen bond.
这种吸引就是氢键。
Each one is weak on its own, far weaker than the bonds inside the molecule.
单独一个氢键很弱，比分子内部的键弱得多。
But there are billions of them, and together they run this entire lesson.
但氢键有数十亿个，合起来就撑起了这一整节课。
Now watch every property fall out of that one bond. Cohesion and adhesion.
现在看每一条性质是怎么从那一种键里长出来的。
Water molecules stick to each other and to other surfaces, giving surface tension and letting water climb a narrow tube.
内聚力和附着力： 水分子彼此相黏，也黏在别的表面上，于是有了表面张力，也能沿着细管往上爬。
That is how water reaches the top of a tall tree.
水就是这样到达高大树木顶端的。
High specific heat. Heating water means breaking hydrogen bonds first, so the temperature rises slowly, stabilising the temperature inside an organism.
高比热容：加热水要先把氢键打断， 所以温度上升得很慢，这就稳定了生物体内部的温度。
High heat of vaporization. Escaping as vapour costs even more energy, so sweating carries heat away.
高汽化热： 变成蒸气逃逸要花更多能量，所以出汗能带走大量热量，让你降温。
Ice is less dense, because the hydrogen bonds lock into an open lattice with gaps, so a frozen pond keeps its life alive underneath.
冰的密度更小，因为氢键把分子锁成一个带空隙的疏松晶格，所以冰会浮起来， 结冰的池塘底下的生命得以存活。
And water is a superb solvent, because its charged ends pull solutes apart.
而水是极好的溶剂， 因为它带电的两端会包围极性物质和离子物质，把它们拆开。
Living matter is built mostly from six elements. Carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur.
生命物质主要由六种元素构成：碳、氢、氧、氮、磷、硫。
Of those, carbon is central, and it earns that place with one property. A carbon atom forms four stable covalent bonds.
其中碳处于核心地位，而它靠一条性质赢得这个位置：一个碳原子能形成四个稳定的共价键。
Four is the sweet spot.
四正好是那个甜蜜点。
With four, carbon can bond to other carbons on several sides at once, building long chains, branches and rings.
有了四个键，碳可以同时在好几个方向上与其他碳相连， 搭出长链、分支和环。
Those are the skeletons that every biological molecule hangs on.
这些就是所有生物分子挂靠其上的骨架。
Nothing else in the periodic table does this as well.
周期表里没有别的元素能把这件事做得这么好。
Big biological molecules are called macromolecules, and almost all of them are built the same way. Take a small repeating unit, called a monomer, and join many of them into a long chain, called a polymer.
大的生物分子叫做大分子，而它们几乎都是用同一种方式搭起来的： 取一个小的重复单元，叫做单体，把很多个连成一条长链，叫做聚合物。
Think of beads on a string.
可以想成串在线上的珠子。
Change which beads you use and you change what the string does.
换掉用的珠子，这串东西的功能就变了。
There are four classes to know. Carbohydrates. Lipids.
有四类要记住：碳水化合物、脂质、核酸、蛋白质。
Nucleic acids. And proteins. Three of those four are true polymers.
这四类里有三类是真正的聚合物。
Lipids are the exception, and we will come back to why.
脂质是例外，我们稍后会回来讲原因。
So how does a cell join two monomers?
那么细胞是怎么把两个单体连起来的？
By removing a water molecule.
靠移走一个水分子。
One monomer gives up a hydrogen, the other gives up a hydroxyl group, those two leave together as water, and the gap that opens becomes the new bond.
一个单体交出一个氢，另一个交出一个羟基，这两者一起以水的形式离开， 空出来的位置就变成新的键。
That is dehydration synthesis.
这就是脱水缩合。
And to take the polymer apart, run it backwards. Add a water molecule across the bond, put the hydrogen back on one side and the hydroxyl on the other, and the bond breaks.
要把聚合物拆开，就把这个过程倒过来： 在键上加一个水分子，把氢放回一边、羟基放回另一边，键就断了。
That is hydrolysis, which literally means splitting with water.
这就是水解，字面意思就是"用水劈开"。
Building and digesting are chemical opposites.
合成和消化是化学上的一对相反过程。
Here is a small counting result the exam likes.
这里有一个考试很喜欢的小计数结论。
Suppose you build a polymer from ten monomers. How many water molecules are released?
假设你用十个单体搭出一条聚合物， 会释放出多少个水分子？
Line the ten beads up.
把十颗珠子排成一排。
To join them into one chain you need nine bonds, because every bond sits in a gap between two neighbours, and ten beads have nine gaps.
要把它们连成一条链，你需要九个键， 因为每个键都位于两个相邻珠子之间的空隙里，而十颗珠子有九个空隙。
Each bond releases exactly one water.
每形成一个键正好释放一个水。
So nine water molecules.
所以是九个水分子。
In general, the number of waters is one less than the number of monomers.
一般地，水的个数比单体的个数少一。
And hydrolysis reverses it exactly. Add those nine waters back and you get your ten monomers again.
而水解正好把它逆转回来： 把那九个水加回去，你又得到十个单体。
Class one. Carbohydrates.
第一类：碳水化合物。
The monomer is a simple sugar, a monosaccharide, and glucose is the one to know.
它的单体是简单的糖，也就是单糖，其中要记住的是葡萄糖。
Join many and you get a polysaccharide, and what you get depends on how they are linked.
把很多个连起来就得到多糖，而得到什么取决于它们是怎么连的。
For storage, plants make starch and animals make glycogen, both coiled and branched so they pack away compactly and break down quickly.
用于储存时，植物做出淀粉，动物做出糖原，两者都是卷曲带分支的， 所以能紧凑地收纳起来，也能很快被分解。
For structure, plants make cellulose, whose straight chains lie side by side and lock into tough fibres.
用于结构时，植物做出纤维素， 它的链是直的，一条挨着一条，互相锁成坚韧的纤维。
That is why wood is strong, and why you cannot digest it.
这就是木材强韧的原因，也是你消化不了它的原因。
And because sugars carry many hydroxyl groups, they dissolve in water.
而由于糖上带有很多羟基，它们是极性的，很容易溶于水。
Class two. Lipids.
第二类：脂质。
These are defined by what they do not do. They are nonpolar, so they do not mix with water.
它们是由"不做什么"来定义的：它们是非极性的，所以不与水混合。
We call that hydrophobic, meaning water fearing.
我们把这叫做疏水，意思是"怕水"。
Fats store energy, far more per gram than carbohydrates, which is why the body uses them for long term storage.
脂肪储存能量，而且每克储存的能量远多于碳水化合物， 这就是身体用它做长期储存的原因。
A fat is one glycerol joined to three fatty acid tails.
一个脂肪是一个甘油连着三条脂肪酸尾巴。
Now the most important lipid of all.
现在讲最重要的那种脂质。
A phospholipid has only two tails, and in place of the third it carries a phosphate group.
磷脂只有两条尾巴，第三条的位置换成了一个磷酸基团。
That makes one end polar and the other nonpolar.
这让它一端是极性的、另一端是非极性的。
Drop those into water and they arrange themselves. The water loving heads face outward, the water fearing tails hide inward, and you get a bilayer.
把它们丢进水里，它们会自己排列起来： 亲水的头朝外对着水，疏水的尾朝内躲开水，于是形成一个双层。
That bilayer is the membrane around every cell.
这个双层就是包裹每一个细胞的膜。
Class three. Nucleic acids, which store and carry genetic information.
第三类：核酸，它储存并传递遗传信息。
The monomer is a nucleotide, and it has three parts. A sugar, a phosphate group, and a nitrogen base.
它的单体是核苷酸，由三部分组成： 一个糖、一个磷酸基团、一个含氮碱基。
The sugars and phosphates link into a backbone, and the bases stick out sideways.
糖和磷酸连成一条骨架，碱基则向侧面伸出。
And here is the key idea. The information is not in the backbone, which is the same all the way along.
这里有一个关键想法：信息不在骨架里，因为骨架从头到尾都是一样的。
The information is in the order of the bases.
信息在碱基的排列顺序里。
In DNA the bases pair in a fixed way. A pairs with T, and C pairs with G, held by hydrogen bonds, and the two strands run in opposite directions, which we call antiparallel.
在 DNA 中，碱基以固定的方式配对： A 与 T 配对，C 与 G 配对，由氢键维系；而且两条链的方向相反，我们称之为反向平行。
DNA and RNA differ in three ways. DNA is double stranded, RNA is single stranded, and RNA uses U where DNA uses T.
DNA 和 RNA 有三点不同：DNA 是双链，RNA 是单链，而且 RNA 用 U 代替 DNA 里的 T。
Class four, and the most versatile of all.
第四类，也是用途最广的一类。
Proteins are polymers of amino acids.
蛋白质是氨基酸的聚合物。
Every amino acid shares the same core and differs only in a side group, and there are twenty of them.
每个氨基酸都有相同的核心，只在一个侧基上不同，一共有二十种。
Joining two by dehydration synthesis makes a peptide bond.
两个氨基酸通过脱水缩合连起来，形成一个肽键。
Then the chain folds, at four levels.
然后这条链会折叠，而且分四个层次折叠。
Primary structure is the sequence.
一级结构是氨基酸的排列顺序。
Secondary structure is local folding into coils and sheets, held by hydrogen bonds along the backbone.
二级结构是局部折成螺旋和折叠片， 由骨架上的氢键维系。
Tertiary structure is the whole chain folding into one three dimensional shape, driven by the side groups.
三级结构是整条链折成一个三维形状，主要由侧基驱动。
And quaternary structure is two or more folded chains fitting together, as in haemoglobin, which is four chains working as one.
四级结构是两条或更多条折好的链拼装在一起，比如血红蛋白，它是四条链作为一个整体工作。
And now the single most important sentence in this unit. For a protein, shape determines function.
现在讲本单元最重要的一句话：对蛋白质来说，形状决定功能。
A protein works by fitting something else, so if the shape is right the job gets done, and if the shape is wrong nothing happens.
蛋白质靠与别的东西相契合来工作，所以形状对了，任务就完成；形状不对，就什么也不会发生。
That one rule covers enzymes, transporters, receptors and structural parts.
这一条规则涵盖了酶、转运蛋白、受体和结构成分。
Which tells you how to break a protein. Do not cut the chain. Just change its shape.
这也就告诉你该怎样破坏一个蛋白质：不用把链剪断，只要改变它的形状就够了。
Raise the temperature, or shift the pH, and the weak bonds holding the fold give way.
升高温度，或者改变 pH，维系折叠的那些弱键就会松开。
The chain unfolds and the protein stops working.
链展开，蛋白质就不再工作。
We call that denaturing, and you have watched it in a frying egg. The clear part turns white, and never turns back.
我们把这叫做变性，而你在煎鸡蛋时就看过它发生： 透明的部分变白变硬，而且再也变不回去。
Before the exam tips, one summary the graders reward. Link each class to its job.
在讲考试提示之前，先做一个阅卷人会给分的小结：把每一类和它的职责对应起来。
Carbohydrates. Quick energy and structure.
碳水化合物：快速供能和结构。
Lipids. Long term storage and membranes.
脂质：长期储存和细胞膜。
Nucleic acids.
核酸：信息。
Information. Proteins. Almost everything else, because shape gives them so many jobs.
蛋白质：几乎其他所有事情，因为形状让它们能担任非常多的角色。
If a question names a molecule, name its class, its monomer, and its role.
如果题目点名一个分子，就说出它的类别、它的单体和它的作用。
Three marks students lose.
学生最常丢分的三个地方。
First, for any property of water, trace it back to polarity and hydrogen bonding.
第一，讲水的任何一条性质，都要追溯到极性和氢键。
Saying that water has a high specific heat is not an explanation.
只说"水的比热容高"并不是解释。
Second, dehydration builds and hydrolysis breaks, so name the right one and say where the water goes.
第二，脱水缩合是合成、水解是分解，要说对是哪一个，并说清楚水去了哪里。
Third, for proteins, shape determines function, and heat or a pH change denatures them and stops them working.
第三，对蛋白质，形状决定功能，而升温或 pH 改变会使它变性、失去功能。