Biological Molecules
A-Level Biology Topic 2 19:33 English narration · English + 中文 subtitles burned in
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Suppose you have an unknown food sample.
假设你有一份未知的食物样品。
How do you tell what it is made of?
怎么知道它由什么构成？
Biology has a beautiful trick: add the right chemical, and the colour tells you the answer.
生物学有一个漂亮的窍门： 加入合适的化学试剂，颜色就会告诉你答案。
Heat a sugar with Benedict's solution, and the blue slowly turns green, then orange, then a rich brick-red — the more sugar, the further it goes.
把糖和本尼迪克试剂一起加热，蓝色慢慢变绿， 再变橙，最后变成浓浓的砖红色——糖越多，颜色变得越深。
One drop of colour reveals the hidden molecule.
一滴颜色，就揭示了隐藏的分子。
So let's learn the tests, and then the molecules themselves.
那我们就先学这些检验，再学分子本身。
Every living thing is built from four families of molecule: carbohydrates, lipids, proteins, and the water that surrounds them all.
每一个生命都由四大类分子构成：碳水化合物、脂质、蛋白质，以及包围着它们的水。
In this lesson we test for each, then look inside — how small monomers join into giant polymers, how a single bent tail changes a fat, how a chain of amino acids folds into a working machine, and why water is so special.
在这节课里，我们会分别检验它们，然后深入内部——小小的单体如何连成巨大的聚合物， 一条弯曲的尾巴如何改变一种脂肪，一串氨基酸如何折叠成一台工作的机器，以及水为什么如此特别。
Let's begin.
让我们开始吧。
Four tests, four molecules — learn each as a reagent and a colour.
四个检验，四种分子——每一个都记作试剂加颜色。
For a reducing sugar, add Benedict's solution and heat: blue turns brick-red.
检验还原糖，加入本尼迪克试剂并加热：蓝色变砖红。
For starch, add orange-brown iodine: it goes blue-black.
检验淀粉，加入橙棕色的碘液：变成蓝黑色。
For a lipid, shake the sample with ethanol and pour into water: a white, cloudy emulsion forms.
检验脂质，把样品与乙醇摇匀，再倒入水中： 形成白色浑浊的乳浊液。
And for a protein, add biuret solution at room temperature: blue turns purple.
检验蛋白质，在室温下加入双缩脲试剂：蓝色变紫色。
Reagent plus colour, every time.
每一次，都是试剂加颜色。
The ordinary Benedict's test only says yes or no.
普通的本尼迪克检验只回答有或没有。
A semi-quantitative version estimates roughly how much reducing sugar is there.
半定量版本则能粗略估计还原糖有多少。
First you standardise the test: run it on solutions of known concentration and record each result.
首先把检验标准化：用已知浓度的溶液做检验，并记下每一种结果。
Then you judge an unknown in one of two ways.
然后用两种办法之一判断未知样品。
Watch the time to the first colour change — more sugar turns colour faster.
看第一次变色的时间——糖越多，变色越快。
Or compare the final colour against your set of standards.
或者把最终颜色和你的标准色样对照。
Blue means none; green, yellow and orange mean rising amounts; brick-red means a lot, and often a precipitate settles.
蓝色表示没有；绿、黄、橙表示含量逐渐升高； 砖红色表示很多，常常还会出现沉淀。
Same reagent, but now the colour scale carries a number, not just a yes.
还是同一种试剂，但颜色刻度现在带着数量，而不只是有或没有。
But some sugars, like sucrose, are non-reducing — they give a negative Benedict's test and stay blue.
但有些糖，比如蔗糖，是非还原糖——它们的本尼迪克检验呈阴性，保持蓝色。
To catch them, use a clever four-step trick.
要抓住它们，用一个巧妙的四步法。
First, a normal Benedict's test stays blue, confirming no reducing sugar is there yet.
第一，普通的本尼迪克检验保持蓝色，确认此时没有还原糖。
Second, take a fresh sample and boil it with dilute acid — this hydrolyses the sugar into smaller reducing ones.
第二，取一份新样品，用稀酸煮沸——这会把糖水解成更小的还原糖。
Third, and crucially, neutralise the acid, because Benedict's only works in alkali.
第三，也是关键， 中和这些酸，因为本尼迪克试剂只在碱性条件下起作用。
Fourth, test again: a brick-red colour now proves a non-reducing sugar was there all along.
第四，再测一次：现在出现砖红色， 就证明一开始就存在非还原糖。
Here is a classic exam story.
这是一道经典的考试题。
A solution gives a negative Benedict's test.
一份溶液的本尼迪克检验呈阴性。
It is boiled with dilute hydrochloric acid, neutralised with sodium hydrogencarbonate, then re-tested — and now it turns brick-red.
再用稀盐酸煮沸， 用碳酸氢钠中和，然后重测——现在变成砖红色。
What was present, and why is each step needed?
存在什么？ 每一步为什么必要？
The first negative result rules out a reducing sugar.
第一次阴性结果排除了还原糖。
Boiling with acid hydrolyses the glycosidic bond, splitting a non-reducing sugar such as sucrose into its reducing monosaccharides.
用酸煮沸会水解糖苷键，把蔗糖这类非还原糖拆成还原性单糖。
Neutralising is essential: Benedict's only works in alkaline conditions — skip it and the test fails even when sugar is present.
中和是必需的：本尼迪克试剂只在碱性条件下起作用——跳过这一步，即使有糖检验也会失败。
The positive re-test therefore shows a non-reducing sugar was there all along.
因此阳性的重测说明一开始就有非还原糖。
Always quote that first, negative test: without it, the final red colour cannot tell a non-reducing sugar from a reducing one.
务必写上那次最初的阴性检验： 没有它，最后的红色分不清非还原糖和还原糖。
Now the molecules themselves, starting with carbohydrates.
现在来看分子本身，先从碳水化合物开始。
They come in three sizes.
它们有三种大小。
A single sugar unit is a monosaccharide, like glucose.
单个糖单元是单糖，比如葡萄糖。
Two joined together make a disaccharide, like maltose.
两个连在一起是二糖，比如麦芽糖。
And many joined into a chain make a polysaccharide, like starch.
许多个连成链是多糖，比如淀粉。
Glucose itself forms a six-sided ring, and here is a subtle but crucial detail: it has two ring forms.
葡萄糖本身形成一个六元环， 这里有一个微妙却关键的细节：它有两种环形。
In alpha-glucose the hydroxyl on carbon one points down; in beta-glucose it points up.
在α-葡萄糖里，碳一上的羟基朝下； 在β-葡萄糖里，它朝上。
That tiny difference decides which polymer the glucose can build.
这个微小的差别，决定了葡萄糖能构建哪种聚合物。
Three words that turn up on every paper: monomer, polymer, macromolecule.
三张考卷上都会出现的词：单体、聚合物、大分子。
A monomer is a small molecule that is a single building unit.
单体是作为单个构建单元的小分子。
A polymer is a long molecule made of many monomers joined together.
聚合物是许多单体连在一起的长分子。
A macromolecule is any very large molecule — starches, proteins and nucleic acids all count.
大分子是任何很大的分子——淀粉、蛋白质和核酸都算。
Among the sugars, glucose, fructose and maltose are reducing: they give a positive Benedict's test.
在糖类里，葡萄糖、果糖和麦芽糖是还原糖：它们的本尼迪克检验呈阳性。
Sucrose is a non-reducing sugar — it stays blue until you hydrolyse it.
蔗糖是非还原糖——在你水解它之前一直保持蓝色。
So when a food sample fails Benedict's first, do not write "no sugar"; write "no reducing sugar", then check for non-reducing ones.
所以当食物样品第一次本尼迪克失败时， 不要写没有糖；要写没有还原糖，然后再查非还原糖。
Look carefully at the two rings.
仔细看这两个环。
Glucose has six carbon atoms and closes into a ring.
葡萄糖有六个碳原子，合拢成环。
The two forms differ only at carbon one.
两种形式只在碳一处不同。
In alpha-glucose the hydroxyl group on carbon one points down, below the ring.
在α-葡萄糖里，碳一上的羟基朝下，在环的下方。
In beta-glucose the same group points up, above the ring.
在β-葡萄糖里，同一基团朝上，在环的上方。
Everything else is the same.
其余部分完全一样。
That small difference is not decoration: alpha-glucose builds starch and glycogen, which coil or branch into compact stores.
这点小差别不是装饰：α-葡萄糖构建淀粉和糖原，它们卷曲或分支成紧凑的储存物。
Beta-glucose builds cellulose, whose chains run straight.
β-葡萄糖构建纤维素，它的链笔直延伸。
Get the orientation wrong and you name the wrong polymer — a favourite trap in structure-and-function questions.
把朝向搞错，你就会把聚合物叫错—— 这是结构和功能题目里常见的陷阱。
How do monomers join?
单体是怎么连起来的？
When two sugars link, a glycosidic bond forms between them — but there is a price.
当两个糖连接时，它们之间形成一个糖苷键——但要付出代价。
A molecule of water is squeezed out each time.
每一次都会挤出一个水分子。
We call this condensation: joining units by removing water.
我们把这叫做缩合：通过去掉水来连接单元。
The reverse is just as important.
相反的过程同样重要。
Add a water molecule back, and the bond breaks apart — that is hydrolysis.
加回一个水分子，键就断开——这就是水解。
This is exactly what acid and heat do to sucrose in the non-reducing test: they hydrolyse it into glucose and fructose.
这正是酸和热对蔗糖所做的事，在非还原糖检验里： 它们把蔗糖水解成葡萄糖和果糖。
Monomers are joined by strong covalent bonds, not loose attractions.
单体由强共价键连接，而不是松散的吸引。
For sugars the covalent link is the glycosidic bond.
对糖来说，共价连接就是糖苷键。
Condensation removes one water molecule every time a bond forms, so building a long chain releases many waters.
每一次成键，缩合都会去掉一个水分子，所以造一条长链会放出许多水。
Hydrolysis is the exact reverse: add water, break the bond, free the monomers.
水解正好相反：加水、断键、释放单体。
Enzymes catalyse both directions in living cells, but in the lab we use dilute acid and heat to force hydrolysis of sucrose.
活细胞里酶催化两个方向， 但在实验室我们用稀酸和加热来强制水解蔗糖。
Remember the pair: condensation builds, hydrolysis breaks — and both involve that same water molecule moving out or back in.
记住这一对：缩合构建，水解拆开—— 两者都涉及同一个水分子出去或回来。
The same glucose builds very different polymers, and the shape always fits the job.
同样的葡萄糖能构建出很不一样的聚合物，而形状总是与功能相配。
Plants store energy as starch — amylose, a coiled chain, and amylopectin, a branched one.
植物把能量储存为淀粉—— 直链淀粉是卷曲的链，支链淀粉是有分支的链。
Animals store glycogen, which is even more branched, so it can be broken down fast when energy is needed.
动物储存糖原，它的分支更多， 所以需要能量时能被快速分解。
All three are compact and insoluble — perfect stores.
这三者都紧凑而不溶——是完美的储存物。
Cellulose is different: made from beta-glucose, its chains run dead straight and pack side by side into strong fibres — the stuff of plant cell walls.
纤维素则不同： 由β-葡萄糖构成，它的链笔直伸展，一根挨一根地堆成坚固的纤维——正是植物细胞壁的材料。
These are real starch grains inside a potato, stained brown by iodine.
这些是马铃薯里真正的淀粉粒，被碘染成棕色。
Each grain is a dense package of amylose and amylopectin — and the largest are only about a tenth of a millimetre across.
每一粒都是直链淀粉和支链淀粉的致密包装—— 最大的直径大约只有零点一毫米。
Storage polysaccharides suit their job for three linked reasons.
储存多糖适合它们的工作，有三个相互关联的原因。
They are compact, so lots of energy fits in a small volume.
它们紧凑，所以大量能量装进很小的体积。
They are insoluble, so they do not leave the cell or dissolve away.
它们不溶，所以不会离开细胞或溶掉。
And because they are insoluble they do not change the water potential of the cell, so they do not pull water in by osmosis and burst it.
正因为不溶，它们不改变细胞的水势，所以不会靠渗透把水拉进来胀破细胞。
The many branches also give many free ends, so glucose can be added or removed quickly when the plant or animal needs fuel.
许多分支也给出许多自由末端，当植物或动物需要燃料时，葡萄糖可以快速添加或移除。
Compare the four polymers side by side.
把四种聚合物并排放在一起比较。
Amylose is a long unbranched chain of alpha-glucose that coils into a spiral — compact and easy to pack.
直链淀粉是α-葡萄糖的长而无分支的链，卷成螺旋——紧凑且易堆叠。
Amylopectin is branched, so enzymes can attack many ends at once.
支链淀粉有分支，所以酶能同时攻击许多末端。
Glycogen is like amylopectin but even more heavily branched, which is why animal cells can release glucose so fast during exercise.
糖原像支链淀粉，但分支更密， 这就是动物细胞在运动时能如此快速释放葡萄糖的原因。
Cellulose is the odd one out: straight chains of beta-glucose that never coil.
纤维素是那个另类： β-葡萄糖的直链，从不卷曲。
Always link the picture to the job — coiled or branched for storage, straight for strength.
永远把图像和功能连起来——卷曲或分支用于储存，笔直用于强度。
That sentence wins marks in almost every polysaccharide question.
这句话几乎能在每一道多糖题里得分。
Zoom in on cellulose.
放大看纤维素。
Because it is built from beta-glucose, every other unit is flipped over.
因为它由β-葡萄糖构成，每隔一个单元就会翻转。
That flip forces the chain to stay long and straight instead of coiling.
这种翻转迫使链保持又长又直，而不是卷曲。
Many straight chains then lie side by side and stick together with hydrogen bonds, forming strong bundles called microfibrils.
许多直链并排躺在一起，靠氢键粘合， 形成叫做微纤丝的坚固束。
Microfibrils give the plant cell wall its tensile strength and stop the cell bursting when water rushes in by osmosis.
微纤丝赋予植物细胞壁抗张强度，并在水靠渗透涌入时阻止细胞胀破。
So the beta orientation, the straight shape, the hydrogen bonds, and the wall's job are one continuous story — not four separate facts.
所以β朝向、笔直形状、氢键，以及细胞壁的功能，是同一个连续故事——不是四条孤立的事实。
Now lipids.
现在来看脂质。
The main fat is a triglyceride, and it hates water.
主要的脂肪是甘油三酯，它厌恶水。
It is built from one glycerol molecule as a backbone, joined to three fatty acid tails.
它由一个甘油分子作为骨架， 连上三条脂肪酸尾巴构成。
Each join is an ester bond, formed by condensation, so three waters are removed.
每一处连接都是一个酯键，由缩合形成，所以去掉了三个水。
The tails come in two kinds.
尾巴有两种。
A saturated tail has no double bonds — it is straight, and packs into a solid fat.
饱和尾巴没有双键——它笔直，堆成固态的脂肪。
An unsaturated tail has a double bond that puts a kink in it, keeping it a liquid oil.
不饱和尾巴有一个双键，把它弯出一个折， 让它保持液态的油。
Fats store twice the energy of carbohydrates, gram for gram.
同样一克，脂肪储存的能量是碳水化合物的两倍。
A triglyceride is non-polar and hydrophobic — it does not mix with water.
甘油三酯是非极性的、疏水的——它不与水混合。
That is why the emulsion test works: shaking with ethanol then water makes a cloudy white emulsion.
这就是乳浊检验有效的原因： 与乙醇摇匀再倒入水中，会形成白色浑浊的乳浊液。
As a long-term energy store, fat is hard to beat.
作为长期能量储存，脂肪难以超越。
It releases about twice the energy per gram as carbohydrate.
每克释放的能量大约是碳水化合物的两倍。
It is insoluble, so it stays where it is put.
它不溶，所以待在放它的地方。
And it holds little extra water, so it stores energy with less mass than a hydrated sugar store.
而且它几乎不携带额外的水，所以储存能量时质量比含水的糖储更轻。
Under the skin, layers of fat also give insulation against the cold and a soft cushion that protects organs from knocks.
在皮下，脂肪层还提供御寒隔热，以及缓冲撞击、保护器官的软垫。
Energy, mass, insulation, protection — four roles from one molecule.
能量、质量、隔热、保护——一个分子的四种角色。
A phospholipid is almost a triglyceride, but with one fatty acid swapped for a phosphate group.
磷脂几乎就是甘油三酯，但把一条脂肪酸换成了一个磷酸基团。
This gives it a split personality.
这让它有了双重性格。
The phosphate head is hydrophilic — it loves water.
磷酸头是亲水的——它爱水。
The two fatty acid tails are hydrophobic — they fear it.
两条脂肪酸尾巴是疏水的——它们怕水。
So when you put many phospholipids in water, they arrange themselves automatically: heads facing out to the water on both sides, tails hiding together in the middle.
所以当你把许多磷脂放进水里， 它们会自动排列：头朝外，面向两侧的水，尾巴一起躲在中间。
This double layer, the bilayer, is the basis of every cell membrane.
这个双层，也就是磷脂双分子层， 是每一层细胞膜的基础。
Here is the bilayer drawn out.
这就是画出来的双分子层。
Each phospholipid has a polar, hydrophilic phosphate head and two hydrophobic fatty acid tails.
每个磷脂都有一个极性的、亲水的磷酸头，以及两条疏水的脂肪酸尾巴。
In water the heads face the watery surroundings on both the outside and the inside of the membrane.
在水中，头朝向膜外侧和内侧两侧的水环境。
The tails hide from water by meeting in the oily middle.
尾巴在油性的中间相遇，躲开水。
No special machinery is needed — the arrangement is spontaneous.
不需要特殊机器——这种排列是自发的。
That double sheet is the foundation of every cell membrane: a barrier that keeps the cytoplasm separate from the outside, yet thin enough for proteins to sit in and control what crosses.
这张双层薄片是每一层细胞膜的基础： 一道屏障把细胞质与外界隔开，却又薄到足以让蛋白质嵌进去，控制什么能穿过。
Swap one tail for phosphate, and a storage fat becomes a membrane builder.
把一条尾巴换成磷酸，储存脂肪就变成了造膜材料。
Finally, proteins — polymers of amino acids.
最后是蛋白质——氨基酸的聚合物。
And one fact about them explains why proteins can do so many different jobs when sugars and fats cannot.
而关于它们有一个事实， 解释了为什么蛋白质能做这么多不同的工作，而糖和脂肪不能。
Every amino acid is built on the same template around a central carbon, and only one part of it varies: same amino group, same carboxyl group, same hydrogen — then a side chain, the R group, which is different in each one.
每个氨基酸都围绕一个中心碳原子建立在同一个模板上，而其中只有一部分是变化的： 同样的氨基、同样的羧基、同样的氢——然后是一条侧链，也就是 R 基团，每一个都不同。
There are twenty of them in living things, so twenty different R groups, and everything a protein turns out to be traces back to which R groups are in its chain and in what order.
生物体里一共有二十种，也就是二十种不同的 R 基团， 而一个蛋白质最终成为什么，全都追溯到它的链上有哪些 R 基团、以及排列的顺序。
Two amino acids join by condensation, forming a peptide bond and releasing a water; link many and you have a polypeptide, the raw chain that folds into a protein.
两个氨基酸通过缩合连接，形成一个肽键，并释放一个水； 把许多个连起来，就得到一条多肽，也就是将要折叠成蛋白质的那条原始链。
The diagram next takes that join apart.
接下来的图会把这个连接拆开来看。
Study the general structure.
研究通式结构。
A central carbon holds four things: the amino group, the carboxyl group, a hydrogen atom, and the R side chain that makes each amino acid unique.
一个中心碳抓住四样东西：氨基、羧基、一个氢原子，以及让每个氨基酸独特的侧链。
When two amino acids join, the amino group of one reacts with the carboxyl group of the next.
当两个氨基酸连接时，一个的氨基与下一个的羧基反应。
Condensation removes a water molecule and leaves a peptide bond between them.
缩合去掉一个水分子， 在它们之间留下肽键。
Many amino acids joined this way make a polypeptide.
许多氨基酸这样连成多肽。
Adding water back — hydrolysis — breaks a peptide bond and frees the units again.
把水加回去——水解——会断开肽键，再次释放单元。
Same condensation and hydrolysis story as sugars and fats, but the bond name is peptide, not glycosidic or ester.
和糖与脂肪是同一套缩合与水解的故事，但键的名字是肽键，不是糖苷键或酯键。
A protein's power comes from its shape, built in four levels.
蛋白质的威力来自它的形状，分四级建成。
The primary structure is simply the order of amino acids in the chain, held by peptide bonds.
一级结构就是链中氨基酸的排列顺序，由肽键维系。
The secondary structure is local folding into a spiral alpha-helix or a pleated sheet, held by hydrogen bonds.
二级结构是局部折叠成螺旋的α-螺旋或折叠片，由氢键维系。
The tertiary structure is the whole chain folded into one precise three-dimensional shape, held by interactions between R groups.
三级结构是整条链折叠成一个精确的三维形状， 由R基团之间的相互作用维系。
And the quaternary structure is two or more chains joined into one protein — like haemoglobin, with its four.
而四级结构是两条或更多条链结合成一个蛋白质—— 就像血红蛋白，有四条链。
The tertiary fold is held by four kinds of interaction between R groups.
三级折叠由侧链基团之间四种相互作用维系。
Hydrophobic interactions: non-polar side chains cluster away from water in the core of the protein.
疏水作用：非极性侧链聚在蛋白质核心，躲开水。
Hydrogen bonds: form between polar groups on different parts of the chain.
氢键：在链的不同部分的极性基团之间形成。
Ionic bonds: form between positively and negatively charged side chains.
离子键：在带正电和带负电的侧链之间形成。
And covalent bonding, including strong disulfide bonds between sulfur-containing side chains.
还有共价键，包括含硫侧链之间的强二硫键。
Secondary structure already uses hydrogen bonds along the backbone; tertiary adds these R-group contacts across the whole molecule.
二级结构已经沿主链使用氢键； 三级结构在整个分子上加上这些侧链接触。
Name the level and the bond type together — that is what the mark scheme wants.
把结构层次和键的类型一起说—— 那正是评分标准想要的。
Proteins fall into two broad shape families.
蛋白质大致分成两种形状家族。
Globular proteins fold into a rounded, compact shape.
球状蛋白质折叠成圆润、紧凑的形状。
They are usually soluble in water and do active jobs in the body — enzymes that catalyse reactions, and haemoglobin that carries oxygen, are classic examples.
它们通常溶于水，并在体内做活跃的工作——催化反应的酶，以及运输氧气的血红蛋白，都是经典例子。
Fibrous proteins form long strands or sheets.
纤维状蛋白质形成长链或薄片。
They are usually insoluble and give structure and support — collagen in skin and tendon is the textbook case.
它们通常不溶，并提供结构与支撑—— 皮肤和肌腱里的胶原蛋白是教科书式的例子。
Shape again matches function: a ball that can dissolve and move is useful for chemistry in solution; a tough cable is useful for holding tissues together.
形状再次匹配功能： 能溶解并移动的球，适合溶液中的化学；坚韧的缆绳，适合把组织固定在一起。
Always say soluble or insoluble when you name the type.
说出类型时，永远要说可溶或不溶。
Haemoglobin is a globular protein that carries oxygen in red blood cells.
血红蛋白是球状蛋白质，在红细胞里运输氧气。
It has quaternary structure made of four polypeptide chains: two alpha-globin chains and two beta-globin chains.
它有四级结构，由四条多肽链构成： 两条α珠蛋白链和两条β珠蛋白链。
Each chain holds a haem group, drawn here in green.
每条链含有一个血红素基团，这里画成绿色。
At the centre of each haem is an iron atom, and that is where one oxygen molecule binds.
每个血红素的中心是一个铁原子，那就是一个氧分子结合的地方。
Four chains mean one haemoglobin molecule can carry four oxygen molecules at once.
四条链意味着一个血红蛋白分子 可以一次携带四个氧分子。
The rounded, soluble shape lets it travel dissolved in the red cell, while the four haem sites give high oxygen capacity — structure and function in one picture.
圆润、可溶的形状让它能溶解在红细胞里旅行， 而四个血红素位点给出高载氧能力——结构和功能在同一幅图里。
Collagen is the fibrous opposite of haemoglobin.
胶原蛋白是血红蛋白在纤维状上的对立面。
It gives strength to skin, tendons, bone and blood vessel walls.
它赋予皮肤、肌腱、骨骼和血管壁强度。
One collagen molecule is three polypeptide chains wound tightly around each other in a triple strand, held by hydrogen bonds.
一个胶原蛋白分子是三条多肽链紧紧缠绕成的三股螺旋，由氢键维系。
Many of these molecules then lie side by side, slightly staggered, and are cross-linked into thick fibres.
许多这样的分子再并排躺着，略微交错，并交联成粗纤维。
The staggered, cross-linked arrangement makes collagen very strong when pulled — rather like a rope made of twisted and glued threads.
交错、交联的排列使胶原蛋白在被拉时非常强壮——很像由拧在一起又粘住的线制成的绳。
So: three chains in a triple helix, hydrogen bonds within, staggered packing and cross-links between molecules.
所以：三股螺旋里的三条链，内部的氢键，分子之间的交错堆叠与交联。
That is the structure behind its mechanical job.
那就是支撑其机械功能的结构。
We finish with water — small, but the molecule of life, and every one of its properties traces back to a single asymmetry.
我们以水收尾——它很小，却是生命之源， 而它的每一项性质都可以追溯到同一个不对称。
Its oxygen pulls the shared electrons harder than its hydrogens do, so one end of the molecule sits slightly negative and the other slightly positive.
它的氧比氢更用力地拉住共用电子，所以分子的一端略带负电，另一端略带正电。
Water is polar.
水是极性的。
And because it is polar, every molecule attracts its neighbours: a weak hydrogen bond forms between the slightly positive hydrogen of one and the slightly negative oxygen of the next.
而正因为它是极性的，每个分子都会吸引它的邻居： 一个分子略带正电的氢与下一个分子略带负电的氧之间，形成一个微弱的氢键。
One hydrogen bond on its own is feeble — far weaker than the covalent bonds inside the molecule.
单独一个氢键很弱——远比分子内部的共价键弱。
But each molecule makes several at once, and a liquid held together by that many weak bonds behaves as though it were far heavier than a molecule this small has any right to be.
但每个分子同时形成好几个，而一种被这么多弱键牵在一起的液体， 表现得就像比它这么小的分子本该有的重得多。
That is the whole explanation behind its useful properties, and the diagram next turns it into the three the exam asks for.
这就是它种种有用性质背后的全部解释， 接下来的图会把它变成考试要的那三条。
The diagram shows why those properties follow.
这张图说明为什么会有那些性质。
Each water molecule has a slightly negative oxygen and slightly positive hydrogens.
每个水分子都有略带负电的氧和略带正电的氢。
Dashed hydrogen bonds link the hydrogen of one molecule to the oxygen of the next.
虚线氢键把一个分子的氢连到下一个分子的氧上。
Solvent action: polar and ionic substances dissolve, so reactions can run and blood can carry solutes.
溶剂作用：极性和离子物质溶解， 所以反应能进行，血液能运送溶质。
High specific heat capacity: lots of energy is needed to raise the temperature, so living tissues stay thermally steady.
高比热容：升高温度需要大量能量，所以活组织温度保持稳定。
High latent heat of vaporisation: lots of energy is needed to evaporate water, so when sweat dries it carries heat away and cools the body.
高汽化潜热：蒸发水需要大量能量，所以汗水变干时带走热量，使身体降温。
One polar molecule, three exam-ready properties.
一个极性分子，三条考试可用的性质。
Before you go, four ways to keep your marks.
结束之前，四个保住分数的办法。
First, give each food test as reagent plus positive colour — Benedict's to brick-red, iodine to blue-black, biuret to purple, emulsion to white.
第一，每个食物检验都写成试剂加阳性颜色——本尼迪克到砖红， 碘到蓝黑，双缩脲到紫，乳浊到白。
Second, name the bond precisely — glycosidic, ester, or peptide — all made by condensation.
第二，精确命名键——糖苷键、酯键或肽键——都由缩合形成。
Third, always link shape to function: straight cellulose is strong, branched glycogen is a fast store.
第三，永远把形状和功能联系起来：笔直的纤维素坚固，有分支的糖原是快速储存物。
Fourth, for proteins, name the bond holding each of the four levels.
第四，蛋白质要说出维系四级结构中每一级的键。