Biological Molecules
IGCSE Biology Topic 4 5:26 English narration · English + 中文 subtitles burned in
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Bread, meat, oil, your own muscles — all of them are built from just a handful of chemical elements.
面包、肉、油，还有你自己的肌肉——它们全都由少数几种化学元素构成。
And the large molecules in your food are not really large at all.
而食物里的大分子其实并不是真的"大"。
Each one is a long chain of small units joined together, like beads on a string.
每一个都是许多小单位连成的长链， 就像串在线上的珠子。
Learn the beads, and the whole topic becomes simple.
记住这些珠子，整个专题就变简单了。
Welcome to biological molecules.
欢迎来到生物分子。
The elements in food, the building blocks, the food tests, and the structure of D N A.
食物中的元素、组成单位、食物检验，还有DNA的结构。
Let's begin.
让我们开始吧。
Start with the chemical elements.
先看化学元素。
Carbohydrates and fats share the same three elements: carbon, hydrogen and oxygen.
碳水化合物和脂肪含有相同的三种元素：碳、氢和氧。
That is all.
就这三种。
Proteins have those three as well, but they carry one extra element — nitrogen.
蛋白质也有这三种，但它还多一种元素——氮。
Some proteins also contain sulfur.
有些蛋白质还含有硫。
So the one that stands out is nitrogen, and it belongs to protein alone.
所以特别的那一个是氮，它只属于蛋白质。
If a question asks which food group contains nitrogen, the answer is always protein.
如果题目问哪一类食物含有氮，答案永远是蛋白质。
Now the building blocks.
现在看组成单位。
Starch, glycogen and cellulose are all built from many glucose units — the same bead, joined in different ways.
淀粉、糖原和纤维素都由许多葡萄糖单位构成—— 同样的珠子，用不同的方式连接。
Proteins are built from amino acids; there are about twenty different kinds, joined in a long chain.
蛋白质由氨基酸构成；大约有二十种不同的氨基酸， 连成一条长链。
And fats are the odd one out, because they are not a single chain.
脂肪是特别的一个，因为它不是单一的链。
One fat molecule is made from three fatty acids joined to one glycerol.
一个脂肪分子由三个脂肪酸连到一个甘油上构成。
Remember that number: three fatty acids, one glycerol.
记住这个数字：三个脂肪酸，一个甘油。
Three molecules are built from the same glucose bead, so the exam wants you to separate them by job.
有三种分子由同样的葡萄糖珠子构成，所以考试要求你按功能把它们分开。
Starch and glycogen are energy stores — starch in plants, glycogen in animals.
淀粉和糖原是能量储存物——淀粉在植物里，糖原在动物里。
Cellulose is different: it is not a store at all.
纤维素不一样：它根本不是储存物。
Cellulose builds the plant cell wall and gives it strength.
纤维素构成植物细胞壁，给它强度。
So think about where each one is found: starch in a potato, glycogen in your liver and muscles, cellulose in every plant cell wall.
所以要想想每一种在哪里找到： 淀粉在马铃薯里，糖原在你的肝脏和肌肉里，纤维素在每一个植物细胞壁里。
Five food tests, and you need every colour change.
五个食物检验，每一个颜色变化你都要记住。
For starch, add iodine solution: orange-brown turns blue-black.
检验淀粉，加碘液：橙棕色变成蓝黑色。
For reducing sugars, such as glucose, add Benedict's solution and heat it: blue turns brick-red or orange.
检验还原糖，比如葡萄糖，加本尼迪特试剂并加热：蓝色变成砖红色或橙色。
For protein, add biuret solution: blue turns purple.
检验蛋白质，加双缩脲试剂：蓝色变成紫色。
For fats and oils, use the ethanol emulsion test: a cloudy white layer forms.
检验脂肪和油，用乙醇乳浊液试验： 形成白色浑浊层。
And for vitamin C, add it to blue D C P I P: the blue turns colourless.
检验维生素C，把它加到蓝色的DCPIP里：蓝色变成无色。
Two details cost marks every year.
有两个细节每年都让学生失分。
First: only the Benedict's test needs heat.
第一：只有本尼迪特试验需要加热。
All the others work cold, so if you write "heat it" for iodine or biuret, you lose the mark.
其他试验都是冷的做，所以如果你在碘液或双缩脲那里写"加热"，就会失分。
Second: D C P I P goes colourless — it loses its colour.
第二：DCPIP变成无色——它失去颜色。
Every other test gains a new colour.
其他每个试验都是得到一种新颜色。
Losing colour and gaining colour are opposite things, and the examiner checks which one you wrote.
失去颜色和得到颜色是相反的，考官会检查你写的是哪一个。
Let's use them.
我们来用一用。
A student tests an unknown food.
一个学生检验一份未知的食物。
Iodine solution stays orange-brown.
碘液仍然是橙棕色。
Benedict's solution, after heating, turns brick-red.
本尼迪特试剂加热后变成砖红色。
Biuret solution turns purple.
双缩脲试剂变成紫色。
What is in the food?
这份食物里有什么？
Iodine staying orange-brown means no change, and no change is a negative result — so there is no starch.
碘液保持橙棕色意味着没有变化，而没有变化是阴性结果——所以没有淀粉。
Brick-red is a positive Benedict's result, so reducing sugar is present.
砖红色是本尼迪特阳性结果，所以有还原糖。
Purple is a positive biuret result, so protein is present.
紫色是双缩脲阳性结果，所以有蛋白质。
So the food contains reducing sugar and protein, but no starch.
所以这份食物含有还原糖和蛋白质，但没有淀粉。
Notice the important idea: no colour change is a real result.
注意这个重要的想法： 没有颜色变化也是一个真正的结果。
It tells you the substance is absent.
它告诉你那种物质不存在。
Finally, for the supplement paper: D N A.
最后，补充部分：DNA。
D N A is the genetic material — it carries the instructions for building and running an organism.
DNA是遗传物质——它携带着建造和运行一个生物体的指令。
Learn four points.
要记住四点。
One: two strands are coiled together to form a double helix, like a twisted ladder.
第一：两条链盘绕在一起形成双螺旋，像一个扭曲的梯子。
Two: each strand carries chemicals called bases.
第二：每条链上带有叫做碱基的化学物质。
Three: bonds between pairs of bases hold the two strands together — those bases are the rungs of the ladder.
第三：成对碱基之间的化学键把两条链连在一起—— 这些碱基就是梯子的横档。
And four: the bases always pair in the same way.
第四：碱基总是以同样的方式配对。
A pairs with T, and C pairs with G.
A和T配对，C和G配对。
Here is why this pairing rule is useful.
这就是配对规则有用的原因。
If you know the bases on one strand, you can always work out the other.
如果你知道一条链上的碱基，你总能推出另一条。
Take a strand reading A, T, G, C, A.
假设一条链读作A、T、G、C、A。
Work along the strand one base at a time.
沿着这条链一个碱基一个碱基地做。
A pairs with T.
A和T配对。
T pairs with A.
T和A配对。
G pairs with C.
G和C配对。
C pairs with G.
C和G配对。
A pairs with T.
A和T配对。
So the other strand reads T, A, C, G, T.
所以另一条链读作T、A、C、G、T。
Check your answer by looking down each pair: every one should be A with T, or C with G.
检查你的答案，看每一对： 每一对都应该是A对T，或者C对G。
If you ever see A opposite G, you have made a mistake.
如果你看到A对着G，那就是做错了。
Three marks to lock in.
锁住三个分。
First: carbohydrates and fats are carbon, hydrogen and oxygen; nitrogen belongs to protein alone.
第一：碳水化合物和脂肪是碳、氢、氧；氮只属于蛋白质。
Second: only Benedict's needs heat, and only D C P I P goes colourless.
第二：只有本尼迪特试验需要加热，也只有DCPIP变成无色。
Third: the bases pair only A with T and C with G, so one strand always tells you the other.
第三：碱基只按A对T、C对G配对，所以一条链总能告诉你另一条。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。