Respiration
A-Level Biology Topic 12 9:49 English narration · English + 中文 subtitles burned in
Chapters
Transcript
At any moment your body holds only about fifty grams of ATP.
在任何一个时刻，你体内只有大约五十克 ATP。
Yet over a single day you will make and spend roughly your own body weight of it.
可是在一天之内， 你会制造并消耗掉大约相当于你自己体重那么多的 ATP。
The same molecules are broken apart and put back together, over and over, thousands of times each.
同样的分子被拆开又装回去，来来回回，每个分子要循环上千次。
That is because ATP is not a store of energy. It is a delivery system.
这是因为 ATP 不是能量的仓库，而是一套配送系统。
In this lesson we follow one glucose molecule all the way down, and count the ATP it pays out.
这节课我们跟着一个葡萄糖分子一路走下去，数一数它付出了多少 ATP。
Start with why a cell needs energy at all.
先说细胞为什么需要能量。
It needs energy for active transport, for movement such as a muscle contracting, and for anabolic reactions — building large molecules like DNA and proteins.
它需要能量进行主动运输，需要能量进行运动（比如肌肉收缩）， 还需要能量来建造像 DNA 和蛋白质这样的大分子。 这就是提供这一切的循环。
Here is the cycle that supplies all of that. Energy from respiration joins ADP and a phosphate group to make ATP.
呼吸作用释放的能量把 ADP 和一个磷酸基团连接起来，形成 ATP。
Later that phosphate is broken off again, and a small burst of energy is released.
之后那个磷酸基团又被断开，释放出一小股能量。
Three things make ATP perfectly suited to the job, which is why we call it the universal energy currency. It releases energy quickly, in small amounts that match what a cell needs. It is easily remade, again and again.
有三点让 ATP 非常适合这项工作：它释放能量很快，量小，正好符合细胞的需要； 它很容易被反复再生；它又小又易溶，能轻松移动到需要它的地方。
And it is small and soluble, so it moves easily to wherever it is wanted. ATP is made in two ways: by transferring a phosphate directly, and by chemiosmosis across a membrane.
ATP 有两种制造方式：直接转移一个磷酸基团，以及跨膜的化学渗透。
A respiratory substrate is simply a molecule that gets broken down to release energy.
呼吸底物就是被分解以释放能量的分子。
They are not all equal.
它们并不都一样。
Per gram, lipids release the most energy, because they carry the most hydrogen.
按每克计算，脂质释放的能量最多，因为它们含有最多的氢。
Proteins come next, and carbohydrates release the least.
蛋白质其次，糖类最少。
Now, how do we tell which one an organism is actually using?
那么我们怎么知道一个生物实际在用哪一种呢？
We use a number called the respiratory quotient, or RQ.
我们用一个叫呼吸商的数值，简称 RQ。
It is the molecules of carbon dioxide produced divided by the molecules of oxygen taken in.
它等于产生的二氧化碳分子数除以吸入的氧气分子数。
For carbohydrate the value is about one point zero.
糖类的数值约为一点零。
For lipid it is about zero point seven, because lipid needs much more oxygen for each carbon dioxide it makes.
脂质约为零点七，因为脂质每产生一个二氧化碳需要多得多的氧。
And for protein it sits in between, at about zero point nine.
蛋白质介于两者之间，约为零点九。
Let us work one out.
我们来算一个。
Take the equation for the aerobic respiration of glucose. One glucose plus six oxygen gives six carbon dioxide plus six water.
取葡萄糖有氧呼吸的方程式： 一个葡萄糖加六个氧，生成六个二氧化碳加六个水。
Now divide the carbon dioxide produced by the oxygen taken in.
现在用产生的二氧化碳除以吸入的氧气。
That is six over six, which is one point zero.
也就是六除以六，等于一点零。
And that number tells you something useful straight away. An RQ near one point zero says the organism is respiring carbohydrate.
这个数字立刻就告诉你一些有用的信息： RQ 接近一点零，说明这个生物在呼吸糖类。
An RQ nearer zero point seven says it is using lipid instead, because lipid takes more oxygen for each carbon dioxide it gives back.
RQ 接近零点七，说明它在用脂质，因为脂质每还回一个二氧化碳要消耗更多的氧。
Aerobic respiration — respiration with oxygen — has four stages, and the examiner wants the place as well as the name.
有氧呼吸——有氧气参与的呼吸——有四个阶段，考官既要名称也要部位。
Glycolysis happens in the cytoplasm.
糖酵解发生在细胞质里。
The link reaction happens in the matrix of the mitochondrion.
连接反应发生在线粒体的基质中。
The Krebs cycle also happens in the matrix.
克雷布斯循环也发生在基质中。
And oxidative phosphorylation happens on the inner membrane.
氧化磷酸化发生在内膜上。
Follow the carbon through and you see how glucose becomes pyruvate, then acetyl coenzyme A, while the energy ends up in ATP.
跟着碳走一遍，你就看到葡萄糖如何变成丙酮酸， 再变成乙酰辅酶A，而能量最终进入 ATP。
Two things to hold on to. Most of the ATP comes from that very last stage.
有两点要记住： 绝大部分 ATP 来自最后那个阶段。
And the first three stages hand their hydrogen to reduced NAD and FAD, which carry it there.
而前三个阶段把氢交给还原型 NAD 和 FAD，由它们运送过去。
Glycolysis happens in the cytoplasm, and it starts by spending energy rather than making it.
糖酵解发生在细胞质中，而且一开始是花能量而不是产能量。
Glucose, which has six carbons, is phosphorylated using two ATP, and becomes fructose bisphosphate — still six carbons, but now unstable.
有六个碳的葡萄糖被磷酸化，消耗两个 ATP，变成果糖二磷酸——仍然是六个碳，但现在很不稳定。
That molecule is then split in two, giving two triose phosphates of three carbons each.
这个分子随后一分为二，生成两个各含三个碳的丙糖磷酸。
Those are then oxidised to become two molecules of pyruvate.
它们再被氧化，变成两个丙酮酸分子。
Now count up. Four ATP are made, but two were spent at the start, so the net gain is two ATP.
现在算一算：制造了四个 ATP， 但开始时花掉了两个，所以净得两个 ATP。
Some NAD is also reduced — NAD is a coenzyme, a helper that carries hydrogen.
同时还有一些 NAD 被还原—— NAD 是一种辅酶，也就是运载氢的辅助分子。
And notice what is missing. No oxygen.
注意这里少了什么：没有氧。
Glycolysis runs perfectly well without it, which matters later.
糖酵解在没有氧的情况下照样进行，这一点后面很重要。
If oxygen is available, pyruvate moves into the mitochondrion, and the link reaction takes place in the matrix.
如果有氧，丙酮酸就进入线粒体，连接反应在基质中发生。
Each pyruvate loses a carbon, which leaves as carbon dioxide, and what remains is a two-carbon acetyl group.
每个丙酮酸失去一个碳，以二氧化碳的形式离开，剩下的是一个两碳的乙酰基。
Coenzyme A picks it up, forming acetyl coenzyme A, and some NAD is reduced.
辅酶A 把它接过去，形成乙酰辅酶A，同时有一些 NAD 被还原。
Now the Krebs cycle, also in the matrix.
接着是克雷布斯循环，同样在基质中。
The acetyl group joins a four-carbon molecule called oxaloacetate, making a six-carbon molecule called citrate.
乙酰基与一个叫草酰乙酸的四碳分子结合， 生成一个叫柠檬酸的六碳分子。
Through a series of small steps the citrate is converted back to oxaloacetate, ready for the next acetyl group.
经过一系列小步骤，柠檬酸又被转回草酰乙酸， 准备接收下一个乙酰基。
As it goes round, two things happen over and over. Decarboxylation removes carbon as carbon dioxide. Dehydrogenation removes hydrogen, reducing NAD and FAD, which carry it to the inner membrane.
在绕圈的过程中，有两件事反复发生： 脱羧把碳以二氧化碳的形式移走；脱氢把氢移走，还原 NAD 和 FAD，由它们运到内膜上。
This last stage makes most of the ATP, and it happens on the inner membrane.
最后这个阶段产生绝大部分 ATP，发生在内膜上。
The hydrogen arriving on the reduced coenzymes splits into protons and energetic electrons.
由还原型辅酶送来的氢分解成质子和带能量的电子。
The electrons pass along the chain of carriers in the membrane, releasing energy at each step.
电子沿着膜上的一串载体传递，每一步都释放出能量。
That energy is used to pump protons out of the matrix, into the space between the two membranes, so protons build up there.
这些能量被用来把质子从基质泵到两层膜之间的空间，于是质子在那里积累起来。
Now they want to come back, and there is only one way through: a channel called ATP synthase.
现在它们想回来，而只有一条通路：一个叫 ATP 合酶的通道。
As the protons flow back through it, that flow provides the energy to join ADP and phosphate into ATP.
质子流回来时，这股流动提供了把 ADP 和磷酸连接成 ATP 的能量。
This is chemiosmosis.
这就是化学渗透。
And at the end of the chain sits oxygen.
而在这条链的末端是氧。
Oxygen is the final electron acceptor: it takes the electrons and protons and forms water.
氧是最终电子受体：它接受电子和质子，形成水。
Without it the whole chain backs up and stops.
没有它，整条链就会堵塞并停下来。
That explains the shape of the organelle.
这就解释了这个细胞器的形状。
A mitochondrion has two membranes.
线粒体有两层膜。
The outer one is smooth.
外膜是光滑的。
The inner one is folded into cristae, and every one of those folds adds more surface for the electron transport chain and for ATP synthase to sit on.
内膜折叠成嵴，而每一道褶皱都为电子传递链和 ATP 合酶提供了更多的附着面积。
More surface means more ATP made each second, which is why cells that work hard, like muscle cells, have mitochondria packed with folds.
面积越大，每秒制造的 ATP 就越多，这就是为什么工作繁重的细胞——比如肌肉细胞—— 里面的线粒体褶皱密布。
The matrix is the space inside the inner membrane, and it holds the enzymes for the link reaction and the Krebs cycle.
基质是内膜以内的空间，里面装着连接反应和克雷布斯循环所需的酶。
Now anaerobic respiration — suppose there is no oxygen.
现在假设没有氧。
The chain stops, so the link reaction and the Krebs cycle stop too, and only glycolysis can keep running.
链停了，于是连接反应和克雷布斯循环也停了，只有糖酵解还能继续。
But glycolysis needs NAD, and the NAD is now stuck in its reduced form.
但糖酵解需要 NAD，而 NAD 现在卡在还原态。
So the cell frees it up again by fermentation.
所以细胞通过发酵把它重新释放出来。
In mammals, pyruvate becomes lactate, which is broken down later when oxygen returns.
在哺乳动物体内，丙酮酸变成乳酸，等氧回来以后再被分解。
In yeast and in plants, pyruvate becomes ethanol and carbon dioxide.
在酵母和植物里，丙酮酸变成乙醇和二氧化碳。
Either way the NAD is freed, so glycolysis carries on.
无论哪种方式，NAD 都被释放出来，糖酵解得以继续。
But this gives far less energy than aerobic respiration, because you collect only the small yield of glycolysis.
但这产生的能量远少于有氧呼吸，因为你只收到糖酵解那一点点产量。
Rice lives with exactly this. It has three adaptations: aerenchyma — air-filled spaces — in the roots to carry air down, ethanol fermentation to keep making some ATP, and faster stems to reach the air above.
水稻正是这样活下来的：根里有通气组织，进行乙醇发酵，茎长得更快去够上面的空气。
Finally, how do we measure any of this?
最后，我们怎么测量这一切呢？
A respirometer measures the oxygen taken in by living things, such as germinating seeds or small invertebrates.
呼吸计测量生物吸入的氧气，比如萌发的种子。
Here is the trick that makes it work. Soda lime sits under the seeds and absorbs all the carbon dioxide they give off.
让它能工作的窍门在这里：种子下面放着碱石灰，把它们放出的二氧化碳全部吸收。
So the only thing changing the gas volume is the oxygen being used up.
于是唯一改变气体体积的就是被消耗掉的氧气。
As the volume falls, the bead moves along the tube towards the flask, and how far it travels tells you how much oxygen was taken in.
体积下降时，小珠沿着管子朝烧瓶方向移动，它移动了多远就告诉你吸入了多少氧。
The other method uses a coloured dye.
另一种方法用有颜色的染料。
A redox indicator such as DCPIP or methylene blue loses its colour when it gains hydrogen from respiring cells.
像 DCPIP 或亚甲蓝这样的氧化还原指示剂， 从呼吸的细胞那里获得氢时会褪色。
So you time how long the colour takes to disappear — the faster it goes, the faster the respiration.
所以你计时看颜色多久消失—— 消失得越快，呼吸就越快。
Four things the examiner keeps asking for.
考官反复要的四点。
First, whenever you name a stage, give the location too: glycolysis in the cytoplasm, link reaction and Krebs in the matrix, oxidative phosphorylation on the inner membrane.
第一，每次说出一个阶段的名字时，也要给出部位： 糖酵解在细胞质，连接反应和克雷布斯循环在基质，氧化磷酸化在内膜。
Second, be precise about what NAD and FAD do.
第二，要说准 NAD 和 FAD 做什么。
They carry hydrogen to the electron transport chain — they do not make ATP themselves.
它们把氢运送到电子传递链——它们自己并不制造 ATP。
Third, know your RQ values: about one point zero for carbohydrate and about zero point seven for lipid.
第三，记住 RQ 的数值：糖类约一点零，脂质约零点七。
Fourth, oxygen's job has a name.
第四，氧的作用是有名字的。
It is the final electron acceptor, and it forms water.
它是最终电子受体，并形成水。
Saying only that oxygen is "needed" earns nothing.
只写"需要氧"是拿不到分的。
So there it is.
就是这样了。
One glucose molecule, four stages, a folded membrane covered in pumps, and oxygen waiting at the end to take the electrons away.
一个葡萄糖分子，四个阶段，一层布满质子泵的折叠膜， 还有在末端等着带走电子的氧。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。