Cellular Energetics
AP Biology Topic 3 7:58 English narration · English + 中文 subtitles burned in
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Transcript
Look at the washing powder in your cupboard.
看看你家柜子里的洗衣粉。
In warm water it removes a food stain in half an hour.
在温水里，它半小时就能洗掉食物污渍。
In a lab, the same job would need strong acid and boiling heat.
在实验室里，同样的工作需要强酸和沸腾的高温。
The powder does it gently, because it contains enzymes.
洗衣粉却做得很温和，因为它含有酶。
Every reaction needs a push to start: the activation energy, the blue hill here.
每个反应开始时都需要一个推动：这就是活化能，图上那座蓝色的山。
An enzyme offers an easier path — the orange curve, a much lower hill.
酶提供一条更容易的路：橙色的曲线，山低得多。
Far more molecules can now cross it, so the reaction runs fast at body temperature.
现在有多得多的分子能翻过去，所以反应在体温下就能很快进行。
Notice what does not change: reactants and products sit at the same levels.
要注意什么没有改变：反应物和生成物的高度完全一样。
Enzymes are biological catalysts. An enzyme is a catalyst: it changes the speed, never the outcome, and it is not used up.
酶改变的是速度，绝不是结果。
Why is each enzyme so specific?
为什么每种酶都这么专一？
Because of a pocket called the active site.
因为它有一个叫做活性位点的口袋。
The substrate must match that pocket in shape and in charge.
底物必须在形状和电荷上都与这个口袋相配。
On the left is the older lock-and-key picture: a fixed shape, one substrate.
左边是较早的锁钥模型：形状固定，只配一种底物。
On the right is the model we use now, induced fit: the site is flexible, so the enzyme closes around the right substrate.
右边是我们现在使用的模型，叫诱导契合：位点是可变的， 所以酶会包住正确的底物。
Watch a single enzyme work.
看一个酶工作。
The substrate drops in, it reacts, and the two products leave.
底物落进去，发生反应，两个产物离开。
The enzyme itself is unchanged, so it starts again at once.
酶本身没有改变，所以它马上又开始下一次。
Now watch the thermometer climb.
现在看温度计升高。
The heat pulls the pocket out of shape, and the next substrate cannot fit.
热量把这个口袋拉变形，下一个底物就放不进去了。
One healthy enzyme runs that cycle thousands of times a second.
一个正常的酶每秒能完成上千次这样的循环。
A denatured one never will.
一个变性的酶再也做不了。
Now the environmental impacts on enzyme function.
酶只在合适的条件下才工作。
Temperature first.
先看温度。
Warming makes molecules collide more often, so the rate climbs to the optimal temperature.
升温让分子碰撞得更频繁， 所以速率上升到最适温度。
Past the optimum the rate drops sharply: heat breaks the bonds holding the fold, and the active site loses its shape.
越过最适温度后，速率骤降： 热量破坏了维持折叠的键，活性位点失去形状。
We say the enzyme is denatured.
我们说这个酶变性了。
pH does the same thing.
pH 的作用是一样的。
Every enzyme has an optimal pH, and moving away from it changes the charges inside the active site.
每种酶都有最适 pH，偏离它就会改变活性位点内部的电荷。
Pepsin needs stomach acid; gut enzymes need a mild base.
胃蛋白酶需要胃里的强酸；肠道里的酶则需要弱碱。
Concentration matters too.
浓度也很重要。
Add substrate and the rate climbs quickly, because many active sites are free.
加入底物，速率上升得很快， 因为还有许多活性位点是空的。
Then the curve flattens: every site is busy, so extra substrate changes nothing.
然后曲线变平：每个位点都在忙， 再加底物也没有用。
The enzyme is saturated.
酶饱和了。
The dashed lines mark the top rate and half of it.
虚线标出最大速率和它的一半。
To go faster, the cell must make more enzyme.
要想更快，细胞必须制造更多的酶。
Some molecules slow enzymes.
有些分子会减慢酶的速度。
A competitive inhibitor is shaped like the substrate, so it sits in the active site and blocks it.
竞争性抑制剂的形状与底物相似， 所以它坐进活性位点，把位点挡住。
Add plenty of substrate and the inhibitor is out-competed.
加入大量底物，抑制剂就被挤掉了。
A noncompetitive inhibitor binds somewhere else, and changes the shape of the active site, so extra substrate cannot help.
非竞争性抑制剂结合在别的地方，改变了活性位点的形状， 所以再多的底物也帮不上忙。
So here is the test the exam wants.
考试要考的判断标准就在这里。
Cells do this on purpose: the end product of a pathway switches off the first enzyme in it.
细胞会故意这样做：一条途径的终产物会关掉这条途径的第一个酶。
Cells do not spend glucose directly. They spend ATP.
细胞不会直接消耗葡萄糖，它们消耗的是 ATP。
ATP carries three phosphate groups, and the useful energy sits in the third bond.
ATP 带有三个磷酸基团，有用的能量就在第三个键里。
Break it with water and you get ADP, a free phosphate, and a usable packet of energy.
用水把它打断，你就得到 ADP、一个游离磷酸，和一小包可用的能量。
Respiration pushes a phosphate back on, and the ATP is remade.
呼吸作用再把一个磷酸装回去，ATP 就重新生成了。
Energy in along the top, energy out along the bottom.
上面的箭头是能量进来，下面的箭头是能量出去。
Why bother with a middle molecule?
为什么要多一个中间分子？
Because ATP lets a cell pair reactions.
因为 ATP 让细胞可以把两个反应配成一对。
Breaking ATP down is downhill: it releases free energy.
分解 ATP 是下坡：它释放自由能。
Building a protein, or pumping ions against a gradient, is uphill: it needs energy put in.
合成蛋白质，或者逆着梯度泵送离子， 是上坡：它需要输入能量。
The cell couples them, so the release pays for the climb.
细胞把两者偶联起来， 于是释放出的能量支付了上坡的花费。
And remember the limit: energy coming in must be more than energy lost, or the cell loses its order.
还要记住一个界限：进来的能量必须多于损失的能量，否则细胞就会失去秩序。
So where does that energy come from?
那么这些能量是从哪里来的？
From light.
来自光。
These green ovals are chloroplasts, packed inside the cells of a leaf.
这些绿色的椭圆就是叶绿体，密集地排布在叶片的细胞里。
Inside each one are stacks of flat sacs called thylakoids, sitting in a fluid called the stroma.
每个叶绿体里面都有一摞摞扁平的囊，叫做类囊体， 它们浸在一种叫做基质的液体中。
Photosynthesis runs in two linked stages.
光合作用分两个相连的阶段。
The light-dependent stage happens in the thylakoid membranes.
光反应阶段发生在类囊体膜上。
Chlorophyll absorbs light and lifts electrons to a higher level.
叶绿素吸收光，把电子提升到更高的能级。
Water is split to replace them, releasing oxygen.
水被分解来补上这些电子， 同时放出氧气。
This stage makes ATP and reduced NADP.
这个阶段制造 ATP 和还原型 NADP。
Follow the top arrow: they travel to the Calvin cycle in the stroma, where their energy fixes carbon dioxide into sugar.
顺着上面的箭头看：它们走到基质里的卡尔文循环， 在那里它们的能量把二氧化碳固定成糖。
How do those electrons make ATP?
这些电子究竟怎样造出 ATP？
Watch closely.
请仔细看。
The electrons are passed along the electron transport chain — carriers in the membrane — losing energy at each step.
电子沿着膜内的一串载体依次传递，每一步都损失一点能量。
That energy is used to pump protons across the membrane.
这些能量被用来把质子泵到膜的另一侧。
Protons pile up on one side, and that difference in concentration stores energy.
质子在一侧越积越多，这个浓度差就是被储存起来的能量。
There is only one way back.
回来的路只有一条好走。
The protons flow back through a channel in ATP synthase, and that spins it like a wheel.
质子经由 ATP 合酶上的通道流回来， 带着它像轮子一样转动。
Every turn joins ADP and a phosphate to make ATP.
每转一圈，就把 ADP 和一个磷酸接在一起，生成 ATP。
This is chemiosmosis, and the same machine works in a chloroplast, a mitochondrion and a bacterium.
这就是化学渗透；同一台机器在叶绿体、线粒体和细菌里都在工作。
Here is a real mitochondrion, under an electron microscope.
这是一个真实的线粒体，在电子显微镜下拍摄。
Look at the inner membrane: folded again and again into shelves called cristae.
看它的内膜： 一层层反复折叠，形成叫做嵴的隔板。
Those folds are not decoration.
这些褶皱不是装饰。
More membrane means more chains and more ATP synthase, so more ATP.
膜越多，电子传递链就越多，ATP 合酶也越多，产生的 ATP 就越多。
Cellular respiration takes that sugar apart again, each step feeding the next.
呼吸作用又把那些糖拆开，每一步的产物都交给下一步。
Glycolysis splits glucose in the cytoplasm, making pyruvate and a little ATP.
糖酵解在细胞质里把葡萄糖劈开，生成丙酮酸和少量 ATP。
No oxygen is needed yet.
这一步还不需要氧。
The pyruvate then moves into the mitochondrion.
丙酮酸接着进入线粒体。
The link reaction and the Krebs cycle, both in the matrix, release carbon dioxide and load the electron carriers.
连接反应和克雷布斯循环都在基质中进行，放出二氧化碳， 并把电子载体装满。
Those carriers feed the inner membrane, where oxidative phosphorylation — the chemiosmosis you just saw — makes most of the ATP.
这些载体把电子送到内膜上， 在那里氧化磷酸化——也就是你刚才看到的化学渗透——制造出大部分 ATP。
Oxygen waits at the end of the chain to accept the used electrons.
氧在链的末端等着，接收用过的电子。
Now take the oxygen away.
现在把氧拿走。
Nothing accepts the electrons at the end, so the chain stops, and the Krebs cycle stops with it.
链的末端没有东西接收电子，所以传递链停下来， 克雷布斯循环也跟着停下。
Only glycolysis can keep going, and only if the cell recycles its electron carrier.
只有糖酵解还能继续， 而且必须靠细胞把电子载体回收再用才行。
That is fermentation.
这就是发酵。
With oxygen, one glucose gives about thirty-two ATP.
有氧时，一个葡萄糖大约给出三十二个 ATP。
Without oxygen, the same glucose gives only two — about sixteen times less.
没有氧时，同样一个葡萄糖只给出两个——大约少十六倍。
That is why a sprinter cannot keep sprinting.
这就是短跑运动员没法一直冲刺的原因。
Here is a real exam question.
这是一道真实的考题。
Oligomycin blocks the channel in ATP synthase.
寡霉素会堵住 ATP 合酶上的通道。
Predict its effect on the proton gradient across the inner mitochondrial membrane, and justify that.
请预测它对线粒体内膜两侧质子梯度的影响，并说明理由。
Think about which part still works.
想一想哪一部分还在运转。
The chain is untouched, so it keeps pumping protons out.
传递链没有受到影响， 所以它继续把质子泵出去。
Only the way back is blocked.
只有回来的路被堵住了。
So the prediction: the gradient gets larger.
所以预测是：梯度会变大。 现在来说明理由。
Now justify it. ATP synthase is the only easy route back, so the protons build up and ATP synthesis stops.
ATP 合酶是唯一好走的回路，所以质子不断堆积，ATP 的合成停止。
Notice the two verbs. Predict, then justify — two separate marks.
注意这两个动词：先预测，再论证——这是两处分开给分的地方。
Three marks students throw away.
三个学生常丢的分。
First, an enzyme lowers the activation energy — it is not used up, and the products are unchanged.
第一，酶降低的是活化能——它自己不会被消耗， 生成物也不会改变。
Second, past the optimum the enzyme denatures: say the active site loses its shape, not just that it stops.
第二，越过最适条件后酶会变性： 要写出活性位点失去了形状，而不只是说它停止工作。
Third, when you explain how ATP is made, name all three parts: the membrane, the proton gradient, and ATP synthase.
第三，解释 ATP 是怎样生成的时候，三个部分都要写到： 膜、质子梯度，还有 ATP 合酶。