I love photosynthesis because it gives me two things that I need. I need to breathe,

所以它给我带来氧气。我需要进食，他会给我带来食物。 我很喜欢光合作用。你也许会认为光合作用只存在在植物里。但是他

so it gives me oxygen. And I need to eat. And so it's going to give me food. And so

在细菌中也被发现了。光合作用也存在于藻类中。所以它在单细胞生物中也存在着。光合作用 无处不在。所有光合作用已经存在了很长一段时间。

I love photosynthesis. You might think it's only found in these things, plants, but it's

理解光合作用如何进行是非常重要的事情。因此，让我们从真核细胞的光合作用开始讲起。 这是叶绿体。 你可以看到在一个典型的细胞里能有多少个叶绿体。里面有很多叶绿体。

also found in bacteria. It's found in algae. And so it's found in protists. It's found

你需要熟知一些术语并且知道他们在哪。 第一个是类囊体膜。类囊体膜是这样构成的。

everywhere. And so photosynthesis has been around a long time. It's super important that

基本上光反应是在这里进行到的。如果一沓类囊体像这样集中在一起， 我们叫它基粒。另外一点我们需要理解的是

you understand how it works. And so let's start with the site in eukaryotic cells of

它被一种液体充斥着。这个液体叫做基质。 这将是卡尔文循环的发生地。如果我们将一片叶子磨碎，我们会

photosynthesis. And that's the chloroplasts. So this is a number of cells. And you can

发现里面不仅仅含有一种色素。叶绿素A，进行光合作用 但是这里有一定数量的叶绿素A一起发生反应。如果你把磨碎的叶子放在色层分析纸上，

see how many chloroplasts we could have in a typical cell. So there's a whole bunch of

然后你把它们放在一种溶剂里，你会得到一个色层分析计。 它会分离成许多不同的部分。这里是叶绿素A

them. There are a few terms you should be familiar with. And where they are. First one

和叶绿素B。这个是胡萝卜素和叶黄素。 他们一起工作。在秋天当叶绿素回到叶子上被重吸收时，你会看到其他的色素。

is a thylakoid membrane. Thylakoid membrane is going to be organized like this. And basically

但是，如果我们看看它吸收的光，这里是叶绿素A 和叶绿素B。这个叫做吸收光谱。

that's where the light reaction is going to take place. If you've got a stack of thylakoids

你可以看到它吸收许多蓝色，许多红色 但是它不怎么吸收在中间的这些颜色。比如绿色

like this together we call that a granum. The other big thing to understand photosynthesis

这里有个问题是它吸收最少的颜色是什么，正确答案是绿色 因为它反射了绿光。这个问题实际上困扰了科学家很长时间

is that this is filled with a liquid. And that liquid is called the stroma. And that's

我们并没有一个明确的答案为什么植物是绿色的。 如果植物是黑色的，可能是因为他们太热了。

going to be the site of the Calvin cycle. If we were to grind up a leaf what we would

他们吸收了太多的光。所以让我们先从一个公式开始。因为它是一个简单的化学反应。 它是一个有很多步骤的化学反应。谁是化学反应物呢？

find is that there's not only one pigment, chlorophyll A that does photosynthesis. But

水和二氧化碳。因此，一个植物是如果生长的？它基本上市从根部吸收水 通过叶子吸收二氧化碳。通过它的气孔。

theres a number of them that are working together. And so if you grind up a leaf into some chromatography

另外，它还需要光。所以，这个过程只需要这些简单的元素。 然后把它们放一起成为葡萄糖，这个奇怪的分子。

paper and then you put it in a solvent, what you'll get is chromatography. It's going to

还有氧气。所以这是我得到的食物，这是我所呼吸的氧气。 所以这些植物很好吗？不是的，他们给自己制造糖，所以它们能把这些糖分解用来细胞呼吸。

separate into all of its different parts. And so this right here would be chlorophyll

实际上，如果我把这个箭头的方向反过来， 就是细胞呼吸的过程。所以他们为自己制造食物。

A and chlorophyll B. And this would be like carotene and xanthaphylls. And they're all

并且制造结构。所以像植物细胞壁上的纤维素也是那样形成的。 所以每当我试着想想光合作用的不同步骤时

working together. You'll see this other pigments in the fall when the chlorophyll moves back

我总是想象这张图片。在光合作用这个词汇里包含着"光合"（光反应阶段）和"作用"（达尔文循环） 光合代表着光，作用代表着制造。所以在光合作用里有两个步骤

into the leaf and is reabsorbed. But if we look at what light they absorb, here's chlorophyll

光反应阶段，在类囊体膜中完成 然后是达尔文循环。我们通常叫它暗反应阶段，

A and here's B. This is what's called their absorption spectrum. And what color of light

其实是个愚蠢的叫法，因为这个过程并不发生在黑暗中。它是发生在有光的环境下。 梅尔文 达尔文发现了这个过程，所以我们把这个过程称作达尔文循环。

they are able to absorb. And you can see that they absorb a lot of the blue. A lot of the

这个过程发生在什么场所？它发生在基质，也就是是这个液体的部分。 让我们做一个卡通版的光合作用。反应物是什么呢？

red. But they don't absorb a lot of this in the middle, this green. And so a quick question

水，光和二氧化碳。那生成物是什么呢？ 生成物是氧气和葡萄糖。让我们看一下这是如何发生的

could be what is their least favorite color, plants? And the the right answer would be

在光反应阶段，水和光进入类囊体膜然后它们生成两种物质。 它们生成了氧气。氧气实际上是浪费的产物。

green. Because the reflect that green light. Now this is actually puzzled scientists for

然后它们生成了这两个化学制品。NADPH和ATP。所以他们现在有能量了。 让我们看看他们之间发生了什么。 这些能量进入到了卡尔文循环，然后二氧化碳进入

a long time. And we really don't have a definitive answer as to why plants are green. Know this

葡萄糖生成。 这是光合作用过程的主要部分。 现在我深入的讲一下光反应阶段。

that if they were black they probably would get a little bit too hot. They would absorb

我们现在在哪？我们现在在类囊体膜。 所以如果我们放大类囊体膜，我们就得到了这个图表。

too much light. And so let' start with an equation. Because this is simply a chemical

哪两个物质首先进入？第一个是光。所以光从这里进入。 下一个进入的物质是什么？ 下一个进入的物质是水。我们现在看一下其他类囊体膜的特征是什么。

reaction. It's a chemical reaction with a number or steps. But what are the reactants?

这是类囊体膜的外面，也就是基质。 这是内腔也就是叶绿体的内部。所以这里面有几个非常重要的东西。

Water and carbon dioxide. And so how does a plant grow? It's basically taking water

这里有什么呢？这个是蛋白质。 所以我们把这些东西合起来叫做光反应系统。

in from its roots and it's taking carbon dioxide in through its leaves. Through its stomata.

所以第一个是光反应系统二，然后是光反应系统一。我们把第一个叫做光反应系统二而把第二个叫做光反应系统一的原因是光反应系统一首先被发现。 所以，什么进来呢？光进入了 光用来做什么呢？光用来给电子提供能量使电子在电子传递链中移动。

The other thing it needs is light. And so it's just taking these simple ingredients.

所以电子穿过蛋白质，载体蛋白。 最终电子要来到这里。电子要到NADPH。

And then it's weaving those together into glucose. This monster molecule here. And then

记住，这是光反应阶段的产物之一。 水发生了什么呢？水会被分解 如果你分解水你会得到什么呢?我们会得到氧气。氧气会被扩散到细胞外面、

oxygen. And so this is the food that I get. And this is the oxygen that I breathe. Now

这实际上就是你现在正在呼吸的氧气。 然后我们剩下了这些质子，也就是这些氢离子。他们是失去了电子的氢原子。

are plants just nice? No. They're making this sugar for themselves so they can break it

所以我们现在看看接下来发生什么。 电子在电子传递链中移动，这个过程是光给予它能量。

down using cellular respiration. And in fact if I put this arrow in the other direction,

电子会从这里一路走到这里。 每次电子经过这些蛋白质的其中一个， 它会向内部射入质子。 质子有正电荷。所以现在发生的是内部正在构建起一个正电荷的环境。

that becomes cellular respiration. So they're making food for themselves and they're also

所以这里是正电荷。如果你知道呼吸作用如何进行。 你会意识到这是相反的。所以现在内部是正电荷环境。

going to make some of the structure. So like the cellulose in the cell walls of a plant

这些正电荷回去哪呢？实际上只有一个孔能让他们穿过。 所以他们穿过这个蛋白质。当这些质子出来的时候，

is made from that as well. Okay, so whenever I try to think what are the different steps

他们穿过了一个蛋白质叫做ATP合成酶。他工作起来想一个小的旋转体。 每次一个质子穿过这个蛋白质，就会有一个ATP生成。所以在光反应阶段下制造出来多少能量呢？

in photosynthesis? I always image this picture right here. There's photo and synthesis in

我们制造了NADPH和ATP。 这个过程非常好，因为这些能量就在基质中，所以他们可以进入到下一步过程，卡尔文循环中。 所以谁在提供能量呢？ 光在提供能量。谁在提供电子呢？水在提供电子。这个过程浪费的产物是氧气。

the word. Photo means light. And synthesis means to make. And so there are two steps

现在我们去看看卡尔文循环。在卡尔文循环中发生了什么呢？ 你可以看到这些事反应物。有ATP和NADPH。

in photosynthesis. The light reaction. And those are going to take place in the thylakoid

他们在提供什么呢？他们提供能量。我们还有这个分子。 它叫做RUBP。实际上它是个含有5个碳的分子。然后二氧化碳进入到这个反应当中。

membrane. And then the Calvin cycle. We used to call this the dark reactions which is a

所以它在叶子的气孔中移动。 二氧化碳是一个含有一个碳的分子。所以这里有一个酶叫做 二磷酸核酮糖羧化酶。这个酶将要把这个含有一个碳的分子依附在另外一个含有5个碳的分子上。

silly term. Doesn't happen during the dark. It happen during the light. And so basically

然后它们立刻分裂成两个分别含有三个碳的分子。然后它们从ATP和NADPH中得到了能量。 下一步形成了一个化学制品，叫做G3P。G3P变成什么呢？

the person who worked this all out is Melvin Calvin and so we named it after him. Where

它可以很快的被聚合成葡萄糖或蔗糖或麦芽糖。 不管它们要形成什么，都要在这里被G3P制造成。

does this take place? You guessed it. It takes place in the stroma or this liquid portion.

换句话说，我们是将碳带入这个过程然后是它变得有用。 现在一些G3P被释放了，但是许多G3P是被回收再使用，用它们形成更多的RUBP。

And so let's kind of do a cartoon version of photosynthesis. What are the reactants

所以这就是为什么这个过程循环一遍又一遍的原因。 如果我们没有ATP，如果我们没有NADPH，那么这个反应将要停止。

again? Water, light and carbon dioxide. What are going to be the products that come out

如果这个过程没有二氧化碳，这个反应也会停止。 所以这就是光合作用的反应过程。这个过程进行了上亿年。

of this? It's going to be oxygen and glucose. So let's watch what happens. In the light

但是这里有个小问题。这个问题叫做光呼吸。什么是光呼吸呢？ 光呼吸只在我们没有足够的二氧化碳的时候发生。

dependent reaction water and light go into the thylakoid membrane and they produce two

如果我们没有足够的二氧化碳。让我把二氧化碳划掉。很确定的是G3P不能形成了。 但是还有更坏的事情，氧气可以直接进入卡尔文循环，

things. They produce oxygen. Oxygen is simply a waste product. And then they're going to

然后利用二磷酸核酮糖羧化酶去形成另外一种化学制品。那个化学制品没有任何作用。换句话说， 这个过程是没有目的的。实际上细胞已经把它打破了。

produce these chemicals. NADPH and ATP. So they have energy now. Let's watch what happens

我们叫大部分的植物C-3植物，我们叫它C-3植物的原因是 G3P是3碳分子。所有，对于这些C-3植物，光呼吸是不好的。

to them. Well the energy is going to transfer to the Calvin cycle where carbon dioxide comes

换句话说，它们从中没有得到任何东西。所有它们在氧气进入卡尔文循环的时候会失去东西。 你会在生物进化方面想 为什么这个过程进化出来了？请记住，光合作用先发生，

in and then glucose goes out. And so this is the big picture of photosynthesis. But

然后大气中的氧气才出现。因此，在最初的时候这不是问题， 但是最终变成了一个问题。你也许还会想，我们在什么情况下会没有足够的二氧化碳。

now let's kind of dig in a little bit deep and talk about the light reaction. Okay, so

我们什么时候没有二氧化碳？它们如果得到二氧化碳？ 植物有气孔，他们周围被保卫细胞包围着

where are we? We're in the thylakoid membrane. So we're in this membrane right here. So if

当植物打开了气孔，二氧化碳会进入。 在大气当中有大量的二氧化碳，当植物没有二氧化碳的时候是因为，

we were to zoom in to that membrane right here, that's what this diagram is. Okay. So

它们把气孔合上了。它们的气孔会在什么时候合上呢？ 它们只会在他们非常非常热的时候把气孔合上，因为植物不想失去水分。

what are the two things coming in? Well the first one is going to be light. So light's

因为在蒸发的过程中，植物在不停的失去水分。如果你是一个植物，如果是一个非常热的天气， 你面临这非常艰难的选择。如果你打开气孔，你会失去水分。

coming in here. Light's coming in here. What's the next things that we're going to have coming

你可能会枯萎。如果你关闭气孔，你得不到二氧化碳， 你就要开始进入光呼吸反应阶段。当然了，大自然找到了解决这个问题的办法。

in? And that's going to be water. Okay, so let's look at some of the other big features

这个只会发生在非常热的环境下。 所以，这是第一个解决方案。这个方法是非常有效的。这个是CAM植物。

in this thylalkoid membrane. So this is the outside, or the stroma. And this is going

植物。CAM植物可以是景天树或者是菠萝。 他们只在夜间打开气孔。所以在夜晚，他们打开气孔，

to be the lumen or the inside. And so there's a couple of big things right here. What's

然后吸收二氧化碳，然后它们会制造出苹果酸。 它们会把它储存在细胞的液泡中。到了白天的时候，

in here? Well these are basically going to be proteins with chlorophyll on the inside

它们会关闭气孔，因为他们不想失去水分。然后现在他们可以 把二氧化碳从苹果酸中取出来，然后在卡尔文循环中利用二氧化碳

of it. And so we call that whole thing together a photo system. So this first one is actually

制造糖。对于CAM植物来说，他们只在夜晚凉快的时候吸收二氧化碳。 然后在白天的时候，他们会关闭自己的气孔。

called photo system II. And then we go to photo system I. And the reason we go backwards

他们不会失去水分。另外一个例子是C4植物。 它们会做些什么呢？它们会做的是他们会把二氧化碳带入，

is that that photo system I was discovered first. So basically what comes in? Light.

然后它们会用酶来制造一个含有四个碳的分子。 那个含有四个碳的分子会移动到在叶子里面的细胞，叫做维管束鞘细胞。

What's that light used to do? Well that light is used to power the movement of an electron

然后它们可以简单的把二氧化碳带入到卡尔文循环中。 所以，所有的这些解决方案都是在可以得到二氧化碳的时候将其带入，

through an electron transport chain. So that electron is going through proteins, carrier

然后制造出一种化学物质。然后它们可以把那个化学物质带入到 卡尔文循环，它们不需要等着二氧化碳的流入。

proteins. And eventually that electron is going to go to here. It's going to go to NADPH.

当然了，这将会有一个额外的步骤，将会需要更多的能量。 我们只会在非常热的地方看到这种情况。

Because remember that's one of the products of the light dependent reaction. Okay. What

C4植物的一个例子是我们经常吃的而且经常用的玉米。 所以这就是光合作用。一个简单的问题是光呼吸

happens to the water then? So the water is going to be split right away. If you split

我希望这对你有所帮助。

water what do you get? Well you get oxygen. So that's the O2 that's going to diffuse out

of a cell. And that's the oxygen that you're actually breathing right now. And then we're

going to have these protons which are simply hydrogen ions. So they're hydrogen atoms that

have lost their electron. Okay, so this is getting kind of messy. So let's look what

happens next. As that electron moves through the electron transport chain, and again it's

powered by the introduction of light here and light here. That electron is going to

be moving all the way down here and every time it goes through one of these proteins,

it's pumping protons to the insides. So it's pumping protons to the inside. Now protons

have a positive charge. So basically what's happening is that you're building up a positive

charge on the inside. So there's a positive charge in here. If you know how cellular respiration

works, you'll realize that this is the opposite of that. So now we have all of these positive

charges on the inside. Where do they go? Well there's only one hole that they can go through.

And that's is to go through this protein here. As those protons move out, they're moving

through a protein called ATP synthase. And it works almost like a little rotor. And every

time a proton goes through we make another ATP. So what have we made in the light dependent

reaction? We've made NADPH and we've made ATP. And what's nice about that is they're

now just sitting right here in the stroma and so they're able to go on to the Calvin

cycle which is going to be the next step in this process. And so who's providing the energy?

Light. Whose providing the electrons? Water. And then a waste product of that is simply

going to be oxygen. Okay. Let's go to the Calvin cycle then. So what's happening in

the Calvin cycle? You can see here's those reactants. So we've got our ATP here, ATP

here and NADPH. What are they providing? Simply energy. We also have this molecule here. It's

called RUBP. Basically it's a five carbon molecule. And then we have carbon dioxide

coming in. So it moves through the stomata of the leaf. And it's going to diffuse its

way in. Carbon dioxide is a one carbon molecule. So basically there's an enzyme here called

rubisco and it's going to attach this one carbon molecule to a five carbon molecule.

It immediately breaks into three carbon molecules. And then it gets energy from ATP and NADPH.

And when we're done it's creating this chemical down here, called G3P. What does G3P become?

Well it can be assembled quickly into glucose or sucrose or maltose or whatever they need

to do, that's going to be produced right in here by the G3P. So that's where we're synthesizing.

In other words we're taking carbon and we're fixing it. We're making it usable. Now some

of that G3P is released. But a lot of it is recycled again to make more of this RUBP.

And so that's why it's a cycle over and over again. What's the big picture? If we don't

have ATP, if we don't have NADPH, then this process is going to shut down. What's the

other thing that could shut it down? If we don't have carbon dioxide. Okay, so that's

basically photosynthesis. And again it's been working for billions of years. But there's

a slight problem. And that problem is called photorespiration. What is photorespiration?

Well photorespiration occurs only when we don't have enough carbon dioxide. So if we

don't have enough carbon dioxide, let me cross that out, well we certainly can't make our

G3P. But something worse happens. Oxygen can actually jump into the Calvin cycle. And using

rubisco can form another chemical. Now that chemical doesn't do anything. In other words

it has no purpose. And the cell actually has to break it down. And so as a result of that

plants, and we call almost all plants C3 plants. And the reason we call them C3 plants is this

G3P is going to be a 3 carbon molecule. So for these C3 plants, photorespiration is bad.

In other words, they don't get anything out of it. And so they're going to lose based

on that oxygen kind of jumping into the Calvin cycle. And so you might think evolutionarily,

why would this have even evolved? Well remember, photosynthesis shows up first. And then oxygen

in the atmosphere shows up much later. And so it wasn't a problem initially, but it became

a problem. Another question you might think is, when are we not going to have enough carbon

dioxide? When wouldn't we have carbon dioxide? Well how do they get carbon dioxide? And plant

is going to have a stomata. And it's surrounded by guard cells. And so basically when a plant

opens up its stomata, carbon dioxide can diffuse in. And so the only time the plant wouldn't

have carbon dioxide, because we have tons of carbon dioxide in the atmosphere, is when

it's actually closed. And when would it be closed in a plant? The only time it's closed

is when it's really, really hot. And a plant doesn't want to lose water. Because through

transpiration you're constantly losing water. And so if you're a plant, if it's a hot day

you have this really tough choice. If you open up your stomata, you're going to lose

water. You could shrivel up. If you close it, you can't get carbon dioxide in and then

you're going to start doing photorespiration. And so of course nature has come up with solutions

to this over time. And it's only going to be found in plants that live in really hot

environment. So here's the first solution. And this totally makes sense. This is in CAM

plants. CAM plant would be a jade plant or like a pineapple. Basically what they do is

they only open their stomata at night. And so at night they open up their stomata. And

then the carbon dioxide will come in and they'll create malic acid out of it. So they're going

to store it in vacuoles inside the cell. Okay. So now when it's day time what they can do

is they can close the stomata because they don't want to lose water. And now they can

actually take that carbon dioxide out of the malic acid and they can use it in the Calvin

cycle to make sugars. So the great thing about CAM plant is again they're only taking in

carbon dioxide at night when it's cool. And then during the day they can close their stomata

and they don't lose water. Another example of this would be in C4 plants. What they do

is instead of doing it day and night, what they'll do is they'll take that carbon dioxide

in and they'll actually use enzymes to make a 4 carbon molecule out of it. That 4 carbon

molecule will move to some cells on the inside of the leaf called the bundle sheath cells.

And then they can simply introduce carbon dioxide into the Calvin cycle here. And so

again, both of these solutions are basically taking in carbon dioxide when you can get

it. Creating a chemical out of it. And then they can introduce that chemical into the

Calvin cycle and they don't have to wait for carbon dioxide to diffuse in. Now of course

there's going to be extra steps in here so it's going to require more energy. And so

we only see this in areas where it's really, really warm. But an example of a C4 plant

that we all eat and use a lot of, in fact most of us are just made out of this stuff

is corn. And so that's photosynthesis. A simple problem is photorespiration, but I hope that's

helpful.