Name: 光合作用。生物學速成班#8 (Photosynthesis: Crash Course Biology #8)
Uploaded: 2021-01-14T04:21:15.000Z
Duration: 13 min 15 s
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Photosynthesis! It is not some kind of abstract scientific thing. You would be dead without

plants and their magical- nay, SCIENTIFIC ability to convert sunlight, carbon dioxide

and water into glucose and pure, delicious oxygen.

This happens exclusively through photosynthesis, a process that was developed 450 million years

It's complicated, inefficient and confusing. But you are committed to having a better,

deeper understanding of our world! Or, more probably, you'd like to do well on your

There are two sorts of reactions in Photosynthesis...light dependent reactions, and light independent

reactions, and you've probably already figured out the difference between those two, so that's

nice. The light independent reactions are called the "calvin cycle"

no...no...no...no...YES! THAT Calvin Cycle.

Photosynthesis is basically respiration in reverse, and we've already covered respiration,

so maybe you should just go watch that video backwards. Or you can keep watching this one.

I've already talked about what photosynthesis needs in order to work: water, carbon dioxide

First, water. Let's assume that we're talking about a vascular plant here, that's

the kind of plant that has pipe-like tissues that conduct water, minerals and other materials

These are like trees and grasses and flowering plants.

In this case the roots of the plants absorb water

and bring it to the leaves through tissues called xylem.

Carbon dioxide gets in and oxygen gets out through tiny pores in the leaves called stomata.

It's actually surprisingly important that plants keep oxygen levels low inside of their

leaves for reasons that we will get into later.

And finally, individual photons from the Sun are absorbed in the plant by a pigment called

Alright, you remember plant cells? If not, you can go watch the video where we spend

the whole time talking about plant cells.

One thing that plant cells have that animal cells don't... plastids.

The chloroplast! Which is not, as it is sometimes portrayed, just a big fat sac of chlorophyl.

Now, the chlorophyll is stashed in membranous sacs called thylakoids. The thykaloids are

stacked into grana. Inside of the thykaloid is the lumen, and outside the

thykaloid (but still inside the chloroplast) is the stroma.

The thylakoid membranes are phospholipid bilayers, which, if you remember

means they're really good at maintaining concentration gradients of ions,

proteins and other things. This means keeping the concentration higher on one side

than the other of the membrane. You're going to need to know all of these things, I'm sorry.

Now that we've taken that little tour of the Chloroplast, it's time to get down to

First thing that happens: A photon created by the fusion reactions of our sun is about

to end its 93 million mile journey by slapping into a molecule of cholorophyll.

This kicks off stage one, the light-dependent reactions proving

that, yes, nearly all life on our planet is fusion-powered.

When Chlorophyll gets hit by that photon, an electron absorbs that energy and gets excited.

This is the technical term for electrons gaining energy and not having anywhere to put it and

when it's done by a photon it's called photoexcitation, but let's just imagine,

for the moment anyway, that every photon is whatever

dreamy young man 12 year old girls are currently obsessed with, and electrons are 12 year old girls.

The trick now, and the entire trick of photosynthesis, is to convert the energy

I mean, electrons, into something that the plant can use.

We are literally going to be spending the entire rest of the video talking about that.

This first Chlorophyll is not on its own here, it's part of an insanely complicated complex

of proteins, lipids, and other molecules called Photosystem II that contains at least 99 different

chemicals including over 30 individual chlorophyll molecules.

This is the first of four protein complexes that plants need for the light dependent reactions.

And if you think it's complicated that we call the first complex photosystem II instead

of Photosystem I, then you're welcome to call it by its full name, plastoquinone oxidoreductase.

Right then, photosystem II, or, if you want to be brief, PSII.

PSII and indeed all of the protein complexes in the light-dependent reactions, straddle

the membrane of the thylakoids in the chloroplasts.

That excited electron is now going to go on a journey designed to extract all of its new

energy and convert that energy into useful stuff. This is called the electron transport

chain, in which energized electrons lose their energy in a series of reactions that capture

the energy necessary to keep life living.

PSII's Chlorophyll now has this electron that is so excited that, when a special protein

designed specifically for stealing electrons shows up,

the electron actually leaps off of the chlorophyll molecule onto the protein, which we call a

The Chlorophyll then freaks out like a mother who has just had her 12 year old daughter

abducted by a teen idol and is like "WHAT DO I DO TO FIX THIS PROBLEM!"

and then it, in cooperation with the rest of PSII does something so amazing and important

that I can barely believe that it keeps happening every day.

It splits that ultra-stable molecule, H2O, stealing one of its electrons, to replenish

Hydrogen ions, which are just single protons, and oxygen. Sweet, sweet oxygen.

This reaction, my friends, is the reason that we can breathe.

Brief interjection: Next time someone says that they don't like it when there are chemicals

in their food, please remind them that all life is

made of chemicals and would they please stop pretending that the word chemical is somehow

Because, I mean, think about how chlorophyll feels when you say that! It spends all of

it's time and energy creating the air we breathe and then we're like

Now, remember, all energized electrons from PSII have been picked up by electron carriers

and are now being transported onto our second protein complex

This little guy does two things...one, it serves as an intermediary between

PSII and PS I and, two, uses a bit of the energy from the electron to

So the thylakoid's starting to fill up with protons. We've created some by splitting water,