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Lecture 4.02: Photosynthesis
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Do curso por Caltech
The Science of the Solar System
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Unit 4: Life in the solar system (week 1)
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Mike BrownProfessor
Planetary Astronomy
The majority of life on Earth is based on energy that's received from the sun.
Not all of it.
We will talk about some of these
alternative energy sources in a little bit.
For the most part, we can think of, terrestrial life is based on solar power.
How do we extract the energy from the sun?
We don't all have the solar panels sitting on our head.
Well, we do it through the process of photosynthesis.
In plants.
In the very simplest form, photosynthesis involves a chemical reaction
which takes carbon dioxide and water, and combines them together.
Now, if you think for a minute, you know that
carbon dioxide is everywhere in the at, in the atmosphere.
Water is everywhere in the atmosphere.
We don't have these chemical reactions happening all over the
place, so, carbon dioxide and water do not react naturally.
They require energy to react.
We call that an endothermic reaction.
That means that these two things will stay in contact
with each other forever, unless you add energy into the system.
When you do add energy into the system, you can get out oxygen.
And, what's left over is CH2O.
This is a carbohydrate.
Where does this energy come from?
I'm going to draw that as the symbol for a photon.
That's the solar energy.
In the planetary science point of view, this is
what enables life here on the surface of the earth.
The energy from the photons.
Are, is added to this chemical reaction, and, as a
waste product, gives out this oxygen, but it makes this carbohydrate.
Why is making carbohydrates important?
Oh, carbohydrates are amazing things.
First off, they are organic molecules.
They have that carbon in them.
Its carbon chemistry is the, one of the richest.
Chemistries that we have for molecules.
Because, you can take any sorts of these carbon molecules with things on them,
and you can build bigger, and bigger, and bigger, carbon molecules out of them.
The other great thing about carbohydrates, as everybody knows,
carbohydrates can be used as a source of energy.
So what you're doing is taking this photon energy.
That it takes to break apart the water molecule, and you're storing it into this
carbohydrate, basically, and allowing that carbohydrate now, that, that molecule.
You, it's, you can think of it as a battery.
It takes the photon and stores it into this carbohydrate battery.
The nice thing about a carbohydrate battery, is,
that you can move it to different places.
So, all of this photosynthesis reaction.
Takes place in the, in the plants cells, like in a
leaf of a plant, and yet, the plant needs energy in many
different places, other than just in its leaves, and so it
can make these carbohydrates, transport these
carbohydrates to places that need energy.
Use the energy there, and we'll talk about how in a minute.
And, it can also take these carbohydrates and turn them into
much more complex long chain hydrocarbons, we call them, where we
have many, many carbons attached to many, many of these hydrogen
and oxygens and, and other things, and you can use those.
To make structure.
Cellulose is one of these long chain hydrocarbons.
You can also put these things into very compact energy storage carbohydrates.
Which have been packed into more compact form.
So the fact that we have abundant CO2 in the atmosphere.
The fact that we have liquid water.
Which can be brought up by the plants and, and put into contact with the CO2.
And the fact that we have sunlight, in
this case, allows us to make these carbohydrates, these
rich hydrocarbons that are both an energy storage source,
and a source of structure that we can use.
For fun, let's look at the chemical
structure of some of these carbohydrate molecules,
just to get a better feel for how they work for energy storage, for.
For structures and for other things.
I'm going to show you these chemical structures if you've never
seen them before, I'll des, describe what's going on here.
Each of these intersections here is a carbon atom.
We don't normally show the carbon atoms, because there's
so many of them, they're all at these intersections.
And each of these carbon atoms, you can see,
are attached to oxygens or hydrogens, in this carbohydrate.
For the chemists out there, you know the size of the.
Line here shows you the type of bond that you have.
But basically, you see that you just have a bunch
of carbons attached together, and you have oxygens and OHs hydroxides.
We'll talk in a little bit about how you take this glucose
and turn it into energy which is what you really want to do.
But this is a simple, sugar, carbohydrate.
You could take all those same atoms and rearrange them differently again, and here
all the carbons that are at these
intersections, and you can form a long chain.
Notice here that, there's a connect here to an identical unit over here.
So you can have as many of these sections as possible.
Going through here, this long chain hydrocarbon is called cellulose.
You could imagine, because you have these very long chains, that you
can actually use them to build things like I don't know, trees, leaves.
There are couple of other interesting things here, cellulose
because it's so big, has such a long chain.
Cellulose is not soluble in water.
You know this.
You can't take a piece of wood and have it very quickly dissolve in water.
And so, cellulose is, is good to be,
for being used as a structure for holding water.
Viewers, we'll talk about, in a little
bit, holding water as a very important point.
Here's another important thing you can do with
these molecules, with these carbohydrates that you first formed.
These are starches.
Starches are what plants use to basically store
their energy of carbohydrates when they don't need them.
And, as you can see this 300 to 600 of these chains, these
individual little units that attach to each other, which are, again, OHs and Cs.
They're just long chains of these carbohydrates.
Like the cellulose, these are big molecules.
They are not soluble in water.
And so, again, the, the carbohydrates, the small carbohydrates, glucose as you know.
You're taking like sugar, it dissolves in water.
Starch does not dissolve in water.
And what that means, practically for a plant,
is that you can store it much more efficiently.
You can't really pack glucose together very efficiently in a plant
that's mostly water, because you have to put it in water.
So there we have it.
From this very simple reaction of CO2, we
have very abundant CO2, and water, very abundant water,
adding in some photons, we get this incredibly
complex chemistry of energy sources, structure sources, energy storage.
It's an amazing story.
It's, it's really hard to not think that this is a way that biology almost.
Has to happen, because it's, it's, it's hard to think of any
other simple chemistry like this, that can lead to such amazing complexity.
It doesn't mean that we're not just being earth-centric when
we say that, but, man, it's a pretty amazing thing.
Now, how this really happens, I can write this chemical formula very simply.
The actual ways in which you take the photon.
You pull up the CO2, you're bringing up the water, involves an incredibly
complex set, set of mechanisms that
biologists have spent centuries really [INAUDIBLE] uncovering.
But, as planetary scientists, we're going to ignore it.
We're going to say, we understand photosynthesis perfectly fine.
There's a flip side to photosynthesis, and that's of course,
I keep on talking about the, the carbohydrates as energy sources.
But I havent talked about what that
chemical reaction is, which gives the energy.
So, let's talk about that now.
[SOUND] This goes by the name respiration.
You've heard that before.
You might even understand what it's going to be
if I just use that word for you.
Respiration involves taking a carbohydrate, and I'm
just going to draw, right, one of these
CH2Os, but the carbohydrates are, in general,
more complex than this very simple one here.
You can imagine this as a glucose molecule.
And you combine that with O2, and what do you get?
Well, it's just the opposite of what we had before, CO2 plus water.
Remember that I told you it took energy to go from this side to this side, and
it releases exactly the same amount of energy,
as it goes from this side to this side.
That energy is released, as heat.
And that heat is what powers life.
It's an interesting symbiotic relationship we're in with plants.
We've always known that plants take in CO2 and give off oxygen, and then
we take in oxygen and give off CO2, there's the CO2 that we're giving off.
But it's more deep than that.
Plants take in CO2 and give off oxygen, but
plants also are the ones that make these carbohydrates.
Are we animals that have eaten plants, we eat something.
We get the carbohydrates eventually from the plants.
And we take those carbohydrates and recombine them with oxygen, that
plants gave off, and give back CO2 which the plants need.
All in all, a pretty amazing system that we have here of photosynthetic based life.
Generally on the surface of the earth.
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