- [Instructor] I hope
you're ready for this one
because we are now entering into probably
what I would deem the two
most difficult lectures
of the entire semester.
Now, I don't want to do
that to scare you too much,
but at the same time, statistically,
these are the lowest scoring quizzes
in any biology class, not
mine, just any biology class.
Because now we're getting
into the processes
of how everything we've
learned up to this point
starts applying,
specially when it comes to
the two major processes of
how energy is first captured
and then undergoes metabolism.
So, remember, these two lectures,
this one and the following one,
are all about how living things use energy
and we talked in the last lecture
about how the cell maintains homeostasis
sometimes with entropy
sometimes against entropy.
But we also talked
about the laws of energy
and when energy is transferred,
you never always fully transfer it,
which is why life on this
planet is completely dependent
upon one process alone,
which is photosynthesis.
Without this process you
and I would not be here
because we don't have the
capability of renewing any energy,
we only can use up the
energy that is being captured
by these photosynthetic organisms.
So, this really also gets into ecosystems
and why it's important that
we maintain stabile ecosystems
because of the chain of food
that goes from these photosynthesizers
up through the heterotrophs,
which are you and I and
all the other organisms
that depend upon them.
So, this lecture is
primarily gonna look at
how any photosynthesizer,
but we're gonna focus primarily on plants,
how any photosynthesizer is
able to capture sunlight,
which is kinetic energy,
and turn it into covalent bond energy,
which biologically is potential energy.
So, the word photosynthesis,
photo means "light"
synthesis, like we think
of dehydration synthesis,
is the storage of that energy
by covalently bonding atoms together
and forming larger structures
like carbohydrates,
and fats, and proteins in the like.
So, plants aren't the majority
of the photosynthesizers
on our planet, in fact,
they're the minority,
the majority are aquatic photosynthesizers
like algae and other such.
There are even some
microorganisms like bacteria
that though they don't have chloroplast,
still can harness solar energy
via other proteins and mechanisms.
So, we're primarily
gonna look at chloroplast
in this lecture, but just be aware
that there are many, many,
many different organisms
on this planet that can
undergo photosynthesis.
Plants, protists, bacteria,
these are kind of the kingdoms
or domains that have organisms
that can undergo photosynthesis.
Animals on the other hand and fungi,
we don't undergo any
type of photosynthesis,
we're the consumers and the decomposers.
So, the only way in which
we can get our energy
is by first allowing the autotrophs,
which are the organisms
that capture sunlight,
harness that energy and then
we can get it biologically.
Now, there are some organisms
down in the deep recesses
of the ocean that can
live off of the energy
that comes through these
thermal aquatic vents,
but those are very few and far between,
so we're primarily gonna focus on the fact
that all life is dependent
upon photosynthesis to survive.
So, little chemistry for you.
The overall process of
photosynthesis is as follows.
And even this isn't the full reaction,
but it's the simplified
version of this reaction.
Basically, any photosynthetic
organism uses carbon dioxide,
water, and then light energy,
as a source of kinetic energy,
remember this is an exergonic reaction
that comes from the sun.
So, this is the part of the
reaction that actually supplies
the kinetic energy.
And then, it uses that
energy to covalently bond
atoms together to form larger structures,
this one right here being glucose.
And then, oxygen is also produced,
so plants and other
photosynthetic organisms
generate oxygen in the process
of consuming carbon dioxide and water.
In fact, we'll show today
that water and oxygen
have a direct relationship
with one another.
The oxygen which you and I
breath and is in the atmosphere
actually comes by the plants ability
to split water into hydrogen
ions and oxygen gas.
The oxygen from the carbon dioxide
and then the carbons themselves,
that goes into the actual
food making process.
So, we're gonna dissect
this overall process down.
But, there is gonna a
question, very simply,
that will test you on
your overall understanding
of this process, meaning what goes in,
what are the substrates,
which is carbon dioxide, water, and light.
We won't worry about the number here,
so don't worry about total amounts,
this is not even fully
chemically balanced.
The products are the glucose and oxygen,
those are the two products
of this overall process.
Like any metabolism,
there's always enzymes
at every step of the way,
but most of those enzymes
we're just gonna ignore.
There are a few, however,
that I'm gonna make mention
of because they play a role
in understanding certain
evolutionary mechanisms, as well.
Now, before we can talk
about the actual structure
of the organelle, the
chloroplast, which is responsible
for the majority of photosynthetic
activity on our planet,
we first have to look at
sunlight as an energy source.
Because energy, as we've talked about,
remember it comes in a wide
variety of manifestations,
well, pure energy, like photons of light,
can actually have higher
or lower strengths,
meaning they're more
energetic or less energetic.
So, let's look at what we call
the electromagnetic spectrum.
You and I, the biology of
our eyes can only detect
or observe a very small
fraction of the amount of energy
that actually exists in our universe.
Notice that all of these
are all different wavelengths of energy.
Now, when we say wavelengths,
when we measure them,
the distance between the
crests of these waves of energy
ultimately tells you how strong they are
and we measure that in
actually nanometers,
which is pretty small.
The visible light, we can
see our eyes and our proteins
are specifically done designed to absorb
wavelengths of light between
400 and 750 nanometers,
that's what we call the
visible light spectrum.
Now, there are other organisms
that can actually see
other things outside of this,
maybe ultraviolet or near-infrared.
In fact, this is where we
look at heat signatures
for near-infrared and whatnot.
And we use, like for your Xbox,
it uses infrared and other things
to ultimately distinguish
who's where and that.
Let's look at the highest level
energy wavelengths, gamma rays.
This will not turn you
into the Incredible Hulk,
it will give you cancer and kill you.
So, gamma rays, which have
the shortest wavelengths
are the most energetic types
of waves that are generated.
Well, the sun's always pumping these out,
but thankfully due to the
Earth's magnetic field,
as well as our ozone layer,
all of the gamma rays
are pretty much redistributed
and don't reach us and
that's a good thing.
X-rays too, most X-rays
that come towards our planet
are scattered away and don't reach us.
X-rays as we know are used a lot of times
when we're looking at bone density,
but too much exposure to
X-rays will mutate your DNA
and ultimately cause you to have cancer.
So, if the gamma rays are strong enough,
I've heard stories of people working
in nuclear reactor facilities
where they were exposed
to just a flash of gamma radiation
and pretty much they
were dead the next day.
So, you get a high enough dose of that
and your cells breakdown
because of the energy
that's absorbed by them and your dead.
Ultraviolet, this is what our body tries
to protect us against, UV radiation.
Now this does reach us,
this actually gets through our ozone layer
and will reach our skin
and this also can mutate
your DNA and your skin,
which is why we usually
wanna wear sunblock
is because though our body
will react and create proteins
like melanin to absorb UV radiation,
in the process of being
producing that melanin,
we are being exposed to UV radiation,
and therefore, have a higher
risk of getting mutated DNA,
and therefore, cancer, which
is why too much UV radiation
will cause skin cancer over time.
And then when we go to
the other end, microwaves,
why do microwaves heat up
your food, do you know why?
Well, the microwaves are set
at a very specific wavelength
that water absorbs, so
when you put your food
into the microwave all you're
doing is causing that water
to absorb that energy and become excited,
thus heating it up, thus
heating your food up.
So, that's really how microwaves work is,
we can use microwaves for communication,
as in some scenarios we do that,
but in your microwave oven,
so to speak in your house,
that's all it's doing
is creating a wavelength
that is specific for
water, water absorbs that,
and that's what heats up your food.
And then, of course, radio waves,
these are the weakest of
all of the energy waves,
they're passing by you all the time.
In fact, the longer they are,
the more you'll reach them out,
when you're in the mountains,
for example, FM and AM,
which one do you reach,
which one reaches you more when your
out away from the cities?
AM, AM has longer
wavelengths, and therefore,
is not absorbed as much by
the mountains and other areas
as the FM radio waves are,
which usually get deflected
by most of the environment,
so that's why AM
typically gets reaches you
and you're able to hear that
for longer periods of time.
So, now we kind of gone
over some of the basics,
let's look at light.
When you see something,
you're actually seeing
the reflection of that
wavelength of light,
so if you see my shirt is
red, which I'm wearing today,
this is the wavelength of
light that you're seeing.
Guess what, the pigments in my shirt
are absorbing these other
wavelengths of light,
same thing for green, same thing for blue.
But, let's look at the
extremes, If you see white,
it's actually the reflection
of all visible light.
So, here, the whiteboard, the pigments,
or the material which this is made up,
actually reflects all
visible wavelengths of light,
and therefore, your eyes pick it up
and interpret it as white.
Guess what black is?
The absorption of all visible light.
That's why when you go out on a hot day,
you'd rather wear a white
shirt than a black shirt.
Why, because the black pigments,
the pigments in your shirt
are absorbing this and
causing you to heat up more
than a white shirt would.
Because the white shirt
is actually reflecting
all of those wavelengths of light.
So, why do I bring that up?
Well, what color are plants
that undergo photosynthesis usually?
Green.
So, guess which color is being reflected
by those photosynthetic
pigments, it's green.
So, the blue and the
red wavelengths of light
are actually what are being absorbed
as a form of kinetic energy
by the plant pigments.
Now, there are some pigments
that are in the leaves
that do absorb green, but they're minimal
compared to the major pigment which absorb
blue and red wavelengths
of light, which is why
pretty much most photosynthetic
organisms are green.
Now, we ask the question just
to help you reiterate on this,
why is the sky blue?
Most people think they
know why the sky is blue,
but most people really don't.
Well, the sky is blue because
as the light hits our ozone,
the major wavelength of
light that gets reflected
in every direction is the
blue wavelength of light.
So, this is why no matter where you look,
wherever you look in the sky,
it's always gonna be blue.
But then, why does, when
the sun starts setting,
why do get orange and
red and yellow colors?
Well, as the sunlight is
traveling through the atmosphere
and as our Earth turns and we
see it across a larger amount
of ozone layer, most of the blue light
is being reflected away
and the only light that
actually gets through the ozone
are the red and the yellow and the orange.
So, that's why when the sun starts setting
we're seeing the light
through more atmosphere,
the blue light's being reflected away
and we see the blue and
the orange and the yellow.
So, now that you have
that little lesson on,
now I will test you on certain dynamics
of what we just went over as
far as the reflection of light
and you'll see how I incorporate in here
as we go through the
physiology of the plant leaves
and the chloroplast.
So, chloroplast, as we've
discussed back in lecture six
on the organelles of the cell,
is the organelle that actually
undergoes photosynthesis,
it's the part of the plant leaves
that are able to capture sunlight
and turn them into sugars.
Now, there's two main
areas in the chloroplast
that are of importance
that you should know.
Now, the chloroplast
itself looks like this,
it has a double membrane around it.
Now, this is a membrane-bound organelle
just like the other
ones we've talked about,
this actually has two membranes around it.
Now, inside the fluid that
is inside of the chloroplast
because we're dealing with
a organelle and not a cell,
we don't call it the cytoplasm,
we call it the stroma.
So, the stroma is the fluid that is
within this double membrane
of the chloroplast.
This is where sugars are made.
So, when we talk about when
glucose and whatnot is made,
that's in the fluid area.
Well, in addition to the fluid,
we have stacks and stacks of membrane
which we call thylakoids.
So, thylakoids, notice
these are the greeny parts
of the chloroplast, this is really
what makes the chloroplast green,
it's not the membrane out here,
it's actually all of these stacks.
Also notice that they're hollow,
these discs have a fluid inside of them
so they're not completely solid,
they're actually have some space inside
and you'll see how that works.
Well, having lots and lots
of stacks of these thylakoids
increases the surface area,
the more stacks you have,
the more space and surface area you have
to under go photosynthesis.
And that's why they're
stacked on top of one another
is because if a light
particle goes through here
and just happens to not
get absorbed by this one,
it'll get absorbed by the next one,
or by the next one,
and so on and so forth.
So, really increases the
amount of light energy
that these little organelles
can actually pick up and absorb,
better than any solar
panels we've created so far.
Now, within the membrane
of the thylakoid discs
are a pigment and this pigment,
the most abundant pigment
that is in the membrane
of the thylakoid discs,
it's called chlorophyll.
Now, there are actually, if I
think off the top of my head,
at least six pigments that can be found
in the thylakoid discs.
We're only gonna talk about two of them
and, in fact, we're not
even gonna talk too much
about those, we're just
gonna mention them briefly.
Chlorophyll is the most abundant pigment
in any photosynthetic organism,
which is why most photosynthetic
organisms are green.
Because if we look at the spectrum
from which this molecule absorbs light,
we find that it absorbs primarily blue
and red wavelengths of light
and it reflects away green.
So, that's why most photosynthetic
organisms are green.
Now, I say most because
there are certain plants
that have purple leaves,
and some almost black,
I mean they're really dark
where they're absorbing
a lot more light energy,
but there's a trade off
usually that comes from that.
So, green is reflected away
by this chlorophyll pigment,
but blue and red is absorbed,
these are the two major
wavelengths of energy
that are absorbed.
Now, I'm not gonna
distinguish between a and b,
we're just gonna call it chlorophyll,
so don't worry about different slight,
because if you look at
it really each version
takes on a different wavelength
of light within that area,
I'm just gonna say blue and red,
we're gonna keep it nice and simple.
But, I do wanna mention one other pigment.
I never know what to call this
whether it's a carotenoid,
I think the carotenoid is
a proper way of saying it.
Well, if you look at
the first five letters,
it looks like carot,
what color are carrots?
Orange, so guess what?
Carotenoids absorb blue and
green wavelengths of light,
but reflect away orange.
Now, this brings us to a concept
of why do leaves turn
different colors in the fall?
We will see that later on this year.
Well, it ultimately comes
down to what pigments
are still surviving in the leaf
as the tree goes into hibernation.
So, as the days get shorter
and as there's less and less
light, some trees, not all,
you know that there are
evergreen trees and what not,
but some trees will go
into a state of hibernation
where they'll essentially
shed their leaves
because the leaves aren't producing
as much as they should be and
so it cuts the leaves off.
Now, as the leaves start dying
the first pigment to
breakdown is the chlorophyll,
but that leaves the
carotenoids and other pigments,
like xanthophyll and other
types of things, behind.
And when those get left behind,
now the chlorophyll's broken down,
it defaults to whatever the
next most abundant pigment is
and if it's a carotenoid
mixed with some other pigments
that usually lasts a little bit longer,
then you're not gonna be able to
reflecting away green anymore,
you're gonna be absorbing blue and green
and reflecting away yellow,
and orange, and red.
And each tree has it's own
composition of pigments,
which is why every tree is going to have
a different assortment or
different color for its leaves
depending upon what pigments
are in the leaves themselves.
Now, eventually they'll
turn completely brown
as all of those pigments have
fully broken down and whatnot.
So, I will test you on
the absorption of light
by the pigments because
this is the key aspect
of photosynthesis in that
it's the absorption of energy,
not the reflection, but
the absorption of energy
that drives this endergonic reaction.
So, this reaction is all
about absorbing energy
and covalently bonding atoms together
to form the organic molecules,
like glucose and sugars and
fats and proteins and such.
So, this first part is probably
the most difficult of it all.
So, I'm gonna go through this
probably five times or so,
different ways, so don't
worry if you don't get it
the first, or the second, or the third,
if you don't get it by
the fifth, then worry,
but don't worry if you don't get it
by the first or second time
around that I go through here.
So, the light reactions are
these membrane-bound portions
of the chloroplast,
which we call thylakoids,
that have the chlorophyll
and carotenoid pigments
embedded in the membrane.
Now, as part of this process
of harnessing the light energy,
the chlorophyll and carotenoid
pigments are arranged
with other proteins in
what we call a photosystem.
Now, when scientists first
discovered the photosystems,
they discovered one and
they called it photosystem I
and they discovered that
there was a second one
that was distinct from the first one,
so they called it photosystem II.
Well, then they discovered
that photosystem II
comes before photosystem I,
but they never changed
their names, morons.
So, when you look at the,
because these do work in tandem
with one another,
photosystem II comes first
and then photosystem I, stupid.
Wish I had a time
machine, ring their necks.
So, it just goes to show
that they're like, well, no,
we're not gonna rename this
one or rename this one, anyway.
So, there are two types of photosystems,
but no matter which one they are,
they have three basic elements to it.
The first is what we call
the antenna pigments,
what are the antenna pigments?
This right here, chlorophyll, carotenoids.
Why do we call them antenna pigments?
'Cause just like the antenna on your car
picks up radio waves
and through various processes
translates that information,
same thing with these,
these absorb the energy,
these have electrons, well
all atoms have electrons,
but these are specially designed
to absorb the photons of
light and energize electrons.
So, that's why we call
them the antenna pigments.
So, both photosystems have
what we call antenna pigments.
Well, like any process
of electrical current,
like you would imagine what's going
through the wall right here,
we can't just tap into any part of that.
So, the way that the
photosystem is arranged
is all of the electrical energy,
'cause that's really what
it becomes at this point
is energized electrons, so
when the photons of light
hit the antenna pigments,
they essentially energize the electrons
and the electrons take
on more potential energy.
As the electrons travel
from protein to protein,
they travel to a central core,
which we call the reaction center,
you can think of it as a plug.
That's where we can
safely harness the energy
that's running through the walls here,
there are special outlets
where we can use that energy.
Well, the same thing
with the reaction center.
The antenna pigments is this huge array
that funnels all of the energy down
to this core set of proteins
called the reaction center.
The last part of the photosystem is called
the electron transport
chain, or ETC for short.
No, not that Heroes of
the Storm character.
So, ETC, electron transport chain.
Well, guess what these
series of proteins do?
They transport electrons,
that's what their name says they do.
But, this is where the two
photosystems become different.
Because in each of the
electron transport chains
each one makes a different
potential energy molecule.
One photosystem makes makes ATP,
the other photosystem makes NADPH.
So, let me give you the
gist of how this works.
Here's the membrane of the thylakoid disc,
remember the phospholipids that make up
all membrane-bound organelles and whatnot,
embedded in this membrane are the proteins
and the pigments that are
necessary for each photosystem.
Well, here, we have photosystem II,
so here's the order of operations,
meaning step by step process.
Sunlight is constantly
bombarding our planet,
well, when the plant, it gets
hit by a photon of energy,
which is blue or red or green or whatnot,
the chlorophyll, just
specifically designed
for different wavelengths of
light, absorbs that light.
That absorption causes the
electrons to become energized,
so you literally ramp up their energy
and the electrons have a
higher potential energy.
The electrons then get
funneled down to what we call
the reaction center where
they then get released
by the reaction center
into what we call the
electron transport chain.
As the electrons travel down
the electron transport chain,
they use up that energy,
and I'll explain how they
do it in both of these,
but they use up that energy.
Well, here is where photosystem II
and photosystem I become connected
because the electrons that
channel from photosystem II,
then go into photosystem I.
Its used up its energy, so
it just gets re-energized
by a new photon of light and
then it does the same thing,
it goes down its own
electron transport chain.
So, that's why these work
in tandem with one another.
Photosystem II funnels electrons
through its electron transport
chain to photosystem I,
photosystem I funnels its electrons
through its electron transport chain,
which then get picked up
by this energy molecule.
So, let's talk a little bit about
the different energy
molecules that they make.
Here's where it hits the fan, so.
Photosystem II makes ATP,
just if you can't remember anything else,
this is the main point,
photosystem II makes ATP.
Photosystem I makes NADPH, just
hammer that into your head,
and If you don't get this next part,
you're not screwed, but.
So, let's talk about photosystem II.
How does it make ATP?
You'll begin to see why
we spent so much time
on the modes of membrane
transport in the previous lecture,
such as osmosis and facilitated diffusion,
and active transport,
and so on and so forth,
it's simple diffusion, they
all come into play right here.
So, the electron,
as it travels down the
electron transport chain
gives this energy and
allows these proteins
to pump hydrogen ions from the stroma
into the thylakoid disc space.
What do we call that,
when you pump a molecule
from a low concentration
to a high concentration
using energy, what do we call that?
Remember, there's simple diffusion,
there's facilitated
diffusion, there's osmosis,
there's active transport,
which one of those is it?
Low to high, against
entropy, using energy,
you better know these for this
weeks quiz, for quiz seven.
Active transport.
So, active transport requires energy, why?
Because you're doing what
doesn't naturally occur,
you're taking molecules,
in this case hydrogen ions,
and pushing them against
their concentration gradient,
you're blowing up the inside
of this thylakoid space
like a balloon, but you're just filling up
with high concentration of hydrogen ions.
So, that's what this does,
this is essentially a proton pump,
this pumps protons, 'cause
that's all hydrogen ions are,
pumps protons into the thylakoid space.
Well, they're trapped in there now
and now you have this high concentration
of hydrogen ions, well, remember we said
that a high concentration is
actually potential energy.
So, guess what?
On the flip side of
this thylakoid membrane,
we have a special enzyme
called ATP synthase.
This enzyme opens up a
channel in the membrane
and allows the hydrogen ions to flow
from a high concentration to a low.
Remember, that occurs naturally,
this works with entropy.
What do we call that, when molecules go
from a high to a low
through a membrane protein?
Facilitated diffusion.
So, here we have active
transport that requires energy,
here we have facilitated
diffusion that occurs naturally,
that occurs just from a high to a low.
Well, remember we said that
movement is kinetic energy.
So, when these hydrogen ions are moving
through this protein, that
kinetic energy is used
to covalently bond this phosphate
to the used up version
of it, which is ADP,
and that's how ATP is made.
Guess what?
This enzyme is also found in mitochondria,
the same enzyme that cyanide
blocks, the same enzyme,
that's why cyanide kills you, remember?
So, this enzyme right here
that allows hydrogen ions out
and uses that energy to
covalently bond the phosphate
with ADP to make ATP, that's ATP synthase.
That's how ATP is made.
So, basically, it blows
it up like a ballon
by pumping hydrogen ions in here
and then as they go out
through facilitated diffusion,
that kinetic energy is turned
back into potential energy
in the form of ATP.
We have a name for this,
a very fancy name that tells
you what I just explained,
it's called chemiosmotic phosphorylation.
I didn't make that up.
So, what's chemiosmosis?
This right here, just pumping
hydrogen ions into here
against the concentration gradient.
What's phosphorylation?
It's when you attach the
phosphate here to make ATP,
that's all it is.
So, you won't have to
explain that back to me,
but you do need to know this,
chemiosmotic phosphorylation makes ATP.
Chemiosmotic phosphorylation is this,
what I just explained to
you, that is how ATP is made.
This is exactly how it's made
in our mitochondria, as well.
The only difference is it's
not made with chloroplast,
it's made with mitochondria,
this process, chemiosmotic
phosphorylation.
So, let's move on to the next
step, this one's so easy.
As the electron go into photosystem I,
they get re-energized 'cause
they used up that energy,
but they've pumped hydrogen ions here
to this membrane space,
so they get re-energized
by a new photon of light.
Now, when these electrons
travel down the photosystem Is
electron transport chain, they
don't do anything special,
they just get picked up by what
we call an electron carrier
and get turned into what we call NADPH.
That's it, see how simple that is?
So, they don't lose their energy,
that, in fact, the
electrons keep that energy.
And when these NADPs pick up the electrons
and some hydrogen,
then they form what we
call an electron carrier.
Notice ADP, these are almost
identical to one another.
So, they're both energy molecules,
you won't even have to worry
about what the shape of NADP is.
Recap, photosystem II makes what?
ATP.
Photosystem I makes?
Good, what's the name of the process
that photosystem II uses to make ATP?
See how easy this is, that's
the hardest part of it all.
One more thing to go over.
The electrons here are actual matter,
remember atoms are made of
protons, neutrons, and electrons.
As electrons go through this process,
they're being consumed here at the end,
the NADPH is picking them
up and pulling them away.
So, then you have to ask the question,
well, where does this get more
because they don't just
pop into existence,
the electrons have to come from somewhere?
Here enters water and this
is why your plants need
a substantial amount of water to occur.
Guess what?
As you water your plants, what
is water for in this process,
what is the whole purpose of water?
The plant wants the electrons
that hold the hydrogen
and the oxygen together to form water.
It doesn't really care about much else,
it just wants the
electrons, so guess what?
Some of the energy in
photosystem II splits water,
breaks the covalent bond,
and steals the electrons
that the hydrogen and the oxygen
were sharing with one another.
So, as it steals those electrons,
those then refuel this process.
That's why if you don't water your plants,
this doesn't occur because
without the electrons
to go through this process and make ATP
and then get over here to make NADPH,
you don't make sugar,
you don't make anything.
Here's also where the oxygen
that you and I breath comes from.
When plants give off oxygen,
that comes from the water we give them,
literally from the water we give them.
Now, it becomes oxygen.
Well, let's talk about the two
other processes we discussed.
As oxygen builds up inside
the thylakoid space,
how does it escape?
Remember, what's the name of the process
where molecules go from a
high to a low concentration
through a membrane?
Hi to low, works with entropy?
Simple diffusion, so all that happens is
as oxygen builds up, it just leaks out
and then leaks out into the air.
So, as it builds up inside,
then it diffuses simple diffusion
out of the thylaloid disc,
out of the chloroplast,
out of the cell membrane, and
goes into the air, that's it.
Water, on the other hand,
as it's getting used up
and you start having less and less water
in the thylakoid space, how
does water get in through?
Osmosis, there you go.
Osmosis just draws more water in,
as the`re's more water on the outside
than on the inside as this is going down,
then water just comes in and
keeps this process going.
We've talked about each one of these now,
osmosis, simple diffusion,
active transport,
facilitated diffusion, see,
it's all there, all relevant.
Now, up to this point,
the plants just made some ATP and NADPH,
but remember those are short
lived energy molecules,
they don't last very long,
they need to be used up right away.
Well, thankfully, there's
a reaction in the stroma
that surrounds the thylakoid disc
that's more than happy
to use that energy up.
We call it the Calvin cycle,
named after the scientist
who discovered it,
that's why it's capitalized.
The Calvin cycle is
essentially glucose synthesis,
however, I want to point out
that the plant doesn't just make sugar.
As we've learned that
simple organic molecules
can be assembled in a
variety of different ways,
well, some of the products
of the Calvin cycle
make sugars, absolutely.
And from there remember that
these are the monosaccharides
that build up the polysaccharides,
like cellulose, which form the cell wall,
or starch, which is the
energy storage polysaccharide
for plants, or even some
disaccharides, like sucrose,
such as the fruit sugars
that a plant would make.
So, again, this is about
not just making sugar,
it's about making everything,
not just carbohydrates,
but also amino acids and
fatty acids and glycerol,
triglycerides, and all those molecules,
organic molecules that we
use for food and energy.
So, though we're gonna talk about glucose,
make it clear that the Calvin cycle
is really about making
all organic molecules,
not just carbohydrates.
Now, this one can be difficult,
but I'm gonna simplify it
down to three main steps.
Three fundamental steps.
The first one is called carbon fixation.
This is the process where the chloroplast
will absorb carbon dioxide
and take it from a gas state
and chemically bond it
to start forming the organic molecules.
That's why we call it carbon fixation,
is when you bind something
and bring it from a gas
state to a solid state.
But, it can't just slap it together
and form glucose right away.
It actually has to go through a number
of metabolic processes before
it's ready to make sugar.
One of those processes
that we're gonna talk about
is called PGAL, or phosphoglyceraldehyde,
PGAL is the shorthand for it.
But, phosphoglyceraldehyde,
these are the building blocks
for glucose, for amino
acids, for triglycerides,
for everything really,
that's what I'm showing you right here.
This G3P in this picture
is the same thing as PGAL.
So, this is phosphoglyceraldehyde,
it just depends upon
who's calling it what,
but it's the same thing,
so this right here is essentially PGAL.
Now, the third step takes the leftovers
from this processes and
remakes an organic molecule
that's necessary for this cycle.
So, let's look at this
cycle and how it goes.
Like I said, I'm gonna skip
almost all metabolic processes
and just keep it simple and
we'll just look at the stages
and what's going on.
But, there are a few key points
that I do want you to know
because one in particular has to do with
an evolutionary mechanism
that plants have evolved
to survive in hot and dry climates.
We're gonna bring this one up quit a bit,
especially when we talk
about evolution later on.
So, let's talk about carbon fixation.
RuBP, which is called
ribulose biphosphate,
remember that ribo sugar,
that five-carbon sugar we've talked about,
the same one that makes up DNA and RNA,
the deoxyribose and the ribose.
Well, ribulose biphosphate
is a five-carbon sugar
that's necessary for this process.
Remember I told you you just
can't slap six-carbon dioxides
together with some oxygen and
hydrogen and form glucose,
it just doesn't work that way chemically.
Well, this molecule is what
we call the starter molecule,
this is what gets the cycle rolling,
so without this you don't make glucose.
Well, here's how carbon fixation works.
This five-carbon gets picked
up with the carbon dioxide
by an enzyme and forms a six-carbon sugar,
not glucose, some other type
of sugar that's not ready yet.
And then it cuts it in half.
Well, this is the only enzyme
whose name you're gonna have to know.
You don't have to know
any of the other enzymes.
Now, the longhand name of it,
don't bother writing this down,
but the longhand name of it
is ribulose biphosphate
oxygenase/carboxylase.
The shorthand is RuBisCo.
So, RuBisCo, nice of them
to shorten that for us,
RuBisCo is the name of the enzyme.
So, RuBisCo comes in, grabs
the RuBP and carbon dioxide,
covalently bonds them, and
then cuts them in half,
that's the first step,
that's carbon fixation.
The next step is what
we call PGAL synthesis.
Well, these are unenergized carbons.
So, the ATP and the NADPH
from the light reactions
come in here and give their
energy to these carbons
and that's what forms what we call PGAL.
Once those use up their energy,
they go back to the light reactions
and they pick up some more,
they get re-energized,
so that's what I showed
you before, right here.
The ATP and the NADPH that are made
in light reactions come here,
get their energy, and go back.
They come here, get there energy,
go back and get re-energized.
That's the connection between the two.
Well, now that the PGALs are energized,
now there's sufficient energy.
Remember that energy of activation,
now there's insufficient
energy for another enzyme
to come in here and combine
them and form glucose.
Now, not all of the PGALs
are used to make glucose,
only a sixth of them are.
So, what happens to the rest,
what happens to five-sixths
of these molecules?
Well, they get turned back in RuBP,
you may think, man, that's wasteful,
well, that's metabolism.
So, metabolism takes these remaining ones,
turns them back into
this five-carbon sugar
and the cycle continues.
Ultimately, six-carbon dioxides come in
and a six-carbon sugar comes out.
That's the Calvin cycle.
Don't worry about the rest of
the metabolism going on here,
it's just metabolism, organic chemistry,
it's beyond what you need
to know for this class.
The reason why I wanted
you to know the enzyme
that catalyzes the reaction
in the Calvin cycle
is because 5% of the plants on
our planet have a big problem
with undergoing the Calvin cycle
when it's really hot and dry,
but they have evolved a
secondary additional pathway
in addition to the Calvin
cycle that allowed them
to survive in those very
hot and dry climates.
All other species, the 95%
of other plant species,
solely use the Calvin
cycle to make sugars,
however, there are a lot
of areas on our planet
that are very hot, very dry.
Well, here's the problem.
When the plant is undergoing
photosynthesis, remember that,
right here, oxygen is
produced in large quantities
as the light reactions make ATP and NADPH.
Well, normally, in normal environments,
the oxygen is able to just easily diffuse
out of the leaf cells
through these little openings
that open and close during the day.
Well, one of the problems
with desert plants is
if they keep those
openings in their leaves
open for too long, not only
do you get oxygen escaping,
which is what it needs to do,
but you also get water evaporating,
which is a bad thing when
you're in the desert.
So, they close those openings
for long periods of time during the day
so that you don't get a
lot of water evaporation
during this process, but
that causes another issue
because now the oxygen
can't escape the cells.
And as oxygen starts building up,
there becomes a big issue
with the Calvin cycle.
Rubisco, that enzyme
that uses carbon dioxide
with ribulose biphosphate sugar,
when oxygen starts becoming more abundant,
then carbon dioxide, this happens,
we call it photorespiration.
So, instead of turning
ribulose biphosphate
into these PGAs and then
converting them into PGALs
and then into sugar, what ends
up happening is the oxygen
out competes the carbon dioxide
because there's more of it,
well, the Rubisco enzyme
can't combine to carbon there
if there is no carbon,
if there's just oxygen,
so it splits it still,
but it's splitting it,
this becomes unstable, this breaks apart,
and you actually release carbon dioxide
instead of capturing it,
that's the complete opposite
of what plants need to
do to make their sugars.
So, this is what we call photorespiration,
where this is a wasteful process
when oxygen starts becoming
too abundant in the cells.
Now, normally, plants don't
have to deal with this
because they live in
air enough environments
where there's enough moisture
that they can release
the oxygen readily enough
that it doesn't build up.
But, like I said, in
these hot, dry environment
this is an issue.
So, how have these desert
plants overcome this problem?
They use what is called the C4 pathway.
Now, here's a key point
and you have to understand
this doesn't replace the Calvin cycle.
All plants use the Calvin cycle.
Let me reiterate that.
All plants use the Calvin cycle,
that is the only method that they can,
that's the organic
chemistry behind glucose,
making glucose and organic molecules.
But, these desert plants
have an additional pathway
called the C4 pathway.
Now, the reason why they
call it the C4 pathway
is because the first step in
this organic chemistry process
creates a four-carbon
molecule, hence the C4 pathway.
That's why the Calvin
cycle's called the C3 pathway
is because it creates C3-carbon
PGAs during its metabolism.
So, what is so special
about the C4 pathway?
Well, there's many different strategies
that different plants can use,
but they all have the fundamental
same organic chemistry
behind the C4 pathway.
And here is the trick.
The enzyme that the C4 pathway uses
to capture carbon dioxide
doesn't care how much oxygen
is present in those cells.
So, it doesn't make the mistake of trying
to combine a sugar with
oxygen like the Rubisco does.
So, in that scenario what happens
is these desert plants can
close their leaf openings
for long periods of time,
oxygen builds up, but the
Calvin cycle just keeps going
because instead of capturing
carbon dioxide with Rubisco,
they capture it with a different enzyme,
which doesn't have that
problem with photorespiration.
Then they just move it
over to the Calvin cycle.
They say, okay, here, here's the carbon
that you need for the Calvin cycle
and they completely bypass the issue
with the Rubisco enzyme.
