Hello and welcome to part 2 of chapter 8. We last left off finishing section 8.4
talking about a photosystem.
Here is a different image of a photosystem from the one I showed you in the first video.
But basically it's the same thing. And this is the beginning, or most of, the light dependent
reactions. The light dependent reactions occur in four stages.
Stage one is the primary photo event. This is when photons of light are
captured by the pigment. So here you see the light coming down and being captured by the pigments in the antenna complex.
Stage two is called charge separation: energy transferred to the reaction center
which sends an electron to an acceptor molecule.
So the energy that builds up in the reaction center,
once it builds up to a critical level, it will eject the electron up and it gets accepted by a primary
electron acceptor.
Stage three is electron transport. The electron is then passed through an electron transport chain
to generate a proton gradient.
The final electron acceptor in this electron transport chain is
NADPH.
Stage 4 is chemiosmosis. That proton gradient is used to generate ATP
using ATP synthase,
the same way it was done in aerobic respiration.
This process can also be called
photophosphorylation.
Phosphorylation we already learned is simply taking the phosphate and sticking it on ADP to make ATP.
In this case the energy used to do that is
light energy, hence the word photo in photophosphorylation.
So there are two types of
photophosphorylation. The first type is cyclic and the second type is non-cyclic.
So let's look at cyclic photophosphorylation.
This involves one photosystem to generate ATP.
Organisms like purple non-sulfur bacteria and green sulfur bacteria do this type.
They eject an electron. As
it's passed through an electron transport system,
the proton gradient drives chemiosmosis, just like I described previously.
But in this case that electron, instead of going into
NADPH, it's recycled back into the reaction center,
so no NADPH is formed and no oxygen is formed. So this method is anoxygenic.
So in this diagram
here's our photosystem one. It absorbs the light, photons of light coming from the Sun.
The
electron is ejected out of the photosystem into an electron acceptor,
which then passes it along an electron transport chain, and it goes back in
to the spot where it held before, instead of having this space filled by
electron donor and
instead of it being accepted by NADPH.
The second type is non-cyclic photophosphorylation.
Let's take a look at how that looks. Okay, so
this
system uses two photosystems. Now some students get confused by the fact that it has
Photosystem II listed here, and then photosystem I listed here.
They were numbered based on when they were discovered not where they're located.
But don't let that confuse you. All of this happens at the same time, not in a stepwise fashion.
So light strikes both
photosystems at the same time.
Excited electrons are ejected out of the photosystems
and
in this case that leaves a hole, the electron needs to be replaced. So the
electron that's ejected out of photosystem II
passes down an electron transport chain and gets accepted by photosystem I to replace the one it lost.
The electron that photosystem I lost
also gets moved down an electron transport chain, but it gets accepted by NADP to become
NADPH.
This still leaves an electron hole in photosystem II. It needs to replace its electron that it lost.
That's done by splitting water.
So water is broken apart, electron is donated to the photosystem, and that's what produces oxygen.
That's why the cyclic photophosphorylation
does not produce oxygen and the non cyclic photophosphorylation
does.
So now let's try to get a complete picture of this on
the thylakoid membrane. Now, I'm afraid this image kind of got distorted, and I have been unable to
undistort it, so I'll have to deal with it as is.
Okay, so let's put the story together. Here's photosystem II, here's photosystem I.
Photons, or light, are striking those photosystems. They are ejecting electrons.
The electron coming out of here is going through this electron transport chain which is pumping hydrogen ions into this
thylakoid space, and
it's replacing the electron lost in photosystem I. The electron from photosystem I is
going out through here and being accepted by NAD to generate
NADPH.
The electron lost from photosystem II is replaced by water, and that's what's generating our oxygen.
Hydrogen ions then build up in this thylakoid space
creating a concentration gradient between that and the stroma outside.
Hydrogen ions will then diffuse through the ATP synthase complex,
generating ATP. And you now have ATP and
NADPH being made. Those are the two products of the light-dependent reactions.
Along with the byproduct of oxygen. Oxygen gets released by the plant to our benefit and the ATP and
NADPH are now holding energy that can be used to run the light independent reactions.
So the light independent
reactions actually take that energy and
grab carbon dioxide gas and take that carbon and convert it into an organic molecule.
So let's look at that.
So this is called the carbon cycle.
It happens in
C3
photosynthesis. It occurs in the stroma of a chloroplast and uses ATP and
NADPH to reduce carbon dioxide
into organic
molecules.
Now I know this diagram can look very intimidating with all these long
complex words, but you don't need to know all of that. We're gonna break it down so that you understand what you need to know.
The calvin cycle has three phases. Phase one is carbon fixation, the incorporation of carbon dioxide
into organic
molecules.
It begins with a
molecule called RuBP.
You don't need to know the full name, that's it written here, but six RuBP
molecules get combined with
six CO2 molecules here and
this produces 12 molecules of
PGA.
Okay, you don't need to know the full words. Just the initials are fine.
This is phase one, carbon fixation, and it uses an enzyme called Rubisco. If you can't read that I'll write it up here.
Rubisco, I always think of Crisco for some reason when I hear that word.
Phase 2 is called the reduction phase. It takes the 12 GPA molecules and
reduces them
into 12 molecules of G3P.
This process uses ATP and NADPH. You can see that here.
Hopefully you'll recognize that G3P,
it was an intermediate, or substrate, in the glycolysis pathway, so it is an organic
sugar.
Two of these
G3P molecules will actually leave the calvin cycle and be converted into glucose or other sugars.
Phase 3 is the regeneration of RuBP. Those 10
G3P molecules are used to
regenerate the six RuBP that can then go back and collect six more carbon dioxide
molecules and keep this cycle going. And this process also uses ATP.
So that's the calvin cycle in a nutshell. It just continues to turn
pumping out G3P molecules that are
used to make sugars.
Please note that glucose is not a direct product of the calvin cycle. It's actually G3P
that's moved into the cytoplasm
where they are then used to produce sugars.
Energy for the calvin cycle is supplied by
18 ATP molecules and twelve
NADPH molecules, which were produced in the light dependent reactions.
So that's photosynthesis, converting light energy
into organic molecules,
trapping the energy in an organic form. So now let's look at the big picture.
Essentially mitochondria
Essentially mitochondria and
chloroplasts are dependent on each other.
Chloroplasts produce glucose and oxygen and
those are the starting substrates of aerobic respiration.
Glucose and oxygen are broken down to generate ATP.
The byproducts of this are
water and carbon dioxide, which are the starting products for
photosynthesis. The water is used to
donate an electron to a photosystem and the carbon dioxide is fixed into glucose.
You can look at this as one big system of transferring energy. Sunlight energy is trapped by the
chloroplasts into an organic molecule and when that's needed for
Use to do work, the organic molecule is broken down in respiration to generate ATP,
Which actually is the form of energy that's used to do work in a cell.
Okay, so this takes us through section 8.6 of
photosynthesis. I'm going to do one final video to cover the remainder of the chapter. Thanks for watching.
