This screencast will discuss chemiosmosis; the process of creating ATP
during the light-dependent reactions of photosynthesis. Remember that in
photosynthesis, our goal is to combine carbon dioxide and water inside the
chloroplast of a leaf in order to ultimately generate glucose and oxygen.
The light-dependent reactions of photosynthesis occur inside the
thylakoid membrane system. With the addition of water and photons of
sunlight, we generate oxygen as well as two products; ATP and NADPH. ATP and NADPH
ultimately enter the stroma, the other compartment inside the chloroplast, where
they are combined with carbon dioxide in order to ultimately generate PGAL a
product that ultimately become glukose. At this point, this should all be review.
Today we're going to focus on the light-dependent reactions and exactly
how it is that they generate ATP and NADPH. Don't forget the light independent
reactions are actually happening inside of plant cells. Specifically they occur
in the organelle called the chloroplast. Within the chloroplast itself, the
light-dependent reactions occur in something called the thylakoid. So
remember this is happening at a very very, very small scale.
Now we're going to talk about the light-dependent reactions, which occur in
the thylakoid. Now, here is my thylakoid, and all the structures that
are embedded in the thylakoid membrane are going to be made of protein. Each one
of them has a very specific function in the light-dependent reactions. The entire
process is kicked off by a photon of light coming from the Sun. An important
note about photons; photons are going to do three separate things during the
light-dependent reactions. Number one activate PS2, number two activate PS1 and
number three they're going to cause a process called photolysis, where water
splits into multiple pieces.
We're going to start by talking about number one where photons
activate PS2.
My photon of light, after traveling 93 million miles from the Sun to the plant
cell, is going to make contact with PS2. PS2 is full of pigment called
chlorophyll, including a specific pigment called p680 which absorbs sunlight of
the wavelength 680 nanometers. The photon contains a significant amount of energy
which will then excite all of the pigments in photosystem 2.
Uou'll notice that I've left the last pigment molecule uncolored and that there's
a small orange dot next to it.
This small orange dot represents an electron which right now is in its low
energy state. Once all of the pigments have been activated by the photon, the
electron will cease to be a low-energy electron and then will become a
high-energy electron. Now this electron has more energy than it normally has. The
electron now has so much energy that will bounce down through a series of
chemical reactions which we call the electron transport chain. During every
step in the electron transport chain, energy is lost from the electron. The
electron has now reached the end of the electron transport chain and is now a
boring low-energy electron once again.
In addition, photosystem 2 is now missing an electron. This electron will need to
be replaced in order to continue photosynthesis.
So now we need to ask ourselves "what do we do with all of the energy that was
released during the electron transport chain?" All of the energy from the
electron transport chain will be used in order to power something called a proton
pump. The proton pump is activated by the harnessed energy from the electron
transport chain. Outside of the thylakoid membrane, out in the stroma, there are
many protons (AKA H+ ions) floating around. There are also a lot of protons
floating around inside the thylakoid membrane. Now most of the time this would
mean that the protons want to get out of the cell membrane, and they do, but the
proton pump is going to cause protons to flow into the cell membrane. So, kind of
like this.
This makes the concentration gradient even stronger than it was before,
rather than making it weaker like it normally would.
After the proton pump has done its work powered by the electron transport chain,
there are now more protons on the INSIDE of the thylakoid then there are on the
OUTSIDE in the stroma. We aren't finished with this tiny electron, however.
It still has more work to do.
While the proton pump is working hard next door, the low-energy electron
is carried over to PS1 and this is where we can see the second part of a photon's
job. The photon will re-energize the electrons in photosystem 1. The electron
will be transformed back into its excited state and will then enter the
electron transport chain again. Notice that in photosystem 1 there's a
different pigment being used. In photosystem 1 we use p700 instead of
p680. The high-energy electron will bounce through the electron transport
chain just like it did in the previous slides. Just as it did previously, it will release
energy with every step of the electron transport chain ending up a low-energy
electron. This time all of the energy from the second electron transport chain
will be used to do something different. This time the energy from the electron
transport chain will be used to combine something called nadp+, two electrons and a
hydrogen ion in order to form nadph. Now we've made one of the two major product
of the light-dependent reactions, nadph, which is going to become very important
as we move into the light independent reactions.
Just to make sure we're on the same page, here's what we've done so far. A photon
of light activated ps2 and triggered the chlorophyll particles inside it.
This causes the release of a high-energy electron whose energy bouncing down the
electron transport chain is used to power a proton pump, which brings in
hydrogen ions from the stroma into the thylakoid.
While that's happening, another photon is activating ps1 which causes
energy to be released. As a high-energy electron bounces down the electron
transport chain, we harness that energy in order to combine nadp+, two electrons,
and a hydrogen ion in order to form nadph. However, we have a small problem with
this. If we go back to PS2, we still have a missing electron which needs to be
replaced. The cycle is going to continue in order to replace this missing
electron. We're going to take a look at the third task of a photon which is to
split water in a process called photolysis. If we look inside of the
thylakoidwe'll see the thylakoid already has some water in it.
When struck by a photon, water will split into three components; it will split into
protons, a molecule of oxygen, as well as three low energy electrons. One of these
three electrons will go on to replace the electron that was lost in ps2. So now
we no longer have a missing electron and PS2, and now we have two electrons
remaining from photolysis. The remaining two electrons from photolysis will transfer
over to the end of PS1 where they will become 2 electrons necessary in order to
form nadph from nadp+. So now we have even more protons on the inside of the
thylakoid and we also have a handy waste product; Oxygen. The oxygen will diffuse
out of the leaf through the stomata. This oxygen, the waste product of photolysis, is
the oxygen that we breathe. So far, we've now created two of the major products of
photosynthesis; we've created oxygen and we've also created the intermediate
product nadph, but we still need to create ATP and that's going to use a
protein called ATP synthase which you can see here in orange at the bottom of
the screen. Thanks to the work of the proton pumps and the product of
photolysis, there are a lot of protons inside the thylakoid. If you
look at the number of protons on the upper half of the screen versus on the
lower half,
it's pretty easy to see that there's a concentration gradient between the upper
half and the lower half. The protons want to move from the crowded upper half of
the screen in order to get onto the lower half of the screen that is less
crowded.
However the only way to get out of the thylakoid is through the ATP synthase
protein, so protons will move through the ATP synthase like this in order to get
onto the less crowded side of the membrane. Every time a proton passes through, the
ATP synthase will combine a molecule called ADP, which has very little
potential energy, with inorganic phosphate. When you combine ADP with an
inorganic phosphate, you get ATP which has a lot of potential energy and ATP is the
other intermediate product of the light-dependent reactions in
photosynthesis.
So, we've covered a lot of material on the screencast so far, so let's make sure
that we're all on the same page. The light-dependent reactions are the first
half of photosynthesis and our goal is to create two products; ATP and NADPH.
Along the way, we will create oxygen as a waste product. The entire process is
triggered by photons of light and photons do three things; they activate
PS2, they activate PS1 and they also split water in a process called
photolysis. As a result of PS1, we have multiple bouts of energy that are
released from a high-energy electron, which allows the proton pump to bring
protons from the stroma into the thylakoid membrane. In PS2, the photosystem is
activated by another photon, which also causes a release of energy through an
electron transport chain. This electron transport chain is harnessed in order to
make a product called nadph. Finally, the results of the proton pumps' work is a
large concentration gradient between the thylakoid inside and the stroma on the
outside. The only way for the protons to exit the thylakoid membrane and travel
down the concentration gradient is through ATP synthase. Everytime protons
pass through it,
it provides the potential to create a molecule of ATP
out of ADP and inorganic phosphate. Our two products, nadph and ATP, are now ready
to travel into the stroma where they can be used in the light independent
reactions. That's it! Bring questions to class and don't forget to subscribe!