Welcome back everyone. In this screencast,
we're going to discuss chloroplasts,
pigments, and photosystems in order to
continue our study of photosynthesis.
Remember, whenever we talk about
photosynthesis, we're referring to the
process where we take photons of light,
carbon dioxide, and water and put them
through a series of chemical reactions
inside the chloroplast of a plant cell.
We then create glucose and oxygen. If we
zoom in even more on photosynthesis, yet
again we'll recall that there are two
separate components to photosynthesis;
the light-dependent reactions and the
light independent reactions. The
light-dependent reactions are triggered
by photons of light released from the
sun, whereas the light independent
reactions are not triggered by photons
of light.
Don't forget that all of this takes
place within the chloroplast. The light
independent reactions take place out in
the middle of the chloroplast in an area
called the stroma, but the
light-dependent reactions take place
inside a special series of compartments
which we're going to learn about later
in this screencast. Let's start by taking
a look at the structure of a chloroplast.
Chloroplasts are organelles that are
found in plant cells, and this is where
photosynthesis takes place. Remember that
organelles are significantly smaller
than cells. Chloroplasts are surrounded
by two membranes. If we were to zoom in
on these two membranes, they would look
just like our old friend the
phospholipid bilayer. A chloroplast has
two layers of membranes; one on the
outside and one on the inside. The two
membranes surrounding chloroplasts are
called the outer and inner membranes, and
the space in between them is called the
lumen. Chloroplasts are highly complex,
but one of the most important structures
inside the chloroplast is something
called the thylakoids. Thylakoids, there
are actually many of them in
chloroplasts, are the sites of the
light-dependent reactions, which is the
first section of photosynthesis. If we
look more closely at a thylakoid, we will
notice that it is also made up of two
memberanes
and this space right in the center
is once again called the lumen. There
isn't just one thylakoid inside a
chloroplast; that would be really
inefficient to only have one site for the
light-dependent reactions. There are
actually stacks of thylakoid spaces
throughout the chloroplast. Each one of
these stacks of thylakoids is called
a granum. If you have more than one
granum we call them
grana. And, they're connected together by
a structure called lamellae, which is
essentially a large membrane containing
more chlorophyll. The area inside the
chloroplast that is not part of the thylakoid
, the grana or the lamellae is
called the stroma. Don't forget that the
thylakoids are the sites of the
light-DEPENDENT reactions, whereas the
stroma is the site of the light
INDEPENDENT reactions. So, somehow the
products of the light-dependent
reactions are going to need to exit the
thylakoids and enter the stroma in order
for the second part of photosynthesis to
occur. We have now discussed chloroplasts,
and so we're going to move on to our
second two topics, which are both
interconnected within the chloroplast.
We're going to discuss pigments and
photosystems, both of which have to do
with the light-dependent reactions. So,
they're going to be doing dealing with
the first part of photosynthesis. If we
look back at this diagram, we now know
that the light-dependent reactions are
taking place in the thylakoids.
However, we need to somehow get the
energy out of the photons of light and
put them into the chloroplasts, and the
next two topics are going to explain how
this works. To discuss both pigments and
photosystems, we're going to take a look
at how the thylakoid is responsible for
absorbing energy from the sun. The sun
releases energy in the form of particles
called photons. Photons of light are not
a form energy that the chloroplast can
absorb directly. The photons must be
absorbed by a pigment contained within a
photosystem. So we're going to zoom in on
the edge of the thylakoid a little.
Ok, now that we've zoomed in on the
edge of the thylakoid a bit more, we can
see how the photon transfers energy to
the chloroplast. So, what I've drawn here,
this funnel shape that looks kind of
like a stentor, is something called a
photosystem. And what it is is a series of
proteins and lipids that helps to
transfer the energy into a form the
plant can actually use. Inside this
funnel that we call a photosystem are
things called pigments. For instance,
chlorophyll which is that green stuff
that you get if you take a really big
fall on grass. There are many different
kinds of pigments, all of which absorb
different wavelengths of light, but this
particular pigment is going to be called
p680.
P for pigment and 680 for the wavelength of
the light in nanometers. The photon
contains a large amount of energy, and
when it makes contact with the
photosystem like this, it's going to
transfer the energy to the pigment
surrounding it. All of the pigment
molecules inside the photosystem are
going to get excited by the contact with
the photon. When all of the pigments have
been excited, it culminates in the
release of a high-energy electron. Now, a
high-energy electron doesn't sound
particularly powerful, but when it runs
through something called an electron
transport chain, it generates a pretty
sizable amount of energy. We will be
discussing this electron transport chain
later on this week. That's it for now!