alright we left off talking about
the structure of the pigments that
are responsible for absorbing the
photons like the energy in the photons
of light
and conveying those to a reaction
center
in a photo system
that will then elevate electrons that will be
then passed through an electron transport
chain
to generate a gradient of hydrogen ions
so let's go through that
now
before doing that though let's look at some
accessory pigments
there are in addition to chlorophyll
which we've just gone over there are
secondary pigments
that can absorb wavelengths that light
that are other than
chorophyll A and that increases the wavelengths of light that can be used in
photosynthesis
so the accessory pigments that can be
found in photo systems are
chlorophyll B, carotenoids, phycobiloproteins
and the cartenoids also act as
anti-oxidants so
carotenoids get there name
from carrots, carrots are orange
as you know and cartenoids are the
yellowish orange in color
these pigments are yellowish orange in color
oppose to the green
chlorophyll pigments and you know that
eating
carrots are good for you and that's
because the carotenoids
present in carrots act as antioxidants
and antioxidants
remove reactive oxygen species from
cells and those reactive oxygen species
can be very damaging
the carotenoids in
in the photosystems act as
antioxidants
as well as light capturing photo
pigments
and that carotenoids are really the reason that
in the fall we have bright orange colors
in leaves
as senescence occurs for
deciduous trees in the fall
the chlorophyll molecules in the photo
systems are degraded before the
carotenoids are so the green the majority
of pigments in the
in the photosystems in leaves are
chlorophylls which are green and as they
are degraded is as senescence
occurs in the fall the carotenoids
are still present and they shine they
show through and so we see
bright orangish red colors in the fall
so now let's look at the photosystems
that contain
these photo pigments that
harvest light energy
the phonesystem consists of several
components
one is an antenna complex and there are
hundreds of pigment molecules found there
chlorophylls chlorophyll B chlorophyll A
other chlorophyll molecules other
accessory pigments I should say
carotenoids fiabilit proteins
and so on and there is a reaction
center that has 2 chlorophyll
A molecules located there and it's the
job of the antenna complex
to funnel energy harvested by
the absorbtion of photons to funnel that energy
into the reaction center and it is at
the reaction center that
electrons will be elevated to higher
energy state
and be passed on to electron carriers and
that passing on
the pass it down electrons will allow
work to be done
and that work just like in cellular
respiration work that we will be done
primarily the pumping of hydrogen ions
across a membrane in this case is the
thylakoid membrane
as we will see so the energy of the electrons
is transferred through the antenna complex
to the reaction
center so here we have with a schematic
of this process we have the absorption of a photon of light
by accessory pigments
in a photosystem so the photosystem
consists of tons
of protein lots of protein molecules
and then pigment molecules
embedded in that protein complex and
the pigments that are
absorbing
the energy and photons of light past
that energy they don't pass electrons on
the past the rnergy to other
accessory pigment molecules and eventually
that energy will be
transferred to the reaction center
containing
2 chlorophyll A molecules and it is
at that point
the absorption of energy by the
the reaction center that elevates
electron to a high energy level
and that high energy electron
will be passed on to
electron acceptor molecules as we will
see
and then those electron acceptors will
pass the electron down a chain
which will then allow the pumping of
hydrogen ions across
the thylakoid membrane where the photosystems
are embedded now what we can do is look
at
an actual three-dimensional structure
of a particular photosystem this is
photosystem 2
involved in photosynthesis oxygen
photosynthesis in land plants and algae
and as you can see this is a very
complex structure
the pigment molecules are
represented here in space
fill and the
protein by grey backbone the pigment
molecules
are color-coded according to their their
chemical identity
and I'm not going to go into too much
detail about this
structure at this point but you can see
that a photosystem is a complex
of pigment molecules and
associated proteins that orient the
pigment molecules
in an antennal configuration that will
funnel
electrons to the reaction center
at the center of the photosystem
so we talked about the transfer of
energy to chlorophyll A at the reaction
center and we talked about the excited
state electron their
in the
reaction center
and we talked about the excited electron
being transferred from chlorophyll A
then to an electron acceptor
now that excited electron has left
a photosystem in this case and
water then donates electrons to replace
the electrons that are lost
in this excitation and transferring
so the ultimate source of the
electrons then
for this whole process of
capturing light energy elevating electrons to high energy and
then passing them on
those electrons are replaced from incoming
water electrons water is then oxidized
to oxygen
and the processes oxygenic then
oxygenic photosynthesis water
donates electrons
to reaction centers which have lost reaction
which lost
the electrons rather, due to this energy
excitation process
and we can schematically look at this
process as follows:
so here's light being absorbed by
photosystem and
especially that energy being transferred
to the reaction center
that results in an elevated electron
electron elevated in energy
being transferred to an electron
acceptor so
that electron acceptor accepts that high
energy electron
and that will pass the electron on to
other acceptors
as a way to do work to pump hydrogen
ions
that will be used for ATP synthesis in
chemiosmosis as we've discussed previously
but that electron that is now lost from
the reaction center
from the photosystem to an acceptor
of that electron that needs to be replaced
and that is replaced from water
when that happens the energy units
the electron rather is in its low energy
state
and water now loses
electrons and will generate oxygen
as that process proceeds it will be
broken into hydrogen ions and
and molecular oxygen and then we're
ready to restart the process by the
absorption of light
that electron being elevated to high
energy levels
this is the cyclic process but it must be
it's not cyclic in the same way as
certain bacteria
run cyclic photosynthesis here we must
replace the electrons that are lost by photosystems
by the oxidation of water by the donation
of electrons
from water that's why we need to water
our plants
now in oxygenic photosynthesis of the
kind we've been talking about
we'll leave some other non oxygenic photosynthesis
for a few minutes we'll come back to
that but in the major forms of
oxygenic photosynthesis we can
break the light dependent reactions
of photosynthesis the part that's
dependent upon light
into four stages we like to break things
in stages so
let's do that we have a primary photo
event
where a photon of light is captured by a pigment
molecule in a photosystem
and then charge separation involves the
transfer of the energy
absorbed by the pigment
molecule from the photon
we transfer that energy to the reaction
center in a series of steps
tranferring that energy to other pigment
molecules which passed the energy on
and such that an excited electron
eventually is generated in the
reaction center
and that excited electron becomes
transfer to
an acceptor molecule
and that acceptor molecule
is going to be NAD
P+ so
you're used to seeing NAD+
not NADP+ but in this case what we
have is NAD
P+ being the electron carrier and that
NAD+ takes electrons
and becomes NADPH
so NADPH is the reduced form of NAD
P+ so NADPH
is going to carry electrons and pick
up those electrons
eventually from
carriers from electron carriers that are
transporting the electrons initially
donated by photosystem
by a phonesystem now in oxygenic photosynthesis where does that
photosystem get electrons to replace
those that are transferred
to the carriers that eventually get
eventually are involved in the reduction of
NADP+
to NADPH where do those electrons how are
they regenerated
those come in from water the electron
donates
those electrons and that generates
oxygen, oxygenic photosynthesis
so in addition to NADPH being
generated
eventually through electron transport
there's also
the production of ATP by chemiosmosis
because as electron transport passes
electrons
down an electron transport chain
this much like cellular respiration that
electron transport
the lost of electron energy in a series
of transport reactions
can be used to do work and that work is
the pumping of hydrogen ions
across the membrane that the photosystem
and accessory electron
transport proteins are embedded in
and that membrane is the membrane of the
thylakoid
deep within the chloroplast so
we can generate not only
reduced NADP+ that is NADPH
but also we can pump hydrogen ions from
the stroma of the chloroplast inside
the thylakoid so into the internal
part of the thylakoid
and then that
proton gradient can be used by chemiosmosis
to produce ATP using ATP synthase we get
to see ATP synthase again one of my
favorite molecules
so here we have two molecules that
are going
to be produced in this process
of the light dependent reactions
one molecule remember is going to be a
NAD-
PH and the other molecules are going to
the ATP
and these two molecules can be used then to
drive the endergonic reactions
of carbon fixation so we can fix carbon
from carbon dioxide into
carbohydrates like glucose
and this process fixation of
carbon
occur in a biochemical reaction, in a series of biochemical reactions
that we call carbon fixation reactions
or
the Calvin cycle you'll remember the Kreb cycle well this is the Calvin cycle
and we will examine this now everything
up
until the Calvin cycle
everything has been light dependent
reactions
but the fixation of carbon and the Calvin
cycle
are light independent reactions so they
are light
independent and we'll cover those shortly
there are bacteria the green
sulfur bacteria and the purple
nonsulfur bacteria that
do not use water as an electron donor
instead they have cyclic
electron transfer in which the electrons
are not replaced by water but rather are
cycle back
to the photosystem and that's where we'll pick
up
within the next part of this lecture
before moving onto
the two photosystems found in
plants and
some bacteria and
algae
