hey guys this is learn biology.net I'm
Frankie and in this revision session
will cover specification reference three
point five point one this is topic five
a and your learning outcomes are number
one explain the steps and the light
dependent reactions of photosynthesis
number two understand the roles of
photosystem two photosystem one NADP
reductase and ATP synthase and number
three understand a proton gradient is
created in the thylakoid which is used
by ATP synthase to generate ATP from ADP
and inorganic phosphate so let's begin
in the light dependent reaction plants
use photosynthesis term one in a
sequence of events to produce ATP and
NADPH this whole process is called non
cyclic photophosphorylation electrons
are replenished via the photolysis of
water chlorophyll in photosystem two
absorbs light energy which excites
electrons to a higher energy level the
electrons that leave the chlorophyll are
replaced with electrons from the
photolysis the oxidation of water these
higher energy electrons are transferred
via electron carriers to other important
proteins embedded in the thylakoid
membrane such as the enzyme nad p
reductase which catalyzes the reduction
of nad P to NADPH reduced nad p ATP
synthase which takes advantage of the
proton gradient created in the thylakoid
and as protons are pumped back into the
stroma down their concentration gradient
adp is phosphorylated to form ATP so
this first diagram is going to show the
events of step one so let's explain this
diagram step number one as light energy
as absorbed by photosystem two which
absorbs light best at a wavelength of
680 nanometers photosynthetic pigments
such as chlorophyll a become excited
that is the electrons now have more
energy and they are released from the
chlorophyll no these high-energy
electrons are passed to a mobile
electron-carrier which begins the
transfer of these electrons down the
electron transport chain water is
oxidized ie it loses electrons from the
photolysis of water when light energy
splits the water molecule into protons
electrons and oxygen the electrons from
water can now enter photosystem 2
replacing the lost electrons from the
chlorophyll so my next diagram showing
step two shows a couple of other players
in these events we have a proton pump we
have another electron carrier to
continue the transfer of these electrons
and we can see that we are creating a
proton gradient a high proton gradient
so step number two a proton pump also
embedded in the thylakoid membrane
receives electrons passed through it
from one electron carrier to the next
this proton pump uses the energy from
the electron transfer to pump protons
from the stroma into the thylakoid
creating a high proton gradient within
the thylakoid the transferred electrons
have no lost energy these electrons
replace the electrons which will be
removed from chlorophyll in photosystem
one which is shown in the next step and
so this diagram is showing the
continuation of these events of the
light dependent reactions where the
electron carrier is replacing the lost
electrons from photosystem 1 and light
energy which re-energizes electrons to
an even higher level which are also
transferred to the electron carrier and
then those electrons get utilized by an
enzyme called nad p reductase to
generate some NADPH for the calvin cycle
so step number three light energy is
absorbed by photosystem one which
absorbs light best at a wavelength of
700 nanometers when photosystem one
absorbs light energy electrons are
excited once more but this time to an
even higher energy state these super
excited electrons are again passed to an
electron carrier which then transfers
these electrons to an enzyme nadph
reductase an ADP reductase donates these
high-energy elect
to a molecule of NADP in the formation
of NADPH the enzyme also catalyzes the
reduction of nad P forming NADPH and in
doing so also contributes to a lower
proton gradient in the stroma as it
takes those protons and my final diagram
here showing the last step is the
utilization of an enzyme called ATP
synthase which takes advantage of the
high proton gradient in the thylakoid to
generate ATP so step number four excited
electrons lose energy as they move down
the electron transport chain as we have
seen some of this energy is used to pump
protons into the thylakoid now the
thylakoid has a higher proton gradient
than the stroma the protons move down
their concentration gradient from the
thylakoid into the stroma via the enzyme
ATP synthase which takes advantage of
the energy from the proton movement and
catalyzes ATP from ADP and inorganic
phosphate ATP and NADPH are the final
products from the light dependent
reactions and they now go on to the
Calvin cycle so I hope my overview of
light dependent reactions have been
useful to you and I'll see you in the
next lesson when we go through the
calvin cycle until then take it easy
