Download PHOTOSYNTHESIS

PHOTOSYNTHESIS
BEGINS
Hello there, I’m Dr. Melvin Calvin speaking from my
famous lab at the University of California, Berkeley. I
will be presenting the process of photosynthesis to you.
You probably already know I hold the 1961 Nobel Prize
for chemistry and thus am quite qualified to present this
lesson to you. I am particularly famous for my work on
the Light independent (Calvin-Benson reaction). So hold
on for a wild ride on the photosynthesis pathway!
CYCLIC ELECTRON FLOW
On the next slide, you will see how the
plant can conduct photosynthesis to
generate ATP. Thus plants can provide
ATP utilizing light energy. The method
utilizes an electron that is boosted to a
higher energy level by the photon. A
model of the reaction center of a
photosystem is shown below.
Photon
energy
captured by
electron
Makes ATP
No OXYGEN produced
No NADPH produced
Electrons flow back to
origin at chlorophyll a
Reaction
center
p700
Photosystem I
group of
chlorophyll called
antenna assembly
p700
ee
Enzyme
1
e
Enzyme
4
Cytochrome
ADP
P
ATP
Lets start with CYCLIC PHOSPHORYLATION or Photosystem I
and watch the electron. The black colour represents the low
energy electron and the red colour the high energy electron.
One more time for CYCLIC PHOSPHORYLATION or Photosystem I
p700
ee
Enzyme
1
e
Enzyme
4
Cytochrome
ADP
P
ATP
Notice how the photon hits P700 boosting the electron to the
higher orbital. It is then carried by enzyme 1 to the cytochrome
where it’s energy is used to convert ADP into the higher energy
ATP. The electron then becomes low energy (black) and is carried
by enzyme 4 back to p700 where it will repeat the process again.
Remember
the sugars we
built together.
We used
carbon and
hydrogen plus
some oxygen
to make
glucose!
Now it is your responsibility to memorize and
understand the cyclic photophosphorylation
pathway. This pathway will generate ATP energy
for cells but it does not provide the most
important function of the photosynthetic
process .. THE ABILITY TO BIOSYNTHESIZE! Plants
must be able to make biomass like sugars that
can be turned into cellulose for their cell walls or
other important chemicals. In order to do this
they need a carbon source and a source of
hydrogens to attach to the carbons .
The next slide describes a process essential to
getting photosystem II started. Photosystem II is
the pathway for generating the hydrogens and
creating the reducing power needed to conduct
biosynthesis reactions.
Recall from your earlier work on acid base
chemistry the ionization of water shown by the
equilibrium equation below.
H2O
In Photosynthesis these H+ ions are
removed by p680 which has a great
affinity for electrons. An enzyme called
the Z protein speeds up the reaction.
Since p680 strongly attracts the
hydrogen, and removes it from the
equation , more water splits in order to
replace the disappearing hydrogen
ions. This results in the continuous
splitting of water molecules called
photolysis.
H+ + OH This happens in normal
water as discussed in acid
base chemistry such that the
H+ concentration is 10-7. This
value remains constant
unless something disrupts
the equilibrium.
Lets take a closer look at how this water is split by PHOTOLYSIS.
H2O
H+ + OH H
Remove 4 H+
to be used
for
reduction.
H
H
H
Then 4 OHremain.
What
happens to
these?
O H
O H
O H
O H
These form
one oxygen
gas molecule
O2
These form 2
water molecules
Powering this whole
process is the p680
which strongly attracts
the low energy
electrons, causing the
water to continually
split apart
H2O
e
p680
Therefore splitting the water
molecule results in the
following:
4H2O
4H+
O2
2H2O
Low energy
electrons
Now we have the starting point for photosystem II.
This cycle starts with the splitting of water and the
p680 chlorophyll molecule located on the far right
hand side of our photosynthesis diagram.
The electrons in the photosynthetic cycles actually
travel as electron pairs (two electrons). For simplicity,
the model only shows one electron.
LIGHT REACTION
p700
e
Now lets follow the pathway for Photosystem II or Noncyclic
Photophosphorylation
Enzyme
1
Enzyme
2
O2
Split
H2O
H2 O
e
Enzyme
4
e
Enzyme
3
Cytochrome
ADP
P
ATP
p680
NADP+
Notice that the low energy electron starts at p680, is hit by the photon and
becomes high energy. It then is carried by enzyme 3 to the cytochromes
where its energy is converted into ATP and the electron becomes low energy.
It is passed to enzyme 4 and then to p700 where it is hit by another photon
and turns high energy. It then progresses through enzyme 1 and enzyme 2 to
be used as energy to attach the hydrogen to NADP which will be used to
reduce carbon later in the Light independent Reaction (Calvin cycle). It then is
low energy and drops below the dotted line, entering the Calvin cycle
e
e
H
NADPH
LIGHT REACTION
p700
e
Lets go through the cycle once again
Enzyme
1
Enzyme
2
O2
Split
H2O
H2 O
e
Enzyme
4
e
Enzyme
3
Cytochrome
ADP
P
ATP
p680
NADP+
Notice that the low energy electron starts at p680, is hit by the photon and
becomes high energy. It then is carried by enzyme 3 to the cytochromes
where its energy is converted into ATP and the electron becomes low energy.
It is passed to enzyme 4 and then to p700 where it is hit by another photon
and turns high energy. It then progresses through enzyme 1 and enzyme 2 to
be used as energy to attach the hydrogen to NADP which will be used to
reduce carbon later in the Light independent Reaction (Calvin cycle). It then is
low energy and drops below the dotted line, entering the Calvin cycle
e
e
H
NADPH
At this point the light reaction
has generated ATP for energy
and NADPH which has vast
reducing power. These two
products will be used in the
Calvin cycle in the
biosynthesis of the organic
molecules.
See the complete cycle in the next lesson!!
End Photosystem 1 and 2
Lesson continues with the Calvin
cycle