Download PHOTOSYNTHESIS

PHOTOSYNTHESIS
BEGINS
Hello there, I’m back again to continue my lesson for
you. We will now be studying the Light independent
reaction and integrating it to the light dependent
reaction. I determined this pathway utilizing the
isotope, carbon 14, as a tracer. I worked together with
my colleagues Andrew Benson and James Bassham to
discover the Light independent now called the CalvinBenson reaction. So lets get started!!
As a review from the last lesson, we recall that 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.
On the next slide we will concentrate on the Light
independent reaction (Calvin cycle) and how carbon
dioxide is fixed from the air into organic molecules, but
first I must mention the enzyme Rubisco.
Hi I’m
Most enzymes will process about 1000
substrate molecules a second. Rubisco
processes only about 3. That makes him
very ineffective, but unfortunately, we
need him, and as a result cells need
much more Rubisco than any other
enzyme to accomplish the Carbon
dioxide fixation task.
In order to fix carbon, the enzyme Rubisco attaches carbon dioxide to
RuDP which forms an unstable six carbon sugar. This unstable sugar
immediately breaks down into two PGA molecules. In this cycle, only
one PGA is shown proceeding through. The reason is simplicity which
will be explained later. PGA or phosphoglyceric acid “uses up the
energy” from the ATP. This again is a simplistic version with the
details to be added later. The reducing power of NADPH is then used
to convert the two PGA to PGAL which is an aldehyde . PGAL is then
converted back into RuDP and the energy of ATP is used once again.
This cycle must repeat three times in order to generate one PGAL.
ADP
ADP
ADP
PP
ATP
P
ATP
LIGHT INDEPENDENT REACTION
RuDP
PGA
ATP
used
CO2
3X
NADP+
H
NADPH
NADPH
used
PGAL
H2 O
ATP
used
energy
Did you know?
Rubisco, being the most plentiful
protein(enzyme) in chloroplasts is
probably the most plentiful protein on
earth. Go ahead you do the math!!
You need me to keep photosynthesis
on the planet going. Almost the
entire Earth depends on the capture
of sunlight to to provide the energy
to power all the food chains on the
planet!!
Lets repeat the process this time with the simplistic version of steps this time:
PHASE 1 a) Carbon dioxide is attached to the five carbon RuDP
b)An unstable six carbon sugar is formed breaking into 2 PGA
PHASE 2 a) ATP is used to add energy to the conversion of PGA
b) NADPH is used up as the hydrogen are used to reduce PGA into PGAL
In this process water and excess heat energy are produced.
PHASE 3 a)PGAL is cycled with the addition of ATP to generate the RuDP back again
ADP
ADP
ADP
PP
ATP
P
ATP
LIGHT INDEPENDENT REACTION
RuDP
PGA
ATP
used
CO2
3X
NADP+
H
NADPH
NADPH
used
PGAL
H2 O
ATP
used
energy
Now it is time to get the details of the
Calvin cycle correct and not just use the
simplistic model. Now when PGA
phosphoglyceric acid is converted to
PGAL phosphoglyceraldehyde. See
below:
PGA contains the
carboxylic acid
functional group
PGAL contains the
aldehyde functional
group
The PGA is more correctly 3-phosphoglyceric acid. When the
ATP is used up, the high energy phosphate joins with 3phosphoglyceric acid to form 1,3-phosphoglyceric acid. Once this
is formed NADPH adds the hydrogen to the molecule to reduce it
to PGAL. In order to power this redox reaction, hydrolysis of the
phosphate bond occurs releasing the phosphate.
Pi
ATP
ADP
3-phosphoglycerate
OH
NADPH
NADP
Glyceraldehyde
phosphate
1,3-diphosphoglycerate
Phosphate
H
.
Since this first
carbon compound
has 3 carbons, this
pathway is called
C3 photosynthesis
Lets repeat the process this time with the simplistic version of steps this time:
PHASE 1 a) Carbon dioxide is attached to the five carbon RuDP
b)An unstable six carbon sugar is formed breaking into 2 PGA
PHASE 2 a) ATP is used to add energy to the conversion of PGA
b) NADPH is used up as the hydrogen are used to reduce PGA into PGAL
In this process water and excess heat energy are produced.
PHASE 3 a)PGAL is cycled with the addition of ATP to generate the RuDP back again
ADP
ADP
ADP
PP
ATP
P
ATP
LIGHT INDEPENDENT REACTION
RuDP
PGA
ATP
used
CO2
3X
NADP+
H
NADPH
NADPH
used
PGAL
H2 O
ATP
used
energy
You have probably noticed that in PHASE 3, the
RuDP generation step, ATP is used. When PGAL is
generated, it must provide for regeneration of the
RuDP to attach to the carbon dioxide. In order to do
this, the cycle must proceed three times .
3X
Is shown in the cycle to represent this requirement.
3 x 2PGAL (3C) =18 C
Minus 3 x RuBP (5C) = 15 C
One 3 C PGAL made
Also, RuDP and RuBP are the same chemical,
they. represent ribulose diphosphate and
ribulose biphosphate respectively. They utilize a
different prefix but both Di and Bi mean two.
So, it takes three turns of
the Calvin cycle to make
one PGAL molecule to be
used for biosynthesis.
In order to run the Calvin cycle three
times, the following energy molecules
from the light reaction are used.
6 ATP used to convert all the PGA to PGAL
6 NADPH used to convert all the PGA to PGAL
3 ATP used to regenerate the RuDP
Since one turn of the noncyclic photosynthetic pathway only generates one
ATP and one NADPH, it must cycle 6 times in order to provide the necessary
energy molecules to make one PGAL. In addition, cyclic electron
photosynthesis could be used to make up the additional ATP requirement.
The enzymes that are numbered in the
photosynthetic cycle have names, they are:
ENZYME 1 FERRODOXIN Fd
Ferro means iron.
Iron in this enzyme
ENZYME 2 FLAVOPROTEIN Fp
ENZYME 3 PLASTOQUINONE Pq
ENZYME 4 PLASTOCYANIN Pc
This is the last addition to the
cycle for you to remember.
LIGHT REACTION
Lets complete the entire cycle .
Ferro
doxin
p700
e
Flavo
protein
O2
Split
H2O
H2 O
e
Plasto
cyanin
e
Plasto
quinone
Cytochrome
ADP
P
RuDP
PGA
p680
ATP
ATPATP
LIGHT INDEPENDENT REACTION
ATP
used
CO2
e
H
e
NADP+
NADPH
NADPH
NADPH
used
PGAL
H2 O
ATP
used
energy
Now I have a question for you . What
happens to the production of carbohydrates
if there is no carbon dioxide, but there is an
ample supply of light energy?
Think carefully!!!
Without carbon dioxide, the Calvin
cycle cannot proceed. Thus no sugar is
produced even though the plant could
build up supplies of ATP and NADPH.
In the next slide you will
see the template for the
entire photosynthetic
pathway. See if you can
remember what goes
where and how the cycle
works.
ADP
P
This is Dr. Melvin
Calvin signing off.
Hope to see you in
my lab again!
THE END
This lesson is dedicated to the
memory of Jeff Ryu’s mother.