Transcript PHOTOSYNTHESIS DETAILS

PHOTOSYNTHESIS DETAILS
BEGINS
Hi there kids. I’m here to talk
about some of the details about
photosynthesis now that you have
a good understanding of the
pathway and how it works.
Before I start, I would like to help to clarify some
chemical names we have or will been using in
photosynthesis and show how they are the same
(reused) to those in metabolism.
PGAL is the same as G-3-P or glyceraldehyde3phosphate of glycolysis
PGA is the same as 3-phosphoglyceric acid of
glycolysis
PEP is phosphoenolpyruvate from glycolysis
NADPH is made up of the same subunits as NADH used
in metabolism. NADPH simply has a phosphate added
onto the end. Both are used as electron acceptors.
Please
tell me
you
recall
these
from
my
lesson.
My job
is on
the
line!
Multiply each
plant
photosynthetic
cell by the
amount of
rubisco and you
get the most
numerous
protein on Earth!
From the previous work of
photosynthesis, you probably remember
that I am the enzyme responsible for
attaching carbon dioxide to the RuBP. I
am extremely important as I am the only
enzyme capable of completing this
operation. It’s like my job is guaranteed.
Oh, and did I also mention that I’m lazy
as far an enzymes go. Most enzymes
work about 1000 substrates a second
while I only work about 3 a second. That
means you need a lot of me to
accomplish carbon fixation in a plant!
Senor, remember how the enzymes work.
LIGHT INDEPENDENT REACTION
PGA
1.
RuDP
CO2
O2 is a competitive
inhibitor of rubisco
PGA
That is what I do. I hook these two together,
but consider this. I have a little problem, well
actually big problem. You see I can’t really
distinguish between oxygen and carbon
dioxide. So my active site can bring 1.carbon
dioxide into position to attach RuDP or it could
bring 2.oxygen in to position to attach to RuDP
PHOTORESPIRATION
2.
O2
3 carbon sugar
(remains in Calvin cycle)
RuDP
GLYCOLIC ACID
(useless and released as CO2)
When I hook O2 to RuBP it is called photorespiration.
When I do this the five carbon RuBP splits into a
useable three carbon molecule and a useless two
carbon compound (glycolic acid). Glycolic acid exits
the Calvin cycle and the chloroplast altogether where
it is broken into CO2.
Glycolic acid leaves the chloroplast and enters a
peroxisome. It is then broken down into two
carbon dioxide units without any ATP energy
produced. This is therefore very unproductive for
the photosynthetic process. This drains away
some of the carbon already fixed by the Calvin
cycle. In the soybean plant about 50% of fixed
carbon is lost this way.
Photorespiration occurs only in
light,
consumes oxygen,
produces carbon dioxide and
produces no ATP energy!
Organisms rely on plants to provide
food as reduced carbons in sugars.
Photorespiration is very wasteful
destroying work already done in
carbon fixation!
C3 plants
soybeans
geranium
trees
cucumber
tomato
Most
plants
•C3 plants make up the
majority of plant species.
• Photorespiration occurs in C3
plants.(they only have Rubisco)
•C3 plants are so named
because the first stable
intermediate formed by the
dark reaction is a three carbon
compound called PGA.
Now given conditions where the weather is cool
moist and cloudy, I will work fairly well. I will attach
carbon dioxide as long as oxygen concentrations do
not get too high. Now you might wonder, how do
oxygen concentrations get too high? The
atmospheric oxygen is 21% whether it is cool or not.
Well let me explain:
Stoma
The process of carbon dioxide fixation actually occurs
in the leaf. At the bottom of the leaf are small holes
called stoma that can open or close to allow more or
less air into the interior of the leaf for carbon fixation
to occur. If it is cool and moist the stoma remain open;
thus O2 levels remain the same. If, however it is hot
and dry the stoma close and O2 levels inside the leaf
increase. This is due to CO2 levels that drop (CO2 used
by Calvin cycle) and lack of air entering through the
leaf stoma.
During cool moist and cloudy conditions, the
stoma are wide open allowing the maximum
amount of air into the leaf and thus the carbon
dioxide levels remain relatively high and oxygen
levels do not rise above 21%. So in this
situation, oxygen levels are low.
Open stoma allows O2 out and CO2 in
Cloudy, Cool
Moist conditions
I’m good!
Open Stoma
Low Oxygen
Little photorespiration!
That means higher
photosynthetic productivity!
During hot dry and sunny conditions, the stoma
close and only allow the minimum amount of
air into the leaf. The carbon dioxide is used up
by the Calvin cycle and oxygen levels rise above
21%. So in this situation, oxygen levels are
high.
Closed stoma decreases CO2 and thus increases the O2
content. Rubisco will begin photorespiration.
Sunny hot and dry weather
Closed Stoma
High Oxygen
I’m bad
I turn traitor, and instead of
attaching CO2 to RuBP I attach
O2. This results in lots of
photorespiration and thus lower
photosynthetic productivity!
PHOTORESPIRATION!!
In order to reduce the loss as a result of my poor performance
in hot sunny and dry conditions, some plants have evolved
different pathway s. There are both C4 and CAM plants.
Hello there. I’m
Hello there. I’m Mrs.
señor cactus.
Maize and I am a C4
That gringo they
plant. I can’t begin to
call Rubisco, if
tell you how
you try to make
disappointed I was with
him work in the
that Rubisco fellow. My
hot desert he is
species and all other
useless . I’m a
members of the C4
member of the
group, have evolved a
CAM plant group
pathway to circumvent
and we have
his bad habit of
developed a
photorespiration. I’ll
better method of
tell you my secret in a
carbon fixation.
minute.
We C4 plants use PEP carboxylase and PEP
instead of using Rubisco to hook CO2. PEP
carboxylase is the enzyme that attaches CO2 to
the 3 carbon sugar, PEP (phosphoenolpyruvate).
I’m so glad we found PEP carboxylase who is
vastly superior to Rubisco because of his
higher affinity to attract CO2 and his loyalty.
He will not turn traitor and bond O2. I don’t
know what I would do without him as I live in
hot dry and sunny environments. So we C4
plants stop photorespiration by using PEP!
Fix
carbon
at
night
Photosynthesis
during
day
We CAM plants are also called
succulent or water storing plants.
We only open our stoma at night
and then we attach CO2 to a variety
of organic acids. Since the only time
evaporation is low in the desert is at
night, this works well for us. We
store the fixed carbon in the organic
acids until daylight when we can use
the light reaction to power up the
Calvin cycle and reduce the carbons
to sugars. Thus water loss is
minimized and photorespiration is
stopped.
Did you know?
Bring a blanket—the desert is not always
hot ---At night the desert is cold
Colder temperatures mean less
evaporation and more moisture in the
air. Less water loss from stoma if they
are open.
We are three from the
East. We travel by night
and follow the star
So in summary and
scientific lingo
Sugar cane is
a C4 plant
1. C4 plants preface the Calvin cycle with reactions that
incorporate CO2 into four carbon compounds. Instead of
using Rubisco and RuBP to hook CO2, they use PEP
(phosphoenolpyruvate) and the enzyme PEP carboxylase.
PEP carboxylase is vastly superior to rubisco because is has a
higher affinity to attract CO2 and it has no affinity to bond
O2. So these plants do not allow photorespiration and
rubisco cannot turn traitor!
2. CAM plants are also called succulent or water storing
plants. These plants only open their stoma at night and
the CO2 is incorporated into a variety of organic acids.
Thus water loss is minimized and photorespiration is
stopped.
Cactus is a
CAM plant
So do you think I got fired from my job by
the C4 and CAM plants? Think again!
We C4 plants still have to do the Calvin cycle, so we still need
Rubisco to fix carbon the RuBP. We accomplish this by locking
Rubisco up in a place where there is no access to oxygen.
Rats, these bundle
sheath cells make it so I
can’t get any oxygen .
C4 plants have a special anatomy. The mesophyll cells
have access to the air and they incorporate our hero PEP
who will reduce CO2. Bundle sheath cells do not have
access to the air and are located beneath the mesophyll
cells. PEP can pass the CO2 to Rubisco who, without
oxygen, behaves himself and conducts the Calvin cycle
without any photorespiration. The sugars produced then
enter the vascular tissue.
This is my C4
or Hatch Slack
pathway.
Notice how
air is only
accessible to
mesophyll
cells. The
bundle sheath
cells contain
Rubisco and
receive CO2
from Malate.
Air here
Carbon dioxide
Oxygen
Carbon dioxide
Oxygen
PEP
carboxylase
Mesophyll
cell
plasmodesmata
Bundle Sheath Cell
Mesophyll
cell
Bundle Sheath Cell
Rubisco
Rubisco
Sugar
Sugar
No oxygen
Vascular tissue contains sugar water
No oxygen
The C4 pathway is not free. CO2 is actually fixed
twice and that costs energy in the form of more
ATP. The C3 pathway requires 18 ATP for the
synthesis of one molecule of glucose while the C4
pathway requires 30 ATP.
But if you use me, you will lose
about 50% of your fixed carbon to
photorespiration in hot dry and
sunny environments !!
Minimizing the loss of fixed carbon makes me
more efficient than C3 plants in hot dry
environments even though additional energy ATP
is required.
I you hear about the Krantz Anatomy,
it is just talking about my bundle
sheath cells and the fact that the
thylakoids are not arranged in grana.
The grana are arranged in a wreath
like appearnace and likely concentrate
the CO2 around Rubisco to enhance
uptake. Since the mesophyll cells do
have grana they made up a special
word to describe this chloroplast
arrangement its called dimorphic
Today, C4 plants, which grow
primarily in the Tropics
(Latitude 450 and less)
represent about 5% of Earth's
plant biomass. C4 plants are
only 1% of the total plant
species, yet they account for
around 30% of terrestrial
carbon fixation. Pretty good
work huh?
Before you go any further, think about evolution of plant
photosynthesis. Photosystem I probably evolved first to produce
ATP followed by Photosystem II and the Calvin cycle to provide
the ability to conduct biosynthesis of chemicals to build cells.
Rubisco is an ancient enzyme likely from a time when the Earth did not possess oxygen in
the atmosphere. Given those conditions, the inability of the enzyme’s active site to
distinguish CO2 from O2 would have not made a difference.
About 2.5 billion years ago, the plants increased atmospheric oxygen content and then
Rubisco could combine oxygen resulting in photorespiration.
Since the Calvin cycle pathway had already been established for billions of years of
plant photosynthesis, plants retained the existing pathway and simply put methods
in place to minimize the photorespiration. It looks like the C4 pathway evolved
several times in different plant species. This is called convergent evolution.
This is a similar pattern to cells retaining the ancient glycolysis pathway and merely
extracting energy more efficiently from the end product via Krebs and ETC.
I’m telling you, there’s really a lot of important and
relevant information in my lesson. Skip ahead, your
teacher won’t mind. Look for the lesson First Cells in
the evolution section
Now, señor, let me tell you
about my CAM pathway
Can you
remember
the Krebs
cycle
chemicals
like malate
and
oxaloacetate
?
•CAM stands for Crassulacean acid
metabolism named after one of my plant
families.
• I close my stoma during the day to
conserve water.
• At night I open my stoma to collect CO2
and attach it to organic acids from the Krebs
cycle and store them in vacuoles.
•When the sun rises and photosynthesis
starts, I use the ATP and NADPH generated
to supply the Calvin cycle with energy. The
organic acids release CO2 to Rubisco to
supply the Calvin cycle.
Hey, what category plant is this Agave
stuff? C3, C4 or CAM?
Agave is a CAM plant.
Pineapple also is a CAM plant.
Jade is a CAM and actually a member of the
Crassulaceae family. Sound familiar?
How about this Jade plant?
How about
this
pineapple?
Now there are two colours on these
graphs. Which do you think belongs to C4
plants like me and which to C3 plants?
C3 is the yellow line
C4 is the green line
Now can you explain this
graph next to me? Please
utilize the information
you now know.
In temperate regions, the lower light intensity, and cooler temperatures act
to the disadvantage of C4 plants. C3 plants have an advantage due to their low
rate of photorespiration and because they need no energy for the preliminary
fixation of CO2 by PEP.
In tropical regions approaching the equator, the higher light intensity
and hot temperatures set the C4 plants at an advantage reflected by
the increase in net yield of fixed carbon sugars. C3 plants suffer from
increasing amounts of photorespiration.
So long!
Good
Bye
Adios
END PHOTOSYNTHESIS DETAILS
Photosynthesis – working everyday
to provide you with food.