All right and let's look at the Calvin cycle directly.
We're going to consider six terms of this cycle in which we take six molecules of carbon dioxide and fix all of them into organic molecules.
So let's go through this cycle first in an overview sense.
We can divide it into phases: the carbon fixation phase which, as we've said, occurs in the first biochemical reaction of the pathway,
and then the use of ATP energy and NADPH oxidation in the reduction phase of the carbon cycle to produce molecules of glyceraldehyde-3-phosphate
and we've glyceraldehyde-3-phosphate; that was an intermediate in the glycolytic pathway
and glycolisis - this was the intermediate produced by clevage of a six carbon compound into two three carbon compounds
and for every six terms of this cycle, we will generate two molecules of glyceraldehyde-3-phosphate that can be converted into glucose and other sugars.
So we need six turns of this cycle, we need to add six carbons coming from 6 carbon dioxide molecules
in order to generate two molecules of three carbons each of glyceraldehyde-3-phosphate that will be then fixed in glucose
and that makes sense because if we have two molecules of glyceraldehyde-3-phosphate - thats two times three carbons equal 6 carbons -
and we're using the six carbons from six
molecules of carbon dioxide.
That is we need six turns of this cycle to generate our two molecules of glyceraldehyde-3-phosphate.
It will then be converted to glucose and other sugars.
Now for every six turns, we'll actually generate 12 molecules of glyceraldehyde-3-phosphate
two of which will be incorporated into sugars and ten of which will continue on in the pathway for every six turns of the cycle
and here we need ATP energy again to produce Ribulose 1-5 biphosphate, that is RuBP,
the molecule to which carbon would be added from carbon dioxide to form 3-phosphoglycerate in the first step of the cycle again.
So the last phase of the Calvin cycle is the regeneration of RuBP, or Ribulose 1-5 bisphosphate
and this instance then represents the fixation of carbon - the whole goal of photosynthesis in the first place -
and the required molecules are 12 ATP here, six ATP here, and 12 NADPH's.
So we have a ratio of 1.5 ATP's used per every one NADPH used to drive the cycle, to this fuel cycle.
And where do these molecules come from?
Of course they came from the light dependent reactions using photo-systems two and then photo-system one
and these reactions are light dependent, as we've already discussed.
Now we're ready to look at the relationship between cellular respiration and photosynthesis that we've covered
and this picture of the Calvin cycle here actually is a good segue into that because if you'll notice
glyceraldehyde-3-phosphate here, 1-3 biphosphoglycerate, and three phosphoglycerate are all intermediates in the glycolytic pathway
but here glycolosis would actually run in this direction.
These three compounds would be produced in glycolosis in this direction but in photosynthesis, in the Calvin cycle, they are being produced in this direction
and this brings up the notion that in fact the biochemistry of photosynthesis and the biochemistry of cellular respiration are intimately related
and in fact we know that photosynthesis evolved before cellular respiration did
and that it's not surprising then that evolution would seize on some of the same enzymes and the same genes that encode those enzymes
to function function in cellular respiration.
So let's look at a few slides here now by way of review.
These are text slides and you can freeze the movie and examine them as long as you'd like
and then we'll move on to the relationship between photosynthesis and cellular respiration.
So by way of review...
And here we have our schematic of the relationship between these.
Notice that cellular respiration produces water, as you know water is released in respiration as is carbon dioxide,
and these are starting compounds that are needed for photosynthesis.
Likewise, photosynthesis produces oxygen and sugars which glucose here enters in the breakdown for respiration.
So if all of this is occurring in a plant cell, if we imagine this in a plant cell,
we have respiration going concurrently photosynthesis and these these two placids, in a way - the mitochandria and the chloroplasts -
are providing each other with the compounds they need to conduct their respective biochemistry.
*coughs* Excuse me...
And you'll note other similarities as well.
You notice that for example the ATP synthase found in the inner mitochandrial membrane is evolutionary closely related to the ATP synthetase
found in the thylakoid membrane of
chloroplasts.
So there are molecular homologies the deep biochemistry of these two processes
and it reflects the ability of evolution to seize upon existing biochemistry is to modify them for new purposes.
In animals, of course, or in cells that can't photosynthesize, they need to obtain their glucose from some other source;
they can't obtain them directly in the same cell from photosynthesis.
So how do they do that? They have to eat things.
They have to consume other animals or plants to obtain the glucose required for cellular respiration.
*coughs* Excuse me...
Aright, now we're ready to consider another feature of photosynthesis and that is photorespiration
and that involves the Rubisco enzyme so let's talk about that in just a moment.
Photorespiration is a result of the enzyme Rubisco catalyzing not the first step of the Calvin cycle
but instead of oxidizing Ribulose 1-5 biphosphate.
In addition to conducting the first step of the Calvin cycle, the Rubisco enzyme is capable of adding oxygen to RuBP and eventually releasing in fact CO2.
CO2 is released and RuBP is oxidized.
CO2 is produced and Rubisco is of capable conducting this biochemistry as well
and in fact Rubisco plus oxygen oxidizes RuBP and produces co2, so it actually removes carbons from RuBP.
So this is counterproductive to fixation of carbon; we're actually releasing carbon in this biochemical reaction
and the oxygen and the CO2 which are the
substrates for either the first of the Calvin cycle
or alternatively Rubisco's catalized oxidation of RuBP releasing carbon dioxide.
The carbon dioxide and oxygen actually compete for the active side for Rubisco
So even under normal conditions, a 25 degree centigrade about close to a quarter or 20%
of the activity of Rubisco actually acts against the fixation of carbon and in fact oxidizes RuBP.
So that's even under normal conditions, but under hot conditions the situations you're so much look at why that might be true.
So if we look at a leaf, for example, the photosynthetic cells are under the cuticle of the leaf
and in order to conduct gas exchange, there are openings in the belief that are regulated by guard cells
and these are called stomata - singular would be stoma, plural would be stomata -
and for example under normal conditions there's gas exchange throuigh these stomata, oxygen is released, and carbon dioxide is brought in.
But water can also leave through these stomata and can dry out the leaf there's loss of water through evaporation and so the stomata close under hot conditions.
So there are plants that have adapted a mechanism to close these guard cells, to constrict these guard cells around these stomata openings
but that leads to a problem under hot
conditions.
When that happens, oxygen, being produced by photosynthesis, builds up in the leaf and carbon dioxide cannot enter the leaf
and as you know, we need carbon dioxide to enter cells in order to be fixed into glucose molecules
in the process the of light independent reactions of photosynthesis.
So we have a build up of oxygen and a deficit, if you will, of carbon dioxide
and from what I've just told you about photorespiration, the Rubisco enzyme under conditions of high oxygen and low carbon dioxide
will favor the oxidation of RuBP and not the fixation of carbon, so on we've got a problem here.
We have photorespiration - this this process is called photorespiration -
so we're oxidizing RuBP by adding oxygen and in fact we're releasing CO2; were doing the opposite of fixing carbon in other words.
And since C02 and 02 are competing for the active site on Rubisco
then we've got a special problem under hot conditions because in the leaf we have high concentrations of oxygen,
lower concentrations of carbon dioxide
and that will favor the oxidation of RuBP.
So plants need to find a way to, at least, have mechanisms that will combat this photorespiration, which decreases photosynthetic yields significantly.
So there are so several adaptations that allow this.
Certain plants use an enzyme other than Rubisco that they use Phosphoenolpyruvate carboxylase,
in which this enzyme catalyzes the addition of carbon dioxide or carbon to Phosphoenolpyruvate, PEP,
and a four carbon compound is produced instead of the normal three carbon compounds that result in the first step in the Calvin cycle.
And then CO2 carbon to be released from that four carbon compound later and can be used by Rubisco in the normal Calvin cycle.
And these types of plants are called "C4 plants."
These are c4 plants because instead of producing a three carbon compund immediately,
a four carbon compound is produced by PEP carboxylase prior to the production of a three carbon compound that can be used by the Calvin cycle.
So c4 plants use PEP carboxylase to capture co2 and temporarily store it and then the carbon dioxide can later be stripped from the four carbon compund
and moved to another cell where the CO 2 is released and used in the Calvin cycle as a normal plant would.
And that's where we'll pick up with in the next part of this lecture.
