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Photosynthesis
Non Cyclic Electron Flow and The Calvin Cycle
Amusical Lecture by Glenn Wolkenfeld
Thanks for joining us for Episode Two,
Wher more ‘bout photosynthesic waits for you.
We last saw how plant cells create, O¬¬2, ATP, and NADPH.
Now we’ll start with a twist on the light reactions,
Called cyclic flow, we’ll see how it happens.
Then we will investigate how the Calvin Cycle makes carbohydrate.
(Part I, Cyclic Electron Flow)
Non – cyclic flow is also called the “Z – scheme” Put Z on its site and see what I mean.
Electrons rise in energy then fall and rise again.
Making ATP and NADPH my friends.
A variation on this is cyclic flow.
Which makes ATP, no NADPH or O.
Cyclic flow results from the Calvin Cycle’s needs.
It uses less NADPH than ATP.
NADPH buildup has the repercussion, of inhibiting NADP+ reduction.
‘cause NADPH clogs up reductase.
Keepping NADP+ from moving into place.
So electrons don’t go to NADP+.
They’re clogged, stuck like a pimple’s pus.
Where can they go from P700?
Nature evolved them a place to flow.
A shunt pathway moves them to the ETC.
I’m talking ‘bout the one in PS II tou see.
These energized alectrons flow and release energy.
Which as you know is used to synthesize ATP.
( Part II: Calvin Cycle)
Now it’s time to learn the cycle named for Calvin.
Which results in the fixation of carbon.
Consider that each carbon in your bones, blood and hair,
Was once in a CO2 in the air.
The Calvin Cycle has three parts.
Fixation of carbon is how it starts.
The molecules produced rae then reduced.
‘cause reduction is the key thing that happens in part 2.
Part 2 is the payoff as you will see.
It produces molecule G3P
A reduced molecule with lots of energy.
And a starting point for all the molecules that cells need.
Part 3 sets up conditions for the cycle’s start.
By recreating what’s at the cycle’s heart.
RuBP (Ribulose Bisphosphate), a five carbon molecule that seems to apparate.
‘Cause RuBP’s at the cycle’s start.
Then reappears at the very last part.
The role of RuBP at the start and end.
Is why this is a cycle like the one named Krebs.
The cycle’s inputs are know to you: NADPH and ATP and CO2
Fixation of carbon is catalyzed by RuBP Carboxylase Enzyme
This enzyme’s nickname you should know.
Sounds like a cookie maker: it’s RUBISCO.
An enzyme so old it was already here,
Before O2 accumulated in the air.
The CO2 enters through leaf stoma, diffusing through the leaf and arriving at the stroma.
There RuBisCO grabs it in its active site, and squeezes it with RuBP tight.
RuBP has carbons five, it has two phosphates, one on each side.
When CO2 combines with RuBP it makes a six carbon compound you won’t see.
It instantly dissociates into two 3 – phosphoglycerates,
Each with three carbons and one phosphate.
Each gets phosphorylated to bisphosohoglycerate.
Phosohorylated means that each receives aphosphate group from ATP.
This phospharylation, is followed by reduction.
And you know there’s an enzyme with the function.
Of taking electrons and H from NADPH and adding them to to bisphosphoglycerate.
Take off a phosphate for Glycealdehyde – Phosphate.
A starting point for compounds like carbohydrates.
(Part III: Calvin Cycle, conclusion)
Glyceraldehyde- 3 – phosphat, known as G3P,
as a product cells harvest, and I hope you see,
how light reactions NADPH and ATP,
are needed to fix CO2 and make organic energy.
And to complete the cycle we must now regenerate.
The starting point of Calvin Cycle ribulose bisphosphate.
The intermediates involved in taking G3Ps,
And recombining them into RuBPs.
Are too much for freshman biology.
You can get the details when you get yors Ph. D.
But know for now that to get 3 RuBPs from 5 G3Ps cost 3 ATPs!
(Part IV: Conclusion)
Now this journey ends whit its emphasis,
On the reaction of photosynthesis.
For over four billion years it’s been the food chains basis.
In fossils from that era you can see the traces,
Of tiny cells with prokaryotic form.
Stromatolites that lived in waters warm.
Through photosynthesis they transformed our planet,
So aerobic life can live upon it.
And so evolved sentient beings who can analyze,
How photoautotrophs can photosynthesize.
You could say that photosynthesis gave birth to codiyions for cons ciousness on Planet Earth!
