This is the third in a three part series about the light reactions of photosynthesis.

Scroll down below the video to see the lyrics.

Now back to our electrons, moved by the sun
Flowing from Photosystem 2 to Photosystem 1,
They get to PS 1 like a used-up battery
The transport chain used up their energy.

But when light hits PS 1’s antenna
Electrons bounce to reaction center
This one’s P700,
That wavelength makes electrons go.

PS 1’s electron acceptor grabs them away
Leaving P700 in an oxidized way
And P-seven-O-O with its oxidized blues
Accepts the electron from PS 2.

Back to electron from PS1
It now goes for a ride having its own fun,
But powering proton pumps ain’t its fate
It travels to NADP+ reductase.

An enzyme chloroplasts count upon
To pass some H and electrons
To NADP+ which gets reduced
It’s how NADPH gets produced.


O2 is the by-product of the light reactions,
Oxygen production, here’s how it happens
Remember back in PS 2 it came to arise
Reaction center P680 got oxidized?

To replace lost electrons P680 liberates
Electrons from water which dissociates
Into one oxygen and protons two
The single O will meet another forming O2.

These protons are formed in the thylakoid space,
Increasing proton concentration in that place.
So we see a side effect of water’s oxidation
Is enhancement of ATP creation!

Non-cyclic flow is also called the Z-scheme
Put “Z” on its side and see what I mean,
Electrons rise in energy, then fall then 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
Keeping 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 2, you see
These energized electrons flow and release energy
Which as you know is used to synthesize ATP!