PHOTOSYNTHESIS
by Arthur W. Siebens, Ph.D., Copyright 1999
(to the tunes of "Parigi, o cara" from LaTraviata by Giuseppe Verdi
and "La donna e mobile" from Rigoletto by Giuseppe Verdi)
(to the tune of Parigi, o cara)
Chloroplasts can take some light, transform its energy
Through the light (-dependent) and dark reactions (light-independent), make glucose for you and me
It starts with photosystem II, 2 electrons get a boost
Chlorophyll steals two more from water, oxygen's produced.

The electrons then pump protons as to photosystem I they flow on
Through photophosphorylation
There's ATP formation.

This ATP is synthesized through chemiosmotic proton flow
From the electron transport chain
Across the thylakoid membrane
By ATP synthase, you know.

Much of the energy remains, now the electrons are boosted high
By photosystem I to an electron acceptor, then they backslide (down a second electron transport chain)
The final carrier of electrons two, is NADPH
The light reaction's through-but we've got glucose to create.

(to the tune of La donna e mobile)
The Calvin cycle takes a CO2 (in the stroma)
Adds it to 5 carbons1 (ribulose bisphosphate) then splits in two (phosphoglycerate, 3C's)
With energy from 3 ATP's
2 NADPH's complete the deed (donating 2 high energy electrons),
Glyceraldyhde phosphate (P-Gal) will emerge (after 3 turns of the Calvin cycle)
Glucose (6 C's) results when two (P-Gals) converge
That's the dark reaction
But O2 can cause a detraction.

Rubisco likes CO2, but it also accepts O2
If CO2 is low and O2 is high (e.g., within the leaf with stomata closed on a hot, dry day)
Carbon fixed is kissed good-bye (up to 50%)
In photorespiration there's fixed carbon elimination
CO2 's released, the chloroplast is fleeced!

(To the tune of Parigi, o cara)
Certain plants avoid this fate-they use isolation
Carbon's fixed in 4-carbon acids2 away from the Calvin cycle's exploitation
C4 plants use space3, CAM plants use time4, they control CO2 release
The Calvin cycle's still used, but fixed carbon's increased.

(to the tune of the end of La donna e mobile)
Pho-to-syn-the-sis is increased!
Pho-to-syn-the-sis is increased!

1 By the enzyme ribulose bisphosphate carboxylase (rubisco), probably the most abundant protein on earth.
2 C4 plants use mainly oxaloacetate and malate, CAM plants mainly malate and isocitrate.
3 C4 plants (e.g. corn) fix carbon into organic acids in cells lacking the Calvin cycle (mesophyll cells), then transport the acids to other cells containing the Calvin cycle (bundle sheath cells).
4 CAM plants (e.g., cacti) fix carbon into organic acids in their mesophyll cells at night with their stomata open. The next day CO2 is released and incorporated into sugars (with stomata closed and the light reactions supplying ATP and NADPH).