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Click here for interactive tutorials (AP Bio level) about photosynthesis

Organisms like plants that photosynthesize
Chemically combine water with carbon dioxide
Making carbohydrates and oxygen too,
That’s why photosynthesis should matter to you!

The light reactions are photosynthesis’s first phase
The output’s ATP and NADPH
O2‘s the by-product, and as you can see
The inputs are water, and light energy.

Phase 2 is the Calvin Cycle makes carbohydrate,
Using up the ATP and the NADPH,
While absorbing CO2 (it’s carbon fixation)
Photosynthesis, what a sensation

Light reactions
happening in leaves
Making oxygen
Through a solar-charged
current of electricity
Converted into chemical energy

Chloroplasts are the photosynthesizing organelle.
You can find them in leaves, in a mesophyll cell,
Inside chloroplasts is a fluid called stroma,
it’s true everywhere, from Tokyo to Roma.

The stroma bathes tiny sacs called thylakoids,
Membrane bound sacs, with a tiny space inside,
The thylakoid’s membrane is loaded with protein
It’s the light reactions’ main scene

It also has pigments like green chlorophylls
Orange carotenoids and yellow xanthophylls
These pigments absorb light that’s mostly red and blue
The green is not absorbed it reflects back to you.

Every chlorophyll has a porphyrin ring,
With magnesium atoms in the that center of that thing.
In the light reactions that magnesium gets pinged,
By photons, making its electrons zing,

Notice the chlorophyll’s tail — a hydrocarbon chain
Which anchors chlorophyll into the thylakoid membrane.
A molecule with style, so pretty,
In the thylakoid, it makes electricity.


Engelmann showed us the action spectrum
He used a prism to break the sun’s
Light into its many different colors,
Shone it on an algae called Cladophora

TheO2 algae makes will stimulate bacteria
Which will grow in any oxygen rich area
Bacteria loved it over blue and red
But hardly grew over green, they might as well be dead!

This line of growth, it’s a reflection,
Of photosynthesis’s action spectrum
Note this doesn’t match exactly
The absorption spectra of chlorophyll a or b.

‘Cause the carotenoids and xanthophylls we´ve met
Absorb light frequencies the chlorophylls can´t get
And in the thylakoid, they all cooperate!
In making ATP and NADPH