Photosynthesis Cheat Sheet - Just the highlights of Photosynthesis
General formula Light (photons) + CO2 + 6 H2O → C6H12O6 + 6 O2 0
Overall Point: Photosynthesis uses CO2 , light, and water to create sugar (C6H12O6 ).
Note: Oxygen gas is just a byproduct, yes it is important to us, but it is just a waste product of this
reaction.
1. Light Reactions (or the Light Dependent Reactions)
Where does this step occur?: On the thylakoid membrane in the chloroplast
Overall Point: To trap energy from sunlight and make NADPH and ATP. These will be used to power the
Dark Reaction.
Light (photons) + H2O→ NADPH + ATP
Blue and Red light is trapped in the chloroplasts, green and yellow light is not used, these colors are
reflected back. This is why you see plants as green, it is the color that is not being used by plants!
A. Photosystem II is first -
1. pigments trap energy and excite electrons. They are passed to an electron transport chain
(same as in cellular respiration).
2. Two water molecules are split into 4 H+ and Oxygen gas (O2)
3. The H ions that have been pulled off of the water are put into the inner membrane space of
the chloroplast . This makes one side of the membrane acidic (lots of H+) and one side less acidic.
Membranes HATE to have one side higher in acid than the other! (remember when we looked at
osmosis and water passes through a membrane to try and even out salt solutions on both side of the
membrane?) WAIT... THIS SOUNDS AWFUL FAMILAR DOESN'T IT!
This difference in acidity (H+) is used to slowly pass some H+ to an Oxygen molecule. As the H+ passes
from one side of the membrane to the other, it spins ATP synthase. (If this does not sound familiar, go
back and look at the last stage of cellular respiration)
B. Photosystem I is second -
1. pigments trap energy and excite electrons (H)
2. The electron carrier NADP+ recives a H (an electron) giving NADPH
At the end of the light cycle - we have created NADPH and ATP
