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
2. Calvin Cycle, the Dark Reactions (or the Light Independent Reactions)
Where does this step occur?: In the stroma (liquid part) in the chloroplast
Overall Point: To use the NADPH and ATP created in the light reaction to trap CO2 from the air and fix it
(build it) into the three carbon molecule G3P. This occurs is three steps.
3 CO2 + NADPH + ATP → G3P
A. Carbon Fixation
RuBisCo (Ribulose-1,5-bisphosphate) grabs a CO2 molecule.
B. Reduction
The Carbon dioxide is built into a larger molecule. Using 6 ATP and 6 NADPH
C. Regeneration
The RuBisCo is rebuilt so that it can grab another CO2.This takes 3 additional ATP.
In order to produce 1 G3P this cycle must happen 3 times (three CO2 are fixed)
In order to produce 1 Glucose this cycle must happen 6 times (six CO2 are fixed)
If the light cycle stops, the dark cycle will not receive the ATP and NADPH that it needs to fix carbon, this
is why plants need sunlight to grow. Actually, if the sun stopped shining, plants would be unable to fix
carbon, so there would be no more sugar, and all the animals would starve as well. (Assuming
everything did not freeze first.) Good thing the sun is going to be shining for the next 5 billion years!