Download Notes on C3 Photosynthesis Examples of C3 plants: lilac, wheat

Notes on C3 Photosynthesis
Examples of C3 plants: lilac, wheat, oak trees, rice…
These pathways are the ones we already learned:
1. Light reactions
2. Dark reactions (Calvin cycle)
The reason we refer to this process as a C3 process is because many of the
intermediates in the Calvin Cycle have 3 carbons! ie phosphoglycerate, glyceraldehyde,
G3P…
But C3 plants have a problem. If it gets too hot, too much water vapor will leave
through the stomata and this threatens the plant with dehydration.
Plants solve this by closing their stomata.
New Problem! This cuts off CO2 from entering the leaf. So oxygen gas increases and
CO2 gas decreases. This is an unfavorable gas ratio in the leaf air spaces.
Rubisco is unfortunately sensitive to both oxygen and carbon dioxide. So if oxygen is in
greater supply Rubisco will bind to it and do Photorespiration; breaking down G3P back
into CO2. This is very wasteful and unproductive for the plant.
Notes on C4 photosynthesis
Examples of C4 plants: Corn, sugarcane, grasses…
They have evolved an extra pathway in their dark reactions in response to dealing with
heat and dehydration.
Light reactions still the same.
If it gets hot
1. Stomata close, O2 increases, CO2 decreases
2. Gases are in contact with mesophyll cells that do not have Rubisco. Instead they
have an enzyme called PEP carboxylase which can only bind to CO2
So: Photorespiration is avoided!