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C3 / C4 plants. (Help..)

Roger Wallsgrove roger.wallsgrove at bbsrc.ac.uk
Fri Jul 18 07:58:02 EST 1997

In article <33ccfc15.89515 at news.telepac.pt>, eowyn at bug.fe.up.pt (Raquel) says:
>Hello there!
>I was wondering if you guys could enlighten me. What exactly *is* the
>difference between C3 and C4 plants, and how does it affect
>photosynthesis and respiration?
>Also, I read somewhere that only C3 plants had photorespiration. Why
>is that?
Might actually be easier for you if you pick up a basic plant physiology
text book, but I'll give you an answer as I understand it.

Basic fact - ALL plants assimilate CO2 via Ribulose bisphosphate 
carboxylase/oxygenase (RUBISCO), leading to "C3" products.  This enzyme 
also catalyses an oxygenase reaction producing C2 products - these are 
recycled via the photorespiration pathway, which recovers 75% of the 
carbon (25% lost as CO2). The split between the two reactions varies with
CO2/oxygen concentrations (more CO2, more C3 products), and temperature.

C4 plants have a way of concentrating CO2 - they have another enzyme
which fixes CO2 into "C4" products, in the mesophyll cells.  These C4
compounds then move to the bundle sheath cells, where they are broken
down to release CO2, which is then assimilated by RUBISCO.  In this 
process the CO2 concentration around RUBISCO is higher, hence less
oxygenase reaction.

Note "less" oxygenase - not zero!!  C4 plants certainly photorespire,
but they don't lose the CO2 released (it is re-fixed in the mesophyll).
C4 plants have all the enzymes for photorespiration, and it is easy to
measure a definite flux through this pathway - but not by looking at the
release of CO2, as one can with C3 plants.

Basic respiration in the two classes of plants is identical, and the
CO2 fixing process has no influence on it.

If you need more information, and details of the chemistry and the 
structural and physiological differences involved - go to a decent 
library and look it up in a good text book!

Roger Wallsgrove

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