anyone with the chemical equation for photosynthesis

Tom Buckley buckley at BIOSERVER.VSB.USU.EDU
Mon Apr 12 14:25:30 EST 1999

The most widely used model for C3 photosynthesis was published by Graham
Farquhar, Susanne von Caemmerer and Joe Berry in 1980 (Planta 149:78)
and improved, modified, etc. in later papers by combinations of the same
authors.  It's also been presented in many modeling papers since then,
so it should be easy to find.

If you need the eqns for some offhand work or teaching, a brief
nonrigorous exposition (given below) suffices, but for publication work
you should definitely read the original papers.

The model is basically a pair of michaelis-menten functions, one
representing  a  RuBP-carboxylation-limited phase (sometimes called
'Rubisco limited', and operative at lower intercellular co2 partial
pressures), and one representing a RuBP-regeneration-limited phase (or
'electron-transport-limited', at higher ci's):

co2 assimilation rate = A = lesser of Av and Aj, where
Av = V(c-g)/(c+k) - r    [Rubisco-limited]
Aj = J(c-g)/(c+2k) - r    [e-tpt-limited]

V and J are essentially Vmax's for the RuBP carboxylation and
regeneration phases of the Calvin cycle, respectively; c is
intercellular co2 partial pressure, essentially the substrate
concentration; g is co2 compensation point in absence of mitochondrial
respiration (i.e. accounts for photorespiration); k is an effective Km
for Rubisco (accounts for o2 competition); and r is the rate of
mitochondrial or 'dark' respiration.  Rate units (A, V, r) are usually
given as flux densities (micromoles per sq.m per s) and concentrations
(c, g, k) are usually partial pressures.


Thomas N. Buckley
Biology Department
Utah State University
Logan, UT 84322-5305
voice: 435-797-3567
fax: 435-797-1575
email: buckley at
"Science is the belief in the
   ignorance of experts."
 - Richard P. Feynman

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