Botany online 1996-2004. No further update, only historical document of botanical science!
The Dark Reactions of Photosynthesis, Assimilation of Carbon Dioxide And The CALVIN Cycle.
Due to the use of isotopes were M. CALVN and his collaborators
at the University of California, Berkeley able to reveal completely
the reactions taking place during the incorporation of carbon
dioxide into carbohydrates in the relatively short period from
1946 - 1953. The quick success was based on the use of sensitive
methods (two-dimensional paper chromatography, autoradiography),
a suitable specimen and the rapid progresses of enzyme biochemistry.
Cultures of the single-celled green alga Chlorella pyrenoidosa
(that was introduced to photosynthetic studies in 1919 by O. WARBURG)
were supplied with light and an even stream of air containing
At a given time (t= 0) was 14CO2 added to
the stream of air for a short time. It was assumed that the labelled
carbon dioxide molecules were successively incorporated into intermediates
of the carbohydrate synthesis. After 3, 5 etc. seconds were the
experiments stopped by adding boiling alcohol and the newly produced
14C-labelled intermediates were separated and identified
by paper chromatography.
In summary, one can describe the end result of the dark reactions
The first stable compound that was labelled radioactively
already after 3 seconds was 3-phosphoglycerate (3-PG), a substance
we got to know previously as an intermediate of glycolysis. 14C
is found in the carboxyl group of 3-PG. At first was it assumed
that the molecule accepting the carbon dioxide would have to be
a C2 unit. But after a futile search was finally ribulose diphosphate
(RuDP), a C5 unit identified as the acceptor
C5 + C1 = 2 C3
6 RuDP + 6 CO2 > 12 3-PG
12 3-PG + 12 NADPH2 + 12 ATP > 12 GAP + 12 ADP + 12 Pi
+ 12 NADP
12 GAP > 1 glucose (net synthesis product of carbon dioxide
+ 10 GAP
10 GAP + 6 ATP > 6 RuDP
NADPH2 and ATP stem, as we will see, from
the light reactions of photosynthesis in which the light energy
is converted into chemical energy.
After understanding the pathway in Chlorella pyrenoidosa
arose the question whether it occurs in all other green plants,
too. It could be shown to be an important pathway of all green
plants. Even isolated chloroplasts (from spinach, for example)
are still fully active and all reactions of the CALVIN cycle take
part within them.
© Peter v. Sengbusch - Impressum