244
J O H N P A U L
catabolism may (in some cells at least) continue unaltered or only
slightly reduced even when the population is stationary. This means
that rapidly-growing cells are more efficient than slow-growing ones as
has been demonstrated directly (Quastel and Bickis, 1959). (Bacteria
behave in the same way: Gunsalus and Shuster, 1961.) The question
arises: what happens to the excess energy? No experimental data are
available but it is commonly thought that phosphorylation becomes
"uncoupled", i.e. that the amount of A T P formed is no longer directly
related to the free energy changes ofthe associated chemical reactions.
The general implication is that there is no simple feedback control of
carbohydrate utilization and that the non-growing cell is less efficient
since the energy made available is presumably dissipated mainly as
heat.
There is one interesting corollary to these considerations. In rapidly
growing cells a very large part of the available energy can be directly
accounted for by synthetic processes. Much ofthe remainder is probably
required for maintenance of concentration gradients, cytoplasmic
movement, cell division and so on, apart from what is inevitably lost.
This means that little energy can be available for other processes, such
as are involved in the functions of differentiated cells. In a stationary
population, however, there may be an enormous surplus of available
energy. Consequently, there would seem to be some thermodynamic
rationale for the widely held impression that growth and differentiation
are generally dissociated from each other.
I I I .
E N E R G Y - Y I E L D I N G R E A C T I O N S
A. U T I L I Z A T I O N OF C A R B O H Y D R A T E
1. Source of Carbohydrate
It has been known for many years that in tissue cultures glucose is the
main source of energy (Astrup, Ehrensvard, Fischer and 0hlenschlager,
1947; Gemmill, Gey and Austrian, 1940; Krontowski and Bronstein,
1926; Krontowski and Jazimirska-Krontowska, 1926; Lewis, 1922;
Willmer, 1942; Wilson, Jackson and Brues, 1942) and that several
other sugars, both monosaccharides and disaccharides, can replace it
(Astrup et al, 1947; Chang and Geyer, 1957; M . Harris, 1950;
Warburg, 1930). Earlier studies employed primary explants growing in
plasma clots but many of the results obtained in this way still stand.
The findings of a few key studies, including some which utilized cell
culture methods, are summarized in Table III. It can be seen that
there is general agreement about the fact that fructose, galactose,
J O H N P A U L
catabolism may (in some cells at least) continue unaltered or only
slightly reduced even when the population is stationary. This means
that rapidly-growing cells are more efficient than slow-growing ones as
has been demonstrated directly (Quastel and Bickis, 1959). (Bacteria
behave in the same way: Gunsalus and Shuster, 1961.) The question
arises: what happens to the excess energy? No experimental data are
available but it is commonly thought that phosphorylation becomes
"uncoupled", i.e. that the amount of A T P formed is no longer directly
related to the free energy changes ofthe associated chemical reactions.
The general implication is that there is no simple feedback control of
carbohydrate utilization and that the non-growing cell is less efficient
since the energy made available is presumably dissipated mainly as
heat.
There is one interesting corollary to these considerations. In rapidly
growing cells a very large part of the available energy can be directly
accounted for by synthetic processes. Much ofthe remainder is probably
required for maintenance of concentration gradients, cytoplasmic
movement, cell division and so on, apart from what is inevitably lost.
This means that little energy can be available for other processes, such
as are involved in the functions of differentiated cells. In a stationary
population, however, there may be an enormous surplus of available
energy. Consequently, there would seem to be some thermodynamic
rationale for the widely held impression that growth and differentiation
are generally dissociated from each other.
I I I .
E N E R G Y - Y I E L D I N G R E A C T I O N S
A. U T I L I Z A T I O N OF C A R B O H Y D R A T E
1. Source of Carbohydrate
It has been known for many years that in tissue cultures glucose is the
main source of energy (Astrup, Ehrensvard, Fischer and 0hlenschlager,
1947; Gemmill, Gey and Austrian, 1940; Krontowski and Bronstein,
1926; Krontowski and Jazimirska-Krontowska, 1926; Lewis, 1922;
Willmer, 1942; Wilson, Jackson and Brues, 1942) and that several
other sugars, both monosaccharides and disaccharides, can replace it
(Astrup et al, 1947; Chang and Geyer, 1957; M . Harris, 1950;
Warburg, 1930). Earlier studies employed primary explants growing in
plasma clots but many of the results obtained in this way still stand.
The findings of a few key studies, including some which utilized cell
culture methods, are summarized in Table III. It can be seen that
there is general agreement about the fact that fructose, galactose,