V.
DETERMINING FACTORS IN CELL GROWTH
219
sugar. Hence the most direct source of carbon for this kind of protein
synthesis is sugar, and the nitrogen is drawn from certain groups which
are able to donate it at receptive sites. The main source of these
nitrogenous groups is conceived to be the products of prior protein
breakdown which are reworked in the cells so that they may be stored
in the form of such nitrogen-rich compounds as glutamine; although,
of course, similar compounds may also arise de novo from exogenous
nutrients. These nitrogen-rich residues not only store nitrogen temporarily but, in the case of glutamine, they also act as readily available
sources for protein synthesis, far more readily available incidentally,
than asparagine which usually occurs in quantity when protein synthesis
is arrested and breakdown is more prominent.
Moreover, the two kinds of protein so synthesized seem to differ in
their subsequent fate. The protein which embodies
14
C-proline directly
and which is rich in hydroxyproline, is characteristically not re-used in
the metabolism of the cell. It represents a storage moiety or a metabolic
point of no return, not unlike that of the protein of many viruses. On
the other hand, the protein manufactured more directly from sugar may
be metabolically used, and is to be regarded as capable of continual
'turn-over', releasing amino acids, from which the nitrogen is re-worked
for renewed protein synthesis, and leaving behind a carbon frame-work
which can be fed into the oxidative cycle and eventually yield carbon
dioxide.
Thus the chemical growth stimulants, such as those now known to be
present in coconut milk, not only alter the course of these metabolic
reactions in the manner that has been indicated, but an important part
of their role is to speed up the pace of metabolic 'turn-over' by reversible
synthesis and breakdown of metabolically active protein. In other words,
the coconut milk and other chemical growth-induction stimuli to cell
division not only put the metabolic engine into gear, they also speed up
the rate at which the metabolic fires burn. To account for these events,
it has also been necessary to invoke different compartments in cells,
compartments in which the same substrates have different metabolic
fates (Steward et al., 1956; Steward and Pollard, 1959). But having
exerted this effect upon some major aspect of metabolism, e.g. protein
metabolism, many other physiological aspects of the growing cells are
taken in stride, and of these water intake and the accumulation of
inorganic ions are probably notable examples.
2. Anatomical and Morphological Considerations
Above it has been shown how the biochemistry and physiology of
growing cells contrast with those of resting cells. There are, however,
also suggestive morphological considerations. As parenchyma cells grow
DETERMINING FACTORS IN CELL GROWTH
219
sugar. Hence the most direct source of carbon for this kind of protein
synthesis is sugar, and the nitrogen is drawn from certain groups which
are able to donate it at receptive sites. The main source of these
nitrogenous groups is conceived to be the products of prior protein
breakdown which are reworked in the cells so that they may be stored
in the form of such nitrogen-rich compounds as glutamine; although,
of course, similar compounds may also arise de novo from exogenous
nutrients. These nitrogen-rich residues not only store nitrogen temporarily but, in the case of glutamine, they also act as readily available
sources for protein synthesis, far more readily available incidentally,
than asparagine which usually occurs in quantity when protein synthesis
is arrested and breakdown is more prominent.
Moreover, the two kinds of protein so synthesized seem to differ in
their subsequent fate. The protein which embodies
14
C-proline directly
and which is rich in hydroxyproline, is characteristically not re-used in
the metabolism of the cell. It represents a storage moiety or a metabolic
point of no return, not unlike that of the protein of many viruses. On
the other hand, the protein manufactured more directly from sugar may
be metabolically used, and is to be regarded as capable of continual
'turn-over', releasing amino acids, from which the nitrogen is re-worked
for renewed protein synthesis, and leaving behind a carbon frame-work
which can be fed into the oxidative cycle and eventually yield carbon
dioxide.
Thus the chemical growth stimulants, such as those now known to be
present in coconut milk, not only alter the course of these metabolic
reactions in the manner that has been indicated, but an important part
of their role is to speed up the pace of metabolic 'turn-over' by reversible
synthesis and breakdown of metabolically active protein. In other words,
the coconut milk and other chemical growth-induction stimuli to cell
division not only put the metabolic engine into gear, they also speed up
the rate at which the metabolic fires burn. To account for these events,
it has also been necessary to invoke different compartments in cells,
compartments in which the same substrates have different metabolic
fates (Steward et al., 1956; Steward and Pollard, 1959). But having
exerted this effect upon some major aspect of metabolism, e.g. protein
metabolism, many other physiological aspects of the growing cells are
taken in stride, and of these water intake and the accumulation of
inorganic ions are probably notable examples.
2. Anatomical and Morphological Considerations
Above it has been shown how the biochemistry and physiology of
growing cells contrast with those of resting cells. There are, however,
also suggestive morphological considerations. As parenchyma cells grow