Life at the Cell and Below-Cell Level. The Hidden History of a Fundamental Revolution in Biology
"Dr. Ling is one of the most inventive biochemist I have ever met." |
Chapter 5. Evidence for the Cell Content as a Dilute Solution (p. 26-28) |
|
The basic tenet of the membrane theory that the cell
interior consists of a dilute aqueous solution can be resolved into the tenet
of free water and the tenet of free Ê+—since, as already mentioned, it was found
at the turn of the century that K+ is the main intracellular cation
of most living cells.73,75,98 pp 232-233 The experimental evidence
for free water and free K+ came in the early 1930's and late 1940's,
and thus years after alternative concepts of what one may loosely refer to as
"bound water" and "bound K+" had already
appeared (see Chapter 7). For this reason, I shall present only a brief sketch
of the early evidence for free water and free K+ here, and delay their
follow-ups until [10.2(3)] and [11.3(5)]—after the opposing concepts have been
reviewed. 5.1 Early
evidence for free cell water In 1930, A.V. Hill (1886-1977), Nobel Laureate, showed
that urea distributes81 equally between muscle cell water and the
surrounding medium. This led him to conclude that all water in living cells is
free and none "bound" or "nonsolvent."
Confirmations followed soon.82 5.2 Early
evidence for free cell K+ In 1953, Professor Alan Hodgkin, Nobel Laureate and
Professor Richard Keynes of the Cambridge University published their findings
on the mobility of K+ in the giant nerve fiber (axon) of the
cuttlefish.263 They concluded that K+ which enters an
axon is found in the axoplasm in much the same state as in a 0.5 M KC1
solution.263 p 526 The importance accorded
this piece of work could be seen from a passage written by another Nobel
Laureate, Professor Bernard Katz in "Muscle, Nerve and Synapse."237 "Moreover, these authors (Ernst, Troshin and Ling) take the view that the potassium ions do
not merely form counterions to the negatively charged colloidal structure but
possess selective affinity and are chemically bound to the proteinates.
{This sentence has not correctly represented my position. In my view, to be
presented in Section [10.1(3)], cell K+ is adsorbed primarily by electrostatic
forces and cannot be regarded as "chemically bound." Added by GL.} It seems, however, very difficult to
support this view in the face of the following pertinent observations by
Hodgkin and Keynes (1953) (Reference 263, added by GL}.... These results
are discussed in detail because they are of crucial importance in the still
persistent argument about the validity of the membrane concepts.... It was
clear therefore that the labeled [K+, added by GL] ions that
had entered the axoplasm continued, inside cells, to behave as free ions with
approximately normal mobility."237 pp 42-44 Sixteen years after the publication of
Hodgkin and Keynes' work summarized above, Kushmerick
and Podolsky presented new findings in support of
Hodgkin and Keynes' conclusion. Kushmerick and Podolsky measured the diffusion coefficients of seven
labeled solutes in the cytoplasm of 3- to 6-mm-long frog muscle segments with
open ends.264 The solutes were K+, Na+, Ca2+
(calcium ion), SO42- (sulfate ion), ATP (adenosine
triphosphate), sucrose and sorbitol. They showed that
all solutes (with the exception of Ca2+) demonstrated diffusion coefficients
lower than their respective counterparts in free solution by a factor of two. From these
findings, they concluded that six of the solutes they studied, including K+
and ATP, exist in the free state in the cell water.
And it was mechanical barriers inside the muscle cell, which caused the
impartial two-fold reduction of their respective diffusion coefficients. Thus
it would seem that Kushmerick and Podolsky
had not only confirmed the free-cell-K+ tenet in particular but the
broader free-cell-solute tenet of the membrane (pump) theory as well. As mentioned
above, further follow-ups for both the free K+ and free water ideas
will not be presented until Chapters 10 and 11 respectively. Nonetheless, it
may be pointed out here that all five investigators and reviewer cited above
had overlooked some earlier relevant information in the literature. In 1913, Rudolf Höber reported on the electrical conductance of
frog muscle measured at a frequency (0.9 x 107 Hz), at which the
membrane resistance is short-circuited and thus no longer interfering. Höber found the electrical conductance of muscle
cytoplasm equal to that of a 0.1 to 0.2% NaCl solution.265
This finding contradicts the free K+ tenet of the membrane theory, according
to which the measured conductance should equal or at least approximate that of
an isotonic 0.7% NaCl solution in equilibrium with a normal frog muscle
[4.1(2)]. Höber's finding has been confirmed
repeatedly by other investigators including Professor A.V. Hill—whose work was
mentioned above—and myself.266; 15 Table 8.1 So far, we have focused on the membrane theory and its underlying assumption that living cells are membrane-enclosed dilute solutions. In what follows, I shall examine theories and facts more closely related to the concept that living cells are solid bodies of protoplasm. I shall begin with the emergence of a new branch of chemistry, called colloid chemistry. |
|
|
Ðàçäåëû êíèãè
Contents (PDF
218 Kb) |
|
|
Íà ñòðàíèöó
êíèãè "Life at the Cell and Below-Cell Level..." |
|