Returning to
the defense table, Mr. Tappin went through the, by now, well-established ritual of
exchanging new material for used material with his colleague. As the lawyer turned toward
the witness, he began speaking.
"Professor
Yardley, during an earlier part of cross-examination, we talked about the difficulty of
plausibly accounting for the generation, and bringing together, of compounds such as fatty
acids, phosphates and glycerol in order to try to synthesize phospholipids, one of the
primary components of many kinds of cell membrane. Before proceeding to talk about cell
membranes in a little more detail, there is one further point which I would like to
address.
"I
believe phosphatidic acids are the simplest class of phospholipids," the lawyer said.
"Is this correct?"
"Yes,"
replied the professor.
"Moreover,"
Mr. Tappin added, is it also the case that derivatives of phosphatidic acid, such as
lecithins, tend to exist in cells primarily in an optical isomeric form that is in a
left-handed, rather than in a right-handed, isomeric configuration?"
"That's
right," the professor confirmed.
"Consequently,
Professor," Mr. Tappin concluded, once again, evolutionary theory is confronted with
the problem of having to come up with an explanation for how such a preference arose with
respect to optical isomers, just as in the case of proteins, as well as of ribonucleic
acids. Would you agree with this assessment of the situation?"
"Yes, I
would," acknowledged the professor.
"If,
Dr. Yardley, as presently seems to be the case based on present knowledge, there is no
readily apparent, natural pathway by which to generate phospholipids, how do evolutionary
biologists propose to account for the development of cell membranes?" inquired the
lawyer.
"There
are a number of different possibilities," the professor stated. "In my earlier
testimony, I touched on a number of these, including carbonaceous chondrites, protenoid
micro spheres and transitional liposome-like structures."
"Would
you expand a little, Dr. Yardley, on the possible role of carbonaceous chondrites with
respect to cell membrane formation?" the lawyer requested.
"There
are several ways to look at the findings vis-a-vis carbonaceous chondrites," the
professor began. "One of these ways involves the discovery of amphiphilic compounds,
and the other possibility deals with the hydrocarbons which are found in some of these
meteorites.
"Amphiphilic
compounds," explained the professor, "have both hydrophilic, or water-loving, as
well as hydrophobic, or water-hating, components. These compounds have been observed to
spontaneously form membranous-like boundary structures when placed in an aqueous
environment.
"When
placed in water, the hydrophobic parts of these compounds tend to curl up in order to
minimize contact with water. In the process of curling up, a vesicle or protected,
interior space is created, within which various kinds of chemical reaction might take
place under the right circumstance."
"Dr.
Yardley, before you continue on," Mr. Tappin interjected, "I would be interested
to know if tests have been conducted to determine if these amphiphilic compounds exhibited
any phospholipid-like properties?"
"Samples
of these compounds were studied by means of an electron microscope," responded the
professor. "One of the purposes of this analysis was to determine if a membranous
structure was present in these compounds.
"These
studies did detect the presence of a membranous structure approximately 10 nanometers, or
10 billionths of a meter, in thickness. This is consistent with the upper boundary size of
the cell membranes of many organisms.
"In
addition to the electron microscope studies, tests were performed in order to examine the
ability of these membranous structures to encapsulate polar solutes, or water- soluble
molecules, in a manner which was the same as, or similar to, cellular membranes in living
organisms. A dye was used in this study, and the researchers found that the amphiphilic
material from carbonaceous chondrites had the ability to encapsulate polar solutes with
approximately one-tenth of one percent of the encapsulation efficiency of the
phospholipids found in living organisms."
"In
other words, Dr. Yardley, although these extraterrestrial compounds could form
membrane-like structures with about the same thickness as the cell membranes of living
organisms, they were almost nothing like phospholipids in this, presumably, important area
of being able to encapsulate polar solutes. Is this correct?" the defense counsel
asked.
"Essentially,
yes," the professor responded.
"You
also mentioned, Professor, the hydrocarbon-related possibility associated with the
carbonaceous chondrites," the lawyer said. "What exactly does this
involve?"
"Around
1970," the professor pointed out, "several researchers studied seven
carbonaceous chondrite meteorites. They discovered chains of hydrocarbons consisting of
between 10 and 23 carbon atoms - a finding which was consistent with what also had been
observed in the Murchison meteorite.
"This
is comparable, in some respects, to the 12 to 20 carbon atoms contained in fatty acids,
one of the main components of the lipids found in the phospholipids that make up most cell
membranes. In the absence of any plausible natural prebiotic method of synthesizing fatty
acids, such chains may have served as a source for the type of hydrocarbons which make up
fatty acids in lipids and, therefore, cell membranes."
"Dr.
Yardley, isn't it the case," asked the defense counsel, "that fatty acids
contain chains of hydrocarbons consisting of even numbers of carbon atoms?"
"That's
right," the professor acknowledged."
"Therefore,"
said the lawyer, "not only are some of the hydrocarbon chains, ranging in length from
10 to 23 carbon atoms, that are found in the meteorites, both too short or too long,
relative to those hydrocarbon chains that range in length from 12 to 20 carbon atoms which
are found in fatty acids, but if the meteorite hydrocarbon chains contain odd numbers of
carbon atoms, then this would be another dissimilarity between the meteorite hydrocarbons
and fatty acid hydrocarbons. Is this correct, Dr. Yardley?"
"Yes,"
the professor replied.
"In
effect, if one tried to view these differences in the best possible light," stated
the lawyer, "one would have to assume that, somehow, carbon atoms either would have
to be added to, or removed from, many of the hydrocarbon chains found in the meteorites.
Would you agree with this, Dr. Yardley?"
"This
seems reasonable," indicated the professor.
"Furthermore,"
Mr. Tappin continued, "isn't it the case that the hydrocarbon chains found in the
meteorites would have to be oxidized before those hydrocarbon chains, with the right
lengths of even numbered carbon atoms, could be considered to be fatty acids?"
"Most
probably," the professor answered.
"In
addition," Mr. Tappin pressed, "even if one were to concede that fatty acids
might arise in the prebiotic Archean era world in this extraterrestrial fashion, one still
would have to find a way to bring these fatty acids together with phosphates and glycerol,
under the right conditions, in order to synthesize phospholipids. And, given that
phosphates, in particular, are likely to be extremely rare compounds in the Archean era,
then, Dr. Yardley, wouldn't one have to consider this whole sequence of events to be very,
very improbable?" the lawyer asked.
"I
imagine this would be the case," affirmed the professor.
"Finally,
in the light of previously established testimony," the lawyer stipulated, "one
cannot assume meteorites would represent a very substantial source of these kinds of
hydrocarbon chains, nor can one assume these hydrocarbon chains necessarily would survive
post-impact, prolonged exposure to ultraviolet photolysis or, perhaps, even heat, in the
form of, possibly, relatively high surface temperatures or volcanic activity. Isn't this
so, Professor?"
"Yes,"
Dr. Yardley agreed, "one cannot assume these sorts of thing to be automatic or
given."
"As far
as the possible role of protenoids is concerned in relation to membrane functioning,"
the defense counsel queried, "is there any evidence, Dr. Yardley, that protenoids
have the necessary properties to form active transport systems, or establish ion pump
mechanisms, or to provide transmembrane channel ways, as proteins do in the membrane
complexes of living organisms?"
"At the
present time, I believe there is little, if any, evidence to suggest protenoids have the
kinds of capability to which you are referring," replied the professor.
"Nevertheless, the absence of evidence in the few laboratory experiments which have
been performed to date does not preclude the possibility that during the Archean era,
protenoids with some of these sorts of functional capacity may have been synthesized
naturally."
"Is
there any evidence, Dr. Yardley, that the protenoids have the necessary sort of sequential
arrangements of hydrophobic and hydrophilic amino acids which, upon folding into their
tertiary or folded structure by means of thermodynamic forces, will enable their folded
hydrophobic portions to be located in the interior portions of the phospholipid bilayer
and, consequently, match up with the hydrophobic hydrocarbons of the lipid molecules, as
is the case in the transmembrane proteins of living organisms?"
"At
this time, I know of no such evidence," Dr. Yardley admitted.
"Previously,
Professor," pointed out Mr. Tappin, "you talked about liposomes. You described
them as small vesicles made up of lipid bilayers which might have served as a transitional
membrane-like structure.
"To
talk about liposomes, of course, is assuming that the issue of lipid formation had been
resolved in the Archean era. Would you agree with this, Dr. Yardley?"
"Yes,"
the professor said.
"While
expanding on the structural character of liposomes," said the lawyer, "you spoke
about properties such as the ability to reverse breakage of the bilayer by spontaneously
resealing any gaps which occur as a result of, say, mechanical agitation or shaking. In
addition, Dr. Yardley, you mentioned liposome properties such as being able to trap
solutes which may happen to be near by when these vesicles are dried, as well as liposome
qualities of growth, division and multiplication, all of which are somewhat reminiscent of
what goes on in living organisms.
"Growth,
division and multiplication, Professor, all suggest having access to a supply, regular or
irregular, of lipid molecules. Consequently, wouldn't you agree these properties of
growth, and so on, all presuppose that additional lipid molecules will be available which,
in turn, means that, once again, the question of lipid availability in the Archean era
would have to be addressed?"
"Yes,"
iterated the professor.
"Do
liposomes control their own growth, division and multiplication, Dr. Yardley, or is this
alleged growth, division and multiplication something which, on occasion, occurs to
liposomes as a result of external forces impinging on the liposomes, or as a result of,
say, osmotic lysis - that is, the rupture of the liposome due to an inward diffusion of
salt and water in the process of establishing an equilibrium between the internal and
external environments of the liposome?"
"If,"
postulated the professor, "you are asking me whether the liposome can be said to be
alive in some sense, then, clearly, the liposome cannot be described as being alive, nor
does it control its growth, division and multiplication in the same sense that a
biological organism actively controls these processes. On the other hand, the capacities
of a membrane structure to reverse breakage, expand in size, and be able to participate in
processes of division and multiplication, are fundamental stepping stones on the road
toward becoming part of the life phenomenon."
