"If my
understanding on the matter is correct, Dr. Yardley, living cells are, within certain
limits, able to maintain an internal electrical or ionic potential that is different from
the surrounding environment. In fact, some people have suggested that the ability of a
bounded, or membrane-enclosed, system to maintain this kind of differentiated energetic
relationship with the environment is one of the most recognizable attributes of a living
organism. Would you agree with this way of characterizing the situation?"
"Yes,"
the professor confirmed.
"Are
liposomes capable of maintaining this kind of differentiated energetic relationship with
the environment?" asked the lawyer.
"No,"
Dr. Yardley stated. "As I indicated before, liposomes are not living organisms."
"What
happens, Professor, if some sort of potential difference arises between the internal and
external regions of a liposome?"
"Lipid
structures," Dr. Yardley stated, "tend to show considerable permeability to
water, as well as a small amount of permeability to positively and negatively charged ions
of low molecular weight, although these ions diffuse across the membrane at a rate which
is about one billion times slower than is the case for water molecules. Therefore,
whenever there is a disequilibrium between the inner and outer environments of the
liposome, osmotic diffusion occurs, and this tends to eliminate the disequilibrium.
"If
these potential differences are slight, then equilibrium may be re-established with no
appreciable effect on the bilayer structure of the liposome's membrane. If the potential
differences are great, say, in favor of the external environment relative to the internal
environment of the liposome, then the liposome will swell with the osmotic
diffusion of ions and water into the vesicle's interior and, eventually, may undergo lysis
or rupture."
"Dr.
Yardley, what would a liposome-like structure need in order to get around this osmotic
problem?" Mr. Tappin inquired.
"One
would need," replied the professor, "either some kind of rigid wall capable of
resisting the stresses of lysis, or one would need a system capable, as required by
circumstances, of pumping ions in an out of the interior of the structure, or one would
need some combination of rigid walls and an ion pump."
"When
you say 'rigid wall', this, presumably, refers to things like cellulose in plants?"
queried the lawyer.
"Yes,"
the professor answered. "However, fungi, bacteria, and algae have evolved a variety
of rigid structures besides cellulose to handle the problem of osmotic lysis.
"Some
of these alternative strategies involve combinations of polysaccharide molecules that are
different from cellulose. Other strategies for creating rigidity in membrane walls also
have arisen, involving, for example, silica, lime and chitin - an amorphous polysaccharide
which is intermediate between proteins and carbohydrates - in conjunction with, say,
various carbohydrate matrices."
"Would
one be fair, Dr. Yardley, if one were to say that phospholipids require the presence of
particular kinds of protein in order to have ion pumping capabilities, so that even if one
were to assume phospholipids were laying around, so to speak, in the Archean era,
nonetheless, the mere presence of phospholipids, in and of themselves, would not solve the
osmosis problem?"
"Yes,
that's right," indicated the professor.
"Therefore,"
the lawyer said, "attaching just any old kind of protenoids, or even proteins for
that matter, to phospholipids will not necessarily establish an ion-pumping capability,
unless these protenoids or proteins have the right kind of sequential, structural and
tertiary folding properties which are suited to transporting particular kinds of ions into
and out of the membrane-enclosed structure. Is this right, Dr. Yardley?"
"I
would say so," the professor replied.
"Presumably,"
Mr. Tappin hypothesized, "various kinds of protenoids or proteins would be necessary
to handle the transport or pumping of different kinds of ions such as sodium, magnesium,
potassium, calcium, and so on. Would you agree with this, Professor?"
"Yes,"
Dr. Yardley said.
"This
capacity of a membrane system to actively participate in accepting some things, while
excluding others, is referred to as 'selective permeability', isn't it?" the lawyer
asked.
"That's
correct," acknowledged the professor.
"Besides
ions, Dr. Yardley, what other kinds of capability," the defense counsel inquired,
"would need to be actively included or excluded if a membrane-enclosed structure were
to possess the full range of functional characteristics exhibited by the membrane systems
of living organisms?"
"Organisms
would need some means of actively transporting nutrients into the interior of the
cell," the professor stated. "Simultaneously, organisms would need a means of
not only getting rid of toxic materials which may be accumulating as a result of the
catabolic and anabolic - that is, the tearing down and synthesizing - processes going on
in the cell in relation to such nutrients, but there would have to be some way for this
active transport system to be able to selectively differentiate toxic materials from
metabolites being used in the cell."
"Presumably,"
the lawyer reasoned, "different kinds of transport mechanisms across the membrane
and/or channel ways through the membrane would be needed in order to bring different kinds
of nutrient into the cell, as well as carry various sorts of toxic material out of the
cell. Would you agree with this, Dr. Yardley?"
"Yes, I
would," affirmed the professor.
Mr. Tappin
asked: "Why couldn't nutrients and toxic substances just enter and leave the cell,
respectively, by means of osmotic diffusion, in the same way water and low molecular
weight ions do in liposomes?"
Dr. Yardley
explained: "The phospholipid molecules that form the bilayer structure characteristic
of membranes, constitute a hydrophobic permeability barrier to all hydrophilic, or water
loving, materials, as well as to high molecular weight ions. Passive diffusion, or
osmosis, will not carry these kind of compounds across the permeability barrier formed by
the phospholipid bilayer, and, therefore, active forms of transport must be used, or
channel ways must be provided that will allow unimpeded passage through the hydrophobic
interior of the bilayer membrane structure."
"What
would happen," Mr. Tappin hypothesized, "if the nutrients transported across the
membrane were not co-ordinate with the organism's ability to catabolically tear down, and,
then, anabolically build up necessary molecules using these kinds of nutrient?"
"The
organism would starve to death," responded the professor.
"In
other words," continued the lawyer, "being able to actively transport nutrients
across the membrane's permeability barrier, is not enough. These nutrients must be of the
right kind, and, therefore, would one be right in supposing, Dr. Yardley, that this
particular transport mechanism must be able to preferentially select those nutrients which
will be of use to the organism?"
"Yes, I
believe this would be the case," said the professor.
"Isn't
it true," queried the lawyer, " that modern bacterial organisms tend to divide
about every twenty minutes or so, and, consequently, they need to transport enough
phosphates, of one sort or another, across their membranes, in the interval between
divisions, to be able to double the supply of these molecules which are crucial to the
process of synthesizing the increased amount of ribonucleic acids required for cell
division?"
"Yes,
that is right," the professor indicated.
"Moreover,
isn't it the case, Dr. Yardley," asked the defense counsel, "that because
phosphates tend to be ionized, a specialized carrier enzyme is necessary for the capturing
and transporting of phosphates across the permeability barrier formed by the cell membrane
of these bacteria?"
"Yes,"
agreed the professor.
"Consequently,"
the lawyer concluded, "to look after processes of selective permeability- such as
ion- pumping, nutrient or toxic transport, along with phosphate acquisition and carrier
requirements, one needs a variety of protenoids or proteins with specialized amino acid
sequences to give one the structural characteristics, hydrophobic or hydrophilic
properties, and tertiary folding patterns which meet such a diverse array of cellular
needs. Therefore, not just any kind of protenoid or protein structure will serve such
purposes, is that right Dr. Yardley?"
"As far
as we know, this is the way things work," the professor confirmed.
"Do
membranes provide functions other than the ones already mentioned, Dr. Yardley - other,
that is, than ion-pumping, and active-transporting, mechanisms of one kind or
another?" the lawyer inquired.
"The
ability to maintain a differentiated energetic potential between the interior and exterior
environments of the cell," pointed out the professor, "establishes an ion
gradient. This gradient represents a mother lode of energy which can be mined in various
ways to serve a number of cell functions, including coupled transport of nutrients, which
already has been touched on to some extent, and the production of compounds like adenosine
triphosphate (ATP), which becomes a mobile means of supplying energy to chemical processes
going on throughout the cell.
"For
many years," the professor added, "scientists have known that if one heats and
then dries a phosphate solution, an anhydride bond forms between pairs of phosphate
molecules. This anhydride bond is able to store the energy which is released by the
heating and drying process.
"The
pair of phosphate molecules which are bonded by the anhydride bond are known as
pyrophosphate molecules. Adenosine triphosphate, along with a number of other kinds of
phosphate compounds such as creatine phosphate and phosphoenolpyruvate, contain
pyrophosphate bonds which are capable of storing energy.
"Essentially,
in the case of the potential electrical difference which has been established across the
membrane's permeability barrier, the ion gradient becomes the source for generating the
energy which is stored in the pyrophosphate bonds of ATP rather than through the energy
which is released by the aforementioned laboratory method of heating and drying of a
phosphate solution."
"So,"
the defense counsel proposed, "in order to have a protocell begin to self-assemble,
not only do we need to come up with a solution of phosphates in the Archean era, we also
need to find a way to generate,at a minimum, the anhydride bonds of pyrophosphates
so that we have a means of storing energy generated by the ion gradient associated with
the cell membrane, providing, of course, we can manage to find a way to get these
pyrophosphate bonds into the interior of the bounded environment formed by a complex of
phospholipids and protenoids. Does the foregoing scenario cover, in broad terms, this
aspect of the evolutionary perspective, Dr. Yardley?"
"In
broad terms, yes," replied the professor.
"Stripped
down to its bare essentials, Dr. Yardley, would one be right to say," Mr. Tappin
asked, "that the mining of the energy contained in the ion gradient being maintained
by the potential electrical difference between the interior and the exterior of the cell
... would one be correct if one were to describe this mining process as the rolling, so to
speak, of electrons and/or protons down the gradient in order to gain the energy generated
by the downhill movement of these charged particles along the ion or proton
gradient?"
"This
is, more or less, accurate," acknowledged the professor, "although, as you
indicated, your description is obviously an extremely simplified version of what actually
occurs in the energy producing reactions which take place along the ion gradient
established by the potential electrical difference across the cell membrane."
"Dr.
Yardley, in living organisms, isn't this process of electron or proton translocation along
the electrical gradient that extends across the membrane, handled by specific enzymes or
proteins?" inquired the lawyer.
"That's
correct," the professor said.
"Therefore,
in addition to the specialized protenoids or proteins needed for the pumping of ions, as
well as the transport of compounds such as phosphates, nutrients and toxic materials, one
also needs specialized protenoids or proteins capable of translocating electrons or
protons across the membrane's ion gradient in order to be able to transfer the energy
potential of that gradient to pyrophosphate bonds in compounds such as adenosine
triphosphate. Is this the case, Dr. Yardley?"
"Yes,
it is," affirmed the professor.
"Given,"
postulated the lawyer, "a phospholipid bilayer which is impenetrable to all ionic
molecules except ones of very low molecular weight, and given that many proteins contain
not only ionic side chains but hydrophilic components, how does evolutionary theory
account for the process that would allow proteins to become embedded in a permeability
barrier which, due to its hydrophobic character, one might assume would be resistant to
such a process?"
"We
believe," Dr. Yardley stated, "there is some sort of thermodynamic driving force
that would allow the proteins and the phospholipids to overcome the repulsive forces
acting between the two kinds of molecule. This chemical antagonism is inherently unstable.
"Conceivably,
this condition of disequilibrium could be resolved if there were some, as yet
undiscovered, thermodynamic process which allowed the energy of the system to be
re-distributed in a more stable arrangement. Presumably, the embedding action might take
place during this process involving the thermodynamically driven - and, therefore,
spontaneous - redistribution of the energy toward a more stable ground state."
"You
did say, Professor, this thermodynamic mechanism for the insertion of proteins into
phospholipid bilayers was both theoretical and, as of yet, undiscovered, is this
right?" queried the lawyer.
"Yes, I
did," Dr. Yardley admitted. "However, the fact proteins are found embedded in
phospholipid bilayers in living organisms, despite the inherent chemical antagonisms which
are involved, and the fact we have not seen any evidence of a kinetic or non-thermodynamic
mechanism to account for this state of affairs, then the thermodynamic hypothesis outlined
above - although theoretical and unproven - is not as speculative and arbitrary as you may
think."
