Roots
Gleanings of a chemiosmotic eye
Franklin M. Harold
Summary
In 1961, an inventive Englishman, named Peter Mitchell,
proposed a radically novel hypothesis to explain how
energy is conserved during respiration and photosynthesis, and applied to the generation of ATP and other kinds
of functional work. The chemiosmotic hypothesis sparked an intense controversy that lasted for 15 years. Today,
Mitchell's conception of proton currents and their role in
phosphorylation and active transport is generally accepted, and has ramified into many corners of cellular
physiology. His most profound contribution may have
been to introduce spatial direction into biochemistry,
and thereby transform our perception of the relationship
between molecules and cells. BioEssays 23:848±855,
2001. ß 2001 John Wiley & Sons, Inc.
A revolution at forty
The chemiosmotic theory of biological energy transduction
turns forty this year, traditionally a milestone in our lives and an
occasion for reflection. When Peter Mitchell first proposed, in
1961, that the mechanism of energy coupling in oxidative
phosphorylation is not chemical in nature but effectively electrical, he stirred up a passionate controversy that embroiled
much of the bioenergetics community for the next fifteen years.
In the upshot, bioenergetics was transformed from a chemical
discipline into one that is indissolubly linked to the structure
and organization of cells. Forty years on, hardly anyone still
doubts that the great highways of metabolism are connected to
a panoply of membrane functions by way of ion currents.
Familiar examples include ATP generation by respiration and
photosynthesis, the transport of solutes into and out of cells,
homeostatic regulation and signaling, even certain kinds of
mechanical work. The molecular mechanisms that underlie
energy transduction, still largely unknown as little as a decade
ago, are rapidly coming into focus. We're all chemiosmoticists
now.
For all of us who helped shape the outcome, there is much
cause for satisfaction in this tale of a genuine scientific
revolution. All the same, it often seems to me that our appreciation of the chemiosmotic theory remains incomplete, even
superficial. We have largely adopted Mitchell's insights into the
nature of biological energy coupling, and are vigorously and
critically pursuing the search for molecular mechanisms and
physiological applications. By contrast, the implications of
Department of Microbiology, University of Washington, Seattle, 98195
226th Street SW, Edmonds, WA 98020.
E-mail: [email protected]
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BioEssays 23.9
membranes, transport and the direction of biochemical reactions for the integration of lifeless molecules into living systems
have drawn little attention. This article, then, has two objectives. One is to celebrate the achievement of the late Peter
Mitchell, and of his disciples, critics and successors, in clarifying how cells harness energy and perform work. The other is to
sketch, in contemporary idiom, something of the significance
of the chemiosmotic viewpoint for our understanding of life
itself.
Vectorial biochemistry
In the 1950s, and for a decade thereafter, there was no more
perplexing problem in all of biochemistry than the nature of
energy coupling. How do living organisms capture the energy
available from the degradation of organic matter, or from the
absorption of light, and harness it to the performance of useful
work such as biosynthesis, membrane transport and movement? The first conceptual answer had been supplied
a decade earlier by Fritz Lipmann.(1) The immediate energy
source for biological work is ATP or some related ``energy-rich''
phosphoryl donor, which participates chemically in the
reaction that it supports. The function of the great metabolic
highways, particularly respiration and photosynthesis, is to
generate and maintain the ATP supply. What no one knew was
just how ATP is produced.
Take mitochondria, which generate the bulk of the ATP
produced by respiration, via a process known as oxidative
phosphorylation. It was well established that respiration is
mediated by a cascade of redox proteins, associated with
membranes, that funnel electrons from NADH to oxygen. The
free energy available from this ``exergonic'' reaction is conserved, and drives the ``endergonic'' synthesis of ATP from
ADP and inorganic phosphate. The enzyme that catalyzes the
latter reaction, the ATP synthase (F1F0-ATP synthase; EC
3.6.1. 34), had also been identified. In its native, membranebound form it can generate ATP, whereas the solubilized
enzyme can only break it down. The open question was how
mitochondria harness the free energy of respiration and make
it drive ATP synthesis up the thermodynamic hill. Analogous
questions had arisen with respect to photophosphorylation,
and the uptake of metabolites by organelles and cells.
Biochemists, fresh from the triumphant elucidation of ATP
generation by glycolysis, expected respiratory energy to be
conserved by analogous mechanisms.(2) In the context of this
framework, the immediate task was to identify the hypothetical
intermediates that transfer energy from respiration to the ATP
BioEssays 23:848±855, ß 2001 John Wiley & Sons, Inc.
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synthase. The search engaged the best efforts of leading
laboratories in the 1950s and 1960s, but remained altogether
fruitless. In 1967, Chance et al.(3) reviewed no fewer than
sixteen proposed intermediates and rejected every one. The
sense of frustration was palpable at conferences of the day.
Peter Mitchell's 1961 proposal(4) sliced clear through that
knot to the very foundations of bioenergetics. The heart of the
matter was his claim that both respiratory chain and ATP
synthase represent a novel kind of enzymology: ``chemiosmotic'' pathways, vectorially positioned within the membrane so
as to mediate concurrently both a chemical reaction and the
translocation of a chemical group or substance from one side
to the other. According to Mitchell, redox chains and the ATP
synthase both translocate protons and are linked only through
the proton current; the search for chemical intermediates is
futile because they do not exist.
Figure 1 summarizes the essential principles of what became known as the chemiosmotic hypothesis, not in its original
formulation but in the canonical version devised some years
later.(5,6) Let me also center the discussion on bacteria rather
than mitochondria, for it is the bacteria that display most
strikingly the economy and versatility of chemiosmotic logic.
Briefly, the respiratory chain of redox catalysts is so arranged
within and across the bacterial plasma membrane that, as
electrons wend their way to oxygen, protons are translocated
out of the cytoplasm. The plasma membrane forms a closed
vesicle that is relatively impermeable to protons. In consequence, proton translocation generates an electrical potential
across the membrane, with the interior negative; in time a pH
difference may also arise, with the interior alkaline. Protons at
the external surface therefore find themselves at a higher
electrochemical potential than those in the cytoplasm. They
are subject to a ``pull'', derived from both the pH gradient and
the electrical potential, which Mitchell dubbed the protonmotive force; it pulls protons back across the membrane, down
the electrochemical gradient established by respiration. The
ATP synthase provides a pathway that allows protons to
traverse the membrane; it is so articulated as to couple the
downhill flow of protons to the uphill synthesis of ATP from
ADP and Pi. Energy is conserved, not chemically but by the
proton-motive force, referred to nowadays as the proton
potential.
It was apparent to Mitchell from the beginning that a proton
circulation could support other kinds of membrane work as
well; two of these are depicted in Figure 1. Rickenberg, Monod
and their associates had just characterized the first bacterial
transport system, the lactose permease.(7) Cells that possess
the permease could accumulate b-galactosides to an internal
concentration nearly a thousand-fold higher than that of the
medium; the energy source was unknown, but presumed to be
ATP. Mitchell(8) put forward an alternative. In his view, the
permease carrier has two substrates, a sugar molecule and
a proton, whose fluxes are obligatory coupled as shown in the
Figure 1. Principles of chemiosmotic energy coupling. The
respiratory chain (RC) translocates protons from the cytoplasm across the plasma membrane, making the cytoplasmic
side electronegative and alkaline. Protons at the exterior
surface find themselves at a higher electrochemical potential
than those in the interior; they are subject to a force, whose
magnitude corresponds to the sum of the electrical and pH
gradients, pulling them back into the cytoplasm. The plasma
membrane is intrinsically quite impermeable to protons, but is
studded with protein complexes that offer passage to the
protons and couple their return to the performance of useful
work. Three such devices are shown. The ATP synthase (AP)
generates ATP from ADP and Pi; the coupled operation of the
respiratory chain and ATP synthase is the essence of
oxidative phosphorylation. Transport systems (TS) link proton
flux to that of substrate S, and mediate the accumulation of
metabolites. The motor at the base of the flagellum (FM)
transduces the flux of protons into rotary motion of the
flagellar filament.
diagram: by co-transport, or symport. Accumulation of the
sugar occurs secondarily, thanks to the electrochemical driving force exerted upon the proton. In principle, any membranelocalized function can be coupled to the proton circulation by
an appropriate molecular device that allows passage to
protons and harnesses their downhill flow to the performance
of work. The second example illustrated in Figure 1 is the electrical motor located at the base of the flagellum; it transduces
the flux of protons into rotary motion and powers motility.
Many elements of the chemiosmotic hypothesis had appeared in the literature earlier, or were conceived by other investigators about the same time.(9,10) The notion that electron
transport brings about the separation of protons and hydroxyl
ions across a membrane had been incubating for decades.
Its application to the ATP synthase was quite novel, but the
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crucial role of protons had been foreshadowed in earlier
papers.(11,12) Robert Crane(13) demonstrated experimentally
that the mammalian intestine absorbs sugar by co-transport
with sodium ions. I cannot fully assess the degree or the
manner in which his contemporaries influenced Mitchell's
thinking, but my impression is that he might well have been
more generous in his acknowledgements (see also Williams
Ref. 14, for a personal and much harsher judgment with which
I do not concur). But it is important to note that Mitchell came to
the puzzle of energy transduction from the study of membrane
transport, and not the other way around.(9,10,15,16) He drew
inspiration from the realization that enzyme-catalyzed reactions intrinsically have a direction in space; this is not visible in
solution, but becomes apparent when the catalysts are
arranged within and across a membrane. When his ruminations on vectorial biochemistry and the molecular basis of
transport converged with the problem of energy transduction,
Mitchell wove the disparate strands into a coherent framework
that made the workings of simple cells and organelles instantly
comprehensible. It is this conceptual synthesis that constitutes
his enduring legacy.
Contemporary scientific fashion is unkind to heroes; the
writers of textbooks appear to believe that the truth is out there,
and it hardly matters who stumbles across it. Reality is more
interesting, and therefore something more must be said about
this extraordinary man who passed away in 1992 at the age of
72.(17) Mitchell obtained his Cambridge degree after a long
passage, and formulated the chemiosmotic hypothesis while
teaching in the Zoology Department of Edinburgh University.
Shortly thereafter, a grave bout of ulcers compelled him to
seek a less stressful climate. With the help of private means,
Mitchell purchased Glynn House, a decaying manor in
Cornwall; and then, acting as his own architect, turned it into
a modern laboratory cum family residence (complete with
a herd of dairy cows to balance the budget). For the next
quarter of a century, Mitchell and his lifelong colleague
Jennifer Moyle served as co-directors of the Glynn Research
Institute: Peter the theoretician, visionary and publicist,
Jennifer the experimentalist and anchor in reality. The green
Cornish pastures and woodland became the setting for a
revolution: anyone with an interest in bioenergetics would
sometime make the five-hour train journey from London to
work at the bench, reason out problems, speculate or debate
the ``Wizard of Bodmin''. The home of Helen and Peter Mitchell
was seldom without visitors, and never dull (though I must
confess that I found cytochrome oxidase at the breakfast table
something of a strain). Peter Mitchell was not infallible, nor was
he a saint. But he was a liberal man of wide interests, imbued
with a social conscience; and he possessed that rarest of
qualities, originality. The role of proton currents in biological
energy coupling would undoubtedly have been discovered by
someone else if Mitchell had not done so; but the chemiosmotic theory as a unifying framework for bioenergetics is
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more like a work of art: it bears the personal stamp of an
uncommon mind.
On the path of experimental opportunity
For a hypothesis to engage the attention of working scientists,
it is not sufficient that it carry the ring of possible truth; the idea
must also be fruitful, in the sense of providing a ``path of
opportunity''(18) for experimental verification and challenge.
The chemiosmotic hypothesis proved to be singularly fertile,
calling forth pertinent observations of two kinds. First, Mitchell
consciously formulated his thesis in accordance with the
principles put forward by the philosopher Karl Popper: he
made it falsifiable. The chemiosmotic hypothesis positively
invited challenge, and the offer was taken up with alacrity by
both opponents and proponents. Second, there turned out to
be numerous variations on the theme of energy coupling by ion
currents, particularly among the bacteria; chemiosmotics
proved to be a way of thinking rather than one particular
mechanism. The period between 1961 and 1975, fondly recalled by veterans of the ``chemiosmotic wars'', saw the
transition from bold speculation to solid science, and from
a tenuous hypothesis to a substantial theory whose reach
extends from the molecular level to the organismic.
Mitchell's ideas were initially dismissed because they
seemed to conflict with certain observations already in the
literature. I suspect, however, that the persistent hostility displayed by a large segment of the biochemical community had
deeper roots. Forty years ago, biochemistry was still in the grip
of the notion that, for practical purposes, a cell is just a bag of
enzymes. Mitchell's entire viewpoint was thus unfamiliar, and
many found it downright distasteful. They did not know what
to make of his claims that enzyme-catalyzed reactions have
a direction in space, that the topological disposition of
membrane proteins is crucial to their function, that enzymes
could be coupled through the electrochemical potential of
some ion, and that the starring role is played by that most
evanescent of ions, the proton. Physiologists, and later on
microbiologists, were more receptive because they already
spoke the language. Mitchell's iconoclastic perspective was,
of course, precisely what made his theory revolutionary.(19)
Chemiosmotic logic turns on the organization of functional
systems, and it challenged the traditional perception of the
relationship between molecules and cells.
The dearth of data was quickly rectified, beginning with the
classic experiments of Jagendorf and Uribe(20) who showed
that membrane vesicles derived from chloroplasts synthesized ATP in the dark when subjected to an abrupt shift of the
pH. The newly-formed Glynn Research Institute chimed in with
evidence that mitochondrial membranes are, indeed, quite
impermeant to protons, that uncouplers of oxidative phosphorylation conduct protons across these membranes, and
that certain preparations that retain the capacity for oxidative
phosphorylation consist, not of open membrane fragments,
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but of tiny closed vesicles.(21,22) Then the pace quickened, as
Glynn became intensely productive and investigators from
outside were drawn into the field. One by one the cardinal
postulates were verified: ion-impermeable membranes, proton-translocation by the respiratory chain and ATP synthase,
proton-linked porters for ions and metabolites, and the ability
of artificial electrochemical gradients to elicit ATP generation,
the accumulation of diverse substrates, even the rotation of
bacterial flagella. Measurements of proton stoichiometry
began to appear, and also cleverly reconstituted systems.
A large and rich literature summarized in numerous review
articles.(6,23 ±25) and several books(26±28) documents the
emergence of a new paradigm firmly rooted in chemiosmotic
principles.
The chemiosmotic theory emerged from this careful scrutiny substantially intact, but not without significant modification. One of the modulations is identified by the phrase
``localized currents''. In Mitchell's original formulation, the ionic
potentials that conserve energy refer to the bulk aqueous
phases on either side of the coupling membrane. This is
usually true for bacteria, especially with respect to the linkage
between ion pumps and the porters for various metabolites. In
the case of mitochondria, however, there is reason to suppose
that respiratory chains and ATP synthases are sometimes so
closely apposed that protons pass from one to the other
without equilibrating with the bulk phase. Recent biophysical
measurements, which suggest that the release of protons into
the bulk phase is much slower than their diffusion along a
membrane surface,(29,30) lend credence to this proposal.
Localization of membrane proteins, and the dynamic state of
membrane compartments, affect the interpretation of various
measurements; but they do not undermine the principle of
energy coupling by ion currents.
A more drastic modulation touches upon a cornerstone of
the theory, the concept of ``vectorial metabolism''. Consider
osmoenzymes, the prime movers of the chemiosmotic circuitry, which couple a chemical reaction to the translocation of
some solute. Since, by Curie's principle, a scalar chemical
reaction cannot give rise to vectorial transport, Mitchell envisaged the chemical pathway to have spatial organization that
loops across the barrier; the solute would be conducted from
one side to the other in close concert with the trajectory
of the reaction.(31) For example, he proposed that the Na ‡ ,
K ‡ -ATPase of animal plasma membranes (EC 3.6.1.37)
carries Na ‡ ions outward and K ‡ inward pari passu with the
phosphorylation and dephosphorylation of the enzyme protein. The alternative possibility is that the chemical and
transport functions are spatially separated, but linked through
conformational transitions within the osmoenzyme protein,
such that the ligand-binding site is exposed alternately on one
membrane surface or the other.(32,33) Mitchell was inclined to
reject ``conformational mechanisms'' on aesthetic grounds: he
thought them uninformative. In this he was mistaken. Vectorial
metabolic reactions in Mitchell's sense probably do exist,
particularly in the redox cascades of respiration and photosynthesis.(25,34) However, they are not common, let alone
universal. Conformational coupling of one sort or another
clearly underlies the operation of the ATP synthase and other
ion pumps. Even the phosphotransferase system for sugar
uptake by bacteria, often cited as a prime illustration of
vectorial metabolism.(24,26) is now known to consist of separate domains for translocation and phosphorylation respectively.(35)
The concept of osmoenzymes and of directional reactions
endures, but the renovations go beyond the trivial. Mitchell
himself thought of his theory first and foremost in molecular
terms; he told me once that his purpose was to lend a spatial
dimension to Fritz Lipmann's classic idea of the group-transfer
potential. In the event, this kind of vectorial metabolism proved
to be the exception rather than the rule. Ironically, Michell's
lasting contribution was to illuminate the way membranes work
at the cellular level.
A broad footprint
By 1978, when Mitchell won the Nobel Prize in Chemistry, the
dispute over energy transduction by ion currents had subsided, and fresh research objectives were emerging both
lower and higher on the scale of organized complexity. In
keeping with the reductionist spirit of our time, it is the quest for
molecular mechanisms that has grabbed the headlines. There
are still open questions galore, but in principle we now understand quite well how cytochrome oxidase, bacteriorhodopsin
and the photosynthetic reaction center conduct electrons and
pump protons, how the ATP synthase and other ATPases
couple ATP chemistry to an ion flux, and how the galactoside
symporter and its fellows transport metabolites. This is
splendid science, recently honored by the award of the 1997
Nobel Prize in Chemistry to Jens Skou, Paul Boyer and John
Walker for their contributions to the study of ion-transport
ATPases. However, as one descends to the level of molecular
structure and mechanics, bioenergetics loses its distinctive
character; I have chosen not to travel that path here. Let me
instead draw your eye up the scale, to the operation of chemiosmotic systems at the level of cells and organisms.
The chemiosmotic theory today is not focused solely on
ATP generation, though textbooks often give that impression;
it is about the modalities of membrane transport and their
functions in energy transduction. The bacterial world displays
the range of variations on this theme, but it is the eukaryotes
that best illustrate the many purposes that can be served by
chemiosmotic coupling.
Figures 1 and 2 sample the diversity of patterns found
among the prokaryotes. Escherichia coli represents the
canonical case, the one that corresponds most closely to
Michell's original conception (Fig. 1). Assorted respiratory
chains carry electrons to oxygen or an alternative electron
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acceptor, and concurrently expel protons to generate the
proton potential. The latter supplies the driving force for the
ATP synthase, flagellar motor and an array of membrane
porters. These carry out the uptake of diverse metabolites by
symport with protons, and also extrude sodium ions by antiport
with protons. The latter process generates a secondary
sodium potential, which supports a number of sodium-linked
porters.(36,37) The sketch omits much else, including a battery
of transport systems for ions and metabolites powered by ATP
hydrolysis.
Bacteria have adapted the principles to a variety of environments (Fig. 2). Photosynthetic bacteria expel protons with
redox cascades linked to bacteriochlorophyl; bacteriorhodopsin of halobacteria (and recently found in other marine organisms) is an altogether different proton pump. The anaerobic
enterococci (formerly streptococci) have no redox chains.
They call on the ATP synthase to expel protons and generate
a proton potential that supports transport and motility, while a
separate ATPase drives a sodium circulation.(36,38) Other
permutations look much odder. In the marine bacterium Vibrio
alginolyticus the redox chains pump both protons and sodium
ions; ATP synthesis requires the proton current but flagellar
rotation is powered by Na ‡ .(39) A similar division of labor
is found in methanogens. Oxalobacter formigenes boasts
a virtual proton pump, in which the electrogenic antiport of
oxalate for formate generates the proton potential,(40) the trick
of using electrogenic transport to augment the proton potential
is widely distributed.(36,41) Finally, the strict anaerobe Propionigenium modestum belongs to the small but growing class of
bacteria that operate entirely on a sodium circulation, generated in the present instance by a sodium-translocating
decarboxylase.(39) Note that Eubacteria and Archaea both
employ chemiosmotic energy coupling, with Na ‡ as well as
proton currents.(42) These evidently are exceedingly ancient
Figure 2. Diversity of chemiosmotic patterns in prokaryotes. (i) Enterococcus hirae lacks respiratory chains and generates ATP by
glycolysis. The ATP synthase operates in reverse, extruding protons to generate the proton potential. This, in turn, supports the uptake
of nutrients. (ii) Propionigenium modestum is a strict anaerobe that lives by a sodium circulation. Sodium extrusion if effected by the
enzyme that decarboxylates methylmalonyl CoA; sodium ions return via an ATP synthase. (iii) In Vibrio alginolyticus, respiratory chains
extrude both protons and sodium ions. The ATP synthase prefers H ‡ , but metabolite uptake and flagellar rotation require Na ‡ . (iv)
Oxalobacter formigenes does not pump protons as such. The proton potential arises from the exchange of oxalate anion (divalent) for
formate (monovalent); protons flow into the cell via the ATP synthase.
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devices, that arose in primordial cells prior to the divergence of
the two domains.
Eukaryotic cells feature multiple compartments, each
equipped with its own ion circulation, as illustrated in Figure 3
by a plant cell. Chloroplasts and mitochondria, descendants of
erstwhile bacterial endosymbionts, still operate in the prokaryotic manner. Plasma membrane and endomembranes
employ a different set of molecules, but to much the same
ends. The plasma membrane generates a large electrical
potential with the aid of a proton-translocating ATPase of the
P-type; P-ATPases are quite unrelated to the prokaryotic
F1F0-ATP synthase. An array of porters powered by the proton
circulation brings in nutrients, especially inorganic ions.
Finally, the central vacuole and other intracellular membranes
come with ATPases of yet another kind, the V-ATPases,
related to the F1F0 class. In this instance, protons are pumped
into the vacuole; their return to the cytosol drives the uptake of
Figure 3. Multiple chemiosmotic compartments in a plant
cell. Choloroplasts (C): In the thylakoids, light absorption
pumps protons inward to make the lumen electropositive and
acidic; their return to the matrix drives ATP synthesis.
Mitochondria (M): As in bacteria, protons are extruded
from the matrix by a respiratory chain and return via the F1F0ATP synthase. Vacuoles (V): A V-type ATPase pumps a
v-type ATPase pumps protons into the lumen. They return to
the cytosol by antiport for Ca2 ‡ and other ions, anion by
antiport for Ca2 ‡ and other ions; anion channels mediate
chloride flux. Plasma membrane (PM): A P-type ATP
expels protons, generating a large membrane potential
(interior negative); return of the protons is linked to the
uptake of metabolites and ions.
calcium and other ions, which are sequestered in the vacuole.
Such proton circulations are common among eukaryotes, but
by no means universal. Animal cells, bathed in a sodium-rich
fluid, drive a circulation of sodium ions across the plasma
membrane; it is powered by the familiar Na ‡ , K ‡ -ATPase,
which supports an array of sodium-coupled porters.(26) In
addition, many animal cells employ V-ATPases and a proton
circulation for special purposes.(43)
A host of cellular functions depends, directly or indirectly,
upon these ion circulations; a few examples must suffice here,
gleaned from the recent literature. Animal cells regulate their
cytoplasmic pH,(44) and respond to changes in the osmolarity
of the medium,(45) by means of controlled ion fluxes linked to
the sodium circulation. Trypanosomes also need to regulate
their internal pH; depending on the growth stage, they utilize
proton-ATPases or pyruvate/proton symport.(46) A protontranslocating V-ATPase also plays a central role in the
operation of contractile vacuoles,(47) but how this ``cellular
kidney'' functions is still far from clear. Plant cells make
extensive use of chemiosmotic devices. Acidification of the cell
wall, a prerequisite to wall extension, is accomplished by the
plasma membrane H ‡ -ATPase.(48) Redox systems are also
present there, but their function has been under debate for
decades;(49) uptake of Fe3 ‡ is a candidate. In addition, calcium fluxes are still turning up everywhere, from the medium
into the cell down the gradient established by calcium
extrusion or sequestration. They control the opening of plant
stomata,(50) and link the eyespot of algal cells to the undulipodium or flagellum.(51) Scientists who study such matters will
not regard themselves as chemiosmoticists, and may know
little or nothing about energy transduction. All the same, their
discoveries conform to the conceptual framework that Mitchell
and his successors began to assemble forty years ago.
Pumps, channels and carriers of eukaryotic cells are
generally not scattered about at random, but occupy definite
locations on the cell surface. When they are segregated in
space, the circulation of ions across the plasma membrane
may be supplemented by a lateral current of ions (and electric
charge) from one end of the cell to the other. The existence of
transcellular electric currents has been known for sixty
years;(52) thanks to the development of the vibrating probe
by Lionel Jaffe and his students, they have been documented
in a variety of polarized cells.(53,54) Transcellular ion currents
support spatially extended chemiosmotic circuits, on a scale of
micrometers and even millimeters.
Do transcellular ion currents serve a biological function,
especially in polarized growth or morphogenesis? The issue is
more subtle than it appears. The idea, popular two decades
ago, that the flow of electric charge per se plays a role in
development has gone out of favor. Currents are a consequence of polarized growth, not necessarily its cause.(54) But
there is a place for electric currents in the development of
certain embryos,(53,55) and mounting evidence that a localized
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flux of calcium ions is crucial to apical growth in some
organisms. The most compelling instance is the extension of
pollen tubes, which depends on the flow of calcium ions into
the apex.(56,57) The same appears to hold for apical growth in
ooÈmycetes, but not in true fungi.(58,59) Morphogenesis is a
province of that large territory that lies between genes
and cells. Much of it remains terra incognita, and will
probably remain so until we outgrow the illusion that the higher
levels of order are fully determined by what is spelled in the
genes.(60)
Molecules into systems
To study life, one must take organisms apart and examine their
components; to understand life, one must put the pieces back
together. The hallmarks of life are properties, not of individual
molecules but of large organized ensembles; each is a dynamic pattern deployed in space and time on a scale orders of
magnitude above the molecular. It takes a whole cell to turn
nutrients and energy into biological building blocks, to persist
over time, to reproduce, develop and evolve. When they become part of a cellular system, molecules operate under social
control; reactions come to have location, direction, timing and
function. One might describe a cell biologist as a biochemist
who remembers what the question was.
For Mitchell, the question was how organisms harness
energy; more specifically how scalar metabolism drives
directional transport. But the reach of his ideas greatly exceeded his grasp. The chemiosmotic theory laid the foundations of
a unifying framework for membrane physiology, that continues
to serve us forty years later (with or without acknowledgment).
On a more conceptual level, the chemiosmotic theory supplied
the first specific insights into how molecules join into coherent
large-scale patterns whose attributes are both spatial and
functional. From this vantage point, one looks beyond the
genes to the next higher levels of biological order: compartments, vectorial chemistry, the transmission of energy and
information, and a great concourse of processes that transport
chemical groups, ions, molecules and larger objects from one
place to another in an orderly manner. The principles foreshadowed in Mitchell's writings remain timely. Cell growth and
morphogenesis, in particular, appear in a new light when
perceived as ``transport processes which, in common with the
more popular membrane transport, must be described by
vector quantities having both magnitude and direction in
space'' .(61)
I suspect, moreover, that the great gulf between molecules
and cells lurks, like a black hole, at the core of that mother of all
problems, the origin of life. The prevailing approach, little
changed over the past sixty years, is resolutely scalar. One is
to envisage a rich broth of precursor molecules generated by
prebiotic chemistry, the chance appearance of self-replicating
molecules (RNA?) that somehow learned to specify proteins,
and some sort of self-assembly into ancestral cells. With
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a handful of honorable exceptions,(62,63) authors make only
passing mention of compartments, energy or the beginnings of
cellular organization. This seems to me to put the cart before
the horse.(60) Starting from the chemiosmotic viewpoint, no
theory of biopoiesis deserves serious attention unless it
outlines a way to convert energy into organization, thus generating mounting levels of structure and function naturally. No
field of biological research is more urgently in need of fresh
ideas; and in all of science there may be no question of greater
consequence than the origin of life. Until we can propound, at
the least, a credible theory to account for the origin of living
things from inanimate matter, we cannot lay to rest the ancient
question of whether life is truly consilient with chemistry and
physics.
References
1. Lipmann F. Metabolic generation and utilization of phosphate bond
energy. Adv Enzymol 1941;1:99±162.
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