Am J Physiol Cell Physiol 298: C1280 –C1290, 2010.
First published March 3, 2010; doi:10.1152/ajpcell.00016.2010.
Historical Perspectives
From protoplasmic theory to cellular systems biology: a 150-year reflection
G. Rickey Welch and James S. Clegg
Department of Biological Sciences and Department of History, University of Maryland, Baltimore County, Baltimore,
Maryland; and Bodega Marine Laboratory and Section of Molecular and Cellular Biology, University of California, Davis,
Bodega Bay, California
Submitted 13 January 2010; accepted in final form 26 February 2010
history; protoplasm; cell theory; cytoplasm; cytomatrix; cell water;
cell metabolism; microenvironments
LARGE-SCALE, HIGH-THROUGHPUT
experimental methods today are
producing massive amounts of data at all levels of complexity
in the living cell. The generic enterprise known as “systems
biology” is striving to establish order in this kaleidoscopic,
information-rich empirical realm, with a newfound view of the
cell as a “system.” There is, without doubt, an acute awareness
of the holistic character of cellular structure-and-function in the
postgenomic era. Notwithstanding the stunning successes of
the modern-day analytical and computational approaches, the
myopia of the data explosion in cellular systems biology has
created a blinding sense of the present and a shadowy consciousness of the past. The portrayal of the cell as a “system,”
in particular, has an illustrious history that bears on today’s
revival. The beginning of this notion, remarkable though it may
seem, coincides with the birth of the protoplasm concept in the
mid-19th century. Though now an archaic expression long
buried in the annals of biology, the epistemological aura
originally surrounding the term “protoplasm” paved the way
for the pursuit of everything that we now associate with
cellular structure-and-function. The story of this word, and the
scientific motivation that it garnered, can enrich and inform
present-day efforts to understand how the cell works.
Every scientific discipline exhibits throughout history a
handful of key words that have iconic status, both in the
designation of important beliefs within the field and in the
Address for reprint requests and other correspondence: G. R. Welch, Dept.
of History and Philosophy of Science, Univ. of Cambridge, Free School Lane,
Cambridge CB2 3RH, UK (e-mail: [email protected]).
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conveyance of scientific knowledge in the public domain.
Some of these expressions come and go, whereas others
endure. In biology today, one such unique term is “cell.” It
entered the scientific lexicon in the 17th century and attained
notoriety in the early 19th century under the aegis of the “cell
theory,” which defined the cell as the fundamental unit of all
life forms. Indeed, the cell is commonly regarded as the
smallest component of an organism that is “alive.” There is
little (if any) mention in textbooks nowadays about one of the
most powerful concepts in the history of biology, protoplasm,
an expression that in times past enjoyed such romantic sobriquets as “stuff of life,” “locus of life,” “living essence,”
“Urschleim,” inter alia. In fact, it was the so-called “protoplasmic theory of life,” not the “cell theory,” that yielded the
original functionalist view of living cells. “Protoplasm” was
the precursor expression for literally the entirety of the active
components of the cell, seen as a dynamic whole. The “protoplasm theory” reached its height 150 years ago, and the
perspective that it engendered lasted for only 50 years or so
thereafter. Usage of the term “protoplasm” itself began to wane
early in the 20th century, as the cellular makeup was subjected
to the empiricism and reductionism of the molecular biology
era. Yet, in no small measure it set in motion the development
of the field of cell biology. With the advent of systems biology
today, it seems an appropriate time to look back at the history
of the inspirational role of protoplasm to appreciate the story of
the first impression of the cell as a “system.”
Protoplasm: Whence It Came
The quest to understand the material basis of life, like so
much of science, originates in Greek antiquity. Such ancient
notions, as “proto-matter” (prote: “first, original”) - the principle that there is a fundamental substance that underlies form
and function in the world around us and “hylozoism” (hyle:
“matter,” zoe: “life”) - the idea that life is identifiable with (or
immanent within) matter itself, have reverberated through the
ages (38). The concept of “protoplasm” became the rational
embodiment of these ideas in modern biology. After the onset
of the Newtonian mechanical worldview, interest in the role of
corpuscular “proto-matter” in the action of inanimate and
animate systems mounted during the 18th century (98). Within
the physical sciences such attention sharpened in the early
1800s, as evidenced markedly by William Prout’s (9) provocative hypothesis that the hydrogen atom represents the “protyle” (or “prote hyle”) of the ancients. Prout, a physician,
physiologist, and chemist, based his suggestion on the finding
that the atomic weights for all of the (known) chemical elements appeared to be integer multiples of that of hydrogen.
Biology was not far behind physics and chemistry in its
later-day search for “proto-matter.” The structural motif called
the “cell” had been known since Robert Hooke’s celebrated
0363-6143/10 Copyright © 2010 the American Physiological Society
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Welch GR, Clegg JS. From protoplasmic theory to cellular systems biology: a 150-year reflection. Am J Physiol Cell Physiol
298: C1280 –C1290, 2010. First published March 3, 2010;
doi:10.1152/ajpcell.00016.2010.—Present-day cellular systems biology
is producing data on an unprecedented scale. This field has generated
a renewed interest in the holistic, “system” character of cell structureand-function. Underlying the data deluge, however, there is a clear
and present need for a historical foundation. The origin of the
“system” view of the cell dates to the birth of the protoplasm concept.
The 150-year history of the role of “protoplasm” in cell biology is
traced. It is found that the “protoplasmic theory,” not the “cell theory,”
was the key 19th-century construct that drove the study of the
structure-and-function of living cells and set the course for the
development of modern cell biology. The evolution of the “protoplasm” picture into the 20th century is examined by looking at
controversial issues along the way and culminating in the current
views on the role of cytological organization in cellular activities. The
relevance of the “protoplasmic theory” to 21st-century cellular systems biology is considered.
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skiöld (59) avowed that Schultze’s work “laid the foundations
on which cell research has since been built, and this marks a
new era in the science of cytology.” Building on Schultze’s
seminal ideas, the noted physiologist Ernst von Brücke (10), in
a widely referenced paper entitled “Die Elementarorganismen,” introduced the heuristic viewpoint that protoplasm must
have a complex infrastructure (like an “elementary organism”
in itself), to provide the seat of all cellular activities (see also
Refs. 31, 72, and 88).
The protoplasmic theory engendered “the final repudiation
of the tradition comparing living units to utricles, sacks, boxes,
bubbles, or any other sort of envelope or container” (38). As
the movement gained momentum, some biologists even argued
that the term “cell” should be dropped, that “the advance
demanded a change in nomenclature, for a cell is a box and
lump of protoplasm is not” (5). The 19th-century plant physiologist Julius von Sachs (74) (obviously harkening back to
Hooke) quipped that, “to call the protoplasmic unit a cell was
about as appropriate as calling a live bee in a honeycomb a
cell.”Thus, some 150 years ago, protoplasm had come to
signify “matter possessing a certain molecular constitution
permitting it to manifest life”; the cell, then, was seen as a mere
housing for this matter (38).
Protoplasm: Where It Went
On 8 November 1868, Thomas H. Huxley (“Darwin’s Bulldog”) gave a public lecture in Edinburgh, entitled “On the
Physical Basis of Life” (published in the popular journal The
Fortnightly Review in 1869), which literally made “protoplasm” a household word, and which drew the attention of
scientists and nonscientists alike to the perceived role of
protoplasm as the “locus of life” (93). The “cell theory” was
supplanted by what came to be called the “protoplasmic theory
of life” (26), with far-reaching influence. Protoplasm, as a
material form and as a philosophical principle, became the
symbol of the functionality of life at the most basic level. In
addition to Huxley, such notables as the pioneering physiologist Claude Bernard (7) embraced the conception, calling
protoplasm “life in the naked state” and declaring that, “In this
amorphous, or rather monomorphous, matter life resides. Here
are to be found all the essential properties of which the
manifestations of the higher beings are only diversified and
definite expressions, or higher modalities.” Beyond the scientific inquiry, the “protoplasmic theory” also generated the final
philosophical battleground between the (ultimately victorious)
advocates of mechanism and materialism and the lingering
proponents of vitalism (93).
The “protoplasmic theory,” as implied by the title of Huxley’s lecture, postulated that the physical basis of life (including the foundation of heredity) must be sought in the properties
of this universal biological substance. As the scientific study of
protoplasm unfolded, critical empirical questions emerged:
How far down the scale of biological magnitude can one push
the idea of “living”? What part/parts of protoplasm is/are
“alive”? “Life” then (as now) eluded precise definition. The
physiology of the day had compiled numerous phenomenological attributes of the living state, including irritability, sensibility, contractility, nutritive digestion/assimilation, respiration, and reproduction (21). Seeking these traits in the microcosm of protoplasm was the aim of the protoplasmic theory.
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microscopic observations (and his coining of the term) in the
Micrographia of 1665. As we see in present-day biology
textbooks, Hooke’s “cell” picture became juxtaposed with the
works of Matthias Schleiden in 1838 and Theodor Schwann in
1839 to produce the “cell theory” (39). However, it is a
historical fallacy to distinguish this “theory” as the primary
19th-century construct leading to the elucidation of life’s
ultimate physiological basis and its elementary material character. From the purely technological perspective, the advancement of the cell description may be seen as an inevitable
consequence of the rapid improvements in light microscopy in
the early 19th century. As to its initial impact, the cell theory
signified the emergence of an ordered plan for understanding
the diversity of histological information on animal and plant
tissues (42). In reality, this proposal represented simply a
unified anatomical view of biological forms, without an immediate concern for the substance of life per se. Originally, cells
were seen simply as pores, cavities, conduits, etc., which
Hooke had likened to a honeycomb. (Note that the word “cell”
comes from the Latin cella meaning “inner chamber” or “small
room,” and the prefix cyto- derives from the Greek for “hollow” or “receptacle.”) As described by Hall (38) (quoting from
Hooke in 1665), “They were, in particular, regarded - by their
discoverer among others - as avenues of communication,
‘channels, provided by the Great and Alwise Creator, for the
conveyance of appropriated juyces to particular parts’.”
At its inception in the 19th century, the “cell theory” purported that all living organisms are constructed from like units,
the cell, which was viewed as a “bladderlike structure with
membrane, contents, and nucleus . . . with primary emphasis on
the nucleus and the cell wall (or membrane), relegating the
cellular contents to a secondary role” (31). The lack of substance in this superficial picture came under widespread attack
in the mid-19th century (93). The zoologist Félix Dujardin in
1835 had already described the physicochemical properties of
the living substance (which he called the “sarcode”) in rhizopods. In 1846, the botanist Hugo von Mohl demonstrated the
importance of this substance in plant cells, and it is he who is
usually credited with the first usage of the term “protoplasm”
(from the Greek plasma: “something formed or molded”).
[Actually, the physiologist Jan Purkinje had used the word in
1840 in reference to the “formative material” of animal embryos (38).] The botanist Ferdinand Cohn in 1850 unified the
terminology, suggesting that “plants and animals were analogous not only because of their construction from cells, but also,
at a more fundamental level, by virtue of a common substance,
protoplasm, filling the cavities of those cells” (31).
Led by the prominent anatomist Max Schultze in the period
1858 –1861, the conviction became entrenched that the true
basis of life is to be found in the study of protoplasm, not the
cell. His article (75), entitled “Über Muskelkörperchen und
dass was Man eine Zelle zu nennen habe” (focusing on the
properties of muscle syncytium), was a veritable milestone. As
expounded by Geison (31), “The publication of this paper,
more than any other single event, marked the birth of the
protoplasmic theory of life. On physiological rather than structural grounds, and with special emphasis on the properties of
contractility and irritability, Schultze demonstrated that a single substance, called protoplasm, was the substratum of vital
activity in the tissues of all living organisms, however, simple
or complex.” Even more emphatically, the historian Norden-
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logical concerns and questions at its inception that remain
unresolved to this day. Enzymes, being the fundamental mechanistic elements that execute cellular processes, became the
center of attention. Throughout the history of biochemistry, it
has been taken as a matter of course that most metabolic
activity of the cell results from the superposition of the action
of individual enzymes dissolved in an aqueous phase, with the
dynamics being governed by simple mass-action laws and
random thermal motions of metabolite molecules in weakelectrolyte solution. Despite the “structural” and “organizational” views of the cell coming from protoplasmic theorists,
the newfound gas-phase kinetics and the in vitro chemical laws
in the late 19th century were grafted on to the analysis of
cellular metabolism. This development was fostered by various
observations, such as those of the botanist Wilhelm Pfeffer on
the osmotic properties of cells. Pfeffer found that the cell
behaves as if a semipermeable plasma membrane (the existence of which he deduced) separates two ordinary aqueous
solutions, one being that inside the cell (64). The influential
studies by the chemist Eduard Buchner (11) on fermentation in
cell-free yeast extracts gave further credence to the view that
the cell is merely a homogeneous “bag of enzymes” in aqueous
solution. His findings seem to set the stage for the use of
“cell-free extracts” in the development of biochemistry that
was to take place during the next 50 years. One might say that
the empirical concept of “soluble enzymes” was born in Buchner’s work (23, 47).
Protoplasmic theorists reacted strongly against the picture of
cell metabolism being procreated by the chemists in the early
days. Perhaps the greatest voice was that of the aforementioned
champion Claude Bernard. Enzymes, originally called “ferments” (reflecting the early interest in the nature of alcoholic
fermentation), had been discovered in the mid-19th century. In
1876 the physiologist Wilhelm Kühne (who trained with Bernard) presented to the Heidelberger Naturhistorischen und
Medizinischen Verein a landmark paper proposing that isolable
“ferments” be called “enzymes” (meaning “in yeast” or “leavened”) (37). So was created a new term in biology, and so was
created a divide that continues to be questioned today. The title
of Kühne’s historic article, “Über das Verhalten verschiedener
organisirter und sogenannt ungeformter Fermente,” is telling:
an enzyme in a test tube is one thing, an enzyme in the cell is
another. Bernard (7) articulated the view that the chemically
purist impression of enzyme action does not bespeak the
relationship of the “ferment” to “the organization, the development, and the multiplication, that is to say the life of the
cell.” He recognized that, “Today, two kinds of ferments are
distinguished, according to the soluble or insoluble nature of
the ferment; the one produced by the intervention of an
organized or structured ferment, the other produced by nonorganized ferments, liquids, soluble products . . .” (Bernard’s italics). Some protoplasmic theorists went so far as to propose
models of cell metabolism based on the structural properties of
protoplasm. There were suggestions that the catalytic agents
(“ferments” or “enzymes”) form part of the integral structure of
a dynamic proteinaceous network in the cell, or that they are
attached to the ends of fibrillar protoplasmic tendrils (48, 51).
Such early hypothetical views would prove to be rather close to
reality.
Protoplasmic theory (and physiology) notwithstanding, the
newly conjoined sciences of biochemistry and enzymology
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Although colloidal chemistry was in its infancy at this time
(76), protoplasm, early on, was known to be a gelatinous
dispersion composed (aside from water) mostly of protein (or
proteid) - itself regarded by some as the ultimate “protomatter” of life. In his late 19th-century review, “On the Modern
Cell Theory,” M’Kendrick (56) informs us that by the 1860s,
protoplasm was defined commonly as “a diaphanous semiliquid, viscous mass, extensible but not elastic, homogenous that is to say, without structure, without visible organisation,
having in it numerous granules, and endowed with irritability
and contractility.” Nutritive matter, it was thought, somehow
becomes “alive” when it enters the physical domain of parts (or
all) of this protoplasmic substance.
Early consideration of the cellular composition led to the
realization that protoplasm must have two primary phases:
fluid and solid. As light microscopy and cytochemical staining
techniques steadily improved during the late 1800s, abundant
internal structure became evident. Three working models of the
protoplasmic solid phase developed: reticular/fibrillar, foam/
alveolar, and microsomal/granular (12, 38, 68, 76). The delineation of parts of the protoplasm as “living” relied on analysis
by “vital staining.” Visibly active portions were labeled with
such names (deriving from their respective Greek etymons) as
“phaneroplasm” (“visible”), “kinoplasm” (“moving”), “archiplasm” (“principal”), and “ergastoplasm” (“working”). Other
suggested “plasms” were “cryptoplasm” (for the “hidden” part
of protoplasm), “trophoplasm” (for the “nutritive” part), and
“hyaloplasm” (for the “clear” part). In 1884 the botanist Karl
Nägeli modified the dualistic view of protoplasm by proposing
that it consists of two fundamental portions: “hygroplasm,” the
fluid region, and “stereoplasm,” the solid part - with the latter,
he suggested, being the site of all vital activities (56). This
particular picture of the cellular essence, as a “proteinaceous,
structured, stereoplasmic” medium, would endure as a farsighted vision.
Many biologists of the late 19th century, alas, were not
content simply with the assignment of the “locus of life” to the
solid or fluid phase of the cell. The science of biology, like
physics at the time, was heading inexorably toward an atomistic foundation. As summarized by Hall (38), protoplasmic
theorists throughout the second half of the 19th century professed prolifically that, “Life is the activity of either i. individual molecules of the proper variety; or ii. an unstructured
molecular complex (hyaloplasm); or iii. subvisible multimolecular structures (’metastructures’) of some sort; or iv. visible
intracellular structures (organelles).” Category iii represented,
by far, the most popular, and speculative, view. A wave of
“multimeric” protoplasmic theories ensued, as the physical
basis of life came to be seen as a conglomeration of particles
going by such names as “biogens,” “gemmules,” “idisomes,”
“pangens,” “physiological units,” “plasomes,” inter alia (38,
93, 99). The underlying belief was that these “living molecules” represent the physical (or physiological) basis of life, as
well as the principle of heredity.
With the rise of biochemistry, the metaphor for the cell as a
“chemical factory” assumed a critical role in the latter part of
the 19th century (71). Analogies with the division of labor in
industrial factories, as well as that seen in the specialized organ
systems in higher life forms, abounded in this period (55). The
interface of the science of biochemistry with protoplasmic
theory, though yielding much fruit over time, raised physio-
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PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
20th-Century Cell Biology: What Is Protoplasm Really Like?
During the 20th century, a wide array of purification methods (including selective protein precipitation, ion-exchange
chromatography, gel filtration, electrophoresis, and ultracentrifugation) and analytical procedures (including high-voltage
electron microscopy, increasingly sophisticated light microscopy, confocal microscopy, fluorescence techniques, and in
situ probe analysis) were brought to bear on the examination of
cells and their components. Such investigations gradually revealed an extensive arrangement of organelles, membranous
structures, and cytoskeletal elements interlaced throughout the
cell. We have only to open the pages of any cell biology
textbook today to appreciate the intricate and highly integrated
structural tapestry of the cellular interior. Among the myriad of
experimental approaches and published studies constituting the
colorful history of the discovery of cellular ultrastructure, one
might identify many singular works that stand out as key
advances in our understanding of how the whole cell is put
together. It is beyond the scope of the present article to review
the entirety of this original literature. Suffice it to say that, what
we find over time is an emerging jigsaw puzzle, whose interlocking pieces began to generate in the holistic eye a sense of
synergy far beyond the properties of the components.
Today’s visualization of the makeup of protoplasm, at face
value, would appear to be a reification of the dualistic view that
had arisen in the late 19th century: there are two distinct phases
of the cell, “solid” and “liquid.” By the turn of the century, the
expression “cytoplasm” was becoming a popular descriptor of
the cellular makeup. This word was coined by Albrecht von
Kölliker in the 1860s to designate the fraction of the cell
outside the nucleus, but it gradually replaced “protoplasm” in
the course of the 20th century (100). Today, the discernibly
solid parts (aside from organelles) have acquired such labels as
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“cytomatrix,” “cytoskeleton,” etc., whereas the purely liquid
portion became known as the “cytosol;” the latter term introduced in the 1960s by Henry A. Lardy, as an operational
expression referring to the in vitro supernatant remaining after
all the solid material in cell extracts is pelleted by high-speed
ultracentrifugation (17, 18).
The physiological oversimplicity of such a biphasic “structuralist” representation of the cell was called into question
early in the 20th century. The leading cell biologist Edmund B.
Wilson (99), in a summary lecture on the “Structure of Protoplasm” (delivered at the Marine Biological Laboratory, Woods
Hole, MA) at the end of the 19th century, concluded that, “If
we except certain highly specialized structures, the hope of
finding in visible protoplasmic structure any approach to an
understanding of its physiological activity is growing more,
instead of less, remote, and is giving way to a conviction that
the way of progress lies rather in an appeal to the ultramicroscopical protoplasmic organization and to the chemical processes through which this is expressed.” Wilson’s monumental
text The Cell in Development and Heredity (100), which went
through three editions (1896, 1900, and 1925, respectively,
ultimately growing to 1,000⫹ pages in length), reconciled the
various views of protoplasmic structure from the late 19th
century and was highly influential in setting the stage for
analysis in the early 20th century. He recognized the importance of the dynamic interplay between the structured and
unstructured “phases” of the cell, prophesying in the third
edition of his book that, “There is reason to conclude that of all
the cell constituents the ‘structureless hyaloplasm’ is the most
constant and active, and may perhaps be regarded as forming
the fundamental basis of the protoplasmic system from which,
directly or indirectly, all other elements take their origin.” If
protoplasm is truly a “system,” then there can be only one
physiological “phase” underlying the visual microscopic representation.
In the early 20th century, the exposition of the “submicroscopic structure” of protoplasm attracted considerable attention, as seen perhaps most distinctly in Albert Frey-Wyssling’s
research and writings (28, 29). As it became apparent that
protoplasmic structures are dynamic and labile, the interrelationship of these entities with the fine-grained “ground substance” of the hyaloplasm was a major concern very early. The
reality of micrographic pictures of cells, subject to fixatives (or
“preservatives,” as they were sometimes called), was subject to
debate. Gwendolen Foulke Andrews (who trained under Wilson), in a turn-of-the-century exposé on the methods for
studying cells micrographically, professed that, “I have convinced myself that ‘preservatives’ fix for us little of the true
structure of the living substance, and can, at best, keep for us
grosser relations, of a mixed sort in point of time; hiding an
infinite complexity of form, and destroying perforce those
infinitely delicate relations whose fleeting harmonies make up
life phenomena” (3). This argument persists today.
Among those contributing in a major way to the pursuit of
the intricacies of cell structure in the 20th century was Keith R.
Porter, whose extensive observations and critical ideas, beginning in the 1940s, have earned him a prominent place in the
history of cell biology. Porter was greatly influenced by the
likes of Wilson and Frey-Wyssling, as he followed the quest to
describe and understand “submicroscopic protoplasm.” Although best known for his description of formed elements like
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stayed on their independent heading as the 19th century came
to a close (48). The isolation of individual enzymes and the
observation of “saturation kinetics” in vitro at the turn of the
century, culminating in the Michaelis-Menten and BriggsHaldane theoretical treatments, strengthened the conviction
that the mechanistic understanding of cellular function was to
be found in physical chemistry. Meanwhile, espousal of the
physiological idea of “living molecules” was gradually straining the limits of credibility of the protoplasmic theory in
scientific circles, with charges of vitalism coming from those in
the physical sciences (31, 48, 93). Early in the 20th century,
with the (re)discovery of Mendel’s work, the atomistic focus in
biology shifted predominantly from the phenotype to the genotype (the “germ-plasm”); and the panoply of protoplasmic
“particles” was swept away and replaced with a new unit: the
gene. Usage of the term “protoplasm” itself began to decline,
due not only to its vitalistic baggage, but, more significantly, to
the increasing reductionistic focus amongst cell biologists on
the intricacies of cellular (or protoplasmic) composition. The
maturation of biochemistry, with the eventual elucidation of
how protoplasm is metabolically built up and torn down,
dispensed with the ambiguous question of where/when do
nutrients become “alive;” and the empirical difference between
living “protoplasm” and nonliving “metaplasm” became
blurred (48, 99). And the philosophical inquiry into “What is
life?” became a hunt for the Jabberwock.
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having studied with Porter, they provide an additional perspective about the MTL. We might note that Wolosewick and
Porter (102) coauthored a paper in 1979 that responded to
criticisms of the MTL, with hope of establishing the MTL as a
“real” structure. A recent opinion paper by Kim and Coulombe
(46) compiles abundant evidence that the spatial organization
and regulation of the protein-synthesizing system of eukaryotic
cells are determined by the cytoskeleton, notably actin microfilaments. Those authors posited that, “The cytoskeleton has
evolved as a scaffold that supports diverse biochemical pathways.” Perhaps Keith Porter would have said: “I told you that
in the mid-1970s.”
The relationship between the “solid” and “liquid” phases of
the cell at the “submicroscopic” level is most surely a dynamic
one, though an exact and encompassing model remains to be
determined. Porter’s interpretation may have been wrong in
detail but right in principle. Because of the continuing controversies underlying empirical fixation procedures, static micrographs alone cannot give us a complete picture of the structureand-function of protoplasm. Porter was a realist as well as
dedicated to describing the MTL. In a 1987 letter to one of us
(accompanying the photographs from which Fig. 1 in this
paper was assembled; J. S. Clegg, personal communication), he
wrote: “Do you really think that we or anyone else can make
sense of such a mess.”
A key objection to such “submicroscopic” structural models
as the MTL concerns the question of “soluble proteins.” What
fraction of the cytoplasmic protein constituency is, in fact,
“soluble”? What is the actual concentration of “free” proteins
in the cell? The standard view today is that the aqueous phase
of cytoplasm (the “cytosol”) contains a very large concentration of “dissolved” proteins of various kinds, as well as a
variety of other macro- and micromolecules. Yet, this description flies in the face of a body of evidence, some of it quite old
(84), indicating that the majority of macromolecules (particularly protein) is normally associated, in some manner, with
cytomembrane and cytomatrix structures. Such support for an
organized rendering of the “submicroscopic” realm has come
from the advancement of biochemistry and enzymology.
While the sundry subcellular structures were being discovered during the progression of 20th-century cell biology, the
major metabolic pathways were gradually revealed by the
Fig. 1. A: electron photomicrograph of whole, cultured, newborn kidney cells (NRK) from the work of Keith R. Porter (see
text). B: image obtained by high-voltage electron microscopy of
a magnified region of these cells, displaying the purported
“microtrabecular lattice” (MTL). Professor Porter did not provide details of the preparations involved here, but they appear
to be similar to those given in Ref. 69. (T, trabeculum; MT,
microtubule; ER, endoplasmic reticulum; P, polysome; M,
mitochondrion). *Volume between the formed elements, denoted by Porter as the “water-rich” space.
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the endoplasmic reticulum, Porter believed that the finer-level
cytomatrix organization would turn out to be of exceptional
importance. For example, Porter (67) wrote that “The matrix
therefore occupies, in present day concepts of the cytoplasm,
the same position as that held by the ground substance or
hyaloplasm of the earlier light microscope image. It is the
‘structureless’ medium in which are suspended all the resolvable elements of the cytoplasm . . .” He expounded further
something that would predict his own quest in the decades to
come: “There is no reason to believe that with improved
methods of microscopy and specimen preparation this part of
the cytoplasm will not in time be shown to contain organizations of macromolecules.” In the early 1970s Porter played a
key role in the installation of a high-voltage electron microscope (HVEM) at the University of Colorado. Training this
powerful instrument on fixed, whole tissue culture cells, he set
out on this mission, soon to describe what he termed the
Microtrabecular Lattice (MTL). Using HVEM images, Porter
described the presence of an extensive network of narrow
cylinders (⬃7 nm in diameter and of variable length), the
“microtrabeculae,” that ramified throughout the cell, seemingly
connecting all the formed elements. These and other features of
the MTL can be seen in Fig. 1 (assembled from photomicrographs that Porter sent to J. S. Clegg in 1987). It is easy to
understand why Porter referred to such a unit structure as the
ultimate “cytomatrix,” the most elementary solid phase of the
cytoplasm.
Porter championed the reality of this structure for the remainder of his career. However, few cell biologists have
accepted the existence of the MTL, believing that the elaborate
edifice Porter described was an artifact of fixation or critical
point drying resulting in the deposition of previously “soluble”
proteins on other cell components, as well as the formation of
artificial protein-protein aggregates. John Heuser (41), who
was a participant in this debate, but not supporting the MTL,
has written a recent history of the subject, entitled “Whatever
happened to the ‘microtrabecular’ concept?” in a special issue
of Biology of the Cell. There the reader can find a variety of
photomicrographs of the MTL, as well as Heuser’s interesting
historical account and micrographs of his own work. Also
contained in this issue of Biology of the Cell are short articles
by Mark McNiven (54) and John Wolosewick (101). Both
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PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
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tion and Cell Function,” to (the current title) “Cellular Systems
Biology.”
Despite the mounting evidence for metabolic organization
and microenvironmental effects in vivo, the vision of soluble
metabolic pathways has remained, chiefly as a time-honored
deduction from high-speed centrifugation experiments on cell
and organelle extracts; and it is still a widespread and firmly
rooted way of thinking in cell biology and biochemistry. Thus
“soluble metabolism” and “dilute aqueous phase properties”
have become the conventional wisdom, forming the basis for
experimental design, interpretation of data, and construction of
models. But we must not lose sight of the fact that these beliefs
are built more on assumption and extrapolation of 19th-century
physical chemistry than on hard evidence. Not to belittle the
ensuing reductionism of 20th-century cell biology and biochemistry, as it has yielded an ever-flowing fountain of information on cellular structure-and-function, from the level of genes
and proteins, to metabolic pathways, to organelles, to the cell
itself. After all, most purified enzymes do “work” in a test tube,
under the dilute aqueous conditions that we define in vitro.
Information on the in vitro properties of isolated enzymes has
been of much utility, for example, in piecing together such
theoretical constructs as “Metabolic Control Analysis,” which
has provided a window (at least qualitatively) into the global
distribution of metabolic control in vivo (24). Aside from basic
knowledge, abundant applications in areas of biomedicine and
biotechnology have sprung from the long history of painstaking study on individual enzymes in vitro. Yet, there remains the
“Humpty-Dumpty” quandary: how do we fit the pieces together into a functional whole? (32, 45). Disparities abound, as
we attempt to reconcile the observed features of isolated
enzymes with the observed behavior of metabolism in the
intact cell. For example, it is often found that the observed
kinetic properties of individual enzymes in vitro simply do not
account quantitatively for metabolic fluxes in vivo (90, 97).
The algebra of holism dictates that 1⫹1⬎2. Enzyme action is,
in reality, part-and-parcel of an intricate structural edifice in
living cells. Albeit complicated physically (and mathematically) to grasp, this is the portrait of “protoplasm” for the 21st
century.
Today, a suited pictorial representation of the global unity of
structure-and-function in cells, as in all organismal levels of
biology, in fact, comes from the realm of fractals (1, 89). It is
becoming apparent that localized fractal dynamics provides the
proper theoretical principles for comprehending the character
of enzyme kinetics, mass transport, and thermodynamic flowforce relationships in the cellular microenvironments (4, 52,
92). The particular fractal image that seems to us most apropos
of cellular protoplasm, as regards its present form as well as its
emergent evolution, is the “Sierpiński sponge” (Fig. 2), which
is a confined geometric object whose progressive “fractalization” results in the surface area increasing to (the theoretical
limit of) infinity as the volume shrinks to zero (53). Had Robert
Hooke’s 17th-century microscope been more powerful (we
might muse), perhaps he would have concluded, as we do
today, that his honeycomb “cells” are actually filled with
vibrant, hydrated sponges?!
With the spotlight on the functional significance of microenvironmental organization in vivo, speculation has arisen as
to whether there is, in fact, some kind of unitary, corpuscular
basis for the physiological activity of cellular processes. For
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application of analytical chemistry, as well as by the isolation
and study of individual enzymes using a “grind-and-find”
approach. The “bag of enzymes” representation of the cell was
modified in the 1940s and 1950s, as electron microscopy and
cell-fractionation techniques began to show that certain enzyme activities are associated with specific organelles that
could be separated and identified. Since then, a variety of more
refined microscopy methods, in situ analyses, and extraction
procedures has provided increasing evidence that the surfaces
of intracellular membranes and cytomatrix structures are the
location for a multitude (perhaps the majority) of intermediary
metabolic reactions, as well as such larger-scale operations as
signal transduction, intracellular “trafficking,” protein synthesis, and DNA/RNA processing (for reviews see Refs. 2, 20, 46,
61, 84, and 86; see also Molecular INTeraction [MINT] database, http://mint.bio.uniroma2.it/mint/Welcome.do). For example, enzymes of the glycolytic pathway were once considered to be classic examples of “soluble proteins,” but we now
know that many of them (possibly all) are not freely diffusing
in three dimensions in eukaryotic cells (see Refs. 18, 19, 60, plus
various chapters in Ref. 86). In reality, the actual aqueous portion
of the cytoplasm may not be so “crowded” after all (17, 80).
Renewed attention of late has been given to the role of
localized, heterogeneous microenvironments in defining the
true kinetic and thermodynamic nature of biochemical events
in living cells (e.g., Ref. 32). The importance of surface
activity in the understanding of subcellular processes, in fact,
was realized long before the nature of cellular infrastructure
was clearly established. A noteworthy discussion of this view
and its implications came in 1930 from the biochemist Rudolph
Peters (62) in a prophetic paper entitled “Surface Structure in
the Integration of Cell Activity” (later refined in Ref. 63).
Modern-day calculations of protein “concentration” in association with cytomembranes and cytomatrix elements indicate
high, crystal-like densities in (on) the particulate structures of
the cell (80, 82, 91). Studies with artificially immobilized
enzyme systems, commencing around 1970, have proved crucial in establishing the importance of the microenvironment in
enzyme action. The ground-breaking works of Ephraim Katchalski (43), Klaus Mosbach (57, 85), and Daniel Thomas (87)
are particularly noteworthy (for reviews see Refs. 78, 90, 92,
and 97).
The evolution of our perception of the microworld of enzyme action in vivo, like the generation of our present-day
holistic understanding of cell ultrastructure noted above, has
been a gradual process that may be likened to a puzzle work,
with the elucidation of many interlocking pieces and many
contributing scientists. The original literature here, as well, is
far too extensive for a review within the scope of the present
article. Though, we should like to single out the personage of
the biochemist Paul Srere for mention. Few scientists have had
as much impact on the discovery landscape of enzyme organization in the living cell, not only in terms of his own
pioneering studies, but also in regard to the number of researchers and the breadth of ideas that he influenced. Over the
course of the last 30 years or so, Srere led the way in planning
a host of scientific meetings on this topic, including an ongoing
biennial Gordon Research Conference founded in 1987, whose
title (and scope) has morphed over the years from “Organization of Metabolic Sequences,” to “Macromolecular Organiza-
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PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
example, in 1985 Srere proposed the term “metabolon” for
complexes of metabolically sequential enzymes that exist in
organized states in vivo (83). Whereas such aggregates vary in
their size and structure, the intent of the expression is to draw
attention to the “particle” aspect of cellular metabolism. Unbeknownst to Srere (G. R. Welch, personal communication),
the word “metabolon” (from the Greek for “change”) had
existed fleetingly in the scientific vernacular in the early 20th
century, having been introduced by the physicist Ernest Rutherford in 1903 to designate sequences of atomic radioactivedecay species (73). Ironically, Rutherford was looking to
biology for metaphorical concepts at the time (13). The symbolism of the ancient term “protyle” lingered into the 20th
century, both in physics and biology. In 1920 Rutherford
named the proton, the newly discovered subatomic particle that
forms the nucleus of the hydrogen atom, in honor of Prout’s
original conception of “protyle” (9). Heretofore, the atom was
envisioned as an amorphous blob (a plasma) of positive charge,
within which freely moving electrons are interspersed - the
so-called “plum pudding” model that enjoyed brief popularity
at the beginning of the 20th century.
In an article entitled “The Particle Size of Biological Units:
A Review” published in 1932, the physical chemist John
Ferguson (27) surveyed the (then) current findings on the sizes
of entities that seemed to be minimally related to the manifestation of “life.” With the use of data obtained via the methods
of ultramicroscopy, gel-membrane ultrafiltration, and (the relatively new area of) ultracentrifugation, he considered a variety
of protein molecules, viruses, and “genes” (whose size, as well
as composition, was a matter of conjecture at the time). Having
drawn no unifying conclusion, Ferguson concluded that, “The
conception of a self-perpetuating catalyst of molecular (protein?) size is an alluring prospect to the materialist in search of
AJP-Cell Physiol • VOL
Water: The Overlooked Component of Protoplasm
Water has been recognized since very distant times as
profoundly important in both living and nonliving systems, as
seen for example in the ancient Hindu scriptures: “About 3,000
years ago the Upanishad Thinker said ‘it is water that assumes
the form of this Earth, midregion, this heaven, these mountains,
these gods and men, cattle and birds, herbs and trees, and
animals together with worms, flies and ants. Water is indeed all
these forms: meditate on water’” (15). Legions of thinkers have
since “meditated” on water, generating a rich history on this
deceptively simple molecule, with its own romantic side (6). In
fact, water was the original “protomatter” (or “protyle”) in
ancient Greek philosophy, having been proposed by Thales of
Miletus as the fundamental material basis of the world and
everything in it. Water is so intrinsic to the living state that it
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Fig. 2. The Sierpiński sponge as a model for the evolving infrastructure of living cells,
highlighting the role of internal surfaces. This is a fractal geometric form whose
progressive “fractalization” results in the surface area increasing to (the theoretical limit
of) infinity as the volume shrinks to zero. (Image created by Moses Boone; see
http://www.mathworks.com/matlabcentral/fileexchange/3524-sierpinski-sponge.)
the ‘Protyle’ of life. Speculation is fruitless, however, until
such entities are definitely established by physicochemical data
on the one hand, while the biologist from his (sic) viewpoint
recognizes their ‘vital’ powers of reproduction, assimilation,
and adaptation.” With the influence of 19th-century thought in
evidence, Ferguson (27) closed by opining that, “Life still is a
great mystery, and in the creed of the biologist its essence is
that peculiar ‘integrative force’ which still defies analysis. For
the biologist, biology, and not materialism, must yet remain the
‘working hypothesis’.”
Some 50 years after Ferguson’s review treatise, there appeared a more penetrating study bearing upon the question of
“biological units” in living cells. In 1980 the cell biologist
Peter Sitte (80) presented an in-depth examination of subcellular structures, motivated by the awareness of the high density
of protein molecules in (on) these structures and the realization
of the importance of surface activity in localized metabolic
processes. He pointed out a remarkable homology in the
surface-area-to-volume ratio (the “specific surface”) for all
membranous cytological substructures, corresponding to an
upper limit of 150 ␮m⫺1 and a “sphere of influence” of about
100 nm in diameter (see Fig. 3). Taking an integrated (what he
viewed as a distinctly “systems”) view of the cell, Sitte (79)
suggested that this homology reflects a common evolution in
the structure-and-function of the microenvironmental domains
that define cellular activities. Theoretical analysis has shown
that, by coincidence (!), the observed size of the homologous
spatial “units” noted by Sitte corresponds to the dimensional
limit for efficient reaction-diffusion coupling in metabolic
processes, based on typical values of enzyme catalytic-turnover constants and diffusion coefficients (91). Accordingly, we
are led to modify the fractal design in Fig. 2 to include
idealized spheres denoting confined regions, ranging from
organelles to protein complexes (“metabolons”), responsible
for the execution of localized metabolic processes in situ (see
Fig. 4). Thus it would seem that we have arrived at a “plum
pudding” picture, of sorts, for the structure-and-function of
protoplasm - 100 years after this transitory icon existed in the
annals of physics. We have also arrived at the realization that
the secret of protoplasm is to be found not in substance but in
process. As recapped fittingly by Sitte (80), “2500 years ago
the Greek philosopher Heraclitus assumed all things to be in a
state of steady motion and change. A living cell would have
given him plenty of satisfaction.”
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PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
C1287
Fig. 3. Surface area-to-volume proportions (“specific surfaces”) of cells,
organelles, and compartments. (Adapted from Ref. 80.)
is easily taken for granted and, alas, ignored. Interesting then,
that early descriptions of protoplasm overlooked water, except
to recognize its presence and obvious importance. Understandable perhaps, because those who studied protoplasm in those
days had no way to actually describe the physical properties
and detailed roles of water. When did the physical study of
water in biological systems begin? Difficult to say, but an early
insight was provided by the remarkable scientist Thomas
Graham (36), who found that some of the water in complex
systems, like gelatinous starch, does not dissolve sugar. Those
studying protoplasm at the time did not pick up on this
empirical fact and draw the inference that some of the water in
colloidal substances like protoplasm could differ from ordinary
water, with potentially significant consequences. Graham did
not use the term “bound water” to describe that fraction of
water, but one can posit that he had, indeed, come across the
idea.
The term “bound water” has had a focal position in biology
since the inventive work of Ross A. Gortner. He and his
students showed that a substantial amount of the water in
certain plant systems does not freeze at temperatures where
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Fig. 4. Modified representation of the Sierpiński sponge (see Fig. 2), with a
hierarchy of idealized spheres designating localized metabolic microenvironments.
(copyrighted image created by Roman Maeder; see http://www.mathconsult.ch/
showroom/pubs/MathProg/htmls/p2-16.htm)
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bulk water should do so (58). That fraction of water was
referred to as “bound,” and research on biological (protoplasmic) water would be influenced down to the present time. (Of
course, we now know that such water is not bound in a static
sense but is in dynamic exchange with the surrounding aqueous
phase.) Gortner’s work attracted the attention of those studying
the properties of colloids, a hot topic of that era. A meeting of
major importance was held at the University of Cambridge 29
Sept-1 Oct 1930 on “Colloid Science Applied to Biology,” and
the matter of cell and tissue water was a significant part.
Although Gortner did not attend (apparently due to ill health),
his paper (33) was communicated at the meeting and published
(along with the extensive discussion that followed it) in the
Transactions of the Faraday Society (vol. 26) that same year.
Although Gortner’s notion of “bound water” attracted considerable attention at the 1930 meeting, it was not completely
adopted. The very marked difference in opinion about protoplasmic water, leading to a polarization of views, was summed
up by two participants. The distinguished physiologist A. V.
Hill wrote, “ . . . in living tissue, at least in muscle, practically
the whole of the water is free . . .” (p. 72). Obviously, Hill was
not a proponent of bound water. In contrast, W. Ramsden
proposed that, “ . . . as regards the water inside a biological cell,
I should expect the whole of it to be ‘modified’ water . . .” (p.
697). However, the arguments of Hill prevailed, and his work
was cited for the next 40 years as evidence that almost all cell
water is that of an ordinary bulk solution.
Gortner was a key actor on the stage of protoplasmic water
during in the early 20th century. He wrote that, “It is my belief
that many of the reactions characteristic of living processes
have to do more with the water relationships of the organism
than any other single factor” (33). After the Cambridge meeting, Gortner published two major reviews in the (then) young
Annual Reviews of Biochemistry (34, 35), one of which focused
on the role of water in protoplasm. But his words apparently
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PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
Protoplasm: Is It Relevant Today?
Amidst the flood of data that has inundated the study of the
living cell in the postgenomic 21st century, there is a grand
synthesis proceeding under the rubric of systems biology. This
burgeoning epistemic framework, while only a decade old, has
rapidly assumed a defining role in the organization and integration of empirical information, as well as in driving knowledge discovery. A large and rapidly expanding taxonomic
terminology has arisen within the systems biology vocabulary,
using the suffix “-ome” to denote component sets for the
specific biological entities that are the subject of data collection, annotation, curation, and interpretation at each level of
organization in the living system. Instead of the perception of
the cell as a family of “-plasms” in 19th-century protoplasmic
theory, we see an assortment of “-omes” in 21st-century
cellular systems biology.
Surprisingly, there is no established definition of “systems
biology”, as is obvious from a simple search on the World
Wide Web. Powell et al. (70) put it simply that, “What is often
meant by ‘systems biology’ today is the attempt to make sense
of the vast amounts of data that have been accumulated by the
genome sequencing projects and other data-gathering exercises.” As suggested by Westerhoff and Alberghina (96), the
definition of systems biology is “heterogeneous” and specified
“by examples,” including many topics “all focusing on the
AJP-Cell Physiol • VOL
mechanics behind the emergence of functionality.” Absent a
standard definition, it seems universally accepted that this field
seeks to study the relationships and interactions among the
parts of biological systems and the integration of such information into a picture of the functioning organism (25). Despite
frequent reference to such pioneering systems theories as that
of Ludwig von Bertalanffy, systems biology in its current guise
focuses heavily on static informational arrays and component
mappings. The incorporation of a dynamical representation
would complete the picture, and this stands as an aspirational
goal at the moment. As expressed by Boguski (8), “The future
may lie in a new vision of annotation that supersedes static,
‘repository biology’ with a dynamic ‘virtual cell’ in which
most properties and behaviors can be quantitatively modeled
and dynamically represented in all of their interconnected
complexity” (for example, see Ref. 81). Such reasoning, of
course, lay at the foundation of the protoplasmic theory in the
19th century. Cellular systems biology would appear, at least
superficially, to be the emperor in new clothes.
The challenge today, daunting though it may be, is to create
a vision of the cell in silico by piecing together the “-omes,” in
conjunction with the vast amount of facts on the organizational
character of the life of the cell. The branch of cellular systems
biology known as “interactomics,” in particular, is generating
inroads here (Ref. 94; see also Molecular INTeraction [MINT]
database, http://mint.bio.uniroma2.it/mint/Welcome.do). Yet,
the data-driven advancement of knowledge continues to spawn an
expansive, hierarchically reductionistic collection of “-omes,” and
the path to the integration of the disparate “-omic” information
into a description of the cell as a functioning whole is not
altogether obvious. As argued cogently by Kell and Oliver (44),
the true course of study demands that “data” and “ideas” be
complementary and iterative partners along the way. The idea of
“protoplasm,” at its height, served a supremely important role in
organizing thought and focusing attention on the cellular
makeup as the fundament of life. The protoplasm principle
died when the inquiry “What is life?” died amidst a haze of
varied “-plasms” and “living particles.” Today, the overarching
idea of the cell as a “system,” though certainly an appropriate
identifier, seems self-evident to the point of being nondescript.
It remains to be seen if this idea will avoid becoming too
suffused with the assortment of “-omes” to achieve the goal of
an integrated view of cell structure-and-function.
Stripped of its vitalistic and esoteric trappings, protoplasmic
theory in its day (like cellular systems biology today) is really
about cell physiology (95). The aforementioned cell biologist
E. B. Wilson, a leading visionary (and transitional figure) in the
history of cell biology, remarked presciently in the opening
pages of the first edition of his text The Cell in Development
and Heredity (100), “No attempt is here made to identify a
living ‘protoplasm’ as such or to distinguish between ‘living’
and ‘non-living’ cell-components. Life is treated as a property
of the cell-system as a whole, the components of that system
differing only in the degree and manner of their activity . . .
Logically, no doubt, this is correct; and from a purely physiological point of view is perhaps the only possible mode of
treatment.” “Physiology,” itself, is one of those deeply rooted
biological subjects whose central role waned [relegated to the
status of “a quaint and vaguely anachronistic” discipline (14)]
in the course of the 20th century, as life’s processes were
dissected with increasing precision by the developing sciences
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had little impact. For example, William Seifriz, a prominent
student of protoplasm at the time, wrote a review with the title
The Properties of Protoplasm (77), in which Gortner’s papers
were not mentioned, and the concept of bound water not
considered at all.
Among those who took up the study of cell and tissue water
in the 1950s and 1960s, the names of Afanasii Troshin, Walter
Drost-Hansen, and Gilbert Ling stand out. They each had
differing ideas about the nature of cell water, but all three
agreed that it was very unlikely to be the same as ordinary bulk
water. But, like Gortner, they failed to convince most of the
scientific community. Then, in 1969, two papers appeared that
would change the balance. Freeman Cope (22) and Carlton
Hazelwood et al. (40) published evidence suggesting that much
(even all) of cellular water has properties markedly different
from “ordinary” bulk water. They interpreted their findings as
evidence for Gilbert Ling’s “Association-Induction Hypothesis” (50), which considers all of the water in cells to be in the
form of polarized multilayers, much different from the ordinary bulk fluid. The vibrant controversy that raged over the
next 20 years can be referred to as the “water wars.” It is
beyond the scope of the present article to review the abundant
literature and the excitement of that period. But we should note
that the battlefield has now grown rather quiet; and, although
the issues have not been resolved, the consensus view seems to
be not much different from that put forth by Hill in 1930 which is to say, that almost all of cellular (protoplasmic) water
is not unusual. Notwithstanding, the discussion continues (16,
30, 65, 66, 103). It seems odd that the most vital and widespread of biological molecules remains to have its physical
properties and participation in the cellular milieu (that is,
protoplasm) defined in detail. Perhaps we should heed the
advice of the ancients and meditate on protoplasmic water and
then return to its study.
Historical Perspectives
PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
of biochemistry, biophysics, genetics, and molecular biology.
Along with this analytical reductionism, though, came a loss in
our appreciation of the holistic function of the life form. While
physiology today may have lost its scientific independence, it
nevertheless represents “a point of view, an attitude toward the
study of ‘functions’ (not merely ‘mechanisms’ or ‘processes’)
that gives meaning to biological systems” (49). The study of
living cells, we would suggest, does not need a term, such as
“system,” to give it direction; rather it requires a physiological
way of thinking. It is in this sense that the word (nay, the idea)
“protoplasm” deserves its rightful place in the history of
biology.
DISCLOSURES
REFERENCES
1. Ainsworth C. Cells go fractal. Nature News (4th September, 2009)
doi:10.1038/news.2009.880.
2. Alberts B. The cell as a collection of protein machines: preparing the
next generation of molecular biologists. Cell 92: 291–294, 1998.
3. Andrews GF. The Living Substance as Such: and as Organism. Boston,
MA: Ginn, 1897 (Supplement to J Morphol, vol. 12, no. 2).
4. Aon MA, O’Bourke B, Cortassa S. The fractal architecture of cytoplasmic organization: scaling, kinetics, and emergence in metabolic
networks. Mol Cell Biochem 256/257: 169 –184, 2004.
5. Baker JR. The cell-theory: a restatement, history, and critique. Part III
The cell as a morphological unit. Q J Microsc Sci 93: 157–190, 1952.
6. Ball P. Life’s Matrix: A Biography of Water. Berkeley, CA: University
of California, 2001.
7. Bernard C. Leçons sur les Phénomènes de la Vie communs aux Animaux
et aux Végétaux. Paris: J-B Baillière et Fils, 1878 (translation by Hoff
HE, Guillemin R, Guillemin, L. Springfield, IL: Charles C. Thomas,
1974).
8. Boguski MS. Biosequence exegesis. Science 286: 453–455, 1999.
9. Brock WH. From Protyle to Proton: William Prout and the Nature of
Matter, 1785–1985. Boston, MA: Hilger, 1985.
10. Brücke (von) E. Die Elementarorganismen. Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften zu Wien. Mathematisch-Naturwissenschaftlichen Classe 44: 381–406, 1861.
11. Buchner E. Alkoholische Gärung ohne Hefezellen. Ber Deutschen Chem
Ges 30: 117–124, 1897.
12. Bütschli O. Investigations on Microscopic Foams and on Protoplasm.
London: Adam and Charles Black, 1894.
13. Campos L. The birth of living radium. Representations 97: 1–27, 2007.
14. Caplan MJ. Physiology and Physiology: Back to the future. Physiology
19: 232, 2004.
15. Chandogya C. 900 BC Upanishad VII. The Upanishads (translated by
Swami Nikhilananda). New York: Harper Torch, 1964.
16. Chaplin M. Do we underestimate the importance of water in cell
biology? Nature Reviews Mol Cell Biol 7: 861–866, 2006.
17. Clegg JS. What is the “cytosol”? Trends Biochem Sci 6: 436 –437, 1983.
18. Clegg JS. Properties and metabolism of the aqueous cytoplasm and its
boundaries. Am J Physiol Regul Integr Comp Physiol 246: R133–R151,
1984.
19. Clegg JS, Jackson SA. Glucose metabolism and the channeling of
glycolytic intermediates in permeabilized L-929 cells. Arch Biochem
Biophys 278: 452–460, 1990.
20. Clegg JS, Kell D, Knull H, Welch GR, Wilson J. Macromolecular
interactions: tracing the roots. Trends Biochem Sci 26: 91, 2001.
21. Coleman W. Biology in the Nineteenth Century: Problems of Form,
Function, and Transformation. New York: Wiley, 1971.
22. Cope F. Nuclear magnetic resonance evidence using D2O for structured
water in muscle and brain. Biophys J 9: 303–319, 1969.
23. Cornish-Bowden A (Editor). New Beer in an Old Bottle: Eduard
Buchner and the Growth of Biochemical Knowledge. València: Universitat de València Press, 1997.
24. Cornish-Bowden A, Cárdenas ML (Editors). Technological and Medical Implications of Metabolic Control Analysis. Dordrecht: Kluwer,
2000.
AJP-Cell Physiol • VOL
25. Cowley AW. The elusive field of systems biology. Physiol Genomics 16:
285–286, 2004.
26. Drysdale JJ. The Protoplasmic Theory of Life. London: Baillère, Tindall, and Cox, 1874.
27. Ferguson JH. The particle size of biological units: a review. J Phys
Chem 36: 2849 –2861, 1932.
28. Frey-Wyssling A. The submicroscopic structure of the cytoplasm. J Roy
Microscop Soc 60: 128 –139, 1940.
29. Frey-Wyssling A. Submicroscopic Morphology of Protoplasm and its
Derivatives. New York: Elsevier, 1948.
30. Fullerton GD, Cameron IL. Water compartments in cells. Methods
Enzymol 428: 1–28, 2007.
31. Geison GL. The protoplasmic theory of life and the vitalist-mechanist
debate. Isis 60: 272–292, 1969.
32. Gierasch LM, Gershenson A. Post-reductionist protein science, or
putting Humpty-Dumpty back together again. Nature Chem Biol 11:
774 –777, 2009.
33. Gortner RA. The state of water in colloidal and living systems. Trans
Faraday Soc 26: 678 –685, 1930.
34. Gortner RA. The role of water in the structure and properties of
protoplasm. Ann Rev Biochem 1: 21–54, 1932.
35. Gortner RA. Water in its biochemical relationships. Ann Rev Biochem 3:
1–22, 1934.
36. Graham T. Liquid diffusion applied to analysis. Phil Trans Roy Soc
London 151: 183–224, 1861.
37. Gutfreund H. Wilhelm Friedrich Kühne: an appreciation. FEBS Lett 62,
Suppl: E1–E2, 1976.
38. Hall TS. Ideas of Life and Matter: Studies in the History of General
Physiology (2 vol.). Chicago, IL: University of Chicago Press, 1969.
39. Harris H. The Birth of the Cell. New Haven, CT: Yale University Press,
1999.
40. Hazelwood CF, Nicholas BL, Chamberlain NF. Evidence for the
existence of a minimum of two phases of ordered water in muscle.
Nature 222: 747–750, 1969.
41. Heuser J. Whatever happened to the “microtrabecular concept”? Biol
Cell 94: 561–596, 2003.
42. Jacyna LS. Romantic thought and the origins of cell theory. In: Romanticism and the Sciences, edited by Cunningham A, Jardine N. Cambridge:
Cambridge University, 1990, p. 161–168.
43. Katchalski E, Silman I, Goldman R. Effect of the microenvironment on
the mode of action of immobilized enzymes. Adv Enzymol 34: 445–536,
1971.
44. Kell DB, Oliver SG. Here is the evidence, now what is the hypothesis?
The complementary roles of inductive and hypothesis-drive science in
the post-genomic era. Bioessays 26: 99 –105, 2004.
45. Kell DB, Welch GR. No turning back: reductionism and biological
complexity. Times Higher Educ Suppl (9th August, 1991), p. 15.
46. Kim S, Coulombe PA. Emerging role for the cytoskeleton as an
organizer and regulator of translation. Nature Rev Mol Cell Biol 11:
75–81, 2010.
47. Kohler R. The reception of Eduard Buchner’s discovery of cell-free
fermentation. J Hist Biol 5: 327–353, 1972.
48. Kohler R. The enzyme theory and the origin of biochemistry. Isis 64:
181–196, 1973.
49. Kremer RL. Physiology. In: The Modern Biological and Earth Sciences,
edited by Bowler PJ, Pickstone JV. Cambridge, UK: Cambridge University Press, 2009, vol. 6, p. 342–366.
50. Ling GN. A Physical Theory of the Living State: The AssociationInduction Hypothesis. Waltham, MA: Blaisdell, 1962.
51. Loew O. The Energy of Living Protoplasm. London: Kegan Paul,
Trench, Trübner, 1896.
52. Losa GA, Merlini D, Nonnenmacher TF, Weibel ER (Editors).
Fractals in Biology and Medicine, Basel: Birkhäuser, 2005, vol. 4.
53. Mandelbrot B. The Fractal Geometry of Nature. San Francisco, CA:
Freeman, 1983.
54. McNiven MA. The solid state cell. Biol Cell 94: 555–556, 2003.
55. Miller JG. Living Systems. New York: McGraw-Hill, 1978.
56. M’Kendrick JG. On the modern cell theory. Proc Philos Soc Glasgow
19: 1–55, 1888.
57. Mosbach K, Mattiasson B. Matrix-bound enzymes. Part II. Studies on
a matrix-bound two-enzyme system. Acta Chem Scand 24: 2093–2100,
1970.
58. Newton R, Gortner RA. A method of estimating hydrophilic colloid
content of expressed plant tissue fluids. Bot Gazette 74: 442–448, 1922.
298 • JUNE 2010 •
www.ajpcell.org
Downloaded from http://ajpcell.physiology.org/ by 10.220.32.246 on June 15, 2017
No conflicts of interest are declared by the author(s).
C1289
Historical Perspectives
C1290
PROTOPLASMIC THEORY AND CELLULAR SYSTEMS BIOLOGY
AJP-Cell Physiol • VOL
81. Slepchenko BM, Schaff JC, Macara I, Loew LM. Quantitative cell
biology with the virtual cell. Trends Cell Biol 13: 570 –576, 2003.
82. Srere PA. Protein crystals as a model for mitochondrial matrix proteins.
Trends Biochem Sci 6: 4 –7, 1981.
83. Srere PA. The metabolon. Trends Biochem Sci 10: 109 –110, 1985.
84. Srere PA. Macromolecular interactions: tracing the roots. Trends Biochem Sci 25: 150 –153, 2000.
85. Srere PA, Mattiasson B, Mosbach K. An immobilized three-enzyme
system: a model for microenvironmental compartmentation in mitochondria. Proc Natl Acad Sci USA 70: 2534 –2538, 1973.
86. Stadtman EA, Chock PB (Editors). Current Topics in Cellular Regulation: From Metabolite, to Metabolism, to Metabolon. New York:
Academic, 1992, vol. 33.
87. Thomas D, Broun G, Sélégny E. Monoenzymatic model membranes:
diffusion-reaction kinetics and phenomena. Biochimie 54: 229 –244,
1972.
88. Thomson JA. The Science of Life. London: Blackie, 1899.
89. Weibel ER. Fractal geometry: a design principle for living organisms.
Am J Physiol Lung Cell Mol Physiol 261: L361–L369, 1991.
90. Welch GR. On the role of organized multienzyme systems in cellular
metabolism: a general synthesis. Prog Biophys Mol Biol 32: 103–191,
1977.
91. Welch GR. An analogical “field” construct in cellular biophysics: history
and present status. Prog Biophys Mol Biol 57: 71–128, 1992.
92. Welch GR. Bioenergetics and the cellular microenvironment. Pure Appl
Chem 65: 1907–1914, 1993.
93. Welch GR. T. H. Huxley and the “protoplasmic theory of life”: 100
years later. Trends Biochem Sci 20: 481–485, 1995.
94. Welch GR. The “fuzzy” interactome. Trends Biochem Sci 34: 1–2, 2009.
95. Welch GR. Physiology, physiomics, and biophysics: a matter of words.
Prog Biophys Mol Biol 100: 4 –17, 2009.
96. Westerhoff HV, Alberghina L. Systems biology: did we know it all
along? In: Systems Biology: Definitions and Perspectives, edited by
Alberghina L, Westerhoff HV. Berlin/Heidelberg: Springer, 2005, p. 3–9.
97. Westerhoff HV, Welch GR. Enzyme organization and the direction of
metabolic flow. Curr Top Cell Regul 33: 361–390, 1992.
98. Westfall RS. The Construction of Modern Science: Mechanisms and
Mechanics. Cambridge: Cambridge University Press, 1977.
99. Wilson EB. The structure of protoplasm. Science 10: 33–45, 1899.
100. Wilson EB. The Cell in Development and Heredity. New York: MacMillan, 1896 (1st ed.), 1900 (2nd ed.), 1925 (3rd ed.).
101. Wolosewick JJ. Joining the trek with Keith up the Serpentine Road: the
lattice from another perspective. Biol Cell 94: 557–559, 2003.
102. Wolosewick JJ, Porter KR. Microtrabecular lattice of the cytoplasmic
ground substance: artifact or reality? J Cell Biol 82: 114 –139, 1979.
103. Zheng JM, Chin WC, Khijniak E, Khijniak E Jr, Pollack GH.
Surfaces and interfacial water: evidence that hydrophilic surfaces have
long-range impact. Adv Colloid Interface Sci 127: 19 –27, 2006.
298 • JUNE 2010 •
www.ajpcell.org
Downloaded from http://ajpcell.physiology.org/ by 10.220.32.246 on June 15, 2017
59. Nordenskiöld E. The History of Biology. New York: Knopf, 1928.
60. Ottaway JH, Mowbray J. The role of compartmentation in the control
of glycolysis. Curr Top Cell Regul 12: 108 –208, 1977.
61. Ovádi J. Cell Architecture and Metabolic Channeling. Austin, TX:
Landes, 1995.
62. Peters R. Surface structure in the integration of cell activity. Trans
Faraday Soc 26: 797–807, 1930.
63. Peters R. The problem of cytoplasmic integration. Proc Roy Soc London
B173: 11–19, 1969.
64. Pfeffer W. Osmotische Untersuchungen. Studien zur Zellmechanik.
Leipzig: Verlag von Wilhelm Engelmann, 1877.
65. Pollack GN, Chin WC. Phase Transitions in Cell Biology. Dordrecht:
Springer, 2008.
66. Pollack GN, Cameron IL, Wheatley DN. Water and the Cell. Dordrecht: Springer, 2006.
67. Porter KR. The ground substance: observations from electron microscopy. In: The Cell, edited by Brachet J, Mirsky AE. New York: Academic, 1961, vol. 2, p. 621–675.
68. Porter KR. The cytomatrix: a short history of its study. J Cell Biol 99:
3s-12s, 1984.
69. Porter KR. Structural organization of the cytomatrix. In: The Organization of Cell Metabolism, edited by Welch GR, Clegg JS. New York:
Plenum, 1986, p. 9 –25.
70. Powell A, O’Malley MA, Müller-Wille SEW, Calvert J, Dupré J.
Disciplinary baptisms: a comparison of the naming stories of genetics,
molecular biology, genomics and systems biology. Hist Phil Life Sci 29:
5–32, 2007.
71. Reynolds A. The cell’s journey: from metaphorical to literal factory.
Endeavour 31: 65–70, 2007.
72. Ritter WE. The Unity of the Organism. Boston, MA: Gorham, 1919.
73. Rutherford E, Soddy F. Radioactive change. Philos Mag 5: 576 –591,
1903.
74. Sachs (von) J. Beiträge zur Zellentheorie. Energiden und Zellen Flora
75: 57–67, 1892.
75. Schultze M. Über Muskelkörperchen und dass was Man eine Zelle zu
nennen habe. Archiv für Anatomie, Physiologie, und wissenschaftliche
Medicin: 1–27, 1861.
76. Seifriz W. Protoplasm. New York: McGraw-Hill, 1936.
77. Seifriz W. The properties of protoplasm. Adv Enzymol 7: 35–64, 1947.
78. Siegbahn N, Mosbach K, Welch GR. Models of organized multienzyme systems: use in microenvironmental characterization and in practical application. In: Organized Multienzyme Systems: Catalytic Properties, edited by Welch, GR. New York: Academic, 1985, p. 271–301.
79. Sitte P. Die lebende Zelle als System, Systemelement, und Übersystem.
Nova Acta Leopoldina 47: 195–216, 1977.
80. Sitte P. General principles of cellular compartmentation. In: Cell Compartmentation and Metabolic Channelling, edited by Nover L, Lynen F,
Mothes K. New York: Elsevier, 1980, p. 17–47.