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Protoplasm

Protoplasm is the living substance within a cell, encompassing the cytoplasm, nucleus, and other organelles, and serving as the site for essential biological processes such as metabolism, growth, and reproduction.^[1] It is a complex, viscous colloid composed primarily of water (approximately 70-90%), along with organic compounds including proteins, lipids, carbohydrates, and nucleic acids, as well as inorganic ions and salts.^[1] This fluid matrix, enclosed by the plasma membrane, exhibits dynamic properties, alternating between sol (liquid-like) and gel (semi-solid) states, which facilitate cellular movement and function.^[2] The concept of protoplasm emerged in the mid-19th century as a foundational idea in cell biology, with the term first coined by Czech physiologist Jan Evangelista Purkinje in 1839 to describe the gelatinous fluid in animal tissues, and later applied to plant cells by German botanist Hugo von Mohl in 1846.^[3] It represented the "physical basis of life," viewed by scientists like Thomas Huxley as the essential matter manifesting vital phenomena, driving the protoplasmic theory that emphasized the cell's contents over its boundaries.^[4] This theory, peaking in the late 1800s, influenced early understandings of cellular dynamics, including streaming and division, before advances in microscopy and staining techniques in the early 20th century revealed detailed subcellular structures.^[4] In contemporary biology, the term protoplasm is considered archaic and has been supplanted by precise descriptors like cytoplasm (the region outside the nucleus) and nucleoplasm (within the nucleus), reflecting advances in molecular and systems biology that dissect cellular components at finer levels.^[4] Nonetheless, the protoplasmic legacy endures in discussions of cellular organization, highlighting the shift from holistic views of life to reductionist and integrative models.^[4]

Terminology and Definition

Etymology

The term "protoplasm" derives from the Greek words prōtos, meaning "first" or "primitive," and plasma, meaning "something formed" or "mold," reflecting its connotation as the primary formative substance of life.^[3] It was coined in 1839 by the Czech physiologist Jan Evangelista Purkinje to describe the viscous, formative material observed in the early stages of animal embryos during his microscopic studies of cellular development.^[5] Purkinje introduced the word in German as Protoplasma, emphasizing its role as the essential, life-sustaining fluid within cells, a term that quickly gained traction in European scientific discourse.^[6] Prior to Purkinje's nomenclature, the French biologist Félix Dujardin had described a similar jelly-like, contractile living substance in protozoa in 1835, naming it sarcode after the Greek for "flesh."^[4] This term highlighted the animated, granular nature of the material exuding from foraminifera and other microorganisms. In 1846, the German botanist Hugo von Mohl extended Purkinje's concept to plant cells, applying protoplasm to the slimy, active contents he observed and explicitly equating it with Dujardin's sarcode to unify descriptions across animal and vegetable kingdoms.^[4] The term's adoption into English scientific literature occurred by the mid-1840s, with early uses appearing in translations and botanical texts around 1848, solidifying protoplasm as a standard descriptor for the living basis of cells.^[3] Its broader popularization came in 1868 through British biologist Thomas Huxley's lecture "On the Physical Basis of Life," where he proclaimed protoplasm as the universal "physical basis of life" in both plants and animals, bridging physiological and materialist views of vitality.^[7] This address marked a pivotal moment in elevating the term from niche microscopy to a cornerstone of 19th-century biology.

Core Concept

Protoplasm refers to the entire living material within a cell, comprising the cytoplasm, which forms the outer portion surrounding the organelles, and the nucleoplasm, which constitutes the contents of the nucleus. This living substance includes the cytosol, organelles, and other dynamic components essential for cellular processes, serving as the physical basis for life activities.^[8]^[9] While protoplasm encompasses all vital cellular contents, it excludes non-living structures such as the rigid cell wall in plant cells, which provides structural support, as well as inclusions like large vacuoles and storage granules. The extracellular matrix, an acellular network outside the cell, is similarly not part of protoplasm, distinguishing the term's focus on intracellular living matter.^[8]^[9]^[10] In the 19th century, protoplasm was conceptualized as a uniform, homogeneous substance forming the foundational matrix of cells, often described as a viscous, nutritive fluid without initial differentiation. Over time, advancements in microscopy revealed its heterogeneous nature, incorporating distinct organelles and molecular complexes rather than a single uniform entity. This evolution has led to a partial transition in terminology, where "cytoplasm" now commonly denotes the non-nuclear living content, though protoplasm retains its broader classical scope.^[11]^[9]

Historical Development

Early Observations

The advent of microscopy in the 17th century enabled the first glimpses of the dynamic internal substances within simple living forms, setting the stage for later recognition of protoplasm. Antonie van Leeuwenhoek, a Dutch tradesman and pioneering microscopist, reported in the 1670s his observations of minuscule "animalcules" teeming in pond water samples, which he examined using single-lens microscopes of his own design achieving magnifications up to 270 times. These organisms appeared as actively moving entities, as detailed in his letters to the Royal Society describing their shapes, speeds, and apparent life processes. Building on such empirical foundations, 18th-century naturalists explored the contractility of simple aquatic organisms, revealing hints of fluid-like interiors capable of coordinated responses. René Réaumur, a French physicist and natural philosopher, corresponded extensively with Abraham Trembley in the 1740s, guiding the latter's experiments on freshwater polyps known as hydra, which displayed remarkable contractility and regeneration when cut or injured. Trembley's meticulous studies, conducted in Réaumur's laboratory, demonstrated how these organisms could constrict and expand, challenging prevailing views of vital forces and implying a substantive, responsive medium within their bodies.^[12] By the early 19th century, improved optical instruments allowed for more precise delineations of cellular interiors in both plant and animal tissues. In 1831, Scottish botanist Robert Brown, while investigating orchid pollen grains and plant cells, identified a distinct, opaque nucleus within the cell interior, describing it as a consistent feature across various plant species and suggesting an organized, substantive framework underlying cellular structure. This observation, though not yet termed protoplasm, underscored the presence of a structured internal material beyond mere cell walls. Concurrent protozoological investigations further illuminated the vital qualities of granular internals in unicellular life. In 1838, German microscopist Christian Gottfried Ehrenberg published his comprehensive classification of Infusoria—microscopic aquatic organisms including protozoa—in which he portrayed them as fully formed animals with complex internal organization, featuring a "granular fluid" essential to their motility and digestion. Ehrenberg's detailed illustrations and descriptions emphasized this fluid as the seat of life processes, marking a pivotal empirical step toward conceptualizing a universal living substance.

Key Formulations

In 1839, Czech physiologist Jan Evangelista Purkinje advanced the concept of protoplasm through his studies on embryonic development, particularly in cleaving eggs of animals such as chicks and frogs. He identified the viscous, granular substance within these early embryos as "protoplasm," viewing it as the fundamental material from which entire organisms arise during cleavage and differentiation. This formulation positioned protoplasm not merely as a cellular component but as the dynamic source of formative processes in animal development.^[6]^[5] Building on Purkinje's work, German botanist Hugo von Mohl extended the term to plant cells in his 1846 publication, where he described the inner content of the "primordial utricle"—previously noted by others—as a tough, slimy, granular, semi-fluid mass distinct from the cell wall and vacuole. Mohl equated this substance with Purkinje's protoplasm, emphasizing its role as the living essence of vegetable cells and thereby unifying the concept across plant and animal kingdoms. His analysis highlighted protoplasm's contractile and streaming properties, solidifying its status as the active, physiological core of cellular life.^[10] A pivotal theoretical elevation came in 1868 with British biologist Thomas Henry Huxley's lecture "On the Physical Basis of Life," delivered to the British Association for the Advancement of Science. Huxley declared protoplasm the "physical basis of life," arguing that this albuminous substance, present in all organisms from amoebae to humans, underpins all vital phenomena including metabolism, irritability, and reproduction. He linked protoplasm's chemical composition—primarily proteins and water—to its ability to perform physiological functions, rejecting vitalistic notions and promoting a materialistic view of biology that influenced subsequent research. Notably, Huxley connected protoplasm to earlier descriptions of "sarcode" by Félix Dujardin in 1835, the living jelly observed in protozoa.^[13] In the early 20th century, German physiologist Max Verworn refined protoplasm theory with his "biogen" hypothesis, proposing that protoplasm functions as an elementary living unit composed of dynamic, protein-based "biogen" molecules capable of assimilation, dissimilation, and response to stimuli. Detailed in his 1903 work Die Biogenhypothese, this idea portrayed protoplasm as a self-regulating chemical system, bridging organic chemistry and physiology while engaging debates between mechanistic and vitalistic interpretations of life. Verworn's formulation emphasized protoplasm's role in elementary life processes, influencing early 20th-century biochemistry. Late 19th-century refinements further integrated dynamic behaviors into protoplasm theory, notably through German botanist Julius von Sachs's work on protoplasm's irritability and its central role in plant physiological processes like photosynthesis. His experimental work, including iodine tests on chloroplasts, underscored protoplasm's excitability and unity as the basis of plant vitality, contributing to a more comprehensive understanding of its living properties.^[14]

Physical Characteristics

Colloidal Nature

Protoplasm exhibits a colloidal state characterized as a complex sol-gel system, in which dispersed proteinaceous particles and water form a semi-fluid sol phase that can reversibly transition to a more structured gel phase under physiological stimuli such as changes in pH, temperature, or ionic concentration.^[15] This dual nature arises from the suspension of macromolecular components, primarily proteins, in an aqueous medium, allowing protoplasm to maintain both fluidity for internal transport and rigidity for structural integrity.^[16] The recognition of protoplasm's colloidal properties dates to the mid-19th century, with Thomas Graham's 1861 proposal identifying it as a homogenous, proteinaceous colloid based on diffusion studies that distinguished colloidal substances from true solutions.^[17] This view was reinforced in the 1880s through osmotic pressure investigations by Wilhelm Pfeffer, who demonstrated that protoplasmic boundaries behave like semipermeable membranes, consistent with colloidal osmotic effects in plant cells.^[18] Thomas Huxley's earlier 1868 conceptualization of protoplasm as the "physical basis of life" aligned with this emerging colloidal framework, portraying it as a dynamic, jelly-like substance akin to a complex cytode.^[13] In terms of particle dispersion, protoplasm consists of micelles formed by proteins such as albumins, suspended in an aqueous continuum, with particle sizes typically ranging from 1 to 100 nm—dimensions that prevent gravitational sedimentation while enabling Brownian motion.^[16] These micelles contribute to the overall colloidal stability, as the charged surfaces of protein aggregates interact electrostatically to form a dispersed phase without coalescing.^[19] Reversible phase transitions between sol and gel states are central to protoplasm's colloidal dynamics, particularly evident in amoeboid movement where the ectoplasm gels for pseudopod extension while the endoplasm remains sol-like for flow.^[20] These shifts, occurring without significant volume change, were elucidated in the early 1900s through colloid chemistry principles advanced by Wolfgang Ostwald, who described gelation as a network formation in lyophilic sols responsive to environmental cues.^[19] Microscopic evidence supporting this colloidal dispersion emerged in the 1920s via dark-field illumination techniques, which revealed the erratic Brownian motion of protoplasmic granules, confirming their submicroscopic size and suspension in a fluid medium rather than a rigid lattice.^[21] Such observations underscored the kinetic equilibrium inherent to protoplasm's sol phase, aligning with theoretical predictions from colloid science.^[15]

Viscoelastic Properties

Protoplasm exhibits a wide range of viscosity, transitioning between a low-viscosity sol state during active phases, comparable to water at approximately 1-10 cP, and a high-viscosity gel state in resting conditions exceeding 1000 cP.^[22] These variations were quantified in the 1930s using capillary flow methods on extruded protoplasm from amoebae and plant cells, revealing dynamic shifts influenced by cellular activity.^[23] The elasticity and contractility of protoplasm allow it to undergo deformation and subsequent recovery, facilitating processes such as cytoplasmic streaming, where flow rates reach up to 100 μm/s in certain plant cells like those of Elodea leaves under optimal conditions.^[24] This mechanical resilience enables the protoplasm to maintain structural integrity while supporting rapid intracellular transport. Flow phenomena in protoplasm, notably cyclosis or circulatory streaming, are propelled by motor proteins interacting with cytoskeletal elements, with early quantifications achieved through microcinematography in the 1890s that captured streaming velocities and patterns in living cells.^[25] Under applied stress, protoplasm displays shear-thinning behavior, wherein viscosity decreases to enable fluid movement, mirroring the properties of non-Newtonian fluids and aiding in cellular deformation.^[26] Experimental studies in the 1920s, particularly through microdissection and extrusion techniques, demonstrated protoplasm's thixotropic properties, where viscosity reduces over time under constant shear and recovers upon rest, as observed in algal and protozoan systems.^[22]

Chemical Composition

Major Components

Protoplasm is predominantly composed of water, which constitutes 70-90% of its total weight and serves as the primary solvent for other components.^[27] This water content varies by cell type; for instance, muscle cells typically contain around 80% water. The organic macromolecules in protoplasm include proteins, which make up 10-20% of the total weight and encompass structural elements such as actin and myosin in contractile tissues.^[27] Carbohydrates account for 1-2%, often in the form of storage polysaccharides like glycogen.^[27] Lipids comprise 2-5%, primarily phospholipids that contribute to membrane structures.^[27] Inorganic ions, including salts such as Na⁺, K⁺, and Ca²⁺, represent approximately 1% of protoplasm's mass and help maintain osmotic balance.^[27] The pH of protoplasm is typically maintained between 6.8 and 7.2.^[28] Nucleic acids, such as DNA and RNA, constitute 1-2% of the mass in eukaryotic cells, primarily concentrated in the nucleoplasm where they support hereditary functions, though they are minor contributors overall; percentages are higher in prokaryotes (up to 6%).^[27]^[29] Carbon, hydrogen, oxygen, and nitrogen comprise about 96% of protoplasm's dry weight, underscoring the dominance of these elements in its organic framework.^[30] These components collectively form a colloidal suspension that defines protoplasm's fluid consistency.^[27]

Molecular Interactions

In protoplasm, protein folding and aggregation are driven primarily by hydrophobic interactions and hydrogen bonding, which enable the formation of stable secondary, tertiary, and quaternary structures such as fibrils, globules, or multimeric assemblies. These non-covalent forces minimize exposure of nonpolar residues to the aqueous environment while stabilizing polar interactions, allowing proteins to adopt functional conformations within the crowded cytoplasmic milieu. A representative example is the hemoglobin tetramer, where hydrophobic contacts between subunits and hydrogen bonds between heme groups and globin chains facilitate oxygen-binding cooperativity and overall structural integrity. Such interactions contribute to the organized architecture of protoplasm, preventing random precipitation while enabling dynamic assembly. Ion-protein binding plays a crucial role in modulating protoplasm's physical state, particularly through calcium ions (Ca²⁺) that bind to specific protein sites, inducing conformational changes that trigger gelation and alter viscosity. This process involves Ca²⁺ coordination with oxygen-containing side chains in proteins analogous to calmodulin, promoting cross-linking and shifting protoplasm from a sol to a gel phase, as observed in contractile responses of cellular structures. Early investigations highlighted how divalent cations like Ca²⁺ enhance cohesion in protein colloids, influencing the reversible sol-gel transformations essential for protoplasmic motility.^[16] Carbohydrate-lipid complexes within protoplasm, notably glycoproteins embedded in membranous components, rely on glycosylation for structural stability and functional versatility. Glycosylation typically constitutes 5-10% of glycoprotein mass by weight, with carbohydrate moieties forming hydrogen bonds and steric barriers that shield lipid cores from hydrolysis and aggregation. These complexes facilitate selective interactions at protoplasmic interfaces, supporting compartmentalization and signaling. The effects of pH and ionic strength on protoplasmic organization are mediated by buffering systems, such as phosphate-based buffers, which maintain near-neutral pH to preserve protein solubility and enzymatic activity while countering fluctuations from metabolic byproducts. Elevated ionic strength can screen charges on proteins, promoting aggregation, whereas optimal conditions sustain electrostatic repulsion for fluidity. The Donnan equilibrium further governs ion distribution across semipermeable boundaries in protoplasm, resulting in unequal ion concentrations that influence osmotic balance and membrane potential.^[31] Early biochemical insights into these interactions emerged from 1910s studies on protein denaturation in protoplasm extracts, where heat or chemical agents disrupted native structures, revealing the sensitivity of hydrogen bonds and hydrophobic associations to environmental perturbations. Chick and Martin's work demonstrated that denaturation precedes coagulation, with rate constants varying by temperature and underscoring the role of unfolding in loss of solubility. Sørensen's contemporaneous advancements in pH measurement enabled quantification of acidity's impact on protein stability in biological extracts, laying groundwork for understanding ionic and pH-dependent dynamics in living matter.

Biological Functions

Metabolic Processes

Protoplasm serves as the primary site for metabolic processes that sustain life, encompassing both catabolic breakdown for energy release and anabolic synthesis for building cellular components. In the 19th century, Justus von Liebig recognized in the 1840s that oxidative metabolism, involving the chemical transformation of nutrients into energy through respiration, occurs within cells.^[32] This view aligned with emerging understandings of cellular chemistry, where protoplasm was seen as the medium for these vital reactions. Thomas Huxley further emphasized this in 1868, describing protoplasm as the physical basis of life and linking its metabolic activities—such as the ongoing chemical changes of assimilation and disassimilation—to what he termed the "living force" animating organisms.^[13] Respiration within protoplasm, particularly in the cytoplasm, begins with glycolysis, where glucose is oxidized to pyruvate, yielding a net of 2 ATP molecules and 2 NADH per glucose molecule.^[33] Pyruvate then enters the mitochondria for the Krebs cycle (also known as the citric acid cycle), generating an additional 2 ATP, 6 NADH, and 2 FADH₂ per glucose, which fuel the electron transport chain to produce approximately 28-30 more ATP through oxidative phosphorylation.^[33] Overall, complete aerobic respiration in protoplasm yields approximately 30-32 ATP per glucose molecule, providing the energy currency essential for cellular functions and historically tied to Huxley's concept of protoplasm as the seat of life's dynamic force./05:_Cells/5.09:_Cellular_Respiration) Protein synthesis, a key anabolic process, occurs on ribosomes distributed throughout the cytoplasm and nucleoplasm of protoplasm, where messenger RNA (mRNA) is translated into polypeptides at rates reaching up to 20 amino acids per second in rapidly growing cells like Escherichia coli.^[34] This high-efficiency translation ensures the production of enzymes and structural proteins necessary for maintaining protoplasmic integrity and function. Complementing this, anabolism in protoplasm involves the biosynthesis of carbohydrates and lipids from simple precursors; for instance, fatty acid synthesis takes place in the cytoplasm and endoplasmic reticulum, utilizing acetyl-CoA and NADPH to elongate carbon chains into triglycerides and phospholipids./06:Metabolism_I-_Oxidative_Reductive_Processes/6.12:_Fatty_Acid_Synthesis) These pathways rely on enzymes embedded in the protoplasmic matrix, drawing from its chemical components like nucleotides and lipids to drive synthesis. Waste management in protoplasm is exemplified by the urea cycle, a catabolic pathway primarily in liver hepatocytes that detoxifies ammonia—a byproduct of protein breakdown—into urea for excretion.^[35] This cycle spans the mitochondria and cytoplasm of protoplasmic liver cells, incorporating reactions that consume 4 ATP equivalents per urea molecule produced while preventing toxic ammonia accumulation.^[35] Through these integrated processes, protoplasm orchestrates energy production, macromolecular assembly, and waste elimination, embodying the core of cellular metabolism.

Dynamic Movements

Protoplasm exhibits dynamic movements essential for cellular motility and intracellular transport, a property first highlighted in the mid-19th century through observations of contractility and irritability. In the 1860s, Max Schultze described protoplasm as the fundamental substance underlying these vital activities, demonstrating its ability to contract and respond to stimuli in various cell types, including amoebae and muscle cells. This contractility was viewed as a hallmark of living matter, enabling protoplasm to drive locomotion and shape changes without rigid structures. Amoeboid locomotion represents a classic example of protoplasmic motility, where pseudopods extend through localized actin polymerization at the cell's leading edge, propelling the organism forward. In Amoeba proteus, this process allows speeds of 10-20 μm/min, with the polymerization of globular actin (G-actin) into filamentous actin (F-actin) generating protrusive forces against the plasma membrane.^[36] The viscoelastic flow of protoplasm facilitates this extension by providing a fluid medium for cytoskeletal reorganization.70487-9) Cytoplasmic streaming, or cyclosis, involves myosin-actin interactions that transport organelles and distribute nutrients within the cell. In characean algae such as Chara corallina, myosin XI motors slide along actin filament bundles, achieving streaming velocities of 50-100 μm/s and enabling efficient organelle movement over long distances.^[37] This process highlights protoplasm's role as a dynamic conveyor, with the endoplasm flowing bidirectionally around the vacuole.^[38] Ciliary and flagellar beating rely on dynein-powered microtubule sliding within specialized protoplasmic structures called axonemes. Dynein arms generate inter-doublet sliding, producing bending waves that propel cells or fluids, with beat frequencies typically ranging from 10-40 Hz in organisms like Paramecium and Chlamydomonas.^[39] This rhythmic motion underscores protoplasm's capacity for coordinated, high-frequency contractions in motile appendages.^[40] Vesicular transport through endocytosis and exocytosis cycles maintains protoplasm's compositional balance and facilitates material exchange. In active cells, such as retinal bipolar neurons, rates exceed 1000 vesicles per second during intense stimulation, involving clathrin-coated pits for uptake and SNARE-mediated fusion for release.^[41] These cycles ensure rapid turnover, supporting protoplasm's adaptability to environmental demands.^[42]

Modern Perspectives

Relation to Cell Biology

In modern cell biology, the historical concept of protoplasm corresponds primarily to the cytoplasm—including organelles such as mitochondria and the endoplasmic reticulum—and the nucleoplasm, encompassing chromatin and the nucleolus within the nucleus.^[43] This mapping reflects the 19th-century view of protoplasm as the fundamental living substance filling the cell, now understood as a dynamic matrix of aqueous cytosol interspersed with membrane-bound and non-membrane-bound structures.^[44] The recognition that protoplasm is not a uniform continuum began in the late 19th century, notably with Richard Altmann's 1890 identification of "bioblasts"—granular elements within the protoplasm that he proposed as autonomous units, later recognized as precursors to mitochondria and other organelles.^[44] This challenged the homogeneity of protoplasm, and contemporary studies confirm that organelles occupy approximately 20% of the cell's volume in typical eukaryotic cells, leaving the remainder as cytosol and other components.^[45] Protoplasm played a pivotal role in refining the cell theory proposed by Matthias Jakob Schleiden and Theodor Schwann in the 1830s, which posited cells as the basic units of life; protoplasm was seen as the essential "living stuff" animating these units, providing a material basis for cellular processes.^[43] However, this perspective was superseded in the 1930s and 1940s by the advent of electron microscopy, which revealed intricate subcellular substructures, including detailed organelle architectures unattainable with light microscopy.^[46] The assumption of protoplasmic homogeneity was further invalidated by 1950s ultrastructure studies using advanced fractionation and imaging techniques, which demonstrated compartmentalization and molecular specificity within cells.^[43] Consequently, the term "protoplasm" fell out of common usage after the 1960s, as it carried connotations of vitalism incompatible with emerging reductionist paradigms.^[43] A key shift occurred in the 1940s and 1950s through biochemical advances, such as Oswald Avery's 1944 demonstration of DNA as the genetic material and the 1953 elucidation of DNA's double-helix structure by James Watson and Francis Crick, which emphasized discrete molecules like DNA and proteins over the bulk properties of protoplasm.^[47] These developments redirected focus toward molecular mechanisms governing cellular function, integrating protoplasm's legacy into a compartmentalized, genetically driven model of the cell.^[43]

Current Relevance

In contemporary biology education, the concept of protoplasm persists as a historical precursor to modern cell biology, often introduced in introductory textbooks to illustrate the evolution of understanding cellular structure from a holistic "living substance" to compartmentalized components. Post-2000 educational resources, including college-level texts, emphasize protoplasm's role in 19th-century theories while transitioning students to precise terms like cytoplasm and cytosol, fostering conceptual development in cellular dynamics. This approach underscores the term's value in teaching the shift from vitalistic views to mechanistic explanations of life processes. Despite its obsolescence in molecular biology—where the term is avoided due to its imprecision in describing fractionated cellular elements like organelles and aqueous phases—protoplasm retains niche applications in fields such as plant physiology. In plant physiology, the related concept of protoplasts (wall-less cells exposing protoplasm) remains vital for genetic transformation and synthetic biology experiments, as seen in recent protocols for crop improvement.^[48] Additionally, in astrobiology, protoplasm-like definitions of "living matter" appear in discussions of protocells, bridging nonliving and living systems in origin-of-life models. In modern curricula, including those aligned with international standards, protoplasm is phased out in favor of "cytosol" for the fluid matrix of the cytoplasm, reflecting a preference for terms that align with biochemical precision.^[49] Recent research in synthetic biology has revived interest in protoplasm-like constructs, particularly in the 2010s, where artificial cell models emulate the dynamic, viscous properties of historical protoplasm to engineer minimal life forms. Papers from this period explore phase-separated "surface protoplasm" in mineral-bound systems to catalyze chemical reactions mimicking early cellular evolution, advancing bottom-up assembly of protocells. This resurgence highlights protoplasm's conceptual utility in designing self-organizing biomolecular networks for therapeutic applications. As of 2025, ongoing research in synthetic biology continues to explore protoplasm-like dynamic systems in bottom-up construction of artificial cells to model early cellular evolution and enable novel therapeutic applications.^[50]^[51] The legacy of protoplasm, notably through Thomas Huxley's 1868 essay "The Physical Basis of Life,"^[13] profoundly influenced the transition from vitalism to mechanistic biology, equating protoplasm with life's material foundation and rejecting supernatural origins. Huxley's ideas echo in 21st-century origin-of-life research, where protoplasm's formulation as ordinary matter underpins models of abiogenesis, such as heterotrophic origins from prebiotic soups, informing current studies on self-replicating chemical systems. This enduring impact reinforces protoplasm's role in framing life as an emergent property of physical processes.^[52]

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In E. coli growing in rich medium, the rate of protein synthesis is around 20 amino acids per second per ribosome (Liang et al., 2000), and this occurs at an ...
[34]
Physiology, Urea Cycle - StatPearls - NCBI Bookshelf
Jul 15, 2018 · The urea cycle begins in the mitochondria of hepatocytes and ends in the cytoplasm. The following reactions will be shown schematically below ...
[35]
Actin polymerization and pseudopod extension during amoeboid ...
The assembly of F-actin that peaks between 40 and 60 sec after the onset of stimulation is temporally correlated with the growth of new pseudopods. F-actin, ...
[36]
A physical perspective on cytoplasmic streaming - PMC
Cytoplasmic streaming has been known for more than two centuries, with the first observations attributed to Bonaventura Corti in 1774 [49]. It occurs in a range ...
[37]
Cytoplasmic Streaming Velocity as a Plant Size Determinant
Nov 11, 2013 · Cytoplasmic streaming in characean algal (Charophyceae) cells is an order of magnitude faster (approximately 70 μm × s−1) than that in higher ...
[38]
A Structural Basis for How Motile Cilia Beat - PMC - PubMed Central
Before fixation, the cilia had a beat frequency of approximately 17 hertz (Hz), so a beat occupied around 60 milliseconds (ms). Thirty-one individual cilia ...
[39]
Energetic considerations of ciliary beating and the advantage of ...
Since the beat duration of the cilia of Paramecium is ≈35 msec (beat frequency is ≈28 Hz), it follows that each dynein arm can be reactivated at most once ...<|separator|>
[40]
Article High Mobility of Vesicles Supports Continuous Exocytosis at ...
The efficient resupply of vesicles to ribbons was confirmed by electron microscopy. A 1 min depolarization, releasing 500–1000 vesicles/s, caused a 70% ...
[41]
Frontiers | What is Rate-Limiting during Sustained Synaptic Activity
Assuming a value of 80 aF for the capacitance of a single vesicle (Sakaba, 2006) this corresponds to 125–250 vesicles per second or 0.08–0.17 vesicles per ...
[42]
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[43]
From Bioblasts to Mitochondria: Ever Expanding Roles of ... - NIH
Jun 15, 2010 · Left panels show histological drawings of “bioblasts” in flight muscle (upper) and liver (lower) cells as described by Altmann (1890). Right ...
[44]
Mechanical forces on cellular organelles | Journal of Cell Science
Oct 29, 2018 · Recent studies show that intracellular organelles are, at times, under extreme mechanical strain both in widely used experimental systems as well as in vivo.Missing: percentage | Show results with:percentage
[45]
Tools of Cell Biology - The Cell - NCBI Bookshelf
Although the electron microscope has allowed detailed visualization of cell structure, microscopy alone is not sufficient to define the functions of the ...
[46]
The Structure and Function of DNA - Molecular Biology of the Cell
Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a ...
[47]
Protozoa - an overview | ScienceDirect Topics
Protozoa. These are non-photosynthetic unicellular organisms with protoplasm clearly differentiated into nucleus and cytoplasm. They are relatively large ...
[48]
Mind the Gap! - PMC - NIH
Jun 2, 2009 · As a number of contributors argue in the book Protocells: Bridging the nonliving and living matter (edited by Steen Rasmussen, Mark A. Bedau ...
[49]
Selection and the Origin of Cells | Request PDF - ResearchGate
Aug 6, 2025 · Another model [42] shows that phase-separated "surface protoplasm" can bind to the surface of a mineral and act on a group of chemicals to ...
[50]
Historical Development of Origins Research - PMC - PubMed Central
This was followed by Thomas Graham's 1861 proposal that the protoplasm was a colloid formed by a homogenous, proteinaceous substance, which was understood ...