Cytoarchitecture and Physical Properties of Cytoplasm: Volume, Viscosity, Diffusion, Intracellular Surface Area

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Classical biochemistry is founded on several assumptions valid in dilute aqueous solutions that are often extended without question to the interior milieu of intact cells. In the first section of this chapter, we present these assumptions and briefly examine the ways in which the cell interior may depart from the conditions of an ideal solution. In the second section, we summarize experimental evidence regarding the physical properties of the cell cytoplasm and their effect on the diffusion and binding of macromolecules and vesicles. While many details remain to be worked out, it is clear that the aqueous phase of the cytoplasm is crowded rather than dilute, and that the diffusion and partitioning of macromolecules and vesicles in cytoplasm is highly restricted by steric hindrance as well as by unexpected binding interactions. Furthermore, the enzymes of several metabolic pathways are now known to be organized into structural and functional units with specific localizations in the solid phase, and as much as half the cellular protein content may also be in the solid phase.

Introduction

Much of the current paradigm for cellular biochemistry has been extrapolated from studies of dilute solutions containing a single enzyme and a single substrate whose interaction is diffusion-limited. While this reductionist approach has led to many valuable insights over the past several decades, measurements of the physical properties of cells indicate that the interior of a cell departs from these ideal conditions in several important ways. Long considered a minority view, there now is a dawning awareness on the part of investigators that the nonideality of the cell interior may necessitate an alternate view of cellular biochemistry. In this chapter, we will address five assumptions implicit in the prevailing paradigm, review the extent to which they are valid for cells, and point out areas in need of further study. These assumptions are that: (1) the reaction volume is infinite, (2) the solution is dilute, (3) the concentrations of substrates are much higher than the concentrations of their enzymes, (4) the solution is well-defined, and (5) the solution is homogeneous. Each of these points has been raised by previous reviewers (for example, Srere, 1967, Fulton, 1982, Agutter et al., 1995). This chapter will provide an overview of the field and focus on the most recent contributions. For the sake of brevity, the reader is referred to recent review articles for more detailed discussion of some areas, and I apologize to any author whose work is not cited individually.

Section snippets

Assumption 1 : Infinite Volume

An underlying assumption of classical physical biochemistry is that the reaction volume is infinite. The familiar concept of concentration depends on this assumption. In reality, because a cell is surrounded by a limiting membrane, which is only selectively permeable, the assumption of infinite volume is patently false. The question is whether the finite volume of cells can be neglected, as it frequently is. Single cells range in size from the smallest mycoplasma (0.3 μm in diameter) to the

Physical Properties of Cytoplasm [Measured]

It is obvious from the foregoing discussion that the nonideality of the cell interior could have profound effects on its physical and chemical properties. In this section, we will survey experimental data aimed at studying these properties in the cytoplasm of intact cells. Three areas of research that have seen the most activity are the behavior of intracellular water, the constraints on diffusion and partitioning of inert tracer particles, and the diffusibility of endogenous proteins and

Concluding Remarks

There appears to be ample evidence that the interior of a living cell is not well described by the dilute solution paradigm. Although the average mobility of cellular water is no more than twofold reduced compared to bulk water, existing data do not rule out a small but significant population of immobilized water molecules immediately adjacent to membrane and cytoskeletal surfaces. The evidence suggests that the aqueous phase is crowded with macromolecules, yet at least 50% of cytoplasmic

Acknowledgments

This chapter is dedicated to the memory of Keith R. Porter (1912–1997), who taught me to question accepted paradigms. I also gratefully acknowledge Dick McIntosh for first pointing out the finite volume problem to me several years ago, Ivan Cameron for helpful discussion of his data, and The National Science Foundation for their generous support of my research (MCB-9604594).

References (145)

  • I.L. Cameron et al.

    A mechanistic view of the non-ideal osmotic and motional behavior of intracellular water

    Cell Biol. Int.

    (1997)
  • M.F. Carlier et al.

    Control of actin dynamics in cell motility

    J. Mol. Biol.

    (1997)
  • Y.H. Chou et al.

    Intermediate filaments and cytoplasmic networking: New connections and more functions

    Curr. Opin. Cell Biol.

    (1997)
  • J.S. Clegg et al.

    Effects of reduced cell volume and water content on glycolysis in L-929 cells

    J. Cell. Physiol.

    (1990)
  • R. Cooke et al.

    The properties of water in biological systems

    Annu. Rev. Biophys.

    (1974)
  • J.E. Eriksson et al.

    Cytoskeletal integrity in interphase cells requires protein phosphatase activity

    Proc. Natl. Acad. Sci. U.S.A.

    (1992)
  • A.B. Fulton et al.

    Cotranslational assembly of some cytoskeletal proteins: Implications and prospects

    J. Cell Sci.

    (1993)
  • D.S. Goodsell

    The Machinery of Life”

    (1993)
  • D. Hamill et al.

    Polyribosome targeting to microtubules: Enrichment of specific mRNAs in a reconstituted microtubule preparation from sea urchin embryos

    J. Cell Biol.

    (1994)
  • H. Hoppeler et al.

    Relationship between mitochondria and oxygen consumption in isolated cat muscles

    J. Physiol. (London)

    (1987)
  • T. Ito et al.

    Regulation of water flow by actin-binding protein-induced actin gelation

    Biophys. J.

    (1992)
  • S. Jaken

    Protein kinase c isozymes and substrates

    Curr. Opin. Cell Biol.

    (1996)
  • Κ.M. Johansen

    Dynamic remodeling of nuclear architecture during the cell cycle

    J. Cell. Biochem.

    (1996)
  • J.D. Jones et al.

    Diffusion of vesicle-sized particles in living cells is restricted by intermediate filaments

    Mol. Biol. Cell

    (1998)
  • J. Kertesz

    Percolation of holes between overlapping spheres: Monte Carlo calculation of the critical volume fraction

    J. Phys. (Paris)

    (1981)
  • E.H. Kislauskis et al.

    Characterization of sequences responsible for peripheral localization of β-actin mRNA

    J. Cell Biol.

    (1991)
  • F. Lanni et al.

    Structural organization of interphase 3T3 fibroblasts studied by total internal reflection fluorescence microscopy

    J. Cell Biol.

    (1985)
  • G.N. Ling

    A physical theory of the living state: Application to water and solute distribution

    Scanning Microsc.

    (1988)
  • D.J. Livingston et al.

    Myoglobin diffusion in bovine heart muscle

    Science

    (1983)
  • C. Masters

    Microenvironmental factors and the binding of glycolytic enzymes to contractile filaments

    Int. J. Biochem.

    (1992)
  • G. Minaschek et al.

    Microcompartmentation of glycolytic enzymes in cultured cells

    Eur. J. Cell Biol.

    (1992)
  • G. O’Reilly et al.

    Identification of an actin binding region in aldolase

    FEBS Lett.

    (1993)
  • L. Pagliaro et al.

    Enolase exists in the fluid phase of cytoplasm in 3T3 cells

    J. Cell Sci.

    (1989)
  • K. Ainger et al.

    Transport and localization of exogenous myelin basic protein mRNA microinjected into oligodendrocytes

    J. Cell Biol.

    (1993)
  • Κ. Ainger et al.

    Transport and localization elements in myelin basic protein mRNA

    J. Cell Biol.

    (1997)
  • B. Alberts et al.

    Molecular Biology of the Cell

    (1994)
  • H. Arnold et al.

    Binding of glycolytic enzymes to structure proteins of the muscle

    Eur. J. Biochem.

    (1968)
  • D. Bray et al.

    The actin content of fibroblasts

    Biochem. J.

    (1975)
  • P.C. Bridgman et al.

    The structure of cytoplasm in directly frozen cultured cells

    II. Cytoplasmic domains associated with organelle movements. J. Cell Biol.

    (1986)
  • T. Chang et al.

    Diffusion through coarse meshes

    Macromolecules

    (1987)
  • J.S. Clegg

    Properties and metabolism of the aqueous cytoplasm and its boundaries

    Am. J. Physiol.

    (1984)
  • J.S. Clegg

    L-929 cells under hyperosmotic conditions: Volume changes

    Cell. Physiol.

    (1986)
  • J.S. Clegg et al.

    Glycolysis in permeabilized L-929 cells

    Biochem. J.

    (1988)
  • J.S. Clegg et al.

    Glucose metabolism and the channeling of glycolytic intermediates in permeabilized L-929 cells

    Arch. Biochem. Biophys.

    (1990)
  • N.S. Cohen

    Intracellular localization of the mRNAs of argininosuccinate synthetase and argininosuccinate lyase around liver mitochondria, visualized by high-resolution in situ reverse transcription-polymerase chain reaction

    J. Cell Biochem.

    (1996)
  • N.S. Cohen et al.

    Argininosuccinate synthetase and argininosuccinate lyase are localized around mitochondria: an immunocytochemical study

    J. Cell Biochem.

    (1996)
  • J. Condeelis

    Elongation factor 1 alpha, translation and the cytoskeleton

    Trends Biochem. Sci.

    (1995)
  • C.C. Cunningham et al.

    Microtubule-associated protein 2c reorganizes both microtubules and microfilaments into distinct cytological structures in an actin-binding protein-280-deficient melanoma cell line

    J. Cell Biol.

    (1997)
  • B.S. Eckert

    Alteration of the distribution of intermediate filaments in PtK1 cells by acrylamide

    II: Effect on the organization of cytoplasmic organelles. Cell Motil. Cytoskel.

    (1986)
  • E.L. Elson et al.

    Interpretation of fluorescence correlation spectroscopy and photobleaching recovery in terms of molecular interactions

    Methods Cell Biol.

    (1989)
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