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