The Cellular Milieu

From an analytical standpoint, the interior of the cell remains an enormous challenge: Even an organism as simple as E. coli contains a few hundred different small (e.g., nonpolymeric) molecules and ions, as well as a few thousand enzymes and a significant complement of nucleic acids. Even a simple eukaryotic cell is 10-fold more complex; furthermore, it is subdivided into compartments such as mitochondria, chloroplasts, lysosomes, and the nucleus. Not only do these intracellular compartments add complexity to the problem of introducing probes, but their interiors may be at a different pH (lysosomes) or propensity for oxidation than the rest of the cytoplasm. It is well to remember, moreover, that a typical cell cytoplasm is not a dilute, homogenous solution of low viscosity where translational diffusion happens rapidly. Rather, it has high concentrations (in g/mL) of proteins, including those that make up the cytoskeleton, in addition to nucleic acids and the aforesaid organelles. In extended cells like neurons, diffusive transport is otherwise so slow that molecular transport along axons is an important field of study. While cells evidently expend substantial energy maintaining intracellular conditions of pH, oxygen tension, and the levels of a host of other small molecules at a stable level, conditions in the cell or within a tissue can, nevertheless, change dramatically. For instance, under conditions of oxygen deprivation, brain and muscle tissue become substantially acidotic, the latter due to lactate buildup. Working in plant cells offers an additional degree of complexity due to the presence of the cell wall. In multicellular organisms, cells are differentiated and play a variety of roles, and thus thay are chemically and morphologically distinct.

Finally, there is a dynamic element that must be taken into account—namely, that many of the most interesting questions in cell biology are those involving changes in the cell and its response to stimuli. The recent fascination with studies of gene expression in response to various stimuli largely overlooks other changes in the cell, because there are few tools to study them. For instance, activity levels of the enzyme HMG CoA reductase (which catalyzes the first committed step in the biosynthesis of cholesterol and is a main control point for cholesterol homeostasis) are modulated not only by expressing its gene to increase amounts of the protein, but also by changing the rate at which the enzyme is degraded, phosphorylating it, or competitively inhibiting it, none of which requires turning a gene on or off (Goldstein and Brown, 1990). Because of its topological as well as chemical complexity, the cell remains a very challenging target for study.

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