The Cell Membrane: Ionic Permeability and Electrotonic Properties

Abstract

All living cells are surrounded by a cell membrane which separates the cellular contents from the outside medium. The organised contents of the cell, including the nucleus and such organelles as the mitochondria, Golgi apparatus, lysosomes, endoplasmic reticulum and the components of the cytoplasmic skeleton, exist in a watery medium — the intracellular fluid or cytoplasm — which differs in composition from the fluid bathing the cell — the extracellular or interstitial fluid. There are differences in the compositions of these fluids between different cells and between different species, but in general the intracellular fluid contains less sodium (Na+) and chloride (Cl-) but more potassium (K+) than the extracellular fluid and also contains various negatively charged ions of a protein nature. The fact that there are these differences in composition between the intracellular and extracellular fluid (ECF) is, of course, evidence for the existence of the cell membrane itself. Table 2.1 lists the distribution of the major ion species across the cell membranes of a number of a number of different cells.