Plasma Membrane and the Endoplasmic Reticulum

One of the most striking features of cellular organiza­tion is the pervasive occurrence of membranes.

Not only are cells bounded at their surface by the plasma membrane, but the membranous organelles compart­mentalize specific cellular functions and activities.

Especially in eukaryotic cells, the cytoplasmic matrix itself is pervaded by membrane-bordered channels that comprise the endoplasmic reticulum or ER.

We focus on the chemical composition of the plasma membrane and the membranes that form the endoplasmic reticu­lum and consider the mechanisms by which the trans­port of materials across these membranes can be achieved. The plasma membrane (or plasma lemma) delimits the cell, physically separating the cytoplasm and cellu­lar organelles from the surrounding cellular environ­ment. This implies that all substances either entering or exiting the cell must pass through the plasma mem­brane.

Although the plasma membrane may play a passive role in the exchange of very small molecules between the cell and its surroundings, the flux of many substances is actually facilitated by continuous molecular changes within the membrane. In many in­stances, transport through the membrane is achieved by the active participation of carrier molecules within the membrane and incurs the expenditure of large amounts of metabolic energy.

The cellular ingestion (or excretion) of some materials is associated with gross movements and separations of fragments of the membrane from the main body. Stages of this activity can be seen and studied with the electron microscope (some stages also with the light microscope).

The membranes of the endoplasmic reticulum di­vide much of the internal cell volume into two phases. The space within the channels of the ER comprises the intracisternal space of the cell or the lumenal phase while the space between channels constitutes the ground substance, cytosol, or cytoplasm.

The plasma membrane and the membranes of the endo­plasmic reticulum have many properties in common. This is not surprising in view of the fact that portions of the plasma membrane are derived from the endo­plasmic reticulum (see below). Both membranes are in a continuous state of flux as new components are added to the membranes and others are lost or de­graded.

Thus, the rather static appearance of these membranes in electron photomicrographs belies their dynamic nature. Moreover, periodic continuities exist between the plasma membrane and the ER with the result that the external cellular milieu is at times con­tinuous with the intracisternal space. Much of our cur­rent knowledge of cellular membranes has been ac­quired through studies of the plasma membrane because plasma membranes are more easily isolated from cells in a pure state than ER membranes.

To be sure, important differences between these mem­branes do exist; however, it is becoming increasingly apparent that their chemical composition, organiza­tion, and functions are similar. Indeed, in prokaryotes (which lack an endoplasmic reticulum), the plasma membrane plays many of the roles of the endoplasmic reticulum of eukaryotic cells.