Bulk Transport of Meterials Into and Out of Cells (With Diagram)

Before we consider several mechanisms responsible for bulk movement of materials into and out of cells, it is important that, what is meant by “inside the cell” and “outside the cell”.

The plasma mem­brane as a more or less continuous smooth sheet en- dosing the cell. In reality, this is an oversimplification, for in most cells the plasma membrane exhibits nu­merous outflings and inholdings.

Outfoldings of the plasma membrane cover microvilli, cilia, flagella, and other cytoplasmic extensions. Infoldings of the plasma membrane form small pockets and narrow channels that descend into the cytosol.

Some of these infoldings may join the network of cisternae that form the endoplasmic reticulum and that furrow the cyto­plasm.

This implies that the intracisternal space of the endoplasmic reticulum may be in direct continuity with the surrounding cell environment and that the movement of materials between the lumenal phase and the cell surroundings does not require passage across (or through) any membranes. Consequently, any materials that are in the cisternae of the endo­plasmic reticulum (i.e., in the lumenal phase) may be regarded as “outside” the cell. To get “inside” the cell, substances in the lumenal phase must pass through the membranous walls of the endoplasmic re­ticulum and into the cytosol.

Many intracellular vesicles appear to be derived from the endoplasmic reticulum by being “pinched off from the latter. For example, transitional vesi­cles that merge with the forming face of the cell’s Golgi bodies and peroxisomes and other micro bodies are believed to be formed in this way.

Technically then, the contents of these vesi­cles are “outside” the cell and are separated from the cytosol by membranes. Some vesicles are derived from invaginations of the plasma membrane; the con­tents of these vesicles are also to be considered as “outside” the cell. These relationships are depicted in Figure 15-42, in which the external (or cisternal) and internal (or cytosol) halves of the membrane are distinguished. Particles within such a vesicle are in con­tact with the external face of the membrane.

From the preceding discussion, it should be clear that though substances may be directly exchanged be­tween the cell surroundings and the cytosol through the plasma membrane, many exchanges also occur across membranes of cytoplasmic vesicles and the en­doplasmic reticulum. These exchanges are mediated by the same mechanisms described above in connec­tion with movements directly through the plasma membrane (i.e., diffusion, facilitated diffusion, active transport, etc.).

The formation of cytoplasmic vesicles from the plasma membrane and the consequent entrapment within these vesicles of substances formerly in the cell surroundings is called endocytosis. Several different kinds of endocytosis have been described, including pinocytosis, phagocytosis, and receptor-mediated endocytosis.

Move­ments of materials from the cell into the surroundings by the fusion of cytoplasmic vesicles with the plasma membrane constitute exocytosis. Endocytosis contin­uously removes small portions of the plasma mem­brane, whereas exocytosis continuously adds to the membrane.

From a quantitative standpoint, the turn­over is quite impressive, for in some cells (e.g., cul­tured fibroblasts and macrophages) a membrane area equal to the total surface area of the cell is turned over every few hours. Because tracer studies indicate that many membrane constituents have a half-life of sev­eral days, it has been suggested that some of the membranous material internalized during endocytosis is returned to the plasma membrane during exocyto­sis. Endocytosis and exocytosis have many character­istics in common.