Vesicle-mediated Cellular Transport

Categories: Cellular biology; physiology; transport mechanisms

Cells use several methods to transport ions, polar molecules, and macromolecules through the cell membrane. Some of these can permeate the membrane via osmosis. Small substances, mostly ions, can diffuse through pores composed of transmembrane proteins. Other substances, however—such as glucose, glycogen, and some amino acids—must be transported by membrane-bound carrier molecules in a process called vesicle-mediated transport.

Also called bulk transport, vesicle-mediated transport is an active process that involves the cell membrane (plasma membrane) and consumes energy. Vesicle-mediated transport also provides a mechanism that enables a cell to “hoard” needed nutrients against a concentration gradient. The product of vesicle-mediated transport is a saclike vesicle, typically about 0.05 to 0.1 micrometers in diameter, comprising a fragmented portion of the cell membrane that bounds and contains the substances being transported. Vesicle-mediated transport of substances into cells is called endocytosis (endo means “within”; cytosis, “cytosol” or “cytoplasm”), and movement of substances out of cells is called exocytosis. The three forms of endocytosis are pinocytosis, phagocytosis, and receptor-mediated endocytosis.

Pinocytosis

Pinocytosis is called “cell drinking” because during the process fluids and dissolved solutes are taken into the cell. Pinocytosis involves the formation of membrane-bound vesicles at the cell membrane surface, called pinocytotic vesicles, which are then taken into the cell interior and released. Under certain circumstances, pinocytosis enables a cell to take in fluid at a much faster rate than during normal osmosis. Pinocytosis may augment osmosis or may function entirely independently; cells in an isosmotic solution, for example, can acquire large volumes of additional fluid via pinocytosis.

Studies of plant cell uptake of heavy metals such as lead have clarified much of the processes involved in pinocytosis, as have studies of giant amoebae and phagocytes. Pinocytosis occurs in special depressions in the cell membrane. Each depression, or pit, consists of one or more proteins called clathrin. Clathrin is a complex protein that consists of three large and three small polypeptide chains bound together to form a tripodlike configuration called a triskelion.

During pinocytosis, clathrin-coated pits form around a droplet of extracellular fluid as well as any ions contained within the fluid droplet. The membrane-bound droplet then invaginates via a deep groove through the membrane, pinching off within the interior as a minute, fluid-filled vesicle. The whole process takes only a few seconds.

Phagocytosis

Phagocytosis is called "cell eating" (phago for “eating,” and cytosis meaning “cell”) and refers to the cellular intake of large and generally insoluble molecules and macromolecules that cannot be taken into the cell using other membrane transport mechanisms. Phagocytosis differs from pinocytosis in that little fluid is taken into the cell.

Many small single-celled organisms such as amoebae feed by phagocytosis. Some of the more specialized forms of phagocytosis in plants include the uptake of food by slime molds and the intake of nitrogen-fixing bacteria such as Rhizobium into the root nodules of legumes as they form. In animal cells, phagocytosis functions as a defensive reaction against infection and antigens (foreign substances).

The products of phagocytosis are typically solid substances rather than fluid and are contained within a vesicle called a phagosome. Some examples of phagocytosis involve extensions of the plasma membrane called pseudopodia, which surround the substance. The ends of the pseudopodia fuse to encircle the substance, which is then transported through the membrane and budded off into the cytoplasm. Inside the cell at least some phagocytic vesicles bind with one or more structures within the cytoplasm for further processing. Others are transported and their contents emptied into cell vacuoles or other cytoplasmic organelles.

Receptor-Mediated Endocytosis

Receptor-mediated endocytosis is an efficient process whereby nutrients and other essential macromolecules are taken into the cell. During receptor-mediated endocytosis, specific receptor sites located on the plasma membrane bind to target molecules in the extracellular fluid medium. An example of this process in humans is the taking in of low-density lipoprotein (LDL) particles, which contain cholesterol. By itself, cholesterol is not soluble in blood; within a layer of phospholipids and proteins in the LDL particle, however, the molecules become soluble for LDL receptors to receive. Most of the receptor sites in plant cell membranes are glycoproteins that bind to specific sites on the target macromolecules, called ligands.

Some receptor sites are located throughout the cell membrane, but others are found in clathrin-coated depressions or pits in the membrane. During receptor-mediated endocytosis, the receptor sites located in membrane depressions selectively bind with target substances to form a receptor-ligand complex. Following this, several receptor-ligand complexes may combine to form clusters around which a portion of the cell membrane encircles, producing a vesicle that invaginates inward to pinch off as a coated vesicle. Once inside, changes in the acidity (pH) within the cytoplasm separate the substance from the protein coating. The substance diffuses or is dissolved within the cytoplasm, and the protein coating is recycled back to the cell membrane.

Exocytosis

Exocytosis is the reverse of endocytosis. During exocytosis a vesicle-bound substance is transported through the membrane from the interior to the exterior of the cell. Exocytosis represents the method by which cells secrete or excrete substances out of the cell by means of membrane-bound sacs. A common example found in most cells during initial growth involves the exocytosis of precursor molecules that will form the cell wall. During the process, the precursor molecules bind to the interior of the plasma membrane, then evaginate into the region where the cell wall is developing.

Exocytosis begins in the cytoplasm when a substance or a membrane-bound substance in the cytoplasm migrates to and fuses with the cell membrane. A pit or groove evaginates outward through the cell membrane, and the membrane-bound substance is transported to the cell surface. In some examples of exocytosis, the membrane opens at the cell surface and the interior substance diffuses into the extracellular fluid. In other cases, the vesicle is secreted or excreted as a membranous sac into the extracellular fluid.

Hormones, other secretory products, and waste are the most common substances removed from the cell by exocytosis. Following exocytosis, the vesicle generally dissolves, and the substance diffuses into the extracellular fluid.

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell. 5th ed. New York: Garland, 2008. Print.

Dowler, Brynn C., ed. Endocytosis: Structural Components, Functions, and Pathways. New York: Nova, 2010. Print.

Lodish, Harvey, et al. Molecular Cell Biology. 7th ed. New York: W. H. Freeman, 2013. Print.

Martini, Frederic. Fundamentals of Anatomy and Physiology. 6th ed. San Francisco: Benjamin Cummings, 2004. Print.

Poste, George, and Stanley T. Crooke, eds. New Insights into Cell Membrane Transport Processes. New York: Plenum, 1986. Print.

Smith, Chris A., and Edward J. Wood. Cell Biology. 2nd ed. New York: Chapman and Hall, 1999. Print.

Stein, Wilfred D. Channels, Carriers, and Pumps: An Introduction to Membrane Transport. New York: Academic, 1990. Print.

Weaver, Robert F. Molecular Biology. 5th ed. New York: McGraw, 2012. Print.