|Curtis, Helena, & Sue N. Barnes.Biology. 5th ed. New York: Worth Publishers, 1989: 104.||"The cell membrane (also called, as we noted previously, the plasma membrane) is only about 7 to 9 nanometers thick and cannot be resolved by the light microscope"||7–9 nm|
|Hine, Robert. "Membrane." The Facts on File Dictionary of Biology. 3rd ed. New York: Checkmark, 1999: 198.||"Membranes are typically 7.5–10 nm in thickness with two regular layers of lipid molecules (a bilayer) containing various types of protein molecules."||7.5–10 nm|
|Chen, Aileen, & Vincent T. Moy. Cross-Linking of Cell Surface Receptors Enhances Cooperativity of Molecular Adhesion. Biophysical Journal. Vol. 78 (2000): 2814-2820.||"Assuming that the potential changes linearly over the entire thickness of the membrane (~3 nm), the force needed to extract a transmembrane protein is ~160 pN, which is much larger than the measured rupture forces."||3 nm|
|Baker, John R. Cytological Technique. 5th ed. New York: Wiley, 1966: 3.||"The cell-membrane is exceedingly thin, far below the limit of measurement by the optical microscope; but whether we can actually see a cell-membrane or not in any particular cell, we are compelled by convincing evidence to postulate the existence of a thin surface layer having different characters from the material within."||<< 200 nm|
|Kuchel, Philip W., & Gregory B. Ralston. Theory and Problems of Biochemistry. New York: Schaum's Outline/McGraw-Hill, 1988: 7.||"The plasma membrane (Fig. 1-4) is the outer boundary of the cell; it is a continuous sheet of lipid molecules (Chap. 6) arranged as a molecular bilayer 4–5 nm thick."||4–5 nm|
One of the key components in the understanding of biological cells is the cell membrane. It is the lipid-based sheath that envelops a cell, encloses the cytoplasm (the viscous fluid of the cell where much of the day to day work of the cell transpires), and creates a selectively permeable barrier. The cell membrane, also known as the plasma membrane, has a crucial significance to life. The membrane can be thought of as a crossing guard. A crossing guard controls the flow of people and traffic, protecting the streets from potential harm. Likewise, the cell membrane acts as a boundary that regulates what enters and leaves the cell. The membrane must retain molecules, such as DNA, RNA, and its variety of proteins from dissipating away, while keeping out foreign molecules that might damage or destroy the cell's contents, including molecules essential for life.
It is hard to believe that until the early 1950s the cell membrane was rarely brought up in scientific literature and studies. It was recognized that there was some sort of barrier between the cell and its environment, but barely any information was known. The focus in biology was on processes and structures that can be seen, and since the thickness of cell membranes are thinner than the wavelength of visible light (about 200 nm); they couldn't be seen with the visible light microscope, or even the ultra-violet microscope. The thickness of a cell membrane is now stated to be from three to ten nanometers based on the advanced technology of today. With the electron microscope, the cell membrane is seen as a continuous, thin double line with globular proteins embedded throughout.
Jennifer Shloming -- 2001