General Functions and Characteristics: Cell Membrane
General Functions and Characteristics: Cell Membrane
General Functions and Characteristics: Cell Membrane
cell membrane
A thin membrane, typically between 4 and 10 nanometers (nm; 1 nm = 10−9 metre) in
thickness, surrounds every living cell, delimiting the cell from the environment around
it. Enclosed by this cell membrane (also known as the plasma membrane) are the
cell’s constituents, often large, water-soluble, highly charged molecules such
as proteins, nucleic acids, carbohydrates, and substances involved in
cellular metabolism. Outside the cell, in the surrounding water-based environment,
are ions, acids, and alkalis that are toxic to the cell, as well as nutrients that the cell must
absorb in order to live and grow. The cell membrane, therefore, has two functions: first,
to be a barrier keeping the constituents of the cell in and unwanted substances out and,
second, to be a gate allowing transport into the cell of essential nutrients and movement
from the cell of waste products.
Internal membranes
The presence of internal membranes distinguishes eukaryotic cells (cells with a nucleus)
from prokaryotic cells (those without a nucleus). Prokaryotic cells are small (one to five
micrometres in length) and contain only a single cell membrane; metabolic functions
are often confined to different patches of the membrane rather than to areas in the body
of the cell. Typical eukaryotic cells, by contrast, are much larger, the cell
membrane constituting only 10 percent or less of the total cellular membrane. Metabolic
functions in these cells are carried out in the organelles, compartments sequestered
from the cell body, or cytoplasm, by internal membranes.