Biomedical Engineering Reference
In-Depth Information
Chapter 4
The Membrane as a Transporter, Ion
Channels and Membrane Pumps
A cell membrane's primary role is to create a barrier against materials transferring
between cellular exterior and interior regions. However, the presence of certain
natural or artificial agents (especially during treatment) such as membrane proteins
(MPs), antimicrobial peptides (AMPs), etc., occasionally induces transient or stable
transport events into cell membranes. These properties are often found to be highly
dynamic, time dependent, and specific to the agents inducing them. The events also
fall into different classes due to the diversity of their structures and mechanisms.
In this chapter, we discuss in detail a few classes of such events with a special focus
on their membrane effects.
The cell membrane's function, in general, depends on the constituent membrane
proteins. Floating around in the cell membrane are various types of proteins, gener-
ally globular proteins. They are not held in any fixed pattern, but instead exhibit a
high degree of mobility in the phospholipid layer. In general, these proteins can be
structurally classified into three categories:
•
carrier proteins that regulate transport and diffusion,
•
marker proteins that identify the cell to other cells,
•
receptor proteins that allow the cell to receive instructions, communicate, transport
proteins to regulate what enters or leaves the cell.
Membrane proteins are either (a) peripheral or (b) integral. Peripheral proteins are
bound electrostatically to the exterior parts of head groups and hence can be easily
extracted. Integral proteins are tightly bound to lipid tails and are insoluble in water.
Steroids are sometimes a component of cell membranes in the form of cholesterol;
when present, it reduces the fluidity of the membrane. However, not all membranes
contain cholesterol.
Transport proteins come in two forms. Carrier proteins are peripheral proteins
that do not extend all the way through the membrane. They bond and drag specific
molecules through the lipid bilayer one at a time and release them on the opposite
side. Channel proteins extend through the lipid bilayer. They form a pore through
the membrane that can move molecules in several ways. In some cases the channel
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