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Chemotherapeutic agents
lipids—phospholipids, glycolipids and cholesterol; the
bipolar arrangement of the lipids monitors substances
passing into and out of the cell. The phospholipids, which
are bipolar in nature, line up with their polar regions
pointing towards the interior or exterior of the cell and
their non-polar region lying within the cell membrane.
The polar regions mix well with water, and the non-
polar region repels water. These properties allow the
membrane to act as a barrier to regulate what can enter
the cell (see Figure 7.2). The freely-moving nature of the
membrane allows it to adjust to the changing shape of
the cell so that areas of the membrane can move together
to repair itself if it should become torn or injured. Some
of the outward-facing phospholipids have a sugar group
attached to them; these are called glycolipids. Choles-
terol is found in large quantities in the membrane, and
it works to keep the phospholipids in place and the cell
membrane stable.
Receptor sites
Embedded in the cell membrane are a series of periph-
eral proteins with several functions. As discussed in
Chapter 2, one type of protein located on the cell
membrane is known as a receptor site. This protein
reacts with specific chemicals outside the cell to stimu-
late a reaction within the cell. For example, the receptor
site for insulin reacts with the hormone insulin to cause
activation of adenosine triphosphate (ATP) within
the cell. This reaction alters the cell’s permeability to
glucose. Receptor sites are very important in the func-
tioning of neurons, muscle cells, endocrine glands and
other cell types, and they play a very important role in
clinical pharmacology.
Identifying markers
Other surface proteins are surface antigens, or genetic
ally determined identifying markers. These proteins are
called
histocompatibility antigens
or human leucocyte
antigens (HLAs), which the body uses to identify a cell
as a self-cell (i.e. a cell belonging to that individual). The
body’s immune system recognises these proteins and acts
to protect self-cells and to destroy non–self-cells. When
an organ is transplanted from one person to another, a
great effort is made to match as many histocompatibility
antigens as possible to reduce the chance that the “new”
body will reject the transplanted organ.
Histocompatibility antigens can be changed in
several ways: by cell injury, with viral invasion of a cell,
with age, and so on. If the markers are altered, the body’s
immune system reacts to the change and can ignore it,
allowing neoplasms to grow and develop. The immune
system may also attack the cell, leading to many of the
problems associated with autoimmune disorders and
chronic inflammatory conditions.
Channels
Channels or pores within the cell membrane are made
by proteins in the cell wall that allow the passage of
small substances into or out of the cell. Specific channels
have been identified for sodium, potassium, calcium,
chloride, bicarbonate and water; other channels may
also exist. Some drugs are designed to affect certain
channels specifically. For example, calcium-channel
blockers prevent the movement of calcium into a cell
through calcium channels.
KEY POINTS
■■
The cell is the basic structure of all living organisms.
■■
The cell membrane features specific receptor sites
that allow interaction with various chemicals,
histocompatibility proteins that allow for
self-identification, and channels or pores that allow
for the passage of substances into and out of the cell.
KEY POINTS
Carbohydrate
Receptor sites
Cholesterol
Protein
channel
Lipid bilayer
Phospholipids
Cytoplasm
Extracellular
fluid
Histocompatibility
antigens
Proteins
Non-polar region,
repels water
Polar regions,
mix well
with water
FIGURE 7.2
Structure of the lipid
bilayer of the cell membrane.
