Page 66 - Demo 1

Figure 33. Structure of a Phospholipid
Source: http://images.tutorvista.com/cms/images/101/phospholipids-bilayer.png

When placed in water, hydrophobic molecules tend to form a ball or
cluster. The hydrophilic regions of the phospholipids tend to form hydrogen
bonds with water and other polar molecules on both the exterior and interior
of the cell. Thus, the membrane surfaces that face the interior and exterior of
the cell are hydrophilic. In contrast, the middle of the cell membrane is
hydrophobic and will not interact with water. Therefore, phospholipids form
an excellent lipid bilayer cell membrane that separates fluid within the cell
from the fluid outside of the cell.

Cholesterol, another non-polar lipid molecule, is made up of four fused
carbon rings. It can be found in the interior poron of the bilayer, and tends to
dampen the effects of temperature on the membrane. Thus, cholesterol
funcons as a buffer, prevenng lower temperatures from inhibing fluidity and
prevenng higher temperatures from increasing fluidity too much. Cholesterol
expands the range of temperatures at which a membrane maintains a
funconal, healthy fluidity.
Cholesterol also serves other funcons, such as organizing clusters of
transmembrane proteins into lipid ras. Although it is important in maintaining

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