What
do we know about the lipids that make membranes?
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- Membranes are thin sheets of fat-like
molecules that wrap around cells and also around many parts (organelles)
within cells.
- Membranes separate the
components, either within a cell or among various cells.
- The fatty molecules ("lipids")
that make up most of a cell occur in two adjacent layers.
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The
+ and - signs in the drawing represent the electrical charge
distribution on the end of the lipid. These are usually carried
by phosphate (PO4) and carbonyl (CO) groups.
In
a real cell membrane, the lipid self-organizes into two layers.
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Lollipop Lipids
Would you believe that chemists represent lipids with little drawings
that look like lollipops?
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The circle
at one end represents an electrically charged "phosphate
group", which is the atom phosphorous bonded to four molecules
of oxygen.
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These
phosphate groups are attracted to water because water is charged
(the + pole of water molecules attracts to the negative
pole of the phosphate.)
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The stem
of the lollipop represents a chain of carbon atoms that is attached
to the phosphate group.
Most drawings of lipid symbols show two stems, because real
lipids have two parallel chains of carbons attached to the phosphate
group. Lipids
line up parallel to each other in the presence of water. In
other words, at water's surface, the lipid molecules are "standing
on their heads."
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