The Cell Membrane
Plasma Membrane –
boundary that separates
cells from their
environment.
Selective Permeability
• Only certain substances are allowed to
pass through the membrane.
• Simple Model
showing
hydrophillic (water
“loving”) head and
hydrophobic
(water “fearing”)
tails.
• Complex model
showing atom
arrangement:
• The phosphate heads are
attracted to water and point
toward the outside of the cell.
• The fatty acid tails are not
attracted to water and point
toward the inside (toward each
other).
• The two layers of phospholipids
form a lipid bilayer.
• The phospholipids are not locked into
one spot, but are able to “float around”.
• This fluidity allows molecules to pass
through and proteins to move around.
• Floating around in the cell
membrane are different kinds
of proteins.
• Generally, these proteins
structurally fall into three
categories.
Carrier Proteins
• Regulate and transport
diffusion.
• Carrier proteins are
peripheral proteins
which do not extend all
the way through the
membrane. They move
specific molecules
through the membrane
one at a time.
Facilitated Diffusion
• Passive transport - Requires no
energy.
• Proteins embedded in the plasma
membrane help particles pass from
a high concentration to a lesser
concentration.
• Channel (integral) proteins extend
through the bilipid layer. They form a
pore through the membrane that can
move molecules in several ways.
Active Transport
• Some proteins actively use energy from the
ATPs in the cell to drag molecules from
area of low concentration to areas of high
concentration (working directly against
diffusion) an example of this is the
sodium/potassium pump. Here the energy of
a phosphate (shown in red) is used to
exchange sodium atoms for potassium
atoms.
• Marker proteins extend across the cell
membrane and serve to identify the
cell. The immune system uses these
proteins to tell friendly cells from
foreign invaders. They are as unique as
fingerprints. They play an important
role in organ transplants. If the marker
proteins on a transplanted organ are
different from those of the original
organ, the body will reject it as a
foreign invader.
Marker Proteins
Receptor Proteins
• Allow the cell to receive instructions
• Steroids are a component of cell
membranes in the form of cholesterol.
• When present they add stability, but
restrict movement of the
phospholipids.
• Even though high levels can clog
arteries, cholesterol is crucial to the
membrane stability.
Cholesterol molecules are
interspersed among
phospholipid tails in the
bilayer.
• The cell membrane can also engulf
structures that are much too large to fit
through the pores in the membrane
proteins this process is known as
endocytosis.
3 kinds of endocytosis that take
materials into the cell:
• Phagocytosis – solids
• Pinocytosis – liquids with small
solutes
• Receptor-aided – particles gathered
to be brought in all at once, such as
in a vessicle.
• The opposite of endocytosis is exocytosis.
Large molecules that are manufactured in
the cell are released through the cell
membrane.