The Structure of the Cell Membrane
• http://www.youtube.com/watch?v=ULR79TiUj
80&feature=related
The Structure of Membrane Lipids
Membrane-forming lipids contain both a polar,
hydrophilic region and a nonpolar, hydrophobic region.
• Phospholipids are amphipathic:
– The “head” region, consisting of a glycerol, a phosphate,
and a charged group, contains highly polar covalent bonds.
– The “tail” region is comprised of two nonpolar fatty acid
• When placed in solution, the phospholipid heads
interact with water while the tails do not, allowing
these lipids to form membranes.
Phospholipid Bilayers
• Phospholipid bilayers form when two sheets
of phospholipid molecules align. The
hydrophilic heads in each layer face a
surrounding solution, while the hydrophobic
tails face one another inside the bilayer.
• Phospholipid bilayers form spontaneously,
with no outside input of energy required.
Phospholipids and Water
• Phospholipids do not dissolve when they are placed in
water.
• Water molecules interact with the hydrophilic heads
but not with the hydrophobic tails.
– This drives the hydrophobic tails together.
• Upon contact with water phospholipids form either:
– Micelles
• Heads face the water and tails face each other.
– Phospholipid bilayers (lipid bilayers)
Selective Permeability of Lipid Bilayers
• The permeability of a structure is its tendency to
allow a given substance to pass across it.
Phospholipid bilayers have selective
permeability.
– Small or nonpolar molecules move across
phospholipid bilayers quickly.
– Charged or large polar substances cross slowly, if at
all.
Many Factors Affect Membrane
Permeability
• Many factors influence the behavior of the
membrane:
– Number of double bonds between the carbons in
the phospholipid’s hydrophobic tail
– Length of the tail
– Number of cholesterol molecules in the
membrane
– Temperature
Bond Saturation and Membrane
Permeability
• Double bonds between carbons in a hydrocarbon chain can
cause a “kink” in the hydrocarbon chain, preventing the close
packing of hydrocarbon tails, and reducing hydrophobic
interactions.
– Unsaturated hydrocarbon chains have at least one double
bond.
– Hydrocarbon chains without double bonds are termed
saturated.
• Saturated fats have more chemical energy than unsaturated
fats.
• Membranes with unsaturated phospholipid tails are much
more permeable than those formed by phospholipids with
saturated tails.
Other Factors That Affect Permeability
• Hydrophobic interactions become stronger as saturated
hydrocarbon tails increase in length.
– Membranes containing phospholipids with longer tails have
reduced permeability.
• Adding cholesterol to membranes increases the density of
the hydrophobic section.
– Cholesterol decreases membrane permeability.
• Membrane fluidity decreases with temperature because
molecules in the bilayer move more slowly.
– Decreased membrane fluidity causes decreased permeability.
Cell Membrane
Review Questions
You want to construct a molecule that will
migrate easily through a cell membrane.
What properties should you give your
molecule?
a. It should be small and charged.
b. It should be large and hydrophilic.
c. It should be hydrophobic.
d. It should be hydrophilic.
Which of the following events would
you expect to be spontaneous in
aqueous (water) solutions?
a. net movement of calcium ions from 1.5 molar CaCl2 to 2.0 molar
CaCl2
b. net movement of sugar molecules from 0.5 molar sugar to 0.4 molar
sugar
c. net movement of water from a 1.2-molar solution of NaCl to a 0.9molar solution of NaCl
d. net movement of water from 0.3 molar sugar to pure water
Yeast cells require a protein to transport
glucose from the environment into the
cell. You would expect that protein to be
_____.
a. a transmembrane (integral) protein
b. a peripheral membrane protein
c. present only in the cytoplasm
d. an ion channel
What does it mean for the membrane
to be selectively permeable?