the structure of the cell membrane
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The Structure of the Cell Membrane
• http://www.youtube.com/watch?v=ULR79TiUj80&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 or
isoprene chains.
• 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.
Fluidity of the Membrane • Individual phospholipids can move
laterally throughout the lipid bilayer.– They rarely flip between layers.
• How quickly molecules move within and across membranes is a function of temperature and the structure of the hydrocarbon tails in the bilayer.
Cell Membrane
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.
Review Questions
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 0.9-molar 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) proteinb. a peripheral membrane proteinc. present only in the cytoplasmd. an ion channel
What does it mean for the membrane to be selectively permeable?
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