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 :
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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 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 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:
Heads face the water and tails face each other.
Phospholipid bilayers (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 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
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.
a. It should be small and charged.
b. It should be large and hydrophilic.
c. It should be hydrophobic.
d. It should be hydrophilic.
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
a. a transmembrane (integral) protein
b. a peripheral membrane protein
c. present only in the cytoplasm
d. an ion channel