Chapter 6- Lipids, Membranes, and the First Cells
Lipid: carbon-containing compounds that are found in organisms and are largely nonpolar and
hydrophobic. Lipids dissolve in liquids consisting of nonpolar organic compounds
-Lipids do not dissolve in water because they have a significant hydrocarbon component. Hydrocarbons
are nonpolar because electrons are equally shared in C-H bonds. This makes them hydrophobic.
-Lipids are defined by solubility (physical property) instead of chemical structure. Three types found in
1. Fats: composed of 3 fatty acids that are linked to a 3-carbon molecule called glycerol.Also called
triglycerides/triacylglycerols. Form when dehydration reaction occurs between hydroxyl group of
glycerol and carboxyl group of fatty acid. Joined by ester linkage. Not polymers/monomers.
2. Steroids: four ring structure. Differ by functional groups attached to the rings. Ex. Cholesterol
3. Phospholipids: glycerol linked to phosphate group (PO ) and4to two chains of fatty acid or isoprene.
Structure of Membrane Lipids:
- Polar, hydrophilic region in addition to non-polar, hydrophobic region found in all lipids.
- The charges and polar bonds in the head region interact with water molecules when a
phospholipid is placed in solution. The long isoprene or fatty acid tails are nonpolar. Water
molecules cannot form hydrogen bonds with the hydrocarbon tail, so they do not interact with
this part of the molecule.
- Phospholipids are amphipathic- compounds that contain both hydrophilic and hydrophobic
6.2 Phospholipid Bilayers
-Lipid bilayers are created when 2 sheets of phospholipid molecules align. Hydrophilic heads in each
layer face a surrounding solution while the hydrophobic tails face one another inside the bilayer. The
hydrophilic heads interact with water while tails interact with each other.
-Micelles are tiny droplets created when the heads face water and tails are forced together. Tend to form
from those with relatively short tails.
-Lipid bilayer formation is a spontaneous procedure because free energy of system decreases. Exergonic.
Loss of potential energy.
-Lipid bilayers are highly selective. Selective permeability- Some substances cross a membrane more
easily than others. Small non-polar substances move across bilayers quickly. In contrast, large molecules
and charged substances cross the membrane slowly, if at all.
-Charged compounds and large, polar molecules can’t pass through the tails. Because of their electrical
charge, ions are more stable in solution where they form hydrogen bonds with water than they are in the
interior of membranes which is electrically neutral.
2 aspects of a hydrocarbon chain could affect the way the chain behaves in a lipid bilayer1)the number of
double bonds it contains 2)its length. -Unsaturated: When a double bond exists between two carbon atoms in a hydrocarbon chain.
-Saturated: hydrocarbon chains without double bonds. More chemical energy
-Phospholipids with long, saturated tails form membranes that are much less permeable than membranes
consisting of phospholipids with shorter, unsaturated tails.
-Permeability is a function of temperature, the amount of cholesterol in the membrane, and the length and
degree of saturation of the hydrocarbon tails in membrane phospholipids.
-Membranes are dynamic. Phospholipid molecules whiz around each layer while water and small,
nonpolar molecules shoot in and out of the membrane. How quickly molecules move within and across
membranes is a function of temperature and the structure of hydroc