Chapter 6 Lipids, Membranes, and the First Cells
● Plasma membrane or cell membrane separates life from non life.
● Plasma membrane serves as selective barrier: keeps damaging compounds out of cell
and allow entry of compounds required by cell.
● Lipids: is a term to describe carboncontaining compounds that are found in organisms
and are largely nonpolar and hydrophobic (do not dissolve readily in water)
● Lipids do dissolve in liquids consisting of nonpolar organic compounds
● Hydrocarbons: molecules containing only carbon and hydrogen (nonpolar, hydrophobic)
● Lipids do not dissolve in water due to the hydrocarbon component.
● Fatty acid: hydrocarbon chain bonded to a carboxyl (COOH) functional group
3 Types of Lipids found in Cells
● Fats, steroids and phospholipids
1. Fats: composed of 3 fatty acids that are linked to a 3 carbon molecule called glycerol.
→ Fats are also called triacylglycerols or triglycerides
→ Fats form when a dehydration reaction occurs between a hydroxyl group of
glycerol and the carboxyl group of a fatty acid.
→ Glycerol and fatty acid molecules are joined by an ester linkage
→ Fats are NOT polymers, fatty acids are NOT monomers.
2. Steroids: family of lipids distinguished by the bulky, fourring structure.
→ Various steroids differ from one another by functional groups or side groups
attached to the rings
→ Cholesterol: has a hydrophilic hydroxyl group attached to the rings and a
hydrocarbon tail formed of isoprene subunits. It’s an important part of plasma
membranes in organisms
3. Phospholipids: consists of a glycerol that is linked to a phosphate group (PO4^3) and to
either 2 chains of isoprene or two fatty acids.
→ Phosphate group is also bonded to a small, organic molecule that is charged or
Structures of Membrane Lipids
● NOT all lipids can form membranes
● Membraneforming lipids have a polar, hydrophilic region and a 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
● Long isoprene or fattyacid tails of a phospholipid are nonpolar and hydrophobic
● Water molecules cannot form hydrogen bonds with the hydrocarbon tail Amphipathic: “dual sympathy” has hydrophilic and hydrophobic elements. Ex. Phospholipids
and steroids can be too. (Fats are not)
6.2 Phospholipid Bilayers
● Phospholipids DO NOT dissolve when placed in water
● Lack of interaction with water drives hydrophobic tails together
● Instead of dissolving in water, the phospholipids form either a micelles or lipid bilayers.
Micelles: tiny droplets created when the hydrophilic heads of phospholipids face the water and
the hydrophobic tails are forced together away from the water.
(Tend to form from phospholipids with relatively short tails)
FORM SPONTANEOUSLY (no energy input) entropy increase when these structures form
Phospholipid Bilayers: created when 2 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. Therefore the hydrophilic heads interact with water and hydrophobic tails
interact with one another
→ THESE BILAYERS FORM SPONTANEOUSLY.
(Tend to form from phospholipids with relatively longer tails)
*Micelles or lipid bilayers are much more energetically stable than an independent phospholipid
in solution. (They have lower potential energy)
→ Amphipathic molecules are much more stable in aqueous solution when their hydrophobic
tails avoid water.
OVERALL, the free energy of the system decreases, lipid bilayer formation is exergonic and
Figure 6.5: Liposomes are artificial membrane bound vesicles
● Water inside and outside because hydrophilic heads of the lipids face outward on each
side of the bilayer
● Using liposomes and planar bilayers, researchers can study what happens when a
known ion or molecule is added to one side of a lipid bilayer.
Selective Permeability of Lipid Bilayers
● some substances cross a membrane more easily than others
● Small, nonpolar molecules move across bilayers quickly and large molecules and
charged substances cross the membrane slowly, if at all.
● Ions cannot cross membranes at all unless they get “help” in the form of membrane
● Charged compounds and large, polar molecules cannot pass through the nonpolar,
hydrophobic tails of a lipid bilayer. Saturated: Hydrocarbons without double bonds
→ Saturated with the max number of hydrogen atoms that can attach to the carbon
Unsaturated: Hydrocarbons with a double bond, since the carbon and carbon cannot rotate
freely due to the double bond, it produces a “kink” in the otherwise straight hydrocarbon chain.
→ Fewer than max number of hydrogen atoms attached
Bond Saturation and Hydrocarbon Chain Length Change Membrane Fluidity and
● When hydrophobic tails are packed into a lipid bilayer, the kinks created by double bonds
produce spaces among the tightly packed tails.
→ These spaces reduce the strength of hydrophobic interactions between the tails
● These interactions are stronger in saturated hydrocarbon tails.
● Hydrophobic interactions also become stronger as saturated hydrocarbon tails increase
● Lipids that have extremely long hydrocarbon tails (waxes) form particularly stiff solids at
● Highly saturated fats: Solid at room temp, Highly unsaturated fats: Liquid at room
● Liquid triacylglycerols: Oils
● Short, unsaturated Hydrocarbon Tails= More permeable and fluid. (Allows more stuff to
pass since interior is held less tightly)
● Long, straight, saturated fatty acid tails= less permeable
Cholesterol Reduces Membrane Permeability
● Steroid rings in cholesterol are bulky, adding cholesterol to a membrane should increase
the density of the hydrophobic section.
● Adding cholesterol to membranes decrease their permeability to glycerol
How Does Temperature Affect the Fluidity and Permeability of Membrane?
● At 25 degrees C, or “room temp” the phospholipids in the plasma membrane are liquid.
● Fluidity as well as membrane permeability decreases as temperature decreases.
● As temp drop, individual molecules in the bilayer move m