• Phospholipids are amphipathic lipid molecules—they are part hydrophobic and part
hydrophilic. Plasma membranes are made up of bilayers of phospholipids. These bilayers are
• Ions and molecules diffuse spontaneously from regions of higher concentration to regions of
lower concentration—a process called diffusion. Movement of water across a plasma membrane
is a special case of diffusion called osmosis.
• In cells, membrane proteins are responsible for the passage across membranes of ions and large
and/or polar molecules in the processes of facilitated diffusion and active transport.
Lipids: What Is a Lipid?
• Lipids are carbon-containing compounds that are found in organisms and that are largely
nonpolar and hydrophobic.
• Hydrocarbons are molecules that contain only carbon and hydrogen.
• Lipids have a major hydrocarbon component called a fatty acid.Afatty acid is a hydrocarbon
chain bonded to a carboxyl (COOH) functional group. Hydrocarbons are the reason that lipids
do not dissolve in water.
Three Types of Lipids Found in Cells
• Lipids are defined by solubility rather than by chemical structure, so their structures vary
• Three types of lipids are the most important found in cells:
1. Fats are composed of three fatty acids linked to glycerol.
2. Steroids are a family of lipids distinguished by a four-ring structure. One important steroid in
mammals is cholesterol.
3. Phospholipids consist of a glycerol that is linked to a phosphate group (PO42-) and to either
two chains of isoprene or two fatty acids.
The Structure of Membrane Lipids
• Membrane-forming lipids are amphipathic, containing both hydrophobic and hydrophilic
• For example, phospholipids are amphipathic. The “head” region contains highly polar covalent
bonds, as well as positive and negative charges. Phospholipids also have a nonpolar fatty acid “tail” region.
• When placed in solution, the phospholipid heads interact with water while the tails do not,
allowing these lipids to form membranes.
• Phospholipid bilayers, or simply lipid 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.
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, but charged or large polar substances cross slowly, if at all.
Types of LipidsAffect Membrane Permeability Differently
• When a double bond exists between two carbons in a hydrocarbon chain, the chain is said to be
unsaturated. Hydrocarbon chains without double bonds are termed saturated.
• Adouble bond in an unsaturated lipid causes a bend or “kink” in the hydrocarbon chain,
preventing the close packing of hydrocarbon tails and reducing hydrophobic interactions.
• Phospholipids with unsaturated tails form membranes that are much more permeable than those
formed by phospholipids with saturated tails.
TemperatureAffects Membrane Fluidity and Permeability
• Membrane fluidity decreases with temperature because molecules in the bilayer move more
• Decreased membrane fluidity causes decreased permeability.
Why Do Solutes Move across Lipid Bilayers?
• Small molecules and ions in solution are called solutes.
• The random movement of solutes due to kinetic energy is known as diffusion.
• Adifference in solute concentrations across a selectively permeable membrane creates a
• When a concentration gradient exists, there is a net movement of solutes from regions of high
concentration across the membrane to regions of lower concentration. Diffusion and Osmosis
• Osmosis occurs when solutions of different concentrations are