Section 5 - Cell Membranes,
Channels, Pumps, and Receptors
● Boundaries of cells are formed by membranes
○ Selective permeability
■ Pumps and channels
● Pumps → active transport
● Channels → passive transport/facilitated diffusion
■ Receptors
● Sense information in the environment and transmit or
transduce the information across the membrane
Chapter 12 - Membrane Structure and Function
● Membranes: the interface of 2 environments
○ The cell interior and exterior in regard to the plasma membrane
○ The cellular compartments and the cell interior in regard to internal
membranes
● Common important attributes of membranes
○ Sheetlike structures
■ 2 molecules thick, that form closed boundaries between different
compartments
■ Thickness: 60-100Å (6-10nm)
○ Consist mainly of lipids and proteins (+carbohydrates linked to lipids and
proteins)
○ Membrane lipids are small molecules that have both hydrophilic and
hydrophobic moieties
■ Lipids → form closed bimolecular sheets in aqueous media
■ Lipid bilayer
○ Specific proteins mediate distinctive functions of membranes
○ Noncovalent assemblies
■ Molecules are held together by many noncovalent interactions
○ Asymmetric
■ The 2 faces of biological membranes always differ from each other
○ Fluid structures
○ Electrically polarized
■ The inside is negative
■ Membrane potential
12.1 - Phospholipids and Glycolipids Form Bimolecular Sheets
● Lipids → amphipathic
○ Hydrophilic (polar) head group + hydrophobic
(nonpolar) tail
, ● Membrane formation is a consequence of the amphipathic nature of the molecules
● The favored structure for most phospholipids and glycolipids in aqueous media is a
lipid bilayer
○ 2 lipid sheets → leaflets
● Lipid bilayers form spontaneously by a self-assembly process
○ The hydrophobic effect is the major driving force for the formation of lipid
bilayers
○ Van der Waals attractive forces between the hydrocarbon tails favor close
packing of the tails
○ There are electrostatic and hydrogen-bonding attractions between the polar
head groups and water molecules
● Lipid bilayer membranes have a very low permeability for ions and most polar
molecules
○ The ability of molecules to move through a lipid environment is varied
■ eg Na+ and K+ traverse these membranes 109x more slowly than H2O
○ The permeability of small molecules is correlated with their relative solubilities
in water and nonpolar solvents
■ A small molecule might traverse a lipid bilayer membrane in the
following way
● It sheds the water with which it is associated (solvation shell)
● It dissolves in the hydrocarbon core of the membrane
● It diffuses through this core to the other side of the membrane
● It is resolvated by water
12.2 - Membrane Fluidity Is Controlled by Fatty Acid Composition and
Cholesterol Content
● The fluidity of the membrane lipids is determined by the properties of fatty acid
chains
○ Fatty acid chains in membrane bilayers may be arranged in an ordered, rigid
state or in a relatively disordered, fluid state
○ If the temperature is raised above Tm (melting temperature) → transition from
the rigid to the fluid state takes place rather abruptly
○ The transition temperature depends on the length of the fatty acid chains and
their degree of unsaturation
Channels, Pumps, and Receptors
● Boundaries of cells are formed by membranes
○ Selective permeability
■ Pumps and channels
● Pumps → active transport
● Channels → passive transport/facilitated diffusion
■ Receptors
● Sense information in the environment and transmit or
transduce the information across the membrane
Chapter 12 - Membrane Structure and Function
● Membranes: the interface of 2 environments
○ The cell interior and exterior in regard to the plasma membrane
○ The cellular compartments and the cell interior in regard to internal
membranes
● Common important attributes of membranes
○ Sheetlike structures
■ 2 molecules thick, that form closed boundaries between different
compartments
■ Thickness: 60-100Å (6-10nm)
○ Consist mainly of lipids and proteins (+carbohydrates linked to lipids and
proteins)
○ Membrane lipids are small molecules that have both hydrophilic and
hydrophobic moieties
■ Lipids → form closed bimolecular sheets in aqueous media
■ Lipid bilayer
○ Specific proteins mediate distinctive functions of membranes
○ Noncovalent assemblies
■ Molecules are held together by many noncovalent interactions
○ Asymmetric
■ The 2 faces of biological membranes always differ from each other
○ Fluid structures
○ Electrically polarized
■ The inside is negative
■ Membrane potential
12.1 - Phospholipids and Glycolipids Form Bimolecular Sheets
● Lipids → amphipathic
○ Hydrophilic (polar) head group + hydrophobic
(nonpolar) tail
, ● Membrane formation is a consequence of the amphipathic nature of the molecules
● The favored structure for most phospholipids and glycolipids in aqueous media is a
lipid bilayer
○ 2 lipid sheets → leaflets
● Lipid bilayers form spontaneously by a self-assembly process
○ The hydrophobic effect is the major driving force for the formation of lipid
bilayers
○ Van der Waals attractive forces between the hydrocarbon tails favor close
packing of the tails
○ There are electrostatic and hydrogen-bonding attractions between the polar
head groups and water molecules
● Lipid bilayer membranes have a very low permeability for ions and most polar
molecules
○ The ability of molecules to move through a lipid environment is varied
■ eg Na+ and K+ traverse these membranes 109x more slowly than H2O
○ The permeability of small molecules is correlated with their relative solubilities
in water and nonpolar solvents
■ A small molecule might traverse a lipid bilayer membrane in the
following way
● It sheds the water with which it is associated (solvation shell)
● It dissolves in the hydrocarbon core of the membrane
● It diffuses through this core to the other side of the membrane
● It is resolvated by water
12.2 - Membrane Fluidity Is Controlled by Fatty Acid Composition and
Cholesterol Content
● The fluidity of the membrane lipids is determined by the properties of fatty acid
chains
○ Fatty acid chains in membrane bilayers may be arranged in an ordered, rigid
state or in a relatively disordered, fluid state
○ If the temperature is raised above Tm (melting temperature) → transition from
the rigid to the fluid state takes place rather abruptly
○ The transition temperature depends on the length of the fatty acid chains and
their degree of unsaturation
