Cell Biology
Chapter 11
Membrane Structure and Function

Membrane Functions
o Cells have membranes so they can:
 Compartmentalize
 Regulate flow of materials and information
o Some membranes have enzymatic, signal transduction, and energy
production systems localized within or upon them. Comprised
primarily of proteins.
o Membrane model is the Fluid Mosaic Model; membrane
comprised of a mixture of lipids, and proteins, free to move
laterally so the membrane is a dynamic structure. Mobility is a
critical membrane characteristic:
 Without it, the membrane is unable to function

Cellular and Intracellular Membranes
o Plasma Membrane:
 Hydrophobic core barrier to polar compounds, ions
 Keeps desirable substances in, undesirable
substances out
 Allows gradients to be built
 Proteins form channels or pumps to regulate transit of polar
molecules, ions across membrane
o Intracellular Membrane:
 Compartmentalize functions
 Enzymes and substrates of those functions isolated,
colocalized
 Facilitate locally high concentrations of enzymes
and substrates so reactions more efficient and faster
 Allows gradients to be built

Transport Regulation
o Lipophilic compounds cross w/o special transporters
o Polar/ionic molecules require transport proteins
o Transport may be passive, crossing membrane from high
concentration to low concentration regions
o Transport may be active, using energy to pump ions, sugars, amino
acids across membranes against electrical and/or concentration
gradients
o Direct Communication:
 Plasma membranes of cells may be fused together forming
gap junctions between animal cells across which signals

and low molecular weight substances freely pass. Cardiac
cells interconnected by gap junctions.
Plasmodesmata:
 Intercellular bridges in plants

Signal Transduction
o Receptors:
 First step in signal transduction. Proteins.
o Signal from outside received on outer plasma membrane surface
by receptor proteins, causing changes in receptors that start signal
transduction cascade resulting in release of secondary messengers
in cytoplasm
o 2md messenger events on inside transmit information that a signal
has been received to internal cellular systems
o Steroid receptors in cytoplasm or nucleus, not on cellular surface,
as steroids are lipid-soluble and diffuse freely across plasma and
nuclear membranes.

Membrane Lipids
o Phospholipids
o Sphingolipids
 Mostly neurons
o Gycolipids
 Neurons, chloroplasts
o Sterols:
 Cholesterol; phytosterols.
 Hopanoids-not sterols
 Similar functions:
 Enhance stability and fluidity of membrane, also
make it a better hydrophobic barrier

Differing Membrane Compositions
o Different cellular membranes have different lipid and protein
compositions
o Different faces of the same membrane can have different lipid
compositions, different proteins/protein orientations
o These differences relate to specific functions of each membrane

Phospholipids
o Amphipathic:
 Polar heads, hydrophobic tails
o Fatty acid tails 12-20 carbons long:
 Usually 16 or 18
 Chains less than 12 C result in unstable bilayer.
 Tail length typically an even number of C’s, some
exceptions
o Chain length determines membrane stability and thickness
 Shorter chains, thinner, more fluid, less stable membranes
o Length and ° of saturation varies, often between chains in the same
phospholipids molecule
o All double bonds in cis conformation. Living organisms do not
make trans fatty acids. Cis conformation puts kink in chain at
double bond, loosening packing which makes the membrane more
fluid but less stable.

Cholesterol
o Up to ½ of animal membrane lipid cholesterol
o Cholesterol has 1 hydrophilic –OH group, 4 hydrophobic rings and
a hydrophobic side chain:
 Extremely hydrophobic
o Effect of these characteristics:
 At high temperatures cholesterol reduces membrane
fluidity, stabilizing membrane.
 Low temperatures:
 Reduces fatty acid chain packing, increasing
membrane fluidity
o Cholesterol also blocks ions and small molecules from crossing
membrane by plugging gaps and improving membrane stability
and flexibility:
 Better insulation
o Phytosterols in plants, hopaoids in bacteria have structures similar
to cholesterol and serve similar functions, though synthesis
pathways are quite different

Importance of Membrane Fluidity
o Fluidity measure of how readily phospholipids and mobile proteins
move laterally. Transverse motion rare
o ΔT means potential change in fluidity:
 Membranes can “freeze”, becoming nonfluid.
 Phase transition in membranes is a change in phase from
liquid to solid or vice versa.
 Fluid state is functional state
o Lipid composition regulated by cell, influencing phase transition
temperature. Kinds/percent of lipid composition, chain length, ° of
saturation, percent cholesterol all regulate membrane fluidity

Factors Influence Fluidity
o Longer chains more rigid, freeze at higher temperatures than
shorter chains but more stable at higher temperatures
o Shorter chains less rigid, fluid at lower temperatures than longer
chains but less stable at higher temperatures
o More double bonds, less saturated, less rigid, more fluid at lower
temperatures, less stable at higher temperatures
o Sterols stabilize membranes at high temperatures, keep membranes
fluid as temperatures drop
o Cell trying to maintain constant fluidity under changing
temperature conditions
o Cell may do this by altering lipid composition of membrane,
percentages of different lipids, or ° of saturation of lipid chains

Homeoviscous Adaptation
o Alteration of lipid content of membranes in response to
temperature fluctuations is homeoviscous adaptation
o Enzyme activation/deactivation as temperature changes involved in
homeoviscous adaptation
o Acetyl CoA tranferase +/- 2 C’s at a time to fatty acyl chains
o Desaturase removes H’s in fatty chains, replaces them with double
bonds between C atoms of backbone. Loosens packing, increasing
membrane fluidity
o Critically important for hibernating/estivating animals, which
experience drop in body temperature, as well as poikilotherms
which do not regulate temperature

Motion of Lipids in Membranes
o Transverse Diffusion not thermodynamically favored; rare but
does happen when facilitated by specific proteins called flippases.
Neither phospholipids nor proteins generally flip orientation
without aid. Enables membrane to maintain asymmetric
distributions of different membrane components on the 2 faces
o Rotation and lateral diffusion common for lipids and proteins
o Model of membrane structure is the Fluid Mosaic Model

Membrane Proteins
o Some proteins have sugars covalently bound:
 Glycoproteins
o Found on outer face of plasma membrane
o Glycosylation occurs in lumen of RER, further processing in the
Golgi
o 2 types of covalent linkage:
 N-linked sugars:
 NH2 on the side group of asparagines is used for the
sugar linkage
 O-linked sugars:
 OH groups of serine, threonine, hydroxylysine, or
hydroxyproline used for sugar linkage
o 2-60 sugars, found on proteins on plasma membrane surface facing
out from cell
o Glycoproteins involved in cellular adhesion, signal recognition,
immune response, tissue lining/protection, and membrane stability