Campbell and Reece
Chapter 7
MEMBRANE
STRUCTURE
&
FUNCTION
EARLY FLUID MOSAIC MODEL
UPDATED MODEL of ANIMAL CELL
PLASMA MEMBRANE
PHOSPHOLIPID BILAYER
HOW TO MAKE A PHOSPHOLIPID
 GLYCEROL
+ phosphate group = “head”
+ 2 fatty acid “tails”
PHOSPHOLIPID
Fluidity of Membranes
Fluidity in Membranes
 The more unsaturated tails the more fluid the
membrane (cannot pack the tails as close
together as straight saturated tails)
Fluidity in Membranes: Cholesterol
 only in animal cell membranes
 wedged in between hydrophobic tails
Cholesterol in Membranes
Cholesterol’s Effect
 @ 37ºC cholesterol makes membrane less
fluid by restraining phospholipid movement
 lowers temp required for membrane to
solidify
Viscous Membrane
Fluid Membrane
Animal Membranes
 Cholesterol reduces
membrane fluidity at
moderate temps by
reducing phospholipid
movement; at low temps
it hinders solidification
by disrupting the
regular packing of
phospholipids
Evolution of Differences in Membrane
Lipid Composition
 Species-specific membranes are evolutionary
adaptations made to maintain appropriate
fluidity to accommodate specific
environmental conditions
Examples
 fish that live in lakes that freeze over in cold
months have higher proportion of
unsaturated hydrocarbon tails  membranes
stay fluid in colder temps
Examples
 certain prokaryotes can change the
composition of membranes depending on the
temperature at which they are growing
Chryseobacterium greenlandensis
Examples: Plant Adaptations
 % of unsaturated phospholipids increases as
temps decrease which keeps membranes
from solidifying
Membrane Proteins
 >50 membrane proteins
 2 main categories:
1. Integral Proteins
penetrate the hydrophobic inside of lipid
bilayer
 most are transmembrane proteins
2. Peripheral Proteins
 appendages loosely bound to either
surface

Membrane Proteins
Membrane Proteins
 on cytoplasmic side some proteins held in
place by attachment to cytoskeleton
 on ECF side some proteins attached to fibers
in extracellular matrix
 both give animal cells stronger framework
Major Functions of Membrane Proteins
1. TRANSPORT


provides hydrophilic channel thru
hydrophobic interior of lipid bilayer
some use passive some active transport
Transport Proteins
Major Functions of Membrane Proteins
2. ENZYMATIC ACTIVITY
 all enzymes are proteins so a membrane
protein could have all or part of its structure
function as an enzyme
 in some membranes several enzymes
organized to carry out sequential steps in a
metabolic pathway
Membrane Protein as Enzyme
Major Functions of Membrane Proteins
3. SIGNAL TRANSDUCTION
 membrane protein acts as receptor has
binding site with specific shape that exactly
fits shape of the chemical messenger (signal
molecule or ligand)
 when signal enters receptor site usually the
membrane protein changes shape
(configuration) which relays message into
cell, usually binding to a cytoplasmic protein
Signal Transduction
Major Functions of Membrane Proteins
4. CELL-CELL RECOGNITION
 some glycoproteins act as ID tags recognized
by membrane proteins of other cells which
may bind to them
 attachment short-lived
Cell-Cell Recognition
Major Functions of Membrane Proteins
5. INTERCELLULAR JOINING
 membrane proteins of adjacent cells may
hook together in different types of cell jcts
 tends to be long-lasting
Cell Junctios
Major Functions of Membrane Proteins
6. ANCHORING
 cytoskeletal elements may be noncovalently
bound to membrane proteins: helps maintain
cell shape & stabilizes location of membrane
proteins
Cell Surface Proteins
 medically important:
1. some pathogens use them to adhere/enter
cell
2. some medications designed to take
advantage of using them
Glycocalyx
 glycoproteins + glycolipids
 usually ~15 sugar units
 exterior surface of cell membrane
 key to cell-to-cell recognition
 sorting
cells  in embryo
 Immune System
Plasma Membrane Asymmetry
like cell membrane exterior surface
Selective Permeability
 plasma membrane example of emergent
properties: each individual membrane
protein, lipid, or carb together become a
“supermolecule”
Selective Permeability
 essential to cell’s existences
 Fluid Mosaic Model helps explain how
regulation occurs
 24/7 steady stream on ions & small molecules
in/out cell; each at their own rate
Selective Permeability
Selective Permeability
 depends on:
1. lipid bilayer
2. specific transport protein built into
membrane
Selective Permeability
 In general:
 small, nonpolar molecules get in
 ions and polar molecules don’t get in
Transport Proteins
 hydrophilic substances get thru hydrophobic
lipid bilayer by going thru center of a
transmembrane, transport protein
Channel Proteins
 hydrophilic channel
 hydrophobic a.a. in portion of protein that
interfaces with lipid bilayer
 Aquaporins: allow water molecules to cross
 channel open, allows up to 3 billion water
molecules/s
 water follows its concentration gradient
by osmosis
Aquaporins
Carrier Proteins
 attach to their “passenger”  change in
shape so that passenger is shuttled thru
membrane
 very specific: 1 substance or small group of
similar substances
Passive Transport
 is diffusion of substance across membrane
w/no nrg investment
Diffusion
 In the absence of other forces, a substance
will diffuse from where it is more
concentrated to where it is less concentrated.
(it will move down its concentration
gradient)
 No work required: spontaneous because
particles have KE and are in constant motion
Oxygen
 higher concentration in air inhaled in
alveolar sacs  diffuses into capillaries in
alveoli  thru circulatory system  diffuses
from capillaries in tissues where there is a
higher concentration  individual cells
where concentration lower than capillaries 
mitochondria where concentration lower still
Osmosis
Water Balance
 Tonicity: ability of a surrounding solution to
cause a cell to gain or lose water
 Depends on:
concentration of solutes that cannot cross
membrane relative to the concentration of
all solutes in cell
Isotonic Solutions
 concentration of solutes same inside as
outside cell
Hypotonic & Hypertonic Solutions
w/out a Cell Wall
Osmoregulation
 control of solute concentrations & water
balance
 http://www.stolaf.edu/people/giannini/movies/par
amecium/para%20cont.mov
Water Balance with Cell Walls
 includes cells of plants, fungi, prokaryotes, &
some protists
 walls inelastic so cells in hypotonic solutions
so wall can expand very little before it exerts
backpressure on cell = turgor pressure which
opposes further water intake
 plants w/out wood require cells to be turgid
for mechanical support
With a Cell Wall
Turgid vs. Flaccid
Cell Walls in Hypertonic Solution
 plasma membrane pulls away from cell wall
(called plasmolysis)  plant wilts  dies if
does not receive water
Facilitated Diffusion
 channel or carrier proteins that allow
hydrophilic substances to cross membranes
moving down their concentration gradients
 if transport ions called ion channels
 many are Gated Ion Channels
open/close mechanism works in response
to stimuli (electrical, specific ligand)
Facilitated Diffusion
 http://programs.northlandcollege.edu/biology/Biolo
gy1111/animations/passive3.swf
Gated Ion Channels
Glucose Transporters
Cystinuria
 example of disorder due to absence of carrier
protein for cysteine & other a.a. in kidney
cells
 normally a.a. reabsorbed in kidneys using
carrier proteins
 in this disorder the a.a. accumulate  kidney
stones
Active Transport
 moves substances against their concentration
gradient
 requires energy
 allows cell to maintain concentration
gradients
Na+/K+/ATPase Pump
Na+/K+/ATPase Pump
 http://www.brookscole.com/chemistry_d/templates
/student_resources/shared_resources/animations/i
on_pump/ionpump.html
How Ion Pumps Maintain Membrane Potential
 all cells have voltages across the plasma
membrane
 (-) because cytoplasmic side (-) relative to
ECF side
 overall inside/outside cell neutral but just
inside (-) & just outside (+)
Gradients across the
Plasma Membrane
 a difference in charge across membrane is called:
membrane potential
range is -50 to -200 mV
Membrane Potential
 like any battery has potential energy
 cell uses it to control movement of all
charged particles across plasma membrane
 inside (-) compared to outside so passive
movement of cations into cell & anions out of
cell favored
Ions Move Down
Electrochemical Gradient
 2 forces drive diffusion:
1. chemical gradient
concentration gradient
2. electrical gradient
 cations move into cell, anions out

Example: Absorption in Small Intestine
Electrogenic Pumps
 transport protein that generates voltage
across a membrane
 major one in animal cells is Na+/K+/ATPase
pump
 major one in plants, fungi, & bacteria is a
proton pump
 actively transports protons (H+) out of
cells
 increases + charge outside and increases
– charge inside cell
Proton Pumps
Electrogenic Pumps
 by generating voltage across a membrane
potential energy is increased
 can be used for cellular work
 used in mitochondria to make ATP
 used in cotransport
cotransport
 a substance that has been pumped against its
concentration gradient holds potential
energy
 that energy can be used to do work as it
moves back across the membrane down its
concentration gradient
 2nd protein (not the pump) called a
cotransporter can couple the downhill
diffusion this substance with a 2nd substance
moving up its own concentration gradient
cotransporters
 Cells use the sucrose-H+ cotransporters to
store sugars made in photosynthesis in veins
of leaves
 Plant will distribute sugar to other parts of
plant as needed
another example
Bulk Transport Across the Membrane
 used by large macromolecules or large
volumes of smaller molecules
1. Exocytosis
2. Endocytosis
Exocytosis
 transport vesicles from Golgi  move along
microtubules to plasma membrane
 membrane of vesicle comes in contact with
plasma membrane
 proteins in membranes rearrange lipids in
vesicle membrane & plasma membrane so
that they fuse
 contents released into ECF
Exocytosis
Exocytosis
Endocytosis
 cell takes in substances  vesicle made with
membrane from cell membrane
 uses different membrane proteins than in
exocytosis but looks like reverse of exocytosis
 3 types:
1. phagocytosis
2. pinocytosis
3. receptor-mediated endocytosis
Phagocytosis
 “cell-eating”
 wraps pseudopods around substance
creating a membranous sac = food vacuole 
lysosome to be digested
Pinocytosis
 “cell-drinking”
 cell takes “gulps” of ECF for solutes
 nonspecific

Receptor-Mediated Endocytosis
 allows cells to take in specifically what it needs
 specific ligands bind to specific membrane proteins
 receptor proteins with ligands in place cluster
together into “coated pits” (on cytoplasmic side)