Chapter 8
 Membrane Structure &
Function
Membrane Structure
Selective permeability
Controls traffic
Known as the plasma membrane
Amphipathic - hydrophobic &
hydrophilic regions
Singer-Nicolson developed the
fluid mosaic model
Membranes are Fluid
Structures related to properties &
function
 Membrane is usually about as fluid
as salad oil.
Fluid Mosaic Model
Lipids
Phospholipids - membrane fluidity
Cholesterol - membrane stabilization
“Mosaic” Structure due to:
Proteins:
Integral proteins - transmembrane proteins
Peripheral proteins - surface /appendages
Attachments-framework for animal cells
Membrane carbohydrates -~ cell to cell
recognition; oligosaccharides (cell markers);
glycoproteins; glycolipids; ABO blood typing
Sidedness of Plasma Membranes
 Carbohydrates only on the outside surface
 Proteins may be anchored
– Inside to cytoskeleton
– Outside to the extracellular matrix
Molecules that start on the inside
face of Golgi Complex end up on the
outside face of the plasma
membrane
 Know positioning of these structures; polar
and nonpolar regions of membrane
Membrane Structure
Membrane protein
functions:
Transport
Enzymatic activity
Signal transduction
Intercellular joining
Cell-cell recognition
ECM attachment
Traffic Across Membranes
 Easily
 hydrophobic
 With Assistance
– Hydrophilc: Polar, charged
 Channels
 Shuttles
Transport
 Passive
– No energy expenditure
– Diffuse through membrane
– Diffuse aided by protein
 Facilitated diffusion
 Active
– Energy expenditure—
 ATP
– Usually against concentration Gradient
– Pumps
 cotransport
– Bulk Transport
 Endocytosis
 Exocytosis
Passive Transport
 diffusion of a substance across a biological
membrane
 No energy exerted
 Diffusion - tendency of any molecule to
spread out into available space
 Concentration gradient – moves from high to
low
Osmosis - the diffusion of water across
a selectively permeable membrane;
DOWN the concentration gradient.
Direction determined only by a
difference in total solute concentration
Rate influenced by
Temperature
Steepness of conc. gradient
Water Balance
Osmoregulation - control of water
balance
Comparison of 2 solutions:
Hypertonic - higher concentration of solutes
Hypotonic - lower concentration of solutes
Isotonic - equal concentrations of solutes
Water Balance
Cells with Walls (plants, bacteria,
fungi):
Require hypotonic external
environments to keep their turgor
pressure (water pressure pushing cell
membrane out against cell wall)
Become limp or flaccid when lose turgor
pressure
Plasmolysis - plasma membrane pulls
away from cell wall
Water Balance
Cells without Walls (animals, most
protist):
Require isotonic external environments
Hypertonic environments – cells swell &
may burst with too much water pressure
(Cytolysis)
May have contractile vacuoles (some
protists; paramecium also have less porous
membrane) to control internal water pressure
Contractile vacuoles &
Osmoregulation in Paramecium
Specialized Transport
UtilizingTransport proteins (with or
without channels)
Facilitated diffusion - passage of
molecules and ions with transport
proteins across a membrane down the
concentration gradient.
Specific for its substrate
Facilitated diffusion
 Transport of water and certain hydrophilic
solutes across; down conc. gradient.
 Transport Proteins
 Most-very specific
 2 types pf proteins
– 1. Channel Proteins
– 2. Transport Proteins
Facilitated diffusion
1. Channel Proteins
 Permits rapid flow across
membrane.
 1. Aquaporins: plants and animals.
Discovered in plants in 1994
 2. Ion Channels
– Many of these are Gated
Channels– Stimulus to open
Electrical or Chemical Stimulus
– Example: Nerve cell
stimulated by a
neurotransmitter molecules
(chemical) , opens gate,
allows Na+ into the cell.
 Diseases linked to Ion Channels
A multitude of human and animal diseases
are caused by dysfunction of ion channels. This may
be genetic, i.e. caused directly by mutations in genes
coding for ion channels. Such diseases are called
‘channelopathies’. Examples of channelopathies are
cystic fibrosis, epilepsy, and arrhythmias, e.g. the
long QT syndrome. Also, diseases may result from
defects caused by mutations in genes coding for
regulatory proteins.

 http://www.sophion.dk/Technology/ion_channels
LE 7-15a
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
Facilitated diffusion
2. Transport Proteins
 Undergoes subtle change in shape.
 Alternates between 2 conformations, moving
molecule across as changes.
Facilated Diffusion
 2. Carrier Proteins: Undergo conformational
change, solute is transported as the protein
changes shape.,
Active transport
Pump substance across membranes,
against its concentration gradient,
through a a Carrier Protein, with the help
of cellular energy (ATP)
Example: How cells maintain a higher
concentrations of K+ inside cell
Active Transport Systems
1. The sodium-potassium pump.
Specific kind of Active Transport
Esp. important in animals
Exchanges Na+ for K+
Transports
– 3 Na+ out for every 2 K+ into the cell.
Restores normal electrochemical gradient following
an action potential.
LE 7-16
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Cytoplasmic Na+ bonds to
the sodium-potassium pump
P
ATP
P
ADP
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
P
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
Maintenance of Membrane Potential
by Ion Pumps
 Membrane Potential: The voltage (charge
separation) across a membrane
– Cytoplasm of a cell is negative
– Extracellular fluid is positive
 Ranges from -50- -200mV
 Favors diffusion of cations into the cell, and anions out of
cells
 .
The electrochemical gradient: a combination
of 2 forces that influence the movement of
ions across membranes.
– A chemical force (the ion’s concentration
gradient)
– An electrical force (the effect of the membrane
potential on the ion’s movement).
– SO…….ions move across membranes down
their electrochemical gradient.
• An electrogenic pump is a transport protein
that generates voltage across a membrane.
 The main electrogenic pump of animals is a
sodium-potassium pump.
 The main electrogenic pump of plants, fungi,
and bacteria is a proton pump—actively
transports H+ out of cells.
LE 7-18
–
–
ATP
EXTRACELLULAR
FLUID
+
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
Cotransport
 An ATP powered pump indirectly drives the
Active Transport of several other solutes.
 Example: Sucrose
-
– Sucrose loading in plants
H+ Cotransporter
 CO-TRANSPORT... movement of 2 solutes together
-

 Often moves 1 solute passively & other actively
Ex: 1) H+ pump coupled with sucrose transport
( H+symport* )
2) epithelial transport* Na+glucose model (
glucose absorption* )
 Replenish ions, sugar, fluid following strenuous
LE 7-18
–
–
ATP
EXTRACELLULAR
FLUID
+
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
LE 7-19
–
+
H+
ATP
H+
–
+
H+
Proton pump
H+
–
+
H+
–
+
H+
Sucrose-H+
cotransporter
Diffusion
of H+
H+
–
–
+
+
Sucrose
Bulk transport across the
plasma membrane occurs by
exocytosis and endocytosis
 Large molecules, such as polysaccharides and
proteins, cross the membrane via vesicles.
1. Exocytosis
 In exocytosis: cell secretes
macromolecules.
 Vessicle buds from Gogi.
 Many secretory cells use this—
– Examples: Pancreas secretes
insulin

Neurons secrete
neurotransmitters
2. Endocytosis
 Reverse process.
 The cell takes in macromolecules by forming vesicles
from the plasma membrane.
• Three types of endocytosis:
1. Phagocytosis (“cellular eating”): Cell engulfs
particle in a vacuole. Not specific.
2. Pinocytosis (“cellular drinking”): Cell creates
vesicle around fluid. Not specific.
3. Receptor-mediated endocytosis:
 Very Specific
 Binding of ligands to receptors triggers vesicle
formation
LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
Familial hypercholesteremia
 Defective gene/genes required for
regulation, synthesis, transport, recycling, or
turnover of LDL receptors.
Exocytosis - secretion of
macromolecules by the fusion of
vesicles with the plasma membrane
Endocytosis - import of
macromolecules by forming new
vesicles with the plasma membrane
Phagocytosis –cell “eating”
Pinocytosis – cell “drinking”
Receptor-mediated endocytosis
(ligands)
Bulk transit across membranes
 Cotransport (symport) involves more than
one type of particle being transported by in
the same direction at the same time by the
same mechanism From http://