Membrane Structure and Function
Membranes Are Boundaries
• The plasma membrane is the boundary that
separates the living cell from its surroundings
• The other membranes bound and define other
compartments and organelles.
• Membranes exhibit selective permeability.
They allow some substances to cross it more
easily than others.
Concept 7.1: Cellular membranes are consist of
lipids and proteins in a fluid mosaic arrangement.
• Phospholipids are the primary lipids in most
membranes.
• amphipathic molecules
• The fluid mosaic model states that a membrane
is a fluid structure with a “mosaic” of various
proteins embedded in it
• Proposed by Singer and Nicholson in 1972.
• Proteins dispersed within the bilayer, with only the
hydrophilic regions exposed to water
Fluid Mosaic ModelLipids
Phospholipids arranged in
a bilayer = a unit
membrane
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Disambiguation
• Two layers of phospholipids = 1 lipid bilayer
• 1 bilayer = unit membrane or single membrane
• 2 bilayers (nucleus, ct, mt) = double membrane
• Hydrophobic parts hate water so they are on
the inside-away from water
• Hydrophilic parts love water so they are on the
outside-close to water
How do the proteins fit into the lipid bilayer?
They are not stuck to the opposite sides like
bread on a sandwich. (at least not most of
them)
EM studies reveal that many proteins are
imbedded in the membrane (like chunks of
peanuts in hot peanut brittle).
Biochemical studies revealed that some
proteins were loosely attached and some
tightly attached.
The Fluid Mosaic Model of Singer and Nicholson
was developed to explain these results
I
Phospholipid
bilayer
P
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Membrane proteins are integral (I) or peripheral (P)
Fluid Mosaic Model-Experimental Evidence
Prediction of the model: some proteins can move
around in the membrane: “fluid” mosaic
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell
Lateral movement is frequent but flip-flop is rare
-
Lateral movement
(107 times per second)
Flip-flop
( once per month)
Types of lipid movement in the fluid mosaic
Many proteins can move or drift laterally as
well but they do not flip-flop
• As temperatures cool, membranes become
less fluid as lipids solidify or “freeze”
• The temperature at which a membrane
solidifies depends on the types of lipids
• Membranes rich in unsaturated fatty acids are
more fluid that those rich in saturated fatty
acids. Membrane composition influences
membrane fluidity
• Membranes must be fluid to work properly;
they have to be about salad oil consistency
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydrocarbon tails
Membrane fluidity and fatty acid saturation
The steroid cholesterol also helps to moderate changes
in fluidity as temperatures fluctuate
Cholesterol
Cholesterol within the animal cell membrane
Membrane Proteins and Their Functions
• A more complete overview of membrane
structure
• A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
• Proteins determine most of the membrane’s
specific functions
Fig. 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
• Peripheral proteins are bound to the surface
of the membrane
• Integral proteins penetrate the hydrophobic
core
• Integral proteins that span the membrane are
called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or
more stretches of nonpolar amino acids
N-terminus
C-terminus
 Helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
The hydrophobic parts are often in alpha helix form
As in this seven-alpha-helix protein
• What do these membrane proteins do?
• We can list six major functions of membrane
proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and extracellular matrix
(ECM)
Some functions of membrane proteins
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
Three more functions of membrane proteins
Glycoprotein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Importance of membrane proteins
•25-35% of genome devoted to membrane proteins
•But membrane proteins are hard to study
• In one early study of human membrane proteins:
1352 receptor proteins
817 transporters
533 enzymes
697 other types
3309 unidentified
The Role of Membrane Carbohydrates in Cell-Cell
Recognition (non-protein, non-lipid components)
• Cells recognize each other by binding to
surface molecules, often carbohydrates, on the
plasma membrane
• Membrane carbohydrates may be covalently
bonded to lipids (forming glycolipids) or more
commonly to proteins (forming glycoproteins)
• Carbohydrates on the external side of the
plasma membrane vary among species,
individuals, and even cell types in an individual
Concept 7.2: Membrane structure results in
selective permeability
• A cell must exchange materials with its
surroundings, a process that controlled by the
plasma membrane
• Plasma membranes are selectively permeable,
regulating the cell’s molecular traffic. They let
some things through and they hold other things
back.
The Permeability of the Lipid Bilayer
• Hydrophobic (nonpolar) molecules, such as
hydrocarbons, can dissolve in the lipid bilayer
and pass through the membrane rapidly
• Polar molecules, such as sugars or amino
acids, do not cross the membrane easily
Cartoon
version:
small nonpolar
molecules
can move
through a
membranewater is an
exception
Transport Proteins
• Transport proteins allow passage of
hydrophilic substances across the membrane
• Some transport proteins, called channel
proteins, have a hydrophilic channel that
certain molecules or ions can use as a tunnel
• Special type of channel proteins called
aquaporins facilitate the passage of water:
water moves freely across the membrane (aka
water channels).
• Other transport proteins, called carrier
proteins, bind to molecules and change shape
to shuttle them across the membrane
• Some transport proteins open and shut rapidly
in response to signals:these are called gates
• A transport protein is specific for the substance
it moves
Review: three classes transport proteins
• Channel proteins
• Carrier proteins
• Gates
Preview for 7.3-7.5: How do substances cross
membranes?
What are the mechanisms-given the general
involvement of proteins and the differing
nature of substances involved?
• Diffusion
• Facilitated Diffusion
• Active Transport
• Endo- and Exocytosis
Concept 7.3: Passive transport is movement of a
substance across a membrane with no energy
investment
• Diffusion is the tendency for molecules to
spread out evenly into the available space
• Although each molecule moves randomly,
diffusion of a population of molecules may
exhibit a net movement in one direction-from
high to low concentration
• At dynamic equilibrium, as many molecules
cross one way as cross in the other direction
Passive Transport involves diffusion across a membrane
Moves solute from high to low concentration
Molecules of dye
Membrane (cross section)
Moves from high to low concentration
WATER
Net
diffusion
(a) Diffusion of one solute
Net
diffusion
Net diffusion stops at equilibrium
Equilibrium
• Substances diffuse down their concentration
gradient, the difference in concentration of a
substance from one area to another
• No work must be done to move substances
down the concentration gradient
• The diffusion of a substance across a biological
membrane is passive transport because it
requires no energy from the cell to make it
happen (only requires a concentration gradient)
Facilitated diffusion
• Substances cross membrane
• Move down concentration gradient: no energy
input
• Rely on transport proteins
Concept 7.4: Active transport uses energy to move
solutes against their gradients
• Facilitated diffusion is still “passive” because
the solute moves down its concentration
gradient
• Some transport proteins, however, can move
solutes against their concentration gradients
The Need for Energy in Active Transport
• Active transport moves substances against
their concentration gradient
• Active transport requires energy, usually in the
form of ATP-the cell’s energy currency
• Active transport is performed by specific
proteins embedded in the membranes
Active transport allows cells to maintain concentration
gradients that differ from their surroundings
• The sodium-potassium pump is one type of active
transport system-probably the best understood
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
CYTOPLASM
Na+
[Na+] low
[K+] high
1 Cytoplasmic Na+ binds to
the sodium-potassium pump.
Na+
Na+
Na+
P
ADP
ATP
2 Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
P
3 Phosphorylation causes
the protein to change its
shape. Na+ is expelled to
the outside.
P
P
4 K+ binds on the
extracellular side and
triggers release of the
phosphate group.
5 Loss of the phosphate
restores the protein’s original
shape.
K+ is released, and the
cycle repeats.
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Summary and Comparison-Diffusion,Passive
Transport, Active Transport
Active transport can establish electric potential
–
ATP
EXTRACELLULAR
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+
The proton pump illustrates how electrogenic pumps work
Cotransport or Coupled Transport
• Cotransport occurs when active transport of a
solute indirectly drives transport of another
solute
• Plants commonly use the gradient of hydrogen
ions generated by proton pumps to drive active
transport of nutrients into the cell
• Symport system-both substances move in the
same direction relative to the membrane
• In an antiport system, substances move in
opposite directions
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
H+
+
–
H+
+
H+ Diffusion
of H+
Sucrose-H+
cotransporter
H+
Sucrose
–
–
+
+
Sucrose
Concept 7.5: Bulk transport across the plasma
membrane occurs by exocytosis and endocytosis
• Small molecules and water enter or leave the
cell through the lipid bilayer or by transport
proteins
• Large molecules, such as polysaccharides and
proteins, cross the membrane by bulk transport
using vesicles
• Bulk transport requires energy
Exocytosis
• In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their
contents
• Many secretory cells use exocytosis to export
their products
• Acinar cells of the pancreas are examples-they
secrete digestive enzymes into the intestine
Endocytosis
• In endocytosis, the cell takes in macromolecules
by forming vesicles from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving
different proteins
• There are three types of endocytosis:
– Phagocytosis (“cellular eating”)
– Pinocytosis (“cellular drinking”)
– Receptor-mediated endocytosis or RME
In phagocytosis a cell engulfs a particle into a vacuole
The vacuole fuses with a lysosome to digest the particle
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
Often used as a defense
In pinocytosis, molecules are taken up when
extracellular fluid is “gulped” into tiny vesicles
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
Non-specific-all dissolved substances enter
• In receptor-mediated endocytosis, binding of
ligands to receptors triggers vesicle formation
• A ligand is any molecule that binds specifically
to a receptor site of another molecule
• Some viruses enter in this way
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