Chapter 3
The plasma membrane
and membrane potential
• Explain how cell membrane constituents function in
creating membrane potentials. This will be
measured by quiz and exam scores.
Review
• Membrane structure and composition
• Cell to cell adhesions
• Membrane transport
New
• Membrane potentials
Plasma Membrane
• Forms outer boundary of every cell
• Controls movement of molecules between the cell
and its environment
• Joins cells to form tissues and organs
• Plays important role in the ability of a cell to respond
to changes in the cell’s environment
Plasma Membrane Structure
• Fluid lipid bilayer embedded with proteins
– Most abundant lipids are phospholipids
• Also has small amount of carbohydrates
– On outer surface only
• Cholesterol
– Tucked between phospholipid molecules
– Contributes to fluidity and stability of cell
membrane
• Proteins
– Attached to or inserted within lipid bilayer
Plasma Membrane Structure
ECF
Integral
proteins
Dark line
Carbohydrate
chain
Appearance
Light
using an electron space
microscope
Dark line
Glycolipid Glycolipid
Phospholipid
molecule
Receptor
protein
Gated channel
protein
Proteins
Lipid Cholesterol Leak
bilayer molecule
channel
protein
ICF
•
•
•
•
Channels
Carrier molecules
Docking marker acceptors
Membrane bound enzymes
•
•
•
Cell adhesion
Carrier Microfilament
molecule (linking protein of cytoskeleton
microtubule to
membrane)
Receptor sites
Cell adhesion molecules (CAMs)
– Integrin, cadherin
Cell surface markers
Fig. 3-3, p. 59
Cell-To-Cell Adhesions
– Extracellular matrix
• Serves as biological “glue”
• Major types of protein fibers interwoven in matrix
– Collagen, elastin, fibronectin
– CAMs in cells’ plasma membranes
– Specialized cell junctions
• Desmosomes
• Tight junctions (impermeable junctions)
• Gap junctions (communicating junctions
Specialized Cell Junctions
Desmosomes
Gap junctions
•
Act like “spot rivets” that anchor two •
Small connecting tunnels formed by
closely adjacent nontouching cells
connexons
•
Most abundant in tissues that are
•
Especially abundant in cardiac and
subject to considerable stretching
smooth muscle
•
In nonmuscle tissues permit
unrestricted passage of small nutrient
molecules between cells
•
Also serve as method for direct
transfer of small signaling molecules
from one cell to the next
Tight junctions
•
Firmly bond adjacent cells together
•
Seal off the passageway between the
two cells
•
Found primarily in sheets of epithelial
tissue
•
Prevent undesirable leaks within
epithelial sheets
•
C
Lumen (contains undigested food
and potent digestive enzymes)
SELECTIVE PASSAGE
THROUGH CELLS
Luminal
membrane
Tight
junction
NO PASSAGE
BETWEEN CELLS
Lateral
membrane
Cell 2
Cell 1
Blood
vessel
Epithelial Basolateral
cell lining membrane
intestine
Fig. 3-5a, p. 63
Membrane Transport
• Unassisted membrane transport
– Diffusion
– Osmosis
• Assisted membrane transport
– Carrier-mediated transport
– Facilitated transport
– Active transport
If a substance can
permeate the membrane
If the membrane is
impermeable to a substance
Membrane
(a) Diffusion occurs
(b) No diffusion occurs
KEY
= Penetrating solute
= Nonpenetrating solute
Fig. 3-8, p. 66
Area A
(a) Diffusion
Area B
Area A
Area B
Diffusion from area A
to area B
Diffusion from area A
to area B
Diffusion from area B
to area A
Diffusion from area B
to area A
Net diffusion
No net diffusion
(b) Equilibrium
KEY
= Solute molecule
Net diffusion = Diffusion from area A to area B minus diffusion from area B to area A
Differences in arrow length, thickness, and direction represent the relative
magnitude of molecular movement in a given direction.
Fig. 3-7, p. 65
Membrane Transport
Factors affecting rate of diffusion
collectively make up Fick’s law
of diffusion:
• Magnitude (or steepness) of the
concentration gradient
• Permeability of the membrane
to the substance
– Charge?
• Surface area of the membrane
across which diffusion is taking
place
• Molecular weight of the
substance
• Distance through which
diffusion takes place
Membrane Transport
• Osmosis
– Net diffusion of
water down its
own concentration
gradient
100% water concentration
0% solute concentration
(a) Pure water
90% water concentration
10% solute concentration
(b) Solution
KEY
= Water molecule
= Solute molecule
Fig. 3-9, p. 67
Normal cell volume
Intracellular fluid 300 mOsm/L
nonpenetrating solutes
H2O
300 mOsm/L
nonpenetrating solutes
H2O
200 mOsm/L
400 mOsm/L
nonpenetrating solutes nonpenetrating solutes
No net movement
of water; no change
in cell volume.
Water diffuses into
cells; cells swell.
Water diffuses out of
cells; cells shrink.
(a) Isotonic
conditions
(b) Hypotonic
conditions
(c) Hypertonic
conditions
Fig. 3-13, p. 71
Membrane Transport
Unassisted membrane transport
Assisted membrane transport
• Carrier-mediated transport
– Accomplished by membrane carrier flipping its
shape
– Can be active or passive
– Characteristics that determine the kind and
amount of material that can be transferred across
the membrane
• Specificity
• Saturation
• Competition
Membrane Transport
Types of assisted membrane transport
• Facilitated diffusion
• Active transport
• Vesicular transport
1 Carrier protein takes
conformation in which solute
binding site is exposed to
region of higher concentration.
Facilitated diffusion
•
•
•
Substances move from a
higher concentration to a
lower concentration
Requires carrier molecule
Means by which glucose is
transported into cells
Direction of
transport
ECF
Plasma
membrane
Solute molecule
to be transported
Carrier protein
Binding site
Concentration
gradient
(High)
(Low)
ICF
4 Transported
2 Solute
solute is released
and carrier protein
returns to
conformation in
step 1.
molecule
binds to
carrier
protein.
3 Carrier protein changes
conformation so that binding
site is exposed to region of
lower concentration.
Fig. 3-14, p. 72
Membrane Transport
Active transport
• Moves a substance against its concentration
gradient
• Requires a carrier molecule
• Primary active transport
– Requires direct use of ATP
• Secondary active transport
– Driven by an ion concentration gradient
established by a primary active transport
system
Na+ concentration
gradient
Active
Transport
1
ECF
High Na+
Low K+
High-affinity
binding
site for Na+
Plasma
membrane
Sodium
Potassium Pump
When open to the ECF,
6
the carrier drops off
Na+ on its highconcentration
side and picks up K+
from its lowconcentration side
Na+–K+ pump
Low Na+
ICF
High K+
Low-affinity
binding
K+ concentration
+
site for K
gradient
3 Na+
2
Direction of
K+ transport
2 K+
3 Na+
Low-affinity binding
site for Na+
High-affinity binding
site for K+
Direction of
Na+ transport
5
3
2 K+
Stepped Art
4
Fig. 3-16, p. 75
Active Transport
• Moves a substance against its
concentration gradient.
• Primary active transport:
– Requires direct use of ATP
• Secondary active transport:
– Driven by an ion concentration gradient
established by a primary active transport
– Two types, symport and antiport
Driving ion
in high
concentration
Transported
solute in low
concentration
Driving ion
in low
concentration
Transported
solute in high
concentration
(a) Symport
Fig. 3-17a, p. 77
Driving ion
in high
concentration
Transported
solute in high
concentration
Driving ion
in low
concentration
Transported
solute in low
concentration
(b) Antiport
Fig. 3-17b, p. 77
Secondary
Active
transport
Active Transport
• Moves a substance against its
concentration gradient.
• Primary active transport:
– Requires direct use of ATP
• Secondary active transport:
– Driven by an ion concentration gradient
established by a primary active transport
– Two types, symport and antiport
Carrier-mediated Transport
Characteristics
•
•
•
•
Specificity: Each carrier transports a specific
substance or a few closely related compounds.
Saturation: A limited number of carrier binding
sites are available.
Transport maximum (Tm): The amount of a
substance transported in a given time.
Competition: Several closely related compounds
may compete for transport on the same carrier.
Rate of transport of molecule into cell
Simple diffusion
down concentration
gradient
Carrier-mediated
transport down
concentration gradient
(facilitated diffusion)
Low
High
Concentration of transported molecules in ECF
Fig. 3-15, p. 73
Membrane Transport
• Vesicular transport
– Material is moved into or out of the cell wrapped in
membrane
– Active method of membrane transport
– Two types of vesicular transport
• Endocytosis
– Process by which substances move into cell
– Pinocytosis – nonselective uptake of ECF
– Phagocytosis – selective uptake of multimolecular
particle
• Exocytosis
– Provides mechanism for secreting large polar molecules
– Enables cell to add specific components to membrane
Table 3-2b p80
What is an excitable cell?
Membranes and their potentials are what make cells excitable.
Membrane Potential
• Plasma membrane of all living cells has a
membrane potential (polarized electrically)
• Separation of opposite charges across
plasma membrane
• Due to differences in concentration and
permeability of key ions
• Separated charges create the ability to do
work like electrons in a battery.
• millivolt- 1/1000 volt
Basic Physics
•
•
•
•
Brownian motion
Electrons protons neutrons
Ohms law E=I*P
Opposites attract, likes repel
(hydrophobic/hydrophyllic)
• Potential and kinetic energy
• Velocity and force, F= MA
– Larger mass requires more force to move
– Objects in motion stay in motion unless there is
friction and drag.
Basic measurements
• Volt – unit of charge
– mv – 1/1000 volt
– Car battery =12V, Cell = -70 mv
• Watt = unit of power
– Kw = 1000 watts, light bulb = 60 watts
Shearon Harris = 900 MW
• Ampere
– Unit of current
• ma = 1/1000 ampere
• http://www.osha.gov/SLTC/etools/construction/electrical
_incidents/eleccurrent.html
Membrane Potential
Which has the greatest membrane potential?
B>A
B<C
Membrane Potential
• Nerve and muscle cells
– Excitable cells
– Have ability to produce rapid, transient
changes in their membrane potential when
excited
• Resting membrane potential
– Constant membrane potential present in cells
of nonexcitable tissues and those of excitable
tissues when they are at rest
– Na+, K+, A-
Membrane Potential
• Effect of sodium-potassium pump on
membrane potential
– Makes only a small direct contribution to
membrane potential through its unequal
transport of positive ions
– The movement of ions and the large
negatively charged proteins (A-)
generate the potential difference.
Na+ concentration 1
gradient
ECF
High Na+ Low K+
Na+–K+ pump
High-affinity
binding
site for Na+
Low Na+
ICF
6
Low-affinity
binding
K+ concentration
+
site for K gradient
High K+
3 Na+
2
Direction of
K+ transport
2 K+
3 Na+
Low-affinity binding
site for Na+
High-affinity binding
site for K+
Direction of
Na+ transport
5
3
2 K+
4
Fig. 3-16, p. 75
ECF
Na+–K+
pump
(Passive)
Na+ channel
K+ channel
(Passive)
(Active)
(Active)
ICF
Fig. 3-23, p. 79
60mv
-90mv
-70mv
•
•
•
•
Rp + -70mv
Variable from one cell to another
Poison eliminates this potential
Generated by the imbalance of ions in the intracellular and extracellular spaces.
Nernst Equation
E=(61) log Co/Ci
Table 3-3 p82
Nernst equation
•
•
•
•
•
•
E=(61) log Co/Ci
For Potassium Ek=(61) log 5mM/150mM
For sodium ENa=(61) log 150mM/15mM
Co concentration in the ECF
Ci concentration in the ICF
Used to calculate the contribution of ions to
the resting potential of -70mv
Resting potential
•
•
•
•
EK = -90mv
ENa = 60mv
ECl = -70mv
K and Na drive Cl gradient
Usefulness?
• Neurons and muscle fibers can alter membrane
potential to send signals and create motion.
Plasma membrane
ECF
ICF
Concentration
gradient for K+
Electrical
gradient for K+
EK+ = –90 mV
Fig. 3-20, p. 83
Plasma membrane
ECF
ICF
Concentration
gradient for Na+
Electrical
gradient for Na+
ECF
anions,
mostly
ENa+ = +60 mV
Fig. 3-21, p. 84
Plasma membrane
ECF
ICF
Relatively
large net
diffusion of
K+ outward
establishes
an EK+ of –90
mV
No diffusion
of A– across
membrane
and associated
Relatively
small net
diffusion of
Na+ inward
neutralizes
some of the
potential
created by
K+ alone
Resting membrane potential = –70 mV
Fig. 3-22, p. 85
ANIMATION: Resting Potential