Chapter 3
The Plasma Membrane
and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Review
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•
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Membrane structure and composition
Cell to cell adhesions
Membrane transport
Pgs 52-73
New
• Membrane potentials
• Pgs 73-83
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Plasma Membrane Structure
• Channels
• Carrier molecules
• Docking marker
acceptors
• Membrane bound
enzymes
• Receptor sites
• Cell adhesion
molecules (CAMs)
– Integrin,
cadherin
• Cell surface
markers
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Specialized Cell Junctions
Desmosomes
Gap junctions
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Act like “spot rivets” that anchor two •
Small connecting tunnels formed by
closely adjacent nontouching cells
connexons
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Most abundant in tissues that are
•
Especially abundant in cardiac and
subject to considerable stretching
smooth muscle
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In nonmuscle tissues permit
unrestricted passage of small nutrient
molecules between cells
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Also serve as method for direct
transfer of small signaling molecules
from one cell to the next
Tight junctions
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Firmly bond adjacent cells together
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Seal off the passageway between the
two cells
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Found primarily in sheets of epithelial
tissue
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Prevent undesirable leaks within
epithelial sheets
•
C
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Membrane Transport
• Unassisted membrane transport
– Diffusion
– Osmosis
• Assisted membrane transport
– Carrier-mediated transport
– Facilitated transport
– Active transport
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Membrane Transport
Diffusion
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Table 3-1, p. 62
Membrane Transport
• Osmosis
– Net diffusion of
water down its
own concentration
gradient
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Fig. 3-9, p. 63
Membrane Transport
• Tonicity of a solution
– Determines whether cell remains same size, swells,
or shrinks when a solution surrounds the cell
• Isotonic solution
– Has same concentration of nonpenetrating solutes as
normal body cells
– Cell volume remains constant
• Hypotonic solution
– Lower concentration of nonpenetrating solutes than
normal body cells
– Water enters cell causing cell to swell or perhaps rupture
• Hypertonic solution
– Higher concentration of nonpenetrating solutes than
normal body cells
– Cells shrink as they lose water by osmosis
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Membrane Transport
Types of assisted membrane transport
• Facilitated diffusion
• Active transport
• Vesicular transport
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Membrane Transport
Facilitated diffusion
• Substances move from a
higher concentration to a
lower concentration
• Requires carrier molecule
• Means by which glucose
is transported into cells
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Active Transport
Concentration
gradient
Na+
Phosphorylated
conformation Y
of carrier
Step 1
Molecule to be
transported
Phosphorylated conformation Y of
carrier has high affinity for passenger.
Molecule to be transported binds to
carrier on low-concentration side.
Dephosphorylated
conformation X
of carrier
ECF
(High)
ICF
(Low)
Direction of
transport
Step 2
Dephosphorylated conformation X
= phosphate
of carrier has low affinity for
passenger. Transported molecule
detaches from carrier on high-concentration side.
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Fig. 3-16, p. 70
Sodium Potassium Pump
When open to the ECF, the carrier drops off Na+ on its high-concentration
side and picks up K+ from its low-concentration side
ECF
ICF
When open to the ICF, the carrier picks up Na+ from its low-concentration
side and drops off K+ on its high-concentration side
Phosphorylated conformation Y
of Na+–K+ pump has high affinity
for Na+ and low affinity for K+
when exposed to ICF
= Sodium (Na+)
Dephosphorylated
conformation X of Na+–K+
pump has high affinity for
K+ and low affinity for Na+
when exposed to ECF
Chapter 3 The Plasma Membrane and Membrane Potential
+
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
= Potassium (K )
= Phosphate
Fig. 3-17, p. 70
Secondary
Active
transport
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Fig. 3-18, p. 72
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Table 3-2c, p. 74
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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Carrier-mediated transport:
(a) involves a specific membrane protein that serves as a carrier molecule.
(b) always moves substances against a concentration gradient.
(c) always requires energy expenditure.
(d) two of these answers.
(e) all of these answers.
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Facilitated diffusion:
(a) involves a carrier molecule.
(b) requires energy expenditure.
(c) is how glucose enters the cells.
(d) both (a) and (c) above.
(e) all of these answers.
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Select the correct statement about diffusion.
(a) It depends on the random motion
(b) It involves active forces.
(c) Its rate increases as the temperature decreases.
(d) Molecules move from a lower concentration to a higher concentration.
(e) The chemical gradient of a substance does not affect it.
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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 ( hydroelectric dam) millivolt- 1/1000
volt
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Membrane Potential
Which has the greatest membrane potential?
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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+, AChapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
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
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
Table 3-3, p. 75
Resting potential (Er)
Rp + -75mv
Variable from one cell to
another
Poison eliminates this
potential
Generated by the
imbalance of ions in the
intracellular and
extracellular spaces.
Ion
Out
mM
In
mM
Pot
mV
K
5.5
150
-90
Na
150
15
60
Perm.
50-75
1
Cl
125
9
Chapter 3 The Plasma Membrane and Membrane Potential
Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning
-70
Ion Concentration
Resting Potential
Fig. 3-22, p. 78
Plasma membrane
ICF
ECF
Electrical
gradient for K+
Concentration
gradient for K+
EK+ = –90 mV
Fig. 3-20, p. 76
Plasma membrane
ICF
ECF
Concentration
gradient for Na+
Electrical
gradient for Na+
ENa+ = +60 mV
Fig. 3-21, p. 78
Plasma membrane
ECF
ICF
Relatively large net
diffusion of K+
outward establishes
an EK+ of –90 mV
No diffusion of A–
across membrane
Relatively small net
diffusion of Na+
inward neutralizes
some of the
potential created by
K+ alone
Resting membrane potential = –70 mV
(A– = Large intracellular anionic proteins)
Fig. 3-22, p. 78
Fig. 3-23, p. 79
ECF
Na+–K+
pump
(Passive)
Na+ channel
K+ channel
(Passive)
(Active)
(Active)
ICF
Fig. 3-23, p. 79
Points to Ponder 3, p. 83
Nernst equation
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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
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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.