Download chapter3_part1 Membrane lecture

Lauralee Sherwood
Hillar Klandorf
Paul Yancey
Chapter 3
Membrane Physiology
Sections 3.1-3.3
Kip McGilliard • Eastern Illinois University
3.1 Membrane Structure and Composition
 Plasma membrane
• Encloses the intracellular contents
• Selectively permits specific substances to
enter or leave the cell
• Responds to changes in cell’s environment
• Trilaminar structure
under electron
microscopy
Cell 1
Intercellular
space
Plasma membranes
Cell 2
Figure 3-1 p71
3.1 Membrane Structure and Composition
 The plasma membrane is a fluid lipid
bilayer embedded with proteins.
• Phospholipids
• Most abundant membrane component
• Head contains charged phosphate group
(hydrophilic)
• Two nonpolar fatty acid tails (hydrophobic)
• Assemble into lipid bilayer with hydrophobic
tails in the center and hydrophilic heads in
contact with water
• Fluid structure as phospholipids are not held
together by chemical bonds
3.1 Membrane Structure and Composition
Choline
Head
(negatively
charged,
polar,
hydrophilic)
Phosphate
Glycerol
Tails
(uncharged,
nonpolar,
hydrophobic)
Fatty acids
(a) Phospholipid
molecule
Figure 3-2a p71
ECF (water)
Polar heads
(hydrophilic)
Nonpolar tails
(hydrophobic)
Polar heads
(hydrophilic)
Lipid bilayer
ICF (water)
(b) Organization of phospholipids
into a bilayer in water
Figure 3-2b p71
Lipid bilayer
Intracellular
fluid
Extracellular
fluid
(c) Separation of ECF and ICF by the lipid bilayer
Figure 3-2c p71
3.1 Membrane Structure and Composition
 The plasma membrane is a fluid lipid
bilayer embedded with proteins.
• Cholesterol
• Placed between phospholipids to prevent
crystallization of fatty acid chains
• Helps stabilize phospholipids’ position
• Provides rigidity, especially in cold
temperatures
• Cold-induced rigidity is countered in some
poikilotherms by enriching membrane lipids
with polyunsaturated fatty acids
3.1 Membrane Structure and Composition
 Membrane proteins
• Integral proteins are embedded in the lipid
bilayer
• Have hydrophilic and hydrophobic regions
• Transmembrane proteins extend through the
entire thickness of the membrane
• Peripheral proteins are found on inner or
outer surface of membrane
• Polar molecules
• Anchored by weak chemical bonds to polar
parts of integral proteins or phospholipids
3.1 Membrane Structure and Composition
 Two models of membrane structure
• Fluid mosaic model
• Membrane proteins float freely in a “sea” of
lipids
• Membrane-skeleton fence model
• Mobility of membrane proteins is restricted by
the cytoskeleton
3.1 Membrane Structure and Composition
 Specialized functions of membrane proteins
• Channels
• Carriers
• Receptors
• Docking-marker acceptors
• Enzymes
• Cell-adhesion molecules (CAMs)
• Self-identity markers
3.1 Membrane Structure and Composition
 Membrane carbohydrates
• Located only on outer surface of membrane
• Short-chain carbohydrates bound to
membrane proteins (glycoproteins) or
lipids (glycolipids)
• Important roles in self-recognition and
cell-to-cell interactions
Extracellular fluid
Integral
proteins
Carbohydrate
chain
Phospholipid
molecule
Dark line
Appearance
using an electron Light space
microscope
Dark line
Glycolipid Glycoprotein
Receptor
protein
Lipid Cholesterol Leak channel
protein
bilayer molecule
Gated channel
protein
Peripheral
proteins
Cell adhesion molecule Carrier
(linking microtubule to protein
lntracellular fluid membrane)
Microfilament
of cytoskeleton
Figure 3-3 p72
ANIMATION: Cell membranes
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3.2 Unassisted Membrane Transport
 The plasma membrane is selectively
permeable
• Permeability across the lipid bilayer depends on:
• High lipid solubility
• Small size
• Force is needed to produce the movement of
particles across the membrane
• Passive forces do not require the cell to expend
energy
• Active forces require cellular energy (ATP)
3.2 Unassisted Membrane Transport
 Diffusion
• Random collisions and intermingling of
molecules as a result of their continuous,
thermally induced random motion
• Net movement of molecules from an area of
higher concentration to an area of lower
concentration
• Equilibrium is reached when there is no
concentration gradient and no net diffusion
3.2 Unassisted Membrane Transport
3.2 Unassisted Membrane Transport
 Fick’s law of diffusion
• The rate at which diffusion occurs depends on:
• Concentration gradient
• Permeability
• Surface area
• Molecular weight
• Distance
• Temperature
3.2 Unassisted Membrane Transport
 The movement of ions across the membrane is
affected by their electrical charge.
• A difference in charge between two adjacent areas
produces an electrical gradient.
• An electrical gradient passively induces ion movement -conduction
• Only ions that can permeate the plasma membrane can
conduct down this gradient.
• The simultaneous existence of an electrical gradient and
concentration gradient is called an electrochemical
gradient.
3.2 Unassisted Membrane Transport
3.2 Unassisted Membrane Transport
 Osmosis
• Water moves across a membrane by osmosis,
from an area of lower solute concentration to an
area of higher solute concentration.
• Driving force is the water concentration gradient
• Hydrostatic pressure opposes osmosis
• Osmotic pressure is the pressure required to
stop the osmotic flow
• Osmotic pressure is proportional to the
concentration of nonpenetrating solute
3.2 Unassisted Membrane Transport
3.2 Unassisted Membrane Transport
 Colligative properties of solutes depend
solely on the number of dissolved particles in
a given volume of solution
• Osmotic pressure
• Elevation of boiling point
• Depression of freezing point
• Reduction of vapor pressure
3.2 Unassisted Membrane Transport
 Tonicity refers to the effect of solute
concentration on cell volume
• Isotonic solution
• Same concentration of nonpenetrating solutes as in
normal cells
• Cell volume remains constant
• Hypotonic solution
• Lower solute concentration than in normal cells
• Cell volume increases, perhaps to the point of
lysis
• Hypertonic solution
• Higher solute concentration than in normal cells
• Cell volume decreases, causing crenation
3.2 Unassisted Membrane Transport
3D ANIMATION: Osmosis
3.3 Assisted Membrane Transport
 Phospholipid bilayer is impermeable to:
• Large, poorly lipid-soluble molecules
(proteins, glucose, and amino acids)
• Small, charged molecules (ions)
 Mechanisms for transporting these
molecules into or out of the cell
• Channel transport
• Carrier-mediated transport
• Vesicular transport
3.3 Assisted Membrane Transport
 Channel transport
• Transmembrane proteins form narrow channels
• Highly selective
• Permit passage of ions or water (aquaporins)
• Gated channels can be open or closed
• Leak channels are open at all times
• Movement through channels is faster than
carrier-mediated transport
3.3 Assisted Membrane Transport
Outside cell
Water
molecule
Lipid bilayer
membrane
Aquaporin
Cytosol
Figure 3-10 p82
Figure 3-10 p82
3.3 Assisted Membrane Transport
 Carrier-mediated transport
• Transmembrane proteins that can undergo
reversible changes in shape
• Binding sites can be exposed to either side of
membrane
• Transport small water-soluble substances
• Facilitated diffusion or active transport
3.3 Assisted Membrane Transport
 Characteristics of carrier-mediated
transport systems
• Specificity -- each carrier protein is
specialized to transport a specific substance
• Saturation -- limit to the amount of a
substance that a carrier can transport in a
given time (transport maximum or Tm)
• Competition -- closely related compounds
may compete for the same carrier
3.3 Assisted Membrane Transport
3.3 Assisted Membrane Transport
 Facilitated diffusion
• Passive carrier-mediated transport from high
to low concentration
• Does not require energy
• Example: Glucose transport into cells
3.3 Assisted Membrane Transport
 Facilitated diffusion
• Molecule to be transported attaches on
binding site on protein carrier
• Carrier protein changes conformation,
exposing bound molecule to the other side of
the membrane (lower concentration side)
• Bound molecule detaches from the carrier
• Carrier returns to its original conformation
(binding site on higher concentration side)
3.3 Assisted Membrane Transport
ANIMATION: Active and Facilitated
Diffusion
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ANIMATION: Passive transport
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3.3 Assisted Membrane Transport
 Active transport
• Carrier-mediated transport that moves a
substance against its concentration gradient
• Requires energy
• Primary active transport
• Energy is directly required
• ATP is split to power the transport process
• Secondary active transport
• ATP is not used directly
• Carrier uses energy stored in the form of an ion
concentration gradient built by primary active
transport
3.3 Assisted Membrane Transport
ANIMATION: Active Transport
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3.3 Assisted Membrane Transport
 Na+-K+ ATPase pump
• Pumps 3 Na+ out of cell for every 2 K+ in
• Splits ATP for energy
• Phosphorylation induces change in shape of
transport protein
• Maintains Na+ and K+ concentration
gradients across the plasma membrane
• Helps regulate cell volume
3.3 Assisted Membrane Transport
3.3 Assisted Membrane Transport
 Secondary active transport
• Simultaneous transport of a nutrient molecule
and an ion across the plasma membrane by a
cotransport protein
• Nutrient molecule is transported against its
concentration gradient
• Driven by simultaneous transport of an ion along
its concentration gradient
• Example: Cotransport of glucose and Na+ across
the luminal membrane of intestinal epithelial cells
3.3 Assisted Membrane Transport
3.3 Assisted Membrane Transport
 Vesicular transport
• Transport between ICF and ECF of large particles
wrapped in membrane-bound vesicles
• Endocytosis -- incorporates outside substances into
cell
• Exocytosis -- releases substances into the ECF
• The rate of endocytosis and exocytosis must be
balanced to maintain a constant membrane surface
area and cell volume
• Caveolae may play a role in transport of substances
and cell signaling