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Chapter 4 Membrane
Structure and
Function
4.1 Plasma Membrane
Structure and Function • Plasma membrane separates interior of a
cell from outside environment
• Is not an impermeable wall!
• Maintains homeostasis (nearly constant
conditions) within the cell
• Molecules can enter and exit in regulated
manner
– Depends on size, charge etc.
4.1 Plasma Membrane
Structure and Function • Structure?
• Mostly lipids and proteins
– Phospholipid bilayer (two layers of phospholipid
molecules)
• Hydrophilic (water-loving) polar heads
• Hydrophobic (water-fearing) nonpolar tails
– Cholesterol in plasma membrane of animal
cells provides slight stiffness and strength
– Various kinds of proteins
4.1 Plasma Membrane
Structure and Function
• Membrane proteins may be
– Peripheral proteins – associated with
only 1 side of membrane
• Provide support and stabilize
– Integral proteins – span the membrane
• Can protrude from 1 or both sides
• Can move laterally
4.1 Plasma Membrane
Structure and Function • Membrane is fluid in nature (light oil)
• Stabilized by:
– Hydrophilic and hydrophobic forces
– Peripheral proteins – Inside and outside surface
– Cytoskeleton from inside
Outside
Inside
plasma membrane
glycolipid
glycoprotein
integral protein
cholesterol
peripheral protein
filaments of cytoskeleton
hydrophobic tails
hydrophilic heads
phospholipid bilayer
carbohydrate
chain
Fluid Mosaic model of plasma
membrane
Mosaic of proteins in
phospholipids bilayer
Membrane is asymmetric
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• Membrane proteins have critical functions
• Repertoire of proteins varies in membranes
• They define functions of various cellular
membranes
• Diseases occur if function fails!
Function of the proteins
• Few categories of membrane proteins
– Channel proteins
– Carrier proteins
– Cell recognition proteins
– Receptor proteins
– Enzymatic proteins
Function of the proteins
Function of the proteins
Channel Protein
Allows a particular molecule or ion to
cross the plasma membrane freely .
Specific for a particular molecule or ion.
Cystic fibrosis, an inherited disorder, is
caused by a faulty chloride (Cl–) channel;
a thick mucus collects in airways and in
pancreatic and liver ducts.
a.
Carrier Protein
Selectively interacts with a specific
molecule or ion so that it can cross the
plasma membrane.
Different form channel protein in that
proteins shape changes while carrying
molecules.
The family of GLUT carriers transfers
glucose in and out of the various cell
types of the body . Different carriers
respond differently to blood levels of
glucose.
b.
Function of the proteins
c.
Cell Recognition Protein
The MHC (major histocompatibility
complex) glycoproteins are different
for each person, so organ
transplants are difficult to achieve.
Cells with foreign MHC
glycoproteins are attacked by white
blood cells responsible for
immunity.
Function of the proteins
d.
Receptor Protein
Shaped in such a way that a specific
molecule can bind to it.
Some types of dwarfism result not
because the body does not produce
enough growth hormone, but because the
plasma membrane growth hormone
receptors are faulty and cannot interact
with growth hormone.
Function of the proteins
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e.
Enzymatic Protein
Catalyzes a specific reaction. The
membrane protein, adenylate
cyclase, is involved in ATP
metabolism.
Cholera bacteria release a toxin
that interferes with the proper
functioning of adenylate cyclase,
which eventually leads to severe
diarrhea.
Function of the proteins 4.2 Permeability of the Plasma
Membrane • Permeability?
– Measure of ability to transmit fluid
• Differentially permeable
– Most molecules can not passage freely
• Factors that determine how a substance may be transported across a plasma membrane:
– Size
– Nature of molecule – polarity, charge
4.2 Permeability of the Plasma
Membrane • Concentration gradient
– More of a substance on one side of the
membrane
– Going “down” a concentration gradient
• From an area of higher to lower concentration
– Going “up” a concentration gradient
• From an area of lower to higher concentration
• Requires input of energy
4.2 Permeability of the Plasma
Membrane • Some molecules freely cross membrane
– Water, small, noncharged molecules
– Water may also use special channels called
aquaporins to cross membrane
• Other molecules need aid of:
– Channel proteins
– Carrier proteins
– Vesicles
• Endocytosis or exocytosis
macromolecule
H2O
protein
+
+ -
- charged molecules
and ions
phospholipid
molecule
noncharged
molecules
4.2 Permeability of the Plasma
Membrane
• Diffusion
–Movement of molecules from an
area of higher to lower
concentration
• Down a concentration gradient
–Solution contains a solute (solid)
and a solvent (liquid)
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• Once the solute and solvent are evenly distributed, their
molecules continue to move about, but there is no net
movement of either one in any direction
water molecules
(solvent)
dye molecules
(solute)
a. Crystal of dye is placed
in water
b. Diffusion of water and
dye molecules
c. Equal distribution of
molecules results
Diffusion is spontaneous
No chemical energy is required
• Gases can diffuse
through a
membrane
• Oxygen enters
blood capillaries
from air sacs
because its
concentration is
high in air sacs
• Carbon dioxide
exits capillaries
because of higher
concentration in
capillaries than air
sacs
capillary alveolus
bronchiole
oxygen
O2
O2 O2
O2
O2
O2
O2 O2
O2
O2
O2
O2
Gas Exchange in Lungs 4.2 Permeability of the Plasma
Membrane
• Osmosis
– Diffusion of water across a differentially
permeable membrane
– Membrane is not permeable to solute
– Water moves across membrane from
the side of dilute solution to the side of
more concentrated solution
• Membrane is not permeable to solute
• Water can move from dilute solution (beaker) to
concentrated solution (thistle tube)
– Level of water in the beaker is reduced
– Level of water in the thistle tube is increased
a.
10%
5%
< 10%
> 5%
solute water
b.
c.
beaker
less water (higher
percentage of solute)
more water (lower
percentage of solute)
more water (lower
percentage of solute)
less water (higher
percentage of solute)
differentially
permeable
membrane
thistle
tube
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4.2 Permeability of the Plasma
Membrane • Put a cell in a solution
• Solution can be: – Isotonic: the solute concentration in solution is
same as inside of a cell
– Hypotonic: a solution has a lower solute concentration than the inside of a cell
– Hypertonic: a solution has a higher solute concentration than the inside of a cell
• Isotonic
– No net gain or loss of water
– 0.9% NaCl
• Hypotonic
– Cell gains water
– Cytolysis or (hemolysis in case of blood cells)
• Hypertonic
– Cell loses water
– Crenation
nucleus
6.6 µm 6.6 µm 6.6 µm
Animal cells
plasma
membrane
In an isotonic solution, there is no net
movement of water .
In a hypotonic solution, water enters the cell,
which may burst (lysis).
In a hypertonic solution, water leaves the
cell, which shrivels (crenation).
• Isotonic
– No net gain or loss of water
• Hypotonic
– Cell gains water
– Turgor pressure keeps plant erect – cell wall
• Hypertonic
– Cell loses water
– Plasmolysis
chloroplast
nucleus
25 µm 40 µm 25 µm
In an isotonic solution, there is no
net movement of water.
In a hypotonic solution, the central vacuole
fills with water, turgor pressure develops, and
chloroplasts are seen next to the cell wall.
In a hypertonic solution, the central vacuole loses
water, the cytoplasm shrinks (plasmolysis), and
chloroplasts are seen in the center of the cell.
central
vacuole
cell
wall plasma
membrane
Plant cells 4.2 Permeability of the Plasma
Membrane
• Diffusion is spontaneous from high to low
concentration
• Osmosis is diffusion of water across a
membrane
• Above two account for transport of very
few molecules across membrane
• Membrane proteins aid transport in many
cases
4.2 Permeability of the Plasma
Membrane • Channel proteins
– Few molecules passage through specific
channel proteins
• Transport by Carrier Proteins
– Carrier proteins are specific
• Combine with a molecule or ion to be transported
across the membrane
– Carrier proteins are required for:
• Facilitated Transport
• Active Transport
• Facilitated transport
– Small molecules that are not lipid-soluble
– Molecules follow the concentration gradient
– Energy is not required
Inside
plasma
membrane carrier
protein
solute
Outside
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4.2 Permeability of the Plasma
Membrane • Active Transport
– Molecules move against the concentration
gradient
• Entering or leaving cell
• Examples
– Glucose accumulates in the cells lining digestive tract
– Iodine accumulates in cells of thyroid glands
– Molecules combine with carrier proteins
• Often called pumps
– Energy is required
Sodium-Potassium Pump
K+
Inside
carrier
protein
Outside K+
K+
K+
1. Carrier has a shape that allows
it to take up 3 Na+.
K+
P
ADP ATP
K+ K+
K+
2. ATP is split, and phosphate
group attaches to carrier.
K+
K+ K+
K+
P
3. Change in shape results and
causes carrier to release 3 Na+
outside the cell.
K+
K+
K+
K+
P
4. Carrier has a shape that
allows it to take up 2 K+.
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K+
K+
K+
K+
P
5. Phosphate group is released
from carrier.
K+
K+
K+ K+
6. Change in shape results and
causes carrier to release 2 K+
inside the cell.
K+
K+
K+
K+
K+
K+ K+
K+
K +
K+
K+
K+
K+
K+
K+
K+
K+ K+
P
P
P
P
Inside
6. Change in shape results and
causes carrier to release 2 K+
inside the cell.
carrier
protein Outside K+
K+
K+
ADP ATP
K+ K+
K+
3. Change in shape results and
causes carrier to release 3 Na+
outside the cell.
2. ATP is split, and phosphate
group attaches to carrier.
4. Carrier has a shape that
allows it to take up 2 K+.
5. Phosphate group is released
from carrier.
1. Carrier has a shape that allows
it to take up 3 Na+.
Na/K pump actively
transports Na+ and
K+ against their
concentration
gradient
Energy is spent in
form of ATP
4.2 Permeability of the Plasma
Membrane
• Vesicle Formation
– Membrane-assisted transport
– Transport of macromolecules
– Requires energy
– Keeps the macromolecule contained
• Exocytosis – exit out of cell
• Endocytosis – enter into cell
4.2 Permeability of the Plasma
Membrane • Exocytosis
– Vesicle fuses with plasma membrane as
secretion occurs
– Membrane of vesicle becomes part of plasma
membrane
– Cells of particular organs are specialized to
produce and export molecules
• Pancreatic cells release insulin when blood sugar
rises
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plasma membrane
Inside
Outside
secretory
vesicle
Vesicle Formation
• Endocytosis
– Cells take in substances by vesicle formation
• Phagocytosis: Large, particulate matter
• Pinocytosis: Liquids and small particles dissolved
in liquid
• Receptor Mediated Endocytosis: A type of
pinocytosis that involves a coated pit
paramecium
solute
solute
a. Phagocytosis
b. Pinocytosis
vacuole
coated vesicle
plasma membrane
coated pit
c. Receptor-mediated endocytosis
399.9 µm
vesicle
vacuole
forming
pseudopod
of amoeba
0.5 µm
vesicles
forming
coated
vesicle
coated
pit
receptor
protein
Outside Inside of cell Endocytosis
Passage of Molecules Into and
Out of the Cell
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