Transcript /cbidot/Biology 1005 Lectures/The Plasma Membrane.ppt

BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Life’s Border:
The Plasma Membrane
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5.1 The Nature of the Plasma Membrane
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The Nature of the Plasma Membrane
• The plasma membrane is a thin, fluid entity that
manages to be very flexible and yet is stable
enough to stay together despite being
continually remade due to the constant
movement of materials in and out of it.
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The Plasma Membrane
•
In animal cells, the plasma membrane has
four principal components:
1. A phospholipid bilayer.
2. Molecules of cholesterol interspersed within the
bilayer.
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The Plasma Membrane
3. Proteins that are embedded in or that lie on the
bilayer.
4. Short carbohydrate chains on the cell surface,
collectively called the glycocalyx, that function in
cell adhesion and as binding sites on proteins.
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The Plasma Membrane
phospholipids
cholesterol
proteins
glycocalyx
cell
exterior
cytoskeleton
Phospholipid bilayer:
a double layer of
phospholipid
molecules whose
hydrophilic “heads”
face outward, and
whose hydrophobic
“tails” point inward,
toward each other.
peripheral
protein
Cholesterol molecules
that act as a patching
substance and that
help the cell maintain
an optimal level of
fluidity.
integral
protein
Proteins, which are
integral, meaning
bound to the
hydrophobic interior of
the membrane, or
peripheral, meaning not
bound in this way.
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cell
interior
Glycocalyx: sugar
chains that attach to
proteins and
phospholipids, serving
as protein binding sites
and as cell lubrication
and adhesion
molecules.
Figure 5.1
The Phospholipid Bilayer
• Phospholipids are molecules composed of two
fatty acid chains linked to a charged phosphate
group.
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The Phospholipid Bilayer
• The fatty acid chains are hydrophobic, meaning
they avoid water, while the phosphate group is
hydrophilic, meaning it readily bonds with
water.
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The Phospholipid Bilayer
(a) Phospholipid molecule
polar
head
(b) Phospholipid bilayer
–
watery
extracellular
fluid
hydrophilic
hydrophobic
nonpolar
tails
hydrophilic
hydrophobic molecules hydrophilic molecules
pass through freely
do not pass
through freely
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watery
cytosol
Figure 5.2
The Phospholipid Bilayer
• Such phospholipids arrange themselves into
bilayers—two layers of phospholipids in which
the fatty acid “tails” of each layer point inward
(avoiding water), while the phosphate “heads”
point outward (bonding with it).
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The Phospholipid Bilayer
• Phospholipids take on this configuration in the
plasma membrane because a watery
environment lies on either side of the
membrane.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
The Phospholipid Bilayer
•
In animal cells, the cholesterol molecules that
are interspersed between phospholipid
molecules in the plasma membrane perform
two functions:
1. They act as a patching material that helps keep
some small molecules from moving through the
membrane.
2. They keep the membrane at an optimal level of
fluidity.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
The Phospholipid Bilayer
• Some plasma membrane proteins are integral,
meaning they are bound to the hydrophobic
interior of the phospholipid bilayer.
• Others are peripheral, meaning they lie on
either side of the membrane but are not bound
to its hydrophobic interior.
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Membrane Protein Functions
•
In animal cells, the cholesterol molecules that
are interspersed between phospholipid
molecules in the plasma membrane perform
two functions:
1. structural support
2. cell identification, by serving as external
recognition proteins that interact with immune
system cells
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Membrane Protein Functions
3. communication, by serving as external receptors
for signaling molecules
4. transport, by providing channels for the
movement of compounds into and out of the cell
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The Plasma Membrane
(a) Structural support
(b) Recognition
(c) Communication
(d) Transport
Membrane proteins can
provide structural
support, often when
attached to parts of the
cell’s scaffolding or
“cytoskeleton.”
Binding sites on some
proteins can serve to
identify the cell to
other cells, such as
those of the immune
system.
Receptor proteins,
protruding out from the
plasma membrane, can
be the point of contact
for signals sent to the
cell via traveling
molecules, such as
hormones.
Proteins can serve
as channels
through which
materials can pass
in and out of
the cell.
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Figure 5.3
The Plasma Membrane
• The plasma membrane today is described by a
conceptualization called the fluid-mosaic model
• It views the membrane as a fluid, phospholipid
bilayer that has a mosaic of proteins either fixed
within it or capable of moving laterally across
it.
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5.2 Diffusion, Gradients, and Osmosis
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Diffusion, Gradients, and Osmosis
• Diffusion is the movement of molecules or ions
from a region of their higher concentration to a
region of their lower concentration.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Diffusion, Gradients, and Osmosis
• A concentration gradient defines the difference
between the highest and lowest concentrations
of a solute within a given medium.
• Through diffusion, compounds naturally move
from higher to lower concentrations, meaning
down their concentration gradients.
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Diffusion, Gradients, and Osmosis
(a) Dye is dropped in
(b) Diffusion begins
(c) Dye is evenly distributed
water
molecules
dye
molecules
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Figure 5.4
Diffusion, Gradients, and Osmosis
• Energy must be expended to move compounds
against their concentration gradients, meaning
from a lower to a higher concentration.
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Diffusion, Gradients, and Osmosis
• A semipermeable membrane is one that allows
some compounds to pass through freely while
blocking the passage of others.
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Diffusion, Gradients, and Osmosis
• Osmosis is the net movement of water across a
semipermeable membrane from an area of
lower solute concentration to an area of higher
solute concentration.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Diffusion, Gradients, and Osmosis
• Because the plasma membrane is a
semipermeable membrane, osmosis operates in
connection with it.
• Osmosis is a major force in living things; it is
responsible for much of the movement of fluids
into and out of cells.
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Diffusion, Gradients, and Osmosis
solute
(a) An aqueous solution
divided by a semipermeable
membrane has a solute
—in this case, salt—
poured into its right
chamber.
solvent
semipermeable membrane
(b) As a result, though
water continues to flow in
both directions through
the membrane, there is a
net movement of water
toward the side with the
greater concentration of
solutes in it.
osmosis
(c) Why does this
occur? Water molecules
that are bonded to the
sodium (Na+) and
chloride (Cl–) ions that
make up salt are not free
to pass through the
membrane to the left
chamber of the container.
pure water
water bound to
salt ions
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Figure 5.5
Osmotic Imbalances
• Osmotic imbalances can cause cells either to
dry out from losing too much water or, in the
case of animal cells, to break from taking too
much water in.
• Plant cells generally do not have this problem
because their cell walls limit their uptake of
water.
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Solute Concentration
• Cells will gain or lose water relative to their
surroundings in accordance with what the
solute concentration is inside the cell as
opposed to outside it.
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Solute Concentration
• A cell will lose water to a surrounding solution
that is hypertonic—a solution that has a greater
concentration of solutes in it than does the cell’s
cytoplasm.
• A cell will gain water when the surrounding
solution is hypotonic to the cytoplasmic fluid.
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Solute Concentration
(b) Isotonic
surroundings
(a) Hypertonic
surroundings
(c) Hypotonic
surroundings
H2O
Animal
cell:
plasma
membrane
H2O
H2O
Plant
cell:
H2O
plasma
membrane
cell wall
H2O
H2O
wilted
Net movement of
water out of cell
turgid
Balanced water
movement
Net movement of
water into cell
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Figure 5.6
Solute Concentration
• Water flow is balanced between the cell and its
surroundings when the surrounding fluid and
the cytoplasmic fluid are isotonic to each
other—when they have the same concentration
of solutes.
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Plasma Membranes and Diffusion
PLAY
Animation 5.1: Plasma Membranes and Diffusion
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5.3 Moving Smaller Substances In and Out
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Moving Smaller Substances In and Out
• Some compounds are able to cross the plasma
membrane strictly through diffusion; others
require diffusion and special protein channels;
still others require protein channels and the
expenditure of cellular energy.
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Transport Through the Plasma
Membrane
Passive transport
simple diffusion
Active transport
facilitated diffusion
ATP
Materials move down
their concentration
gradient through the
phospholipid bilayer.
The passage of
materials is aided both
by a concentration
gradient and by a
transport protein.
Molecules again move
through a transport
protein, but now energy
must be expended to
move them against their
concentration gradient.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 5.7
Transport Through the Plasma
Membrane
• Active transport is any movement of
molecules or ions across a cell membrane that
requires the expenditure of energy.
• Passive transport is any movement of
molecules or ions across a cell membrane that
does not require the expenditure of energy.
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Types of Passive Transport
• There are two forms of passive transport:
simple diffusion and facilitated diffusion.
• For either form of transport to bring about a net
movement of materials into or out of a cell, a
concentration gradient must exist.
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.
Types of Passive Transport
• A concentration gradient is all that is required
for simple diffusion to operate.
• Facilitated diffusion, however, requires both a
concentration gradient and a protein channel.
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Facilitated Diffusion
• In facilitated diffusion, transport proteins
function as channels for larger hydrophilic
substances—substances that, because of their
size and electrical charge, cannot diffuse
through the hydrophobic portion of the plasma
membrane.
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Facilitated Diffusion
glucose
outside
cell
plasma
membrane
inside cell
1. The transport protein has a binding
site for glucose
that is open to the
outside of the cell.
2. Glucose binds
to the binding
site.
3. This binding causes
the protein to change
shape, exposing
glucose to the inside
of the cell.
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4. Glucose passes
into the cell and
the protein returns
to its original shape.
Figure 5.8
Active Transport
• Cells cannot rely solely on passive transport to
move substances across the plasma membrane.
• A cell may need to maintain a greater
concentration of a given substance on one side
of its membrane.
• Yet, passive transport equalizes concentrations
of substances on both sides of the plasma
membrane.
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Active Transport
• To deal with such needs, cells use active
transport.
• Chemical pumps move compounds across the
plasma membrane against their concentration
gradients.
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Active Transport
• One example of such transport is the pumping
of glucose into cells that line the small
intestines.
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5.4 Getting the Big Stuff In and Out
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Getting the Big Stuff In and Out
• Larger materials are brought into the cell
through endocytosis and moved out through
exocytosis.
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Exocytosis and Endocytosis
• Both mechanisms employ vesicles, the
membrane-lined enclosures that alternately bud
off from membranes or fuse with them.
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Exocytosis
• In exocytosis, a transport vesicle moves from
the interior of the cell to the plasma membrane
and fuses with it, at which point the contents of
the vesicle are released to the environment
outside the cell.
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Exocytosis
(a) Exocytosis
extracellular fluid
transport vesicle
(b) Micrograph of exocytosis
protein
cytosol
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Figure 5.9
Endocytosis
• There are two principal forms of endocytosis:
pinocytosis and phagocytosis.
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Endocytosis
1.
Pinocytosis is the movement of moderatesized molecules into a cell by means of the
creation of transport vesicles produced
through an infolding or “invagination” of a
portion of the plasma membrane.
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Endocytosis
2.
Phagocytosis is when certain cells use
pseudopodia or “false feet” to surround and
engulf whole cells, fragments of them, or
other large organic materials.
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Endocytosis
(a) Pinocytosis
receptors
captured
molecules
coated
pit
vesicle
Formation of a pinocytosis vesicle.
(b) Phagocytosis
bacterium
(or food particles)
pseudopodium
vesicle
A human macrophage (colored blue) uses
phagocytosis to ingest an invading yeast cell.
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Figure 5.10
Endocytosis
• In pinocytosis, materials are brought into the
cell inside vesicles that bud off from the plasma
membrane.
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