Download The Plasma Membrane

Biology
A Guide to the Natural World
Chapter 5 • Lecture Outline
Life’s Border: The Plasma Membrane
Fifth Edition
David Krogh
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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
glycocalyx
phospholipids
cholesterol
proteins
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.
cell interior
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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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
P
-
hydrophilic
hydrophobic
hydrophilic
nonpolar
tails
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.
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The Phospholipid Bilayer
• In animal cells, the cholesterol molecules
that are interspersed between phospholipid
molecules in the plasma membrane perform
two functions:
• They act as a patching material that helps keep
some small molecules from moving through the
membrane.
• They keep the membrane at an optimal level of
fluidity.
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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:
• structural support
• cell identification, by serving as external
recognition proteins that interact with immune
system cells
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Membrane Protein Functions
• communication, by serving as external
receptors for signaling molecules
• 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
cell exterior
cell interior
Membrane proteins
can provide structural
support, often when
attached to parts of
the cell’s scaffolding
or “cytoskeleton.”
Protein fragments
held within
recognition proteins
can serve to identify
the cell as “normal” or
“infected” to immune
system cells.
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.
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Proteins can serve
as channels
through which
materials can pass
in and out of
the cell.
Figure 5.3
The Plasma Membrane
• The plasma membrane today is described
by a conceptualization called the fluidmosaic 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.
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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.
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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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solute
(a) An aqueous solution
divided by a semipermeable
membrane has a solute
—in this case, salt—
poured into its right chamber.
(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.
solvent
semipermeable membrane
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
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water bound to
salt ions
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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(b) Isotonic surroundings
(a) Hypertonic surroundings
(c) Hypotonic surroundings
H2O
Animal cell:
plasma membrane
H2O
H2O
Plant cell:
H2O
plasma membrane
H2O
cell wall
H2O
wilted
Net movement of
water out of cell
turgid
Balanced water
movement
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Net movement of
water into cell
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
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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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.
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Molecules again move
through a transport
protein, but now energy
must be expended to
move them against their
concentration gradient.
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.
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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
cell exterior
plasma
membrane
cell interior
2. Glucose binds
1. The transport
to the binding
protein has a
site.
binding site for
glucose that
is open to the
outside of the cell.
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 Moving Larger Substances
In and Out
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Moving Larger Substances
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
(b) Micrograph of exocytosis
(a) Exocytosis
extracellular fluid
transport vesicle
protein
cytosol
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Figure 5.10
Endocytosis
• There are two principal forms of
endocytosis: pinocytosis and phagocytosis.
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Endocytosis
• 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
• 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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(a) Pinocytosis
receptors
captured
molecules
1
2
3
4
coated
pit
vesicle
In this form of pinocytosis, called clathrin-mediated endocytosis, cell-surface
receptors bind to individual molecules of the substance to be taken into the cell and
then move laterally across the plasma membrane to a pit, coated on its underside
with the protein clathrin, that will become a vesicle that moves into the cell.
Formation of a pinocytosis vesicle.
(b) Phagocytosis
bacterium
(or food particles)
pseudopodium
vesicle
In phagocytosis, food particles—or perhaps whole organisms—are taken in by means
of “false feet” or pseudopodia that surround the material. Pseudopodia then fuse
together, forming a vesicle that moves into the cell’s interior with its catch enclosed.
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A human immune system cell called a
macrophage (colored blue) uses
phagocytosis to ingest an invading yeast
cell.
Figure 5.11
Endocytosis
• In pinocytosis, materials are brought into
the cell inside vesicles that bud off from the
plasma membrane.
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