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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
7
Membrane
Structure and
Function
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Life at the Edge
• The plasma membrane is a
boundary separates the
living cell from its
surroundings.
• It controls traffic into and
out of the cell.
• The cell must be able to be
selective in its chemical
exchanges with its
environment.
• Like all biological
membranes, the
plasma membrane is
selectively permeableallowing some
substances to cross
more easily than
others.
© 2014 Pearson Education, Inc.
Figure 7.1
Membrane Models: Scientific Inquiry
• Membranes have been
chemically analyzed and
found to be made of
proteins and lipids
• Scientists studying the
plasma membrane
reasoned that it must be
a phospholipid bilayer
© 2014 Pearson Education, Inc.
Cellular membranes are fluid mosaics of
lipids and proteins
• Phospholipids are the
most abundant lipid in the
plasma membrane.
• Phospholipids are
amphipathic molecules,
containing hydrophobic
(fatty acid tails) and
hydrophilic (polar head)
regions.
• A phospholipid bilayer
can exist as a stable
boundary between two
aqueous compartments.
© 2014 Pearson Education, Inc.
Fig. 7.2
• In 1935, Hugh Davson and James Danielli
proposed a sandwich model in which the
phospholipid bilayer lies between two layers of
globular proteins
• Later studies found problems with this model,
particularly the placement of membrane
proteins, which have hydrophilic and
hydrophobic regions
• In 1972, J. Singer and G. Nicolson proposed
that the membrane is a mosaic of proteins
dispersed within the bilayer, with only the
hydrophilic regions exposed to water
© 2014 Pearson Education, Inc.
Fluid Mosaic Model
• The fluid mosaic model
states that a membrane is
a fluid structure with a
“mosaic” of various
proteins embedded in it.
• Proteins are not randomly
distributed in the
membrane.
• The hydrophilic regions of
proteins and
phospholipids are in
maximum contact with
aqueous environments,
and the hydrophobic
regions are in a nonaqueous environment
within the membrane.
© 2014 Pearson Education, Inc.
Fig. 7.3
Figure 7.3
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein CYTOPLASMIC
SIDE OF
MEMBRANE
The Fluidity of Membranes
• Phospholipids in the plasma membrane can
move within the bilayers.
• Most of the lipids, and some proteins, drift
laterally.
• Rarely does a molecule flip-flop transversely
across the membrane
– The lateral movements of phospholipids are rapid.
– Adjacent phospholipids switch positions about 107
times per second.
– Some large membrane proteins drift within the
phospholipids bilayers, although they move more
slowly than the phospholipids.
© 2014 Pearson Education, Inc.
Fig. 7-4
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell
Protein of two organism of Class- Mammals are very similar in structure
both are mixed after few hours .
© 2014 Pearson Education, Inc.
Fig. 7.5
Lateral movement
(∼107 times per second)
(a) Movement of phospholipids
© 2011 Pearson Education, Inc.
Flip-flop
(∼ once per month)
The Fluidity of Membranes
• Membrane fluidity is
influenced by temperature. To
work properly with active
enzymes and appropriate
permeability, membranes must
be about as fluid as salad oil.
• As temperatures cool,
membranes switch from a fluid
state to a solid state as the
phospholipids pack more closely.
• Membrane fluidity is also
influenced by the components of
the membrane.
• Membranes rich in
unsaturated fatty acids are
more fluid that those
dominated by saturated fatty
acids because kinks in the
unsaturated fatty acid tails at the
locations of the double bonds
prevent tight packing.
© 2014 Pearson Education, Inc.
Unsaturated tails
prevent packing
Saturated tails pack
together.
Fig. 7.5 a
The Fluidity of Membranes
• The steroid cholesterol is
wedged between phospholipid
molecules in the plasma
membrane of animal cells.
• At moderate temperatures(warm temperatures such as
37°C),Cholesterol reduces
membrane fluidity
but at low –cool temperatures
cholesterol hinders
solidification and maintains
fluidity by preventing tight
packing.
• Thus, cholesterol acts as a
“temperature buffer” for the
membrane, resisting
changes in membrane
fluidity as temperature
changes.
•
.
© 2014 Pearson Education, Inc.
Fig. 7.5b
Evolution of Differences in
Membrane Lipid Composition
• Variations in lipid composition of cell
membranes of many species appear to be
adaptations to specific environmental
conditions
• Ability to change the lipid compositions in
response to temperature changes has evolved
in organisms that live where temperatures vary
© 2014 Pearson Education, Inc.
Membrane Proteins and Their Functions
• A membrane is a collage of different proteins
embedded in the fluid matrix of the lipid bilayer
• Proteins determine most of the membrane’s
specific functions
• - Integral Proteins the membrane
penetrate the hydrophobic core
• - Peripheral Proteins are bound to the
surface of the membrane
© 2014 Pearson Education, Inc.
Integral Proteins
Fig. 7.3
• Integral proteins penetrate the hydrophobic core of the lipid bilayer,
often completely spanning the membrane as transmembrane proteins.
• Other integral proteins extend partway into the hydrophobic core.
• The hydrophobic regions embedded in the membrane’s core consist of
stretches of nonpolar amino acids, usually coiled into alpha helices.
• The hydrophilic regions of integral proteins are in contact with the
aqueous environment.
• Some integral proteins have a hydrophilic channel through their center
that allows passage of hydrophilic substances.
© 2014 Pearson Education, Inc.
Transmembrane Proteins
Fig. 7.6
• Integral proteins that span the entire lipid bilayer are
transmembrane proteins.
– N-terminus outside cell, C-terminus inside cell
– Hydrophobic areas orient themselves towards lipid
core (inside) of the membrane and hydrophilic regions
of protein orient themselves towards outside/inside of
bilayer, or inside of channel/pore.
© 2014 Pearson Education, Inc.
Peripheral and Intracellular/Extracellular Proteins
Fig. 7.3
• Peripheral proteins are not embedded in the lipid bilayer at all.
– Instead, peripheral proteins are loosely bound to the surface of
the membrane, often to integral proteins.
• On the cytoplasmic side of the membrane, some membrane proteins
are attached to the cytoskeleton.
• On the exterior side of the membrane, some membrane proteins
attach to the fibers of the extracellular matrix.
– These attachments combine to give animal cells a stronger
framework than the plasma membrane itself could provide.
© 2014 Pearson Education, Inc.
Six functions of membrane proteins
a. Transport of specific solutes into or out of cells
b. Enzymatic activity- sometimes catalyzing one
of a number of steps of a metabolic pathway
c. Signal transduction-relaying hormonal
messages to the cell
d. Cell-cell recognition- allowing other proteins
to attach two adjacent cells together
e. Intercellular joining of adjacent cells with gap
or tight junctions
f. Attachment to the cytoskeleton and
extracellular matrix- maintaining cell shape
and stabilizing the location of certain membrane
proteins
Fig. 7.7
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Glycoprotein
(d) Cell-cell recognition
Let us check details of each in next slides
Transport and Enzymatic Activity
a. Transport:
Channels that are selective
for a particular solute
Some transport proteins
require ATP to actively
shuttle substances across
the membrane
b. Enzymatic Activity:
Enzymes may be
sequestered in the
membrane with their active
sites exposed in the
cytoplasm. Several enzymes
may be located in close
proximity to each other and
function sequentially
Fig. 7.7a
© 2014 Pearson Education, Inc.
Signal Transduction and
Cell:Cell Recognition
c. Signal Transduction:
A chemical messenger binds to
a membrane receptor resulting
in a shape change of the
receptor that then triggers a
series of events within the cell
d. Cell:Cell Recognition:
Membrane glycoproteins serve
as ID tags that are recognized
be other cells
Fig. 7.7b
© 2014 Pearson Education, Inc.
Membrane carbohydrates are important for cell-cell recognition
Fig. 7.3
•
•
•
•
Cell-cell recognition- the ability of a cell to distinguish one type of neighboring
cell from another.
– Cell-cell recognition is the basis for the rejection of foreign cells by the
immune system.
Cells recognize other cells by binding to surface molecules, often
carbohydrates, on the plasma membrane.
– Membrane carbohydrates are usually branched oligosaccharides(sugar
groups).
Membrane carbohydrates may be covalently bonded to lipids, forming
glycolipids, or more commonly to proteins, forming glycoproteins.
The sugar groups on the extracellular side of the plasma membrane vary from
species to species, from individual to individual, and even from cell type to cell
type within an individual.
– This variation distinguishes each cell type.
– The four human blood groups (A, B, AB, and O) differ in the external
carbohydrates on red blood cells.
Figure 7.8
HIV
Receptor
(CD4)
Co-receptor
(CCR5)
HIV must bind to the immune
cell surface protein CD4 and
a “co-receptor” CCR5 in order
to infect a cell. HIV can infect
a cell that has CCR5 on its
surface, as in most people.
Receptor (CD4)
but no CCR5
Plasma
membrane
HIV cannot infect a cell lacking
CCR5 on its surface, as in
resistant individuals.
Intercellular Joining and ECM
Attachment
e. Intercellular Joining:
Membrane proteins on
adjacent cells may
attach to form gap or
tight junction
f. Attachment to ECM or
Cytoskeleton:
Membrane proteins can
form non-covalent
bonds with
microfilaments to
maintain cell shape and
stabilize proteins in a
certain location
© 2014 Pearson Education, Inc.
Fig. 7.7c
The inside and outside faces of membranes may
differ in lipid composition
• Each protein in the membrane has a directional
orientation in the membrane so they have distinct
inside and outside faces.
• The asymmetrical arrangement of proteins, lipids,
and their associated carbohydrates in the plasma
membrane (PM) is determined as the membrane
is built by the endoplasmic reticulum (ER) and
Golgi apparatus.
Membrane lipids and proteins are synthesized in
the ER.
Carbohydrates are added to proteins in the ER,
and the resulting glycoproteins are further
modified in the Golgi apparatus.
Glycolipids are also produced in the Golgi
apparatus.
Figure 7.9
Transmembrane
glycoproteins
Secretory
protein
Golgi
apparatus
Vesicle
ER
ER lumen
Glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane
glycolipid
Synthesis and Sidedness of Membranes
1.
2.
3.
4.
•
Membrane proteins and lipids are
synthesized in the ER and
carbohydrates are added to the
proteins.
Glycoproteins undergo
carbohydrate modifications in the
Golgi, and lipids acquire
carbohydrate chains.
Transmembrane proteins,
membrane glycolipids, and
secretory proteins leave the Golgi
in vesicles and travel to the PM.
At the PM the vesicles fuse with
the PM and secretory proteins are
released from the cell. Vesicles
fuse to the PM and the
carbohydrate chains are positioned
so that they are on the outside of
the PM. The outside layer of the
vesicle becomes continuous with
the cytoplasmic (inner) layer of the
plasma membrane.
Molecules that originate on the
inside face of the ER end up on the
outside face of the plasma
membrane.
• Carbohydrate
• Proteins
Membrane structure results in selective
permeability
• A cell must exchange materials with its surroundings, a
process controlled by the plasma membrane
– sugars, amino acids, and other nutrients enter cells and
metabolic waste products leave.
– Inorganic ions such as Na+, K+, Ca2+, and Cl− are
shuttled in and out of cells.
• Plasma membranes are selectively permeable,
regulating the cell’s molecular traffic
– Hydrophobic (nonpolar) molecules, such as
hydrocarbons, can dissolve in the lipid bilayer and pass
through the membrane rapidly
– Polar molecules, such as sugars, do not cross the
membrane easily-they require transport proteins to
enter the cell.
Transport Proteins and Selective
Permeability
• Transport proteins
allow passage of
hydrophilic substances
and specific ions across
the membrane
• Some transport
proteins, called channel
proteins, have a
hydrophilic channel that
certain molecules or
ions can use as a tunnel
© 2014 Pearson Education, Inc.
Aquaporin is a Channel Protein
• The passage of water
through the membrane is
facilitated by channel
proteins known as
aquaporins.
• Each aquaporin allows entry
of as many as 3 billion (109)
water molecules per second,
passing single file through
its central channel, which
fits 10 at a time.
• Without aquaporins, only a
tiny fraction of these water
molecules would diffuse
through the same area of the
cell membrane in a second,
so the channel protein
greatly increases the rate of
water movement.
Carrier Proteins
•
•
•
•
Carrier proteins bind to molecules and change shape to shuttle them
across the membrane.
Each transport protein is specific for the substance that it translocates,
carries across the membrane.
For example, the glucose transport protein in the liver carries glucose into
the cell but does not transport fructose, its structural isomer.
The glucose transporter causes glucose to pass through the membrane
50,000 times as fast as it would diffuse through on its own.
Passive transport is diffusion of a
substance across a membrane with no
energy investment
• Diffusion is the tendency for molecules to spread
out evenly into the available space
– Diffusion is driven by the intrinsic kinetic energy
(thermal motion or heat) of molecules.
– The movements of individual molecules are random.
However, the movement of a population of molecules
may be directional.
• At dynamic equilibrium, as many molecules cross
one way as cross in the other direction
© 2014 Pearson Education, Inc.
Diffusion of One Solute
Fig.7.10
•
•
•
•
•
•
Example: A permeable membrane with microscopic pores separating a
solution with dye molecules from pure water.
Each dye molecule wanders randomly, but there is a net movement of the
dye molecules across the membrane to the side that began as pure water.
The net movement of dye molecules across the membrane continues until
both sides have equal concentrations of the dye.
At this dynamic equilibrium, as many molecules cross one way as cross in
the other direction.
In the absence of other forces, a substance diffuses from where it is more
concentrated to where it is less concentrated, down its concentration
gradient.
No work must be done to move substances down the concentration
gradient; diffusion is a spontaneous process, needing no input of energy.
© 2014 Pearson Education, Inc.
Diffusion of Two Solutes
Fig.7.10a
• Each substance diffuses down its own concentration
gradient, independent of the concentration gradients of
other substances.
– Example: There is a net diffusion of the purple dye to the left,
even though the total solute concentration (orange + purple) is
greater on the left side.
© 2014 Pearson Education, Inc.
The diffusion of a substance across a
biological membrane is passive
transport because it requires no
energy from the cell to make it happen.
The diffusion of water across a selectively
permeable membrane is called osmosis.
•
Imagine that two sugar solutions differing in
concentration are separated by a membrane
that allows water through, but not sugar.
How does this affect the water
concentration?
• Water diffuses across the
membrane from the region of
lower solute concentration to the
region of higher solute
concentration until the solute
concentrations on both sides of
the membrane are equal.
•
•
The movement of water across cell
membranes and the balance of water
between the cell and its environment
are crucial to organisms.
Both solute concentration and
membrane permeability affect tonicity,
the ability of a solution to cause a cell to
gain or lose water.
© 2014 Pearson Education, Inc.
Fig.7.11
Water Balance of Cells Without Walls
• Tonicity is the ability of a solution to cause a cell to gain
or lose water
• Hypotonic solution: Solute concentration is less outside
than that inside the cell; cell gains water and lyses
• Isotonic solution: Solute concentration is the same both
outside and inside the cell; no net water movement
across the plasma membrane
• Hypertonic solution: Solute concentration is greater
outside than that inside the cell; cell loses water and
becomes shriveled
© 2014 Pearson Education, Inc.
Figure 7.12
Hypotonic
Isotonic
(a) Animal cell
H2O
H2O
Lysed
H2O
Shriveled
Normal
Cell
wall
H2O
H2O
H2O
Plasma
membrane
H2O
(b) Plant cell
Plasma
membrane
H2O
Hypertonic
Turgid (normal)
Flaccid
Plasmolyzed
Water Balance in Cells with Walls
• The cells of plants, prokaryotes, fungi, and some protist
have walls.
• A plant cell in a solution hypotonic to the cell contents
swells due to osmosis until the elastic cell wall exerts a
back-pressure on the cell that opposes further uptake.
– At this point the cell is turgid (very firm), a healthy
state for most plant cells. Turgid cells contribute to the
mechanical support of the plant.
• If a plant cell and its surroundings are isotonic, there is
no movement of water into the cell. The cell becomes
flaccid (limp), and the plant may wilt.
• The cell wall provides no advantages when a plant cell is
immersed in a hypertonic solution.
– As the plant cell loses water, its volume shrinks.
Eventually, the plasma membrane pulls away from
the wall. This plasmolysis is usually lethal.
– The walled cells of bacteria and fungi also
plasmolyzed in hypertonic environments.
Osmoregulation-The control of Water
Balance
• Organisms without rigid cell walls have osmotic
problems in either a hypertonic or a hypotonic
environment.
• Water balance is not a problem if such a cell
lives in isotonic surroundings, however.
– Seawater is isotonic to many marine invertebrates.
– The cells of most terrestrial animals are bathed in
extracellular fluid that is isotonic to the cells.
• Animals and other organisms without rigid cell
walls living in hypertonic or hypotonic
environments must have adaptations for
osmoregulation, the control of water balance.
Osmoregulation
• The protist Paramecium
is hypertonic to the pond
water in which it lives.
• In spite of a cell
membrane that is less
permeable to water than
other cells, water
continually enters the
Paramecium cell.
• To solve this problem,
Paramecium cells have
a specialized organelle,
the contractile vacuole,
that functions as a bilge
pump to force water out
of the cell.
© 2014 Pearson Education, Inc.
Fig.7.13
Proteins facilitate the passive transport
of water and selected solutes
• Many polar molecules and ions that can’t easily pass
through the lipid bilayer of the membrane diffuse
passively with the help of transport proteins that span the
membrane.
• The passive movement of molecules down their
concentration gradient via transport proteins is called
facilitated diffusion.
• Most transport proteins are very specific: They transport
only particular substances but not others.
• Two types of transport proteins facilitate the movement
of molecules or ions across membranes:
channel proteins and carrier proteins.
Figure 7.14
EXTRACELLULAR
FLUID
(a) A channel
protein
Channel protein
Solute
CYTOPLASM
Carrier protein
(b) A carrier protein
Solute
Facilitated Diffusion: Passive Transport
Aided by Proteins
• Channel proteins provide
hydrophillic corridors that
allow a specific molecule
or ion to cross the
membrane
• Channel proteins include
– Aquaporins, for
facilitated diffusion of
water
– Many Ion channels
are Gated they open
or close in response to
a stimulus
© 2014 Pearson Education, Inc.
Carrier Proteins and Passive Transport
• Carrier proteins bind to molecules and change shape to
shuttle them across the membrane.
• Each transport protein is specific for the substance that it
translocates.
• For example, the glucose transport protein in the liver
carries glucose into the cell but does not transport fructose, its
structural isomer.
• The glucose transporter causes glucose to pass through the
membrane 50,000 times as fast as it would diffuse through on
its own.
• Some diseases are caused by malfunctions in specific transport
systems, for example the kidney disease-cystinuria.
Active transport uses energy to move
solutes against their gradients
• Active transport moves substances against their
concentration gradient which requires energy ATP.
• ATP supplies the energy for most active transport
by transferring its terminal phosphate group
directly to the transport protein.
• Active transport is performed by specific carrier
proteins embedded in the membranes
• Active transport usually induces a conformational
change in the transport protein, translocating the
bound solute across the membrane.
© 2014 Pearson Education, Inc.
Active Transport: The Sodium-Potassium
Pump
• The sodium-potassium pump exchanges
sodium ions (Na+) for potassium ions (K+) across
the plasma membrane of animal cells.
• The plasma membrane helps maintain these
steep gradients by pumping 3 sodium ions out
of the cell and 2 potassium ions into the cell.
© 2014 Pearson Education, Inc.
Figure 7.15a
EXTRACELLULAR [Na+] high
FLUID
[K+] low
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
Na+
P
[Na+] low
[K+] high
ADP
1 Cytoplasmic Na+
2 Na+ binding
binds to the sodiumpotassium pump. The
affinity for Na+ is high
when the protein
has this shape.
stimulates
phosphorylation
by ATP.
ATP
Figure 7.15b
Na+
Na+
K+
Na+
K+
P
3 Phosphorylation
leads to a change in
protein shape, reducing
its affinity for Na+, which
is released outside.
P
Pi
4 The new shape has a
high affinity for K+, which
binds on the extracellular
side and triggers release
of the phosphate group.
Figure 7.15c
5 Loss of the
phosphate group
restores the protein’s
original shape, which
has a lower affinity
for K+.
6 K+ is released;
affinity for Na+ is
high again, and the
cycle repeats.
Figure 7.16
Passive transport
Diffusion
Active transport
Facilitated diffusion
ATP
How Ion Pumps Maintain Membrane
Potential
• Membrane potential is the voltage difference
across a membrane and ranges from −50 to −200
millivolts (mV).
• Voltage is created by differences in the distribution
of positive and negative ions The inside of the cell
is negative compared to the outside.
– The cytoplasm of a cell is negative in charge relative to
the extracellular fluid because of an unequal distribution
of cations and anions on opposite sides of the
membrane.
– Because the inside of the cell is negative compared with
the outside, the membrane potential favors the passive
transport of cations into the cell and anions out of the
cell.
© 2014 Pearson Education, Inc.
• Two combined forces, collectively called the
electrochemical gradient, drive the diffusion of ions
across a membrane:
– A chemical force (the ion’s concentration gradient)
– An electrical force (the effect of the membrane potential
on the ion’s movement)
An ion does not simply diffuse down its concentration
gradient but diffuses down its electrochemical gradient.
– For example, there is a higher concentration of Na+
outside a resting nerve cell than inside.
– When the neuron is stimulated, gated channels open
and Na+ diffuses into the cell down the electrochemical
gradient.
– The diffusion of Na+ is driven by the concentration
gradient and by the attraction of cations to the negative
side of the membrane.
© 2014 Pearson Education, Inc.
Active Transport by three different types of
Electrogenic Pump
1. An electrogenic pump is a transport protein that
generates voltage across a membrane.
Electrogenic pumps help store energy that can be
used for cellular work.
2. The sodium-potassium pump is the major
electrogenic pump of animal cells.
3. The proton pump main electrogenic pump of
plants, fungi, and bacteria is actively transports H+
out of the cell and transferring positive charge
from the cytoplasm to the extracellular solution.
Electrogenic pumps help store energy that can be
used for cellular work.
© 2014 Pearson Education, Inc.
Proton Pump-Plants and Bacteria
Fig.7.17
• The proton pump uses ATP for energy to
translocate H+ (positive charge) out of the cell.
This creates membrane voltage, or stored
energy. The voltage and H+ concentration
gradient are dual energy sources that can be
utilized to drive the uptake of nutrients.
© 2014 Pearson Education, Inc.
Cotransport: Coupled Transport by a
Membrane Protein
• Cotransport occurs when active transport of a
solute indirectly drives transport of another solute.
As the solute that has been actively transported
diffuses back passively through a transport
protein, its movement can be coupled with the
active transport of another substance against its
concentration gradient.
© 2014 Pearson Education, Inc.
Cotransport is active transport driven by a
concentration gradient
Plants use the gradient of hydrogen ions
generated by proton pumps to drive the
active transport of amino acids, sugars,
and other nutrients into the cell.
A carrier protein such as the sucrose-H+
cotransporter uses the diffusion of H+
down its electrochemical gradient to drive
sucrose uptake into the cell. The H+
gradient is maintained by an ATP-driven
proton pump that expels H+ from the cell,
thus storing potential energy that can be
used for the active transport of sucrose.
© 2014 Pearson Education, Inc.
Fig.7.18
Bulk transport across the plasma
membrane occurs by exocytosis and
endocytosis
• Small molecules and water enter or leave
the cell through the lipid bilayer or by
transport proteins
• Large molecules, such as polysaccharides
and proteins, cross the membrane through
bulk transport via vesicles
• Bulk transport requires energy
© 2014 Pearson Education, Inc.
Exocytosis
• In exocytosis, transport vesicles that have
budded off the Golgi migrate to the
membrane, fuse with it, and release their
contents
• Many secretory cells use exocytosis to
export their products
– Pancreatic cells release insulin
– Neuronal cells release norepinephrine
and acetylcholine
© 2014 Pearson Education, Inc.
Endocytosis
• In endocytosis, the cell takes in
macromolecules by forming vesicles from the
plasma membrane
• Endocytosis is a reversal of exocytosis,
involving different proteins
• There are three types of endocytosis:
– Phagocytosis (“cellular eating”)
– Pinocytosis (“cellular drinking”)
– Receptor-mediated endocytosis-allows the bulk
transport of molecules that might not be
abundant in the environment
© 2014 Pearson Education, Inc.
Figure 7.19
Phagocytosis
Pinocytosis
Receptor-Mediated
Endocytosis
EXTRACELLULAR
FLUID
Solutes
Pseudopodium
Receptor
Plasma
membrane
Coat
protein
“Food”
or
other
particle
Coated
pit
Coated
vesicle
Food
vacuole
CYTOPLASM
Phagocytosis
• In phagocytosis
a cell engulfs a
particle in a
vacuole
• The vacuole
fuses with a
lysosome to
digest the particle
• For exampleDigestion of food
of Amoeba and
• WBC engulf
bacteria.
© 2014 Pearson Education, Inc.
Pinocytosis
• In pinocytosis the cell
gulps droplets of
extracellular fluid into
tiny vesicles.
The cell utilizes the
molecules dissolved in
the extracellular fluid,
not the fluid itself.
© 2014 Pearson Education, Inc.
Receptor-Mediated Endocytosis
• Example: Human cells use a
RME process to take in
cholesterol for use in the
synthesis of membranes and
as a precursor for the
synthesis of steroids.
• Cholesterol travels in the blood
in low-density lipoproteins
(LDL), complexes of protein
and lipid.
• These lipoproteins act as ligands
by binding to LDL receptors on
membranes and entering the cell
by endocytosis.
• In an inherited disease called
familial hypercholesterolemia, the
LDL receptors are defective,
leading to an accumulation of
LDL and cholesterol in the blood.
• This condition contributes to early
atherosclerosis.
© 2014 Pearson Education, Inc.