aid in movement, or cilia, which can have many different functions.
The fluid mosaic model
Nucleus
Ribosomes
Nuclear envelope
Rough endoplasmic reticulum
Nucleolus
Smooth endoplasmic reticulum
Nuclear pore
Intermediate filament
Microvilli
Cytoskeleton
Actin filament
(microfilament)
Microtubule
Ribosomes
Intermediate
filament
Biology of Cell
Centriole
Cytoplasm
Lysosome
Exocytosis
Vesicle
Golgi apparatus
Plasma membrane
Peroxisome
Mitochondrion
The cell membrane
The fluid mosaic model shows proteins embedded in a fluid lipid bilayer. All eukaryotic
cell membranes are formed by Phospholipid stratus arranged in a bilayer and
Membrane proteins inserted in the lipid bilayer
e fluid mosaic
mbranes.
rotrude through the
with nonpolar
them to the
phobic interior.
ns are often bound
portion of these
glycoproteins.
ane proteins are
surface of the
rane phospholipids
the addition of
orm glycolipids.
n filaments and
ents interact with
s. Outside the cell,
have an elaborate
x composed
proteins.
Extracellular
matrix protein
Biology of Cell
Glycoprotein
Glycolipid
Integral
proteins
Glycoprotein
Cholesterol
Actin filaments
of cytoskeleton
Peripheral
protein
Intermediate
filaments
of cytoskeleton
The lipid layer that forms the foundation of a cell’s membranes
is a bilayer formed of phospholipids (figure 5.1). For many
years, biologists thought that the protein components of the
cell membrane covered the inner and outer surfaces of the
phospholipid bilayer like a coat of paint. An early model portrayed the membrane as a sandwich; a phospholipid bilayer between two layers of globular protein.
In 1972, S. Jonathan Singer and Garth J. Nicolson revised
the model in a simple but profound way: They proposed that
the globular proteins are inserted into the lipid bilayer, with
their nonpolar segments in contact with the nonpolar interior
of the bilayer and their polar portions protruding out from the
membrane surface. In this model, called the fluid mosaic model, a
mosaic of proteins floats in or on the fluid lipid bilayer like
boats on a pond (figure 5.2).
We now recognize two categories of membrane proteins
based on their association with the membrane. Integral membrane proteins are embedded in the membrane, and peripheral
proteins are associated with the surface of the membrane.
Phospholipid bilayer
A eukaryotic cell contains many membranes. Alt
not all identical, they share the same fundamen
Cell membranes are assembled from four compon
1. Phospholipid bilayer. Every cell membra
posed of phospholipids in a bilayer. The o
nents of the membrane are embedded wit
which provides a flexible matrix and, at th
imposes a barrier to permeability. Animal
branes also contain cholesterol, a steroid w
hydroxyl group (–OH). Plant cells have
cholesterol content.
2. Transmembrane proteins. A major com
every membrane is a collection of protein
the lipid bilayer. These proteins have a va
functions, including transport and commu
the membrane. Many integral membrane
not fixed in position. They can move abou
phospholipid molecules do. Some membr
Every cell membrane is composed of phospholipids in a bilayer.
The other components of the membrane are embedded within the bilayer, which provides
a flexible matrix and, at the same time, imposes a barrier to permeability. Animal cell
membranes also contain cholesterol, a steroid (lipid) with a polar hydroxyl group (–OH).
The partially hydrophilic, partially hydrophobic phospholipid spontaneously forms a
bilayer: fatty acids are on the inside, phosphate groups are on both surfaces of the
bilayer
O
OJPJ O:
O
H
J J J
Polar Hydrophilic Heads
CH2
J J
H2CJJCJJCH2
O
Nonpolar Hydrophobic Tails
J J J J JJJ
CH2JN;(CH3)3
O
CJO CJO
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH
CH2 CH
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH2 CH2
CH3 CH3
a. Formula
b. Space-filling model
c. Icon
Membrane proteins
Membrane proteins have various functions:
1.Transporters. Membranes are very selective, allowing only certain chemicals to enter
or leave the cell, either through channels or carriers composed of proteins.
2.Enzymes. Cells carry out many chemical reactions on the interior surface of the
plasma membrane, using enzymes attached to the membrane.
3.Cell-surface receptors. Membranes are sensitive to chemical messages, which are
detected by receptor proteins on their surfaces. The message is then sent to the
interior of the cell.
Outside
Outside
Outside
cell
cell
cell
Transporter
Transporter
Transporters
Transporter
Inside
Inside
Inside
cell
cell
cell
Enzyme
Enzyme
Enzymes
Enzyme
Cell-surface
receptor
Cell-surface
receptor
Cell-surface
receptors
Cell-surface
receptor
Membrane proteins
Membrane proteins have various functions:
4.Cell-surface identity markers. Membranes carry cell-surface markers that identify them
Outside
Outside
Inside
Inside
Outside
Inside
to
cell
cellother cells. Most cell
cell
celltypes carry their own ID tags, specific combinations of protein
cell
cell
complexes such as glycoproteins (proteins + carbohydrates) that are characteristic of
that cell type.
5.Cell-to-cell adhesion proteins. Cells use specific proteins to glue themselves to one
another. Some act by forming temporary interactions, and others form a more
permanent bond.
6.Attachments to the cytoskeleton. Some surface proteins are anchored to the
microfilaments of the cytoskeleton by linking proteins.
Transporter
Transporter
Transporter
Enzyme
Enzyme
Enzyme
Cell-surface
Cell-surface
identity
identity
marker
marker
Cell-surface
identity
markers
Cell-surface
identity
marker
Cell-to-cell
Cell-to-cell
adhesion
adhesion
Cell-to-cell
adhesion
Cell-to-cell
adhesion
Cell-surface
Cell-surface
receptor
receptor
Cell-surface
receptor
proteins
Attachment
Attachment
toto
the
the
cytoskeleton
cytoskeleton
Attachments
tocytoskeleton
the
cytoskeleton
Attachment
to
the
Figure
Figure5.5
5.5
5.5Functions
Functions
Functionsof
ofofplasma
plasma
plasmamembrane
membrane
membraneproteins.
proteins.
proteins.Membrane
Membrane
Membraneproteins
proteins
proteinsact
act
actas
asastransporters,
transporters,
transporters,enzymes,
enzymes,
enzymes,cell-surface
cell-surface
cell-surfacereceptors,
receptors,
receptors,and
and
and
Figure
cell-surface
cell-surfaceidentity
identity
identitymarkers,
markers,
markers,as
asaswell
well
wellas
asasaiding
aiding
aidingin
inincell-to-cell
cell-to-cell
cell-to-celladhesion
adhesion
adhesionand
and
andsecuring
securing
securingthe
the
thecytoskeleton.
cytoskeleton.
cytoskeleton.
cell-surface
Inquiry
Inquiryquestion
question
question
Inquiry
The anchoring of proteins
Pores in the bilayer
Membrane proteins
Some
transmembrane
proteinsthe
have
extensive
nonpolarof
regions
Some
membrane
proteins
are
attached
the
surface
the
helical
segments
that traverseto
membrane,
forming a strucsecondary
configurations
of
β-pleated
sheets
instead
Membrane proteins also have common structural featureswith
related
to
their
role.
They
can
ture within the membrane through which protons pass during of
α helices
(chapterpumping
3). Theassociate
β protons
sheets form
characteristic motif,
membrane by special molecules
that
with
the light-driven
of
(figureastrongly
5.7).
be classified considering their position.
folding back and forth in a cylinder so the sheets arrange themphospholipids. Like a ship
tied
a floating
these
anchored
Pores
selves
like to
a pipe
through the dock,
membrane.
This forms
a polar
Some transmembrane
proteins
nonpolar regions
in the
interior
of have
the βextensive
sheets
spanning
the memPeripheral
membrane
proteins
are free toproteins
moveenvironment
about
on
the
surface
of
the
membrane
with This
secondary
configurations
of β-pleated
instead
of
brane.
so-called
β barrel, open
on bothsheets
ends, is
a common
α
helices
(chapter
3
)
.
The
β
sheets
form
a
characteristic
motif,
feature
of thethey
porin class
of proteins
that
are found
the
tethered
phospholipid.
The
anchoring
molecules
arewithin
modiAnchored to a phospholipid in one
layertoofa the
membrane,
possess
nonpolar
folding
back
and
forth
in
a
cylinder
so
the
sheets
arrange
themouter membrane of some bacteria. The openings allow moleselves like a pipe
through the
membrane.
Thisinto
forms athe
polar infied
lipids
that
have
(1)
nonpolar
regions
that
insert
to
pass through
the membrane
(figure
5.8).
regions that are inserted in the lipid bilayer and are free cules
to
move
throughout
one
layer
of
environment in the interior of the β sheets spanning the memternal
brane.bilayer
This so-calledand
β barrel,(2)
open chemical
on both ends, is a bonding
common
a.
b. portion of the lipid
the
bilayer
feature of the porin class of proteins that are found within the
Learning
Outcomes Review 5.3
Figure 5.6 Transmembrane
domains. Integral
membrane
domains
that link
directly outer
to proteins.
membrane of some bacteria. The openings allow moleproteins have at least one hydrophobic transmembrane domain
(shown in blue) to anchor them in the membrane. a. Receptor
a.
b.
Protein
anchored
protein with seven transmembrane
domains. b.
Protein with to
Figure 5.6 domain.
Transmembrane
domains. Integral membrane
single transmembrane
phospholipid
proteins have at least one hydrophobic transmembrane domain
(shown in blue) to anchor them in the membrane. a. Receptor
protein with
sevensome
transmembrane
b. based
Proteinon
with
Biologists
classify
types of domains.
receptors
the
single
transmembrane
domain.
number of transmembrane domains they have, such as
G protein–coupled receptors with seven membrane-spanning domainsBiologists
(chapter classify
9). These
receptors
respond based
to extersome
types of receptors
on the
nal molecules,
as epinephrine, domains
and initiate
cascade
of as
numbersuch
of transmembrane
theya have,
such
events inside
the cell.
G protein–coupled
receptors with seven membrane-spanAnother
example(chapter
is bacteriorhodopsin,
onerespond
of thetokey
ning domains
9). These receptors
external molecules,
suchthat
as epinephrine,
initiate a cascade
transmembrane
proteins
carries outand
photosynthesis
in of
events
inside
the
cell.
halophilic (salt-loving) archaea. It contains seven nonpolar
Proteins
the through
membranethe
confer
the main diff
erences
between
cules
to in
pass
membrane
(figure
5.8).
membranes of different cells. Their functions include transport, enzymatic
action, reception of extracellular signals, cell-to-cell interactions, and cell
Learning
Review
identity
markers.Outcomes
Peripheral proteins
can be5.3
anchored in the membrane by
modifi
ed
lipids.
Integral
membrane
proteins
span thebetween
membrane and have
Proteins in the membrane confer the main differences
onemembranes
or more hydrophobic
regions,
transmembrane
domains,
that
of different cells.
Theircalled
functions
include transport,
enzymatic
action,
reception of extracellular signals, cell-to-cell interactions, and cell
anchor
them.
identity markers. Peripheral proteins can be anchored in the membrane by
■
areIntegral
transmembrane
domains
hydrophobic?
modifiWhy
ed lipids.
membrane proteins
span the
membrane and have
one or more hydrophobic regions, called transmembrane domains, that
anchor them.
■
?
Why are transmembrane domains hydrophobic?
Inquiry question
Based only on amino acid sequence, how would you
recognize an integral membrane protein?
Inquiry question
Another example
is bacteriorhodopsin, one
of the key
Transmembrane proteins (or Integral
membrane
proteins)
only on amino acid sequence, how would you
transmembrane proteins
carries out
photosynthesis
in
? Based
94that part
Biology
of the Cell
II
recognize
an integralat
membrane
protein?
halophilic (salt-loving)and
archaea.span
It containsacross
seven nonpolar
Permanently attached to the membrane
the membrane
least
once.
They differ in the way they traverse the lipid bilayer.
rav32223_ch05_088-106.indd 94
Retinal chromophore
Retinal chromophore
b-pleated sheets
b-pleated sheets
Passive Transport Across Membranes - Simple diffusion
Many substances can move in and out of the cell without the cell’s having to expend
energy. This type of movement is termed passive transport.
Passive transport can occur by simple diffusion
Some ions and molecules can freely pass through the membrane and do so because of
a concentration gradient (a difference between the concentration on the inside of the
membrane and that on the outside).
This random motion causes a net movement from regions of high concentration to
regions of lower concentration, this process is called diffusion. The movement will
continue until the concentration is the same in all regions.
Diffusion
If a drop of colored ink is dropped into a
beaker of water its molecules diffuse.
Eventually, diffusion results in an even
distribution of ink molecules throughout
the water
Passive Transport Across Membranes - Simple diffusion
The major barrier to crossing a membrane is the hydrophobic interior that repels polar
molecules but not nonpolar molecules. If a concentration difference exists for a nonpolar
molecule (e.g. O2) it will move across the membrane until the concentration is equal on
both sides
outer
environment
Nonpolar
region
Cytoplasm
Passive Transport Across Membranes - Selective diffusion
Many important molecules (macro-molecules, polar molecules etc) required by the cell
cannot easily cross the membrane. These molecules can still enter the cell by selective
diffusion through specific channel proteins or carrier proteins embedded in the plasma
membrane.
We call this process of diffusion mediated by a membrane protein facilitated diffusion.
Channel proteins have a hydrophilic interior that Extracellular
Extracellular
fluid
fluid
provides an aqueous channel through which polar
molecules can pass when the channel is open.
Extracellular fluid
Cytoplasm
Cytoplasm
Extracellular
fluid
Extracellular
fluid
Cytoplasm
Cytoplasm
a.a.
Carrier
proteins, bind specifically to the
Figure5.10
5.10Facilitated
Facilitated
diff
usion.
Diffusion
can
facilitated
Figure
usion.
Diffusion
bebe
facilitated
b
molecule
they
assist,diff
much
like
an can
enzyme
shown.
On
the
left
the
concentration
higher
outside
the
cell,
the
shown.
On
the
left
the
concentration
is is
higher
outside
the
cell,
soso
the
ion
binds
to
its
substrate
cases,
transport
continues
until
the
concentration
equal
both
side
cases,
transport
continues
until
the
concentration
is is
equal
onon
both
sides
both
directions,
but
there
net
movement
either
direction.
C
both
directions,
but
there
is is
nono
net
movement
inin
either
direction.
b. b.
Car
case,
the
concentration
higher
outside
the
cell,
molecules
bind
case,
the
concentration
is is
higher
outside
the
cell,
soso
molecules
bind
toto
th
molecule
cross
the
membrane.
This
reversible,
net
movement
molecule
toto
cross
the
membrane.
This
is is
reversible,
soso
net
movement
coc
Cytoplasm
thestate
stateofofthe
thegate
gate(open
(openororclosed).
closed).
voltage
difference
(3)(3)the
A Avoltage
difference
is is
electricalpotential
potentialdifference
differenceacross
acrossthe
themembrane
membranecalled
calleda a
ananelectrical
membranepotential.
potential.Changes
Changesininmembrane
membranepotential
potentialform
formthe
the
membrane
basisfor
for
transmission
signals
the
nervous
system
and
some
basis
transmission
ofof
signals
inin
the
nervous
system
and
some
Passive Transport Across Membranes - Selective diffusion
Aquaporins (proteins): an example of water channels
Aquaporins are integral membrane proteins that form pores in the membrane of biological
cells.
Genetic defects involving aquaporin genes have been associated with several human
diseases.
The plasma membranes contain aquaporins through which water can flow more rapidly
inside the cell than by diffusing through the phospholipid bilayer.
Passive Transport Across Membranes - Osmosis
Osmosis is the movement of water across membranes
The cytoplasm of a cell contains ions and molecules (sugars, amino acids) dissolved in water.
The mixture of these substances and water is called an aqueous solution. Water is termed the
solvent, and the substances dissolved in the water are solutes.
Both water and solutes tend to diffuse from regions of high concentration to ones of low
concentration.
H2O molecules move down their concentration gradient, toward the higher solute
concentration. This net diffusion of water across a membrane toward a higher solute
concentration
is called osmosis
A cell in any environment can be thought of as a plasma membrane separating two
solutions: the cytoplasm and the extracellular fluid
Passive Transport Across Membranes - Osmosis
If two solutions have unequal osmotic concentrations, the
solution with the higher concentration is hypertonic (Greek
hyper, “more than”), and the solution with the lower
concentration is hypotonic (Greek hypo, “less than”).
When two solutions have the same osmotic concentration,
the solutions are isotonic (Greek iso, “equal”).
Human Red Blood Cells
Hypertonic, hypotonic and isotonic solutions
Hypertonic
Solution
Isotonic
Solution
Hypotonic
Solution
Shriveled cells
Normal cells
Cells swell and
eventually burst
0.55 µm
0.55 µm
Plant Cells
0.55 µm
Cell body shrinks
from cell wall
Flaccid cell
Normal turgid cell
Figure 5.12 How solutes create osmotic pressure. In a
hypertonic solution, water moves out of the cell, causing the cell to
shrivel. In an isotonic solution, water diffuses into and out of the cell
at the same rate, with no change in cell size. In a hypotonic solution,
water moves into the cell. Direction and amount of water movement
is shown with blue arrows (top). As water enters the cell from a
hypotonic solution, pressure is applied to the plasma membrane until
the cell ruptures. Water enters the cell due to osmotic pressure from
the higher solute concentration in the cell. Osmotic pressure is
measured as the force needed to stop osmosis. The strong cell wall of
plant cells can withstand the hydrostatic pressure to keep the cell
from rupturing. This is not the case with animal cells.
The cell’s cytoplasm is hypertonic relative to the extracellular fluid, that is, water
diffuses into the cell from the environment, causing the cell to swell. The amount of water
that enters the cell is measured as osmotic pressure, defined as the force needed to stop
osmotic flow. Prokaryotes, fungi, plants, and many protists are surrounded by strong cell
walls, which can contrast high internal pressures without bursting
outside, so these cells need to be able to transport glucose
against its concentration gradient. This requires energy and a
different transporter than the one involved in facilitated diffusion of glucose.
The active glucose transporter uses the Na+ gradient produced
by the sodium–potassium
pumpHowever,
as a source of energy
Diffusion, facilitated
diffusion,
and osmosis
passive
transport
processes.
cellsto
+
Step 1. Three Na
bind to the cytoplasmic
side of theare
protein,
power the movement of glucose into the cell. In this system,
causing
the
protein
to
change
its
conformation.
+ their concentration
can also actively move substances across a cell both
membrane
up
glucose and Na
bind to the transport protein, which alStep 2. In its new conformation, the protein binds a molecule
+
Na to pass
into the
cell down
its concentration
gradient,
gradients. This
process
requires
the
expenditure
oflows
energy
(from
ATP),
and
is therefore
of ATP
and cleaves it
into adenosine
diphosphate
(ADP)
capturing the energy and using it to move glucose into the cell.
and phosphate (Pi). ADP is released,
but
the
phosphate
called active transport.
In this kind of cotransport, both molecules are moving in the
group is covalently linked to the protein. The protein is
same direction across the membrane; therefore the transporter
now phosphorylated.
is a symporter (figure 5.14).
Step
3.
The
phosphorylation
of
the
protein
induces
a
second
Active transport involves highly selective carrier proteins that bind to the substance (an
conformational change in the protein. This change
+
ion or a simple
molecule),
such
astheamembrane,
sugar,soan
translocates
the three Na
across
theyamino acid, or a nucleotide. These carrier
now face the exterior. In this new conformation, the
proteins are:
protein has a low affinity for Na+, and the three bound
Outside
Na+ break away from the protein and diffuse into the
of cell
extracellular fluid.
Step 4. The new conformation has a high affinity for K+, two
of which bind to the extracellular side of the protein as
Na;
Glucose
soon as it is free of the Na+.
Step 5. The binding of the K+ causes another conformational
Coupled
Na;/ K; pump
transport
in the protein,
this time resulting
in the
Uniporters:change
transport
a single
type of
molecule.
protein
hydrolysis of the bound phosphate group.
Symporters:
transport
two molecules
inreverts
the tosame
Step 6. Freed
of the phosphate
group, the protein
its
original shape,direction
exposing the two K+ to the cytoplasm.
This conformation has a low affinity for K+, so the two
Antiporters:
transport
in opposite
bound
K+ dissociatetwo
frommolecules
the protein and diffuse
into the
ATP
interior of the cell. The original conformation has a high
directions
affinity for Na+. When these ions bind, they initiate
ADP+Pi
another cycle.
transport by diffusion; it is achieved only by the constant
expenditure of metabolic energy. The sodium–potassium
pump works through the following series of conformational
changes in the transmembrane protein (summarized in
figure 5.13):
Active Transport Across Membranes
In every cycle, three Na+ leave the cell and two K+ enter. The
changes in protein conformation that occur during the cycle
are rapid, enabling each carrier to transport as many as 300 Na+
per second. The sodium–potassium pump appears to exist in all
animal cells, although cells vary widely in the number of pump
proteins they contain.
Inside
of cell
K;
Figure 5.14 Coupled transport. A membrane protein
transports Na+ into the cell, down its concentration gradient, at the
2.
tertransport, the
potential
energy
in the
gradient
of
concentration
gradient.
They
differ
onlyconcentration
in the direction
that the
one molecule
is moves
used torelative
transport
another
second
molecule
to the
first. molecule against its
concentration
They
differ only
Learning gradient.
Outcomes
Review
5.5in the direction that the
second molecule moves relative to the first.
Active transport requires both a carrier protein and energy, usually in
Learning
Review
the
form of ATP,Outcomes
to move molecules
against a 5.5
concentration gradient. The
+ energy, usually in +
Active transport requires
carrier
protein
in one direction and K in
sodium–potassium
pumpboth
usesaATP
to moved
Naand
Learning
Outcomes
Review
5.5
form of
to move
molecules
against a concentration
gradient.
The
the other
toATP,
create
and maintain
concentration
differences of
these ions.
+ energy, usually in +
Active
transport
requires
both
a
carrier
protein
and
in
one
direction
and
K in
sodium–potassium
pump
uses
ATP
to
moved
Na
In coupled transport, a favorable concentration gradient of one molecule
the
form
of
ATP,
to
move
molecules
against
a
concentration
gradient.
The
theused
other
to create
and
maintain
concentration
differencessuch
of these
ions.
is
to move
a diff
erent
molecule
against its gradient,
as in the
+
in
one
direction
and
K+ in
sodium–potassium
pump
uses
ATP
to
moved
Na
+
In coupledoftransport,
concentration gradient of one molecule
transport
glucose bya favorable
Na .
Explain how endocytosis can be specific.
lem. The substances cells require for growth are mostly large,
The
nature of
cellcannot
plasmacross
membranes
raises a second
prob-a
polarlipid
molecules
that
the hydrophobic
barrier
lem.
The
substances
cells
require
forsubstances
growth areget
mostly
lipid
bilayer
creates.
How
do membranes
these
into large,
cells?
The
lipid
nature
of
cell
plasma
raises
a
second
probpolar
moleculesare
that
cannotincross
the
hydrophobic
barrier a
Two
processes
involved
thisfor
bulk
transport:
endocytosis
lem.
The
substances
cells
require
growth
are
mostly
lipid
bilayer creates. How do these substances get into large,
cells?
and
exocytosis.
polar
molecules
that
cannot
cross
the
hydrophobic
barrier
Two processes are involved in this bulk transport: endocytosisa
lipidexocytosis.
bilayer creates. How do these substances get into cells?
and
Two processes are involved in this bulk transport: endocytosis
Bulk
material enters the cell in vesicles
and exocytosis.
In endocytosis,
plasmathe
membrane
Bulk
materialthe
enters
cell in envelops
vesiclesfood particles
and fluids. Cells use three major types of endocytosis: phagoother
to create
and
maintain
concentration
differences
of these
ions.
istheused
to move
a diff
erent
molecule
against its gradient,
such
as in the
In
endocytosis,
the
plasma
membrane
particles
Bulk
material
enters
cell inenvelops
vesiclesfood
In
coupled
transport,
a
favorable
concentration
gradient
of
one
molecule
cytosis,
pinocytosis,
andthe
receptor-mediated
endocytosis
■ Can
active by
transport
involve a channel protein. Why or
transport
of glucose
Na+.
and
fluids.
Cells
use three major
types of endocytosis:
phagois used to
move
a different molecule against its gradient, such as in the
why
not?
(figure
5.15).
Like
processes
also
reIn endocytosis,
theactive
plasmatransport,
membranethese
envelops
food particles
+
cytosis,
pinocytosis,
and
receptor-mediated
endocytosis
transport
of glucose
Na .
■ Can
active by
transport
involve a channel protein. Why or
quire
energy
expenditure.
and fluids.
Cells
use three major types of endocytosis: phagowhy not?
(figure 5.15). Like active transport, these processes also recytosis, pinocytosis, and receptor-mediated endocytosis
■ Can active transport involve a channel protein. Why or
quire energy expenditure.
why not?
(figure 5.15). Like active transport, these processes also reFigure 5.15 Endocytosis. Both
Bacterialquire energy expenditure.
Bulk Transport: Endocytosis
Some substances are large or polar molecules that cannot cross the hydrophobic barrier.
This bulk material enters the cell in vesicles. Two processes are involved in this bulk
transport: endocytosis and exocytosis.
In endocytosis, the plasma membrane envelops food particles and fluids. Cells use three
major types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated
phagocytosis
and (b) pinocytosis
are forms ofthese processes
cells
endocytosis.(a)
Like
active
transport,
also require energy.
Figure
5.15
Endocytosis.
Both
endocytosis. c. In receptor-mediated endocytosis,
(a) phagocytosis
and (b)
pinocytosis
are forms of
cells
have pits
coated
with
the protein
Figure
5.15
Endocytosis.
Bothclathrin
endocytosis.
c. In receptor-mediated
endocytosis,
that
initiate endocytosis
target are
molecules
(a) phagocytosis
and (b) when
pinocytosis
forms of
cells
have
pits
coated
with
the
protein
clathrin
bind
to
receptor
proteins
in
the
plasma
If the material the cell takes
in isendocytosis
(particles),
endocytosis.
c.particulate
In receptor-mediated
endocytosis,
that initiate Photo
when
target
molecules
membrane.
inserts
(false
color
has
been
cells
have
pits
coated
with
the
protein
clathrin
such as an organism or some
other
fragment
of
organic
bind
to
receptor
proteins
in
the
plasma
added
to enhance
distinction
structures):
that initiate
endocytosis
whenoftarget
molecules
membrane.
Photo
inserts
(false
color
has been
(a)
A
TEM
of
phagocytosis
of
a
bacterium,
matter, the process is called
phagocytosis
(Greek
bind to receptor proteins in the plasma
added to enhance
distinction
of structures):
Rickettsia
tsutsugamushi,
by(false
a mouse
membrane.
Photo inserts
colorperitoneal
has been
phagein, “to eat,”
+
cytos,
“cell”).
(a) A TEM of
phagocytosis
of a enters
bacterium,
mesothelial
cell.
The
bacterium
the
added to enhance distinction of structures):
Rickettsia
tsutsugamushi,
by
a
mouse
peritoneal
host
by phagocytosis
andofreplicates
in
(a) Acell
TEM
of phagocytosis
a bacterium,
mesothelial
cell.
The
bacterium
enters
the
the
cytoplasm.
(b) A TEM
pinocytosis
in is
If the material the cell takes
intsutsugamushi,
is liquid,
the
process
Rickettsia
by
aofmouse
peritoneal
host
cell
by
phagocytosis
and
replicates
in
amesothelial
smooth muscle
cell. bacterium
(c) A coatedenters
pit appears
cell.
The
the
called pinocytosis in
(Greek
pinein,
“to
drink”).
the
cytoplasm.
(b)
A
TEM
of
pinocytosis
in
the
plasma
membrane
of
a
developing
host cell by phagocytosis and replicates inegg
a smooth
muscle
cell. (c) A
pit When
appears
cell,
covered
with
ofcoated
proteins.
the cytoplasm.
(b)aAlayer
TEM
of
pinocytosis
in
in the
plasma membrane
of
amolecules
developing
egg
an
appropriate
collection
of
gathers
a smooth muscle cell. (c) A coated pit appears
cell,
covered with
a layer
of proteins.
in
the
pit deepens
and When
willegg
in the
the coated
plasma pit,
membrane
of a developing
an appropriate
collection
ofa molecules
gathers
eventually
seal
off atolayer
formof
vesicle. When
cell, covered
with
proteins.
in the coated pit, the pit deepens and will
Molecules are often transported
into eukaryotic
cells
an appropriate collection
of molecules gathers
eventually seal off to form a vesicle.
in the coated
pit, the pit deepens and
will
through receptor-mediated
endocytosis.
These
eventually seal off to form a vesicle.
molecules first bind to specific receptors in the plasma
membrane.
Different cell types contain a characteristic set of
receptors, each for a different kind of molecule in their
membranes.
Plasma
membrane
Bacterial
cells
Bacterial
cells
Plasma
membrane
Plasma
a.membrane
Phagocytosis
a. Phagocytosis
a. Phagocytosis
Cytoplasm
Solute
Cytoplasm
1 µm
Cytoplasm
1 µm
1 µm
Solute
Plasma
membrane
Solute
Plasma
membrane
b.Plasma
Pinocytosis
membrane
Cytoplasm
0.1 µm
b. Pinocytosis
Target molecule
Cytoplasm
0.1 µm
Cytoplasm
b. Pinocytosis
Target molecule
Receptor protein
0.1 µm
Target
Coated
pit molecule
Receptor protein
Coated pit
Receptor protein
Coated pit
Clathrin
Clathrin
c. Receptor-mediated endocytosis
Clathrin
Coated vesicle
Coated vesicle
Coated vesicle
0.093 µm
Receptor-mediated
endocytosis
Receptor-mediated
endocytosis
terialterial
fromfrom
vesicles
at the cell surface (figure 5.16). In plant
vesicles at the cell surface (figure 5.16). In plant
cells,cells,
exocytosis
is anisimportant
means
of exporting
the the
materiexocytosis
an important
means
of exporting
materials needed
to
construct
the
cell
wall
through
the
plasma
memals needed to construct the cell wall through the plasma membrane.
Among
protists,
contractile
vacuole
discharge
is is
brane.
Among
protists,
contractile
vacuole
discharge
considered
a form
of exocytosis.
In animal
cells,cells,
exocytosis
pro-proconsidered
a form
of exocytosis.
In animal
exocytosis
videsvides
a mechanism
for secreting
many
hormones,
neurotransa mechanism
for secreting
many
hormones,
neurotransmitters,
digestive
enzymes,
and
other
substances.
mitters, digestive enzymes, and other substances.
The The
mechanisms
for transport
across
cell cell
membranes
are are
mechanisms
for transport
across
membranes
summarized
in table
5.2. 5.2.
summarized
in table
Molecules
are often
transported
into eukaryotic
cells cells
through
Molecules
are often
transported
into eukaryotic
through
receptor-mediated
endocytosis.
These
molecules
first
bind
receptor-mediated endocytosis. These molecules first bind
to specific
receptors
in thein plasma
membrane—they
have have
a
to specific
receptors
the plasma
membrane—they
a
conformation
that
fits
snugly
into
the
receptor.
Different
cell
conformation that fits snugly into the receptor. Different cell
types types
contain
a characteristic
battery
of receptor
types,types,
each each
for for
contain
a characteristic
battery
of receptor
a different
kind of
molecule
in their
membranes.
a different
kind
of molecule
in their
membranes.
The portion
of theofreceptor
molecule
that lies
the the
The portion
the receptor
molecule
thatinside
lies inside
membrane
is trapped
in aninindented
pit coated
on the
membrane
isof
trapped
an indented
pit exocytosis,
coated
on cytothe cyto- the discharge of material from vesicles at the
The
reverse
endocytosis
is
plasmic
side with
clathrin.
EachEach
pit acts
moplasmic
side the
withprotein
the protein
clathrin.
pit like
acts alike
a molecular
mousetrap,
closing
over
to
form
an
internal
vesicle
cell surface.
In plant
exocytosis
is an important
means
of
exporting
the materials
lecular mousetrap,
closing cells,
over to form
an internal vesicle
Learning
Outcomes
Review
5.6
Learning
Outcomes
Review
5.6
when when
the right
molecule
entersenters
the pit
(figure
5.15c).
The The
trig- trigthe right
molecule
the
pit (figure
5.15c).
needed
to
construct
the
cell
wall
through
the
plasma
protists,
Large Large
molecules
and other
bulkybulky
materials
can enter
a cellabycell
endocytosis
molecules
andmembrane.
other
materials
canAmong
enter
by endocytosis
ger that
the trap
theisbinding
of theofproperly
fittedfitted
gerreleases
that releases
the istrap
the binding
the properly
and leave
cellthebycell
exocytosis.
TheseThese
processes
require
energy.
Endocytosis
and the
leave
by exocytosis.
processes
require
energy.
Endocytosis
targettarget
molecule
to thetoembedded
receptor.
When
binding
oc- ocmolecule
the embedded
receptor.
When
binding
contractile
vacuole
discharge
is considered
a
form
of
exocytosis.
may bemay
mediated
by specifi
c receptor
proteins
in theinmembrane
that trigger
be mediated
by specifi
c receptor
proteins
the membrane
that trigger
curs, curs,
the cell
by initiating
endocytosis;
the process
is is
thereacts
cell reacts
by initiating
endocytosis;
the process
the formation
of
vesicles.
the formation of vesicles.
highlyhighly
specific
and very
is now
insideinside
the cell
specific
and fast.
veryThe
fast. vesicle
The vesicle
is now
the cell
carrying
its cargo.
■ What
feature
unites
transport
by receptor-mediated
carrying
its cargo.
■ What
feature
unites
transport
by receptor-mediated
endocytosis,
transport
by a by
carrier,
and and
catalysis
by by
One One
type type
of molecule
that that
is taken
up by
endocytosis,
transport
a carrier,
catalysis
of molecule
is taken
upreceptorby receptorIn
animal
cells,
exocytosis
provides
a
mechanism
for
secreting
many
hormones,
an enzyme?
an enzyme?
mediated
endocytosis
is low-density
lipoprotein
(LDL).
LDLLDL
mediated
endocytosis
is low-density
lipoprotein
(LDL).
molecules
bringbring
cholesterol
into the
it canitdigestive
be
molecules
cholesterol
intocell
the where
cell where
caninbe inneurotransmitters,
enzymes, and other substances.
Bulk Transport: Material can leave the cell by exocytosis
PlasmaPlasma
membrane
membrane
Secretory
product
Secretory
product
Secretory
Secretory
vesiclevesicle
Cytoplasm
Cytoplasm
a.
Proteins and other molecules are secreted from cells in small packets called vesicles, whose
membranes fuse with the plasma membrane, releasing their b.
contents
outside the cell.0.0690.069
a.
b.
µm µm
Figure
Exocytosis.
a. Proteins
and other
molecules
are secreted
in small
packets
called
vesicles,
whose
membranes
Figure
5.165.16
Exocytosis.
a. Proteins
and other
molecules
are secreted
fromfrom
cells cells
in small
packets
called
vesicles,
whose
membranes
fusefuse