membrane structure and function...diffuse from where it is more concentrated to where it is less...
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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 7
Membrane Structure and
Function
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• Essential knowledge 2.B.1: Cell membranes
are selectively permeable due to their
structure.
• a. Cell membranes separate the internal environment of the cell from the external
environment.
• b. Selective permeability is a direct consequence of membrane structure, as described
by the fluid mosaic model
• c. Cell walls provide a structural boundary, as well as a permeability barrier for some
substances to the internal environments
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• Essential knowledge 4.C.1: Variation in
molecular units provides cells with a wider
range of functions.
• a. Variations within molecular classes provide
cells and organisms with a wider range of
functions.
– Different types of phospholipids in cell
membranes
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Overview: Life at the Edge
• The plasma membrane is the boundary that
separates the living cell from its surroundings
• The plasma membrane exhibits selective
permeability, allowing some substances to
cross it more easily than others
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Fig. 7-7
Fibers of extracellular matrix (ECM)
Glyco- protein
Microfilaments of cytoskeleton
Cholesterol
Peripheral proteins
Integral protein
CYTOPLASMIC SIDE OF MEMBRANE
Glycolipid
EXTRACELLULAR SIDE OF MEMBRANE
Carbohydrate
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Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
• The main macromolecules in membranes are lipids and
proteins, but carbohydrates are also important.
• Phospholipids
–Are the most abundant lipid in the plasma membrane
–Are amphipathic, containing both hydrophobic and
hydrophilic regions
• The fluid mosaic model of membrane structure
• States that a membrane is a fluid structure with a “mosaic”
of various proteins embedded in it
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Membrane Models: Scientific Inquiry
• Membranes have been chemically analyzed
• And found to be composed of proteins and
lipids
• The arrangement of phospholipids and proteins
in biological membranes is described by the
fluid mosaic model
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Fig. 7-2
Hydrophilic head
WATER
Hydrophobic tail
WATER
Scientists studying the plasma
membrane reasoned that it must
be a phospholipid bilayer
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Fig. 7-3
Phospholipid
bilayer
Hydrophobic regions of protein
Hydrophilic regions of protein
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(a) Movement of phospholipids
Lateral movement
(107 times per second)
Flip-flop
( once per month)
The Fluidity of Membranes-
-Most everything is in motion
Phospholipids in the plasma membrane
–Membrane molecules are held in place by relatively weak
hydrophobic interactions.
–Can move within the bilayer
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Fig. 7-5
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
(b) Membrane fluidity
Fluid Viscous
Unsaturated hydrocarbon tails with kinks
Saturated hydro- carbon tails
(c) Cholesterol within the animal cell membrane
Cholesterol
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Fig. 7-6
RESULTS
Membrane proteins
Mouse cell Human cell
Hybrid cell
Mixed proteins after 1 hour
Shows that surface proteins move
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(b) Membrane fluidity
Fluid
Unsaturated hydrocarbon tails with kinks
Viscous
Saturated hydro- carbon tails
Fluidity
•Membrane fluidity is influenced by temperature.
•As temperatures cool, membranes switch from a fluid state to a solid state
as the phospholipids pack more closely.
•The type of hydrocarbon tails in phospholipids affects the fluidity of the
plasma membrane
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Fig. 7-5c
Cholesterol
(c) Cholesterol within the animal cell membrane
The steroid cholesterol
–Has different effects on membrane fluidity at different temperatures
–At warm temperatures (such as 37°C), cholesterol restrains the
movement of phospholipids and reduces fluidity.
–At cool temperatures, it maintains fluidity by preventing tight packing.
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• As temperatures cool, membranes switch from a fluid state to a solid state
• The temperature at which a membrane solidifies depends on the types of lipids
• The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma
membrane
• Membranes rich in unsaturated fatty acids are more fluid that those rich in
saturated fatty acids
• Cells can alter the lipid composition of membranes to compensate for changes
in fluidity caused by changing temperatures.
• For example, cold-adapted organisms such as winter wheat increase the
percentage of unsaturated phospholipids in their membranes in the autumn.
• This prevents membranes from solidifying during winter.
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Fluidity
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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
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• Peripheral proteins are bound to the surface
of the membrane
• Integral proteins penetrate the hydrophobic
core
• Integral proteins that span the membrane are
called transmembrane proteins
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• There are two major populations of
membrane proteins.
1. Integral proteins
• Penetrate the hydrophobic core of
the lipid bilayer
• Are often transmembrane proteins,
completely spanning the membrane
Proteins
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Fig. 7-8
N-terminus
C-terminus
Helix CYTOPLASMIC SIDE
EXTRACELLULAR SIDE
How is this protein arranged?
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Proteins
• Peripheral proteinsare not embedded in the lipid bilayer at all. instead,
they are loosely bound to the surface of the protein, often connected to
integral proteins.
• Are appendages loosely bound to the surface of the membrane
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• Six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and
extracellular matrix (ECM)
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Proteins
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Fig. 7-9ac
(a) Transport (b) Enzymatic activity (c) Signal transduction
ATP
Enzymes
Signal transduction
Signaling molecule
Receptor
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Fig. 7-9df
(d) Cell-cell recognition
Glyco-
protein
(e) Intercellular joining (f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
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The Role of Membrane Carbohydrates in Cell-Cell Recognition (more on this later)
• Cells recognize each other by binding to surface
molecules, often carbohydrates, on the plasma
membrane
• Membrane carbohydrates may be covalently
bonded to lipids (forming glycolipids) or more
commonly to proteins (forming glycoproteins)
• Carbohydrates on the external side of the plasma
membrane vary among species, individuals, and
even cell types in an individual
• (unity and diversity)
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Concept 7.2: Membrane structure results in selective permeability (What)
• A cell must exchange materials with its
surroundings, a process controlled by the
plasma membrane
• Plasma membranes are selectively permeable,
regulating the cell’s molecular traffic because
of their composition
• A steady traffic of small molecules and ions
moves across the plasma membrane in both
directions.
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The Permeability of the Lipid Bilayer What types of substance can pass through the membrane?
• Hydrophobic molecules are lipid soluble and
can pass through the membrane rapidly
• –Small molecules like oxygen and carbon
dioxide
• –Polar molecules-Do not cross the membrane
rapidly
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What cannot pass through?
• The hydrophobic core of the membrane
impedes the direct passage of ions and polar
molecules, which cross the membrane with
difficulty.
• •This includes small molecules such as glucose
and other sugars.
• •An ion, whether a charged atom or molecule,
and its surrounding shell of water also has
difficulty penetrating the hydrophobic core.
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Getting other “stuff “ across Transport Proteins
• Things that cannot pass thru: ions, water,
other large or polar molecules
• Transport proteins allow passage of hydrophilic
substances across the membrane
– Channel proteins, have a hydrophilic channel that
certain molecules or ions can use as a tunnel
– Carrier proteins, bind to molecules and change
shape to shuttle them across the membrane
• A transport protein is specific for the
substance it moves.
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Aquaporins: Special transport protein
For many years, scientists assumed that water
leaked through the cell membrane, and some
water does.
The very rapid movement of water through
some cells was not explained by this theory
Water molecules traverse through the pore of
the channel in single file. The presence of
water channels increases membrane
permeability to water.
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Aquaporin
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Transport
• Two Types
– Passive Transport-
• No energy required
• Moves substances with the gradient (swimming
with the current)
– Active Transport-
• Energy required
• Moves substances against the gradient (swimming
with the current)
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Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment (How)
There are three types of passive transport
Diffusion is the tendency for molecules to
spread out evenly into the available space
Osmosis- The diffusion of water
Facilitated diffusion-Diffusion of a substance
aided by a protein
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Concentration gradient
• In the absence of other forces, a substance will
diffuse 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.
• •Each substance diffuses down its
ownconcentration gradient, independent of the
concentration gradients of other substances.
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Fig. 7-11a
Molecules of dye Membrane (cross section)
WATER
Net diffusion
Net diffusion
(a) Diffusion of one solute
Equilibrium
Diffusion
–Is the tendency for molecules of any substance to
spread out evenly into the available space
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Fig. 7-11 Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
(b) Diffusion of two solutes
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Effects of Osmosis on Water Balance
• Osmosis is the diffusion of water across a selectively
permeable membrane
• Is the movement of water across a
semipermeablemembrane down it’s concentration gradient
• –It is affected by the concentration gradient of dissolved
substances
• –The direction of osmosis is determined only by a
difference in totalsolute concentration.
• –The kindsof solutes in the solutions do not matter.
• –Look at water only to determine which direction it will
move
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Lower
concentration of solute (sugar)
Fig. 7-12
H2O
Higher
concentration of sugar
Selectively permeable
membrane
Same concentration
of sugar
Osmosis
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Water Balance of Cells Without Walls
• Tonicity is the ability of a solution to cause a
cell to gain or lose water
• It has a great impact on cells without cell walls
• Isotonic solution: Solute concentration is the same as that inside the
cell; no net water movement across the plasma membrane
• Hypertonic solution: Solute concentration is greater than that inside
the cell; cell loses water
• Hypotonic solution: Solute concentration is less than that inside the
cell; cell gains water
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Types of Solutions and water movement
• If a solution is isotonic
–The concentration of
solutes is the same
as it is inside the cell
–There will be no net
movement of water
–Water will move
equally both
directions
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Types of Solutions and water movement
a solution is hypertonic
• –The concentration of
solutes is greater than
it is inside the cell
• –Water concentration
is greater inside the
cell
• –Water will move from
high to low
concentration
• –The cell will lose
water
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Types of Solutions and water movement
• If a solution is hypotonic
–The concentration of solutes is
less than it is inside the cell
–The concentration of water is
greater outside the cell
–Water will move from high
concentration to low
concentration
–The cell will gain water
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Hypotonic solution
(a) Animal
cell
(b) Plant
cell
H2O
Lysed
H2O
Turgid (normal)
H2O
H2O
H2O
H2O
Normal
Isotonic solution
Flaccid
H2O
H2O
Shriveled
Plasmolyzed
Hypertonic solution
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• Hypertonic or hypotonic environments create
osmotic problems for organisms
• Osmoregulation, the control of water balance,
is a necessary adaptation for life in such
environments
• The protist Paramecium, which is hypertonic to
its pond water environment, has a contractile
vacuole that acts as a pump
• http://www.youtube.com/watch?v=GAmOrIRslg
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Fig. 7-14
Filling vacuole 50 µm
(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.
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Water Balance of Cells with Walls
Cell walls
• –Help maintain water balance
If a plant cell is turgid
–It is in a hypotonic environment
–It is very firm, a healthy state in most plants
–This is a normal environment for plants
–Turgid cells contribute to the mechanical support of the plant.
If a plant cell is flaccid
–It is in an isotonic or hypertonic environment
–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 plasmolysisis usually lethal.
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What types of solutions are these cells in?
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Facilitated Diffusion: Passive Transport Aided by Proteins
• In facilitated diffusion, transport proteins
speed the passive movement of molecules
across the plasma membrane
• Channel proteins provide corridors that allow a
specific molecule or ion to cross the membrane
• Channel proteins include
– Aquaporins, for facilitated diffusion of water
– Ion channels that open or close in response
to a stimulus (gated channels)
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EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute CYTOPLASM
Solute Carrier protein
(b) A carrier protein
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• Carrier proteins undergo a subtle change in
shape that translocates the solute-binding site
across the membrane
• Some diseases are caused by malfunctions in
specific transport systems, for example the
kidney disease cystinuria
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Concept 7.4: Active transport uses energy to move solutes against their gradients
• Active transport
• –Moves substances against their concentration
gradient
• –Requires energy, usually in the form of ATP
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The Need for Energy in Active Transport
• Active transport moves substances against
their concentration gradient
• Active transport requires energy, usually in the
form of ATP
• Active transport is performed by specific
proteins embedded in the membranes
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• Active transport allows cells to maintain
concentration gradients that differ from their
surroundings
• The sodium-potassium pump is one type of
active transport system
• http://www.youtube.com/watch?v=SByeTZKAR
1Q
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2
EXTRACELLULAR
FLUID [Na+] high [K+] low
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM ATP
ADP P
Na+ Na+
Na+
P
3
6 5 4
P P
1
Fig. 7-16-7
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Fig. 7-17 Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
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How Ion Pumps Maintain Membrane Potential
• Membrane potential is the voltage difference across a membrane
• Voltage is created by differences in the distribution of positive and negative
ions
• All cells maintain a voltage across their plasma membranes.
• •Voltage is electrical potential energy due to the separation of opposite
charges.
• •The cytoplasm of a cell is negative in charge compared to the extracellular
fluid because of an unequal distribution of cations and anions on opposite
sides of the membrane.
• •The voltage across a membrane is called a membrane potential,and ranges
from −50 to −200 millivolts (mV). The inside of the cell is negative
compared to the outside.
• •The membrane potential acts like a battery.
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• 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)
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Electrogenic pumps
• An electrogenic pump is a transport protein that
generates the voltage across a membrane
EXTRACELLULAR
FLUID
H+
H+
H+
H+
Proton
pump
+
+
+
H+
H+
+
+
H+
–
–
–
–
ATP
CYTOPLASM
–
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Cotransport: Coupled Transport by a Membrane Protein
• Cotransport
• –A single ATP-powered pump that transports
one solute can indirectly drive the active
transport of several other solutes in a
mechanism called cotransport.
• –Occurs when active transport of a specific
solute indirectly drives the active transport of
another solute
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Fig. 7-19
Proton pump
–
–
–
–
–
–
+
+
+
+
+
+
ATP
H+
H+
H+ H+
H+
H+
H+
H+
Diffusion
of H+ Sucrose-H+
cotransporter
Sucrose
Sucrose
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Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
• Large proteins cross the membrane by different
mechanisms
• Large molecules, such as polysaccharides and
proteins, cross the membrane in bulk via
vesicles
• Bulk transport requires energy
• http://www.youtube.com/watch?v=FJmnxbYBlr
4
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Fig. 7-20 PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
Pseudopodium
“Food”or other particle
Food vacuole
PINOCYTOSIS
1 µm
Pseudopodium
of amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)
Plasma membrane
Vesicle
0.5 µm
Pinocytosis vesicles forming (arrows) in a cell lining a small
blood vessel (TEM)
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor
Coat protein
Coated vesicle
Coated pit
Ligand
Coat protein
Plasma membrane
A coated pit
and a coated vesicle formed during receptor- mediated endocytosis (TEMs)
0.25 µm
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Exocytosis
• In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their
contents
• Many secretory cells use exocytosis to export
their products
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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
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• In phagocytosis a cell engulfs a particle in a
vacuole
• The vacuole fuses with a lysosome to digest
the particle
• In pinocytosis, molecules are taken up when
extracellular fluid is “gulped” into tiny vesicles
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Fig. 7-20a
PHAGOCYTOSIS
CYTOPLASM EXTRACELLULAR
FLUID Pseudopodium
“Food” or
other particle
Food vacuole Food vacuole
Bacterium
An amoeba engulfing a bacterium
via phagocytosis (TEM)
Pseudopodium
of amoeba
1 µm
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Fig. 7-20b
PINOCYTOSIS
Plasma membrane
Vesicle
0.5 µm
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
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• In receptor-mediated endocytosis, binding of
ligands to receptors triggers vesicle formation
• A ligand is any molecule that binds specifically
to a receptor site of another molecule
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Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor
Coat protein
Coated pit
Ligand
Coat protein
Plasma membrane
0.25 µm
Coated vesicle
A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs)