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The Cell
• Common cell structures
• Protein Trafficking basics
• Cytoskeleton
• Extracellular Matrix
Reading
Ch 6: The Cell
Ch 7: Membranes / Transport
Homework
Diffusion & Osmosis
Membrane Transport / Permeability Ch 8 Prequiz
Membrane proteins & Transport
Ch 9 Prequiz
•Pores/channels, Carriers, Pumps Cell Respiration prequiz
•Endocytosis & Exocytosis
Passive vs Active
Extra Credit Science Seminar
cytoskeleton
• Fiber network throughout the cytoplasm
• Organizes cell’s structures and activities,
anchoring many organelles
– support the cell and maintain its shape
– interacts with motor proteins to produce motility
– May regulate
biochemical activities
• composed:
– Microtubules
– Microfilaments
– Intermediate filaments
Microtubules
• The thickest
– ~25 nm in diameter & ~200 nm to 25 microns long
• Functions:
– Shaping the cell
– Guiding movement of organelles
– Separating chromosomes during cell division
Cilia and Flagella
• Microtubules control
the beating of cilia and
flagella, locomotor
appendages of some
cells
• Cilia and flagella differ
in their beating
patterns
Microfilaments (Actin Filaments)
• Thinnest - solid rods ~7 nm in diameter,
• built as a twisted double chain of actin subunits
• structural role - to bear tension, resisting pulling
forces within the cell
• Microfilaments that function in cellular motility
contain the protein myosin in addition to actin
Intermediate Filaments
• range in diameter from 8–12 nanometers
• support cell shape and fix organelles in place
• more permanent cytoskeleton fixtures than the
other two classes
The Extracellular Matrix (ECM) of
Animal Cells
• Animal cells lack cell walls but are covered by an
elaborate extracellular matrix (ECM)
• made of glycoproteins such as collagen (fibrous),
proteoglycans, and fibronectin
• ECM proteins bind to receptor proteins in the
plasma membrane called integrins
Cells are not just floating around
Fig. 6-30a
Collagen
Cells are not just floating around
Proteoglycan
complex
EXTRACELLULAR FLUID
Fibronectin
Integrins
Plasma
membrane
Microfilaments
CYTOPLASM
The plasma membrane is a collage spotted with
many different proteins (mosaic) in the fluid
matrix of the lipid bilayer
~45% protein 45% phospholipid 10% other
• Proteins determine most
of the membrane’s
specific functions
Phospholipid
bilayer
Fig. 7-3
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Membrane Proteins Characteristics
• Peripheral – bound (loosely) to surface
• Integral = transmembrane proteins (tightly bound)
 hydrophobic regions - stretches of nonpolar amino
acids, often coiled into alpha helices
Often span multiple times
N-terminus
C-terminus
 Helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
Fig. 7-8
Which of the following
is likely to be in the
transmembrane
region of
bacteriorhodopsin?
a)Tryptophan
b)Serine
c) Aspartic acid
d)Lysine
Membrane Transport
• Plasma membranes are selectively permeable
• Some molecules easily pass through the
membrane; others do not
aka penetrating
Mixtures
• Most matter exists as mixtures
– Two or more components physically intermixed
• Three types of mixtures
– Solutions
– Colloids
– Suspensions
Solutions
• Homogeneous mixtures
• Usually transparent, e.g., atmospheric air or
seawater
– Solvent
• Present in greatest amount, usually a liquid (usually
H2O, but also oil, alcohol and others)
– Solute(s) (small molecules, ie-ions, glucose, aa’s
proteins)
• Present in smaller amounts
• In H2O solvent solutions, excess solutes
usually crystalize
Solution
Colloids and Suspensions
• Colloids (emulsions) proteins & lipids
– Heterogeneous translucent mixtures, e.g., cytosol
– Large solute particles that do not settle out
– Undergo sol-gel transformations
• Suspensions: cells
– Heterogeneous mixtures, e.g., blood
– Large visible solutes tend to settle out
Colloid
Solutions
Suspension
Solutions
Colloids
Suspensions
Concentration of Solutions
• Expressed as
– Percent, or parts per 100 parts
– Milligrams per deciliter (mg/dl)
– Molarity, or moles per liter (M)
• 1 mole = the atomic weight of an element or molecular
weight (sum of atomic weights) of a compound in
grams
• 1 mole of any substance contains 6.02  1023 molecules
(Avogadro’s number)
Concentration
•Percent (by volume), or parts per 100 parts
1gram/100ml = 1% or
g/ml x 100%
•Molarity, or moles per liter (M)
A mole = molecular weight (sum of atomic weights) in
grams
1 Mole = 6.02 x 1023 molecules
Atomic weight of Na is 22.99
1 mole of Na = 22.99g
1 M (molar) solution of Na = 1mole/Liter or
= 22.99g/Liter
What is the % conc of
5g NaCl in 50ml of water?
a) 1%
b) 5%
c) 10%
d) 50%
Concentration
•Molarity, or moles per liter (M)
A mole = molecular weight (sum of atomic weights) in
grams
1 Mole = 6.02 x 1023 molecules
Atomic weight of Na is 22.99
1 mole of Na = 22.99g
1 M (molar) solution of Na = 1mole/Liter or
= 22.99g/Liter
Solutes
Diffusion: If solutes permeable (penetrating):
Solutes: High  Low
No net H2O movement
(Because of hydrostatic pressure)
H2O
Initial Movement
Osmotic Pressure = Attraction (snapshot)
Osmolarity
H2O: Low  High (solutes)
(based on all solute particles)
Tonicity
LONG TERM / NET
2. Tonicity (overall / NET H2O movement)
If solutes Impermeable (non-penetrating):
H2O: Low  High (solutes)
(based on non-penetrating particles only)
“Random” Movement
Brownian collisions
30
Process of diffusion
Solutes follow their
concentration gradient
= High  Low
Diffusion
Rate of diffusion increases as a function of conc.
larger gradient = faster rate of diffusion
2000 mM  10 mM Faster
100 mM  50 mM Slower
Note: NET?
describes overall movement not all movement
Really molecules move back and forth, but overall high low
Also: NET can be final long-term movement  overall
Diffusion of 1 solute does not affect
diffusion of another
Diffusion: between 2 compartments
Dependent on:
1. Concentration gradient
2. Permeability
Membrane permeable to sugar (penetrating)
Permeable membrane.
What is the NET
movement of Na+?
15mM Na+
a)Inward
b)Outward
150mM Na+
c) No Net movement
d)Not enough info
What is the NET movement of Na+?
Trick!
15mM Na+
a)Inward
b)Outward
150mM Na+
c) No Net movement
d)Not enough info
Osmosis = solvent (water) movement
Importance: as water enters or leaves a cell
 Changes in cell water disrupts cell function by altering
– Volume
– concentrations
Crenation
Water movement also base on solutes
Each particle
needs water to
surround it
Na
Attracts water
Doesn’t necessarily take up space
H2O drawn to solutes
1. Solutes: High  Low
2. H2O: Low  High
(solutes)
Each particle
needs water to
surround it
Solutes act like
sponges and soak up
/attract water
Water Movement
Permeable to solute
Inside
4
6
7 solutes
8
12HH
2O’s
2O’s
Concentration
7/8 solutes
1/2
Outside
4
2
1 solutes
8
4 H2O’s
Concentration
1/2
1/8 solutes
Osmotic Pressure vs Tonicity
Terms to describe what happens to water
Osmotic Pressure = force (attraction)
= initial movement
Tonicity predicts Long-term (seconds)
movement of water
Osmotic pressure:
Initially there are more solutes inside
Permeable
to solute
Solutes are attracting water
Osmotic pressure is inward
After time
Permeable
to solute
Concentrations have changed, and osmotic
pressure changes, so osmotic pressure is an
instant in time
Calculating Osmotic Pressure = Osmolarity
Each particle needs water to surround it
So each individual particle contributes to
osmotic pressure
Expressed as
OSMOLARITY
= Total # of solute particles
100 mM Na+ = 100 mOsmoles (mOsm)
100 mM Na+ & 100mM K+ = 200 mOsm
NaCl Na+ Cl-
Calculating Osmotic Pressure = Osmolarity
Consider Disassociation of ionic compounds
Solid
NaCl Na+ Cl-
100 mM NaCl
= 100 mM Na+ & 100mM Cl= 200 mOsm
aqueous
Hyper-osmotic = Higher [solute]
 stronger pressure
Hypo-osmotic
= lower [solute]
 weaker pressure
Iso-osmotic
= equal [solute]
 equal pressure
Hyper-osmotic
Hypo-osmotic
Solutes
Diffusion: If solutes permeable (penetrating):
Solutes: High  Low
H2O
Osmotic Pressure / Osmolarity = Attraction = Initial
H2O: Low  High (all solutes)
Hypo-osmotic  Hyper-osmotic
Calculating Osmolarity
ALL SOLUTE PARTICLES
1
HypOosmotic
2
permeable - Na+ ClImpermeable - K+
Solution 1
Solution 2
100 mM K+
50 mM NaCl
50 mM K+
100 mM NaCl
200 mOsm
HypERosmotic
250 mOsm
Osmotic pressure: There is a pressure for water to move OUT
Diffusion: Na & Cl follow gradient  move IN
This is only temporarily / initially (While this difference lasts)
Eventually Na & Cl will even out
What is the osmolarity inside?
15mM Na+
a) 0 mOsmols
150mM Na+
b)15 mOsmols
c) 150 mOsmols
d)165 mOsmols
Which solution is hypo-osmotic?
15mM Na+
150mM Na+
a)Inward
b)Outward
c) No osmotic pressure
What is the osmotic pressure?
15mM Na+
150mM Na+
a)Inward
b)Outward
c) No osmotic pressure
Long term Water Movement / Tonicity
Permeable to solute
Inside
4
6
7 solutes
8
12HH
2O’s
2O’s
Concentration
7/8 solutes
1/2
Outside
4
2
1 solutes
8
4 H2O’s
Concentration
1/2
1/8 solutes
Tonicity / Long-Term Movement?
If water moved then solution should change volume
What prevents movement?
Hydrostatic Pressure
There is usually a force that resists changes in volume
Initial
Long-term
Note: gravity
Permeable
membrane
Blood vessel pressure
no Long term
(NET) water
movement
What happens if membrane is
impermeable / non-penetrating?
What determines permeability later…
Impermeable (non-penetrating) membrane
Initial
Long-term
Impermeable
membrane
Because solutes are impermeable, the only way to
equilibrate the concentration is for water to move
Osmosis
Impermeable to solute
Lysis
Permeability affects long-term water movement
Tonicity is determined ONLY by impermeable solutes
Initial
Permeable to solutes
• Solutes diffuse
• Osmotic pressure right
• Long-Term –
 no H2O movement
Impermeable to solutes
• Solutes don’t diffuse
• Osmotic pressure right
• Long-Term –
 H2O right
Long-term
Permeable
membrane
Impermeable
membrane
Tonicity
• Tonicity: potential water movement
= measure of non-penetrating solutes
• Isotonic: A solution with the same solute
concentration
• Hypertonic: A solution having greater solute
concentration
• Hypotonic: A solution having lesser solute
concentration
Commonly in reference to a solution’s ability to cause a
cell to shrink or swell (net water movement)
Calculating Tonicity
Only NON-Penetrating (impermeable) particles
1
2
permeable - Na+ ClImpermeable - K+
Solution 1
Solution 2
100 mM K+
50 mM NaCl
50 mM K+
100 mM NaCl
100 mM
50 mM
HypER tonic
HypOtonic
Long-term Movement of water is INWARD
Solutes
Diffusion: If solutes permeable (penetrating):
Solutes: High  Low
H2O
Osmolarity / Osmotic Pressure = [solute]
= Attraction = Initial
H2O: Low  High (all solutes)
Hypo-osmotic  Hyper-osmotic
Tonicity =LONG TERM = [Impermeable solute]
H2O: Low  High (IMPERMEABLE solutes ONLY)
hypotonic  hypertonic
Semi-Permeable
Impermeable to Na+, Permeable to H2O
What is the long-term movement of H2O?
15mM Na+
a)Inward
b)Outward
150mM Na+
c) No movement
d)Not enough info
Impermeable to Na+, Permeable to K+
Which is hypertonic?
15mM Na+
150 mM K+
150mM Na+
15mM K+
a)Inside
b)Outside
c)Neither
Impermeable to Na+, Permeable to K+
Long-term Water movement?
15mM Na+
150 mM K+
a)Inward
b)Outward
150mM Na+
15mM K+
c) No movement
d)Not enough info
Calculating Osmolarity
ALL SOLUTE PARTICLES
1
HypOosmotic
2
permeable - Na+ ClImpermeable - K+
Solution 1
Solution 2
100 mM K+
50 mM NaCl
50 mM K+
100 mM NaCl
200 mOsm
HypERosmotic
250 mOsm
Osmotic pressure: There is a pressure for water to move OUT
Diffusion: Na & Cl follow gradient  move IN
This is only temporarily / initially (While this difference lasts)
Eventually Na & Cl will even out
What you will need to be able to do
Predicting movement
Permeable: Na+ & HCO3Impermeable to Glucose
NaHCO3 moves:
200 mM Glu
50 mM NaHCO3
a)Inward
b)Outward
100 mM Glu
100 mM NaHCO3
c)Neither
d)Not enough info
Permeable: Na+ & HCO3Impermeable to Glucose
Glu moves:
200 mM Glu
50 mM NaHCO3
a)Inward
b)Outward
100 mM Glu
100 mM NaHCO3
c)Neither
d)Not enough info
Permeable: Na+ & HCO3Impermeable to Glucose
Osmolarity inside is:
200 mM Glu
50 mM NaHCO3
a)50 mOsm
b)200 mOsm
100 mM Glu
100 mM NaHCO3
c)250 mOsm
d)300 mOsm
Permeable: Na+ & HCO3Impermeable to Glucose
The Hyperosmotic solution is:
200 mM Glu
50 mM NaHCO3
a)Inside
b)Outside
100 mM Glu
100 mM NaHCO3
c)Neither
Iso-osmotic
d)Not enough info
Permeable: Na+ & HCO3Impermeable to Glucose
Osmotic pressure is:
200 mM Glu
50 mM NaHCO3
100 mM Glu
100 mM NaHCO3
a)H2O in
b)H2O out
c)Neither
d)Not enough info
Permeable: Na+ & HCO3Impermeable to Glucose
Hypertonic solution is:
200 mM Glu
50 mM NaHCO3
100 mM Glu
100 mM NaHCO3
a)Inside
b)Outside
c)Neither
d)Not enough info
Permeable: Na+ & HCO3Impermeable to Glucose
NET H2O movement is:
200 mM Glu
50 mM NaHCO3
a)Inward
b)Outward
100 mM Glu
100 mM NaHCO3
c)Neither
d)Not enough info
Permeable: Na+ & HCO3Impermeable to Glucose
What will happen to the cell?
200 mM Glu
50 mM NaHCO3
a)Lysis
b)Crenation
100 mM Glu
100 mM NaHCO3
c)No change
d)Not enough info
Estimate tonicity
Before:
Given permeability & 2 solution concentrations
 Determine movement
Now:
given 1 solution concentration and movement
 Determine concentration of 2nd solution
Impermeable to Na+
permeable to Ethanol
The osmotic pressure is:
300 mOsm
a)Inward
b)Outward
c)Neither
300 mOsm
d)Not enough info
RBCs have a tonicity equivalent to 0.9% NaCl
or 0.3 M Na+
(impermeable particle concentration)
Cell crenates when placed in
an unknown mystery solution,
The unknown solution is:
a)0% NaCl
0.9% NaCl
or
300 mM Na+
b)0.9 % NaCl
c)10 % NaCl
d) 300 mM NaCl
Unknown solution
e)0.3 M NaCl
A cell of unknown solute concentration (potato)
Cell loses weight when put in 500 mM Na+.
The cytosol could possibly be:
a)0 mM NaCl equivalent
Unknown
b)500 mM NaCl equivalent
c)1000 mM NaCl equivalent
500 mM NaCl
d)1 M NaCl equivalent
Determine permeability
Given 2 concentrations and movement
 Determine permeability
RBCs - 0.3 M Na+ - impermeable
Cell placed in 300 mM urea
Outside solution is
300 mM Na+
a)Hyper-osmotic
b)Hypo-osmotic
c)Iso-osmotic
300 mM urea
RBCs - 0.3 M Na+ - impermeable
Cell placed in 300 mM urea  Cell explodes
Water moved:
300 mM Na+
a)In
b)Out
c)Neither
300 mM urea
d)unknown
RBCs - 0.3 M Na+ - impermeable
Cell placed in 300 mM urea  Cell explodes
Outside solution must be:
300 mM Na+
a)Hypertonic
b)Hypotonic
c)Isotonic
300 mM urea
d)unknown
RBCs - 0.3 M Na+ - impermeable
Cell placed in 300 mM urea  Cell explodes
Urea is:
300 mM Na+
a)Impermeable
b)Permeable
c)unknown
300 mM urea
What Determines Permeability?
Based on Plasma Membrane
• Separates intracellular fluid (ICF) from extracellular
fluid (ECF)
– Interstitial fluid (IF) = ECF that surrounds cells
Compartmentalization
Passive Processes: Simple Diffusion
Passive = no energy
Lipidsoluble
solutes
Lipids
Water
molecules
CO2 & O2
Simple diffusion: directly through the membrane
What? Simple Based on lipid bilayer – lipids, H2O, O2, CO2
Direction? diffusion Based on Concentration Gradient
Passive Processes: Osmosis
• Movement of solvent (water) across a selectively
permeable membrane
• Water diffuses through plasma membranes:
– Through the lipid bilayer
– Through water channels
called aquaporins (AQPs)
Facilitated and/or simple
Passive Processes: Facilitated Diffusion
• Certain lipophobic molecules (e.g., glucose,
amino acids, and ions) use carrier proteins or
channel proteins, both of which:
– Exhibit specificity (selectivity)
– Are saturable; rate is determined by number of
carriers or channels
– Can be regulated in terms of activity and quantity
Specificity
Only K fits
Confromational change
of a K+-carrier
Facilitated Diffusion Using
Channel Proteins
• Aqueous channels formed by transmembrane
proteins selectively transport ions or water
• Two types:
– Leakage channels
• Always open
– Gated channels
• Controlled by chemical or electrical signals
Open channels are free flowing
Facilitated Diffusion
Facilitated = uses membrane protein
diffusion  Conc Gradient
Channel – Free Diffusion
Ex: Na+-channel (leakage)
Or
Acetylcholine-gated Na+-channel
Facilitated Diffusion - Channels
mostly ions
selected on basis of size and charge
500 mM Na+
10 mM Cl-
Na+
Na+/Cl- Channel
50 mM Na+
100 mM ClFree flow - Na doesn’t affect Cl
but depends on conc. gradient
Cl-
Gated Channels
Receptor
closed
ligand
open
Receptor – anything that can bind another molecule
Can refer to protein as a whole or only a specific region of the protein
Ligand – something that binds to a receptor
Usually a small molecule (ie- not a protein)
Facilitated Diffusion: Carrier Proteins
• transport specific polar
molecules (e.g., sugars
and amino acids)
• Binding causes shape
change in carrier
Carrier – set ratios
Ex: Na+-carrier
Na+/Glucose-symport
3Na+/2K+-antiport
X # of molecules each time
Symport or antiports
Glucose
Symport – carrier that
transports substrates in the
same direction
Antiport – carrier that
transports substrates in
opposite directions
Can be sequential or simultaneous
Facilitated Diffusion - Carriers
500 mM Na+
10 mM Cl-
Na+
Na+/Cl- antiport
50 mM Na+
100 mM Cl-
Fixed Ratio: For every Na In one Cl out
& vice versa For every Na Out one Cl In
Depends on conc. gradient
Cl-
Facilitated Diffusion - Carriers
500 mM Na+
10 mM ClNa+/Cl- symport
Na+
Cl-
No movement
Or impaired
Na wants In
Cl wants out
50 mM Na+
100 mM ClFixed Ratio: For every Na In one Cl In
& vice versa For every Na Out one Cl Out
Depends on conc. gradient
A cell has Mg2+-channels. Which of
the following is NOT permeable?
a)Estrogen
b)CO2
c) Ca2+
d)Mg2+
A cell has Ach-gated Ca2+-channels.
Which of the following is
permeable in the presence of ACh
a)Na+
Ca2+
b)Clc) Ca2+
d)Mg2+
e) None of the above
A cell has Ach-gated Ca2+-channels.
Which of the following is permeable
in the absence of ACh
a)Na+
Ca2+
b)Clc) Ca2+
d)Mg2+
e) None of the above
A cell has Ach-gated Na+-channels.
150mM Na+ outside, 10 mM Inside
What happens in the presence of ACh?
150 Na+
Na+
a)Na+ influx
b)Na+ outflux
10 Na+
c)No movement
A cell has Ach-gated Na+/K+-channels.
150mM Na+ outside, 10 mM Inside
20mM K+ outside, 80mM Inside
What happens in the presence of ACh?
150 Na+
20mM K+
Na+
/K+
a)Na+ influx, K+ influx
b)Na+ outflux, K+ influx
c) Na+ influx, K+ outflux
10 Na+
80mM K+
d)Na+ outflux, K+ outflux
e)None of the above
A cell has Ach-gated Na+/K+-channels.
150mM Na+ outside, 10 mM Inside
80mM K+ outside, 20mM Inside
What happens in the presence of ACh?
150 Na+
80mM K+
Na+
/K+
a)Na+ influx, K+ influx
b)Na+ outflux, K+ influx
c) Na+ influx, K+ outflux
10 Na+
20mM K+
d)Na+ outflux, K+ outflux
e)None of the above
A cell has Na+/K+-symports.
150mM Na+ outside, 10 mM Inside
80mM K+ outside, 20mM Inside
What happens in the ABSENCE of ACh?
150 Na+
80mM K+
a)Na+ influx, K+ influx
Na+/K+
symport
b)Na+ outflux, K+ influx
c) Na+ influx, K+ outflux
10 Na+
20mM K+
d)Na+ outflux, K+ outflux
e)None of the above
A cell has Na+/K+-antiports.
150mM Na+ outside, 10 mM Inside
80mM K+ outside, 20mM Inside
150 Na+
80mM K+
a)Na+ influx, K+ influx
Na+/K+
antiport
b)Na+ outflux, K+ influx
c) Na+ influx, K+ outflux
10 Na+
20mM K+
d)Na+ outflux, K+ outflux
e)None of the above
Active Transport- Primary
PUMP – Like Carrier but uses ATP
Independent of conc. gradient
Active Transport- Primary
Na+-K+ pump
= ATPase = enzyme
Uses ATP to drive
Ion movement
Antiport
Transported substances move in
the opposite direction
Adenosine Triphosphate (ATP)
source of usuable energy = “molecular currency”
The cell’s rechargable battery
Adenine RNA nucleotide
with two additional
phosphate groups
Active Transport- Primary
500 mM Na+
10 mM Cl-
50 mM Na+
100 mM Cl-
Na pump that pumps Na out
Na+
Na pump that pumps Na In
Independent of conc. gradient
Na+
A cell has 3Na+/2K+-pump.
150mM Na+ outside, 10 mM Inside
80mM K+ outside, 20mM Inside
a)Na+ influx, K+ influx
b)Na+ outflux, K+ influx
c)Na+ influx, K+ outflux
d)Na+ outflux, K+ outflux
e)not enough info
A cell has 3Na+/2K+-pump.
Given time, the concentration of
Na and K will be:
a)Na+ high outside, K+ High Outside
b)Na+ high out, K+ High In
c)Na+ high In, K+ High out
d)Na+ high In, K+ High In
Membrane Potential
Typical conc’s – Electrochemical gradient
Secondary active transport use of an exchange
pump (such as the Na+/K+-pump) indirectly to drive the
transport of other solutes
Extracellular fluid
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loading
glucose from
ECF
Na+-glucose
symport transporter
releasing glucose
into the cytoplasm
Symport
Transported substances
move in the same direction
Endocytosis – enables large particles and
macromolecules to enter the cell
Receptor mediated Endocytosis – specific macromolecules
Pinocytosis – nonspecific small
Phagocytosis – whole cells / solid particles
PHAGOCYTOSIS
• In phagocytosis a cell
engulfs a particle
• fuses with a lysosome to
digest the particle
1
Extracellular fluid
Plasma
membrane
Cytoplasm
Transport
vesicle
Uncoated
endocytic vesicle
Lysosome
(a)
(b)
Figure 3.12
PINOCYTOSIS
• In pinocytosis, molecules are
taken up when extracellular fluid
is “gulped” into tiny vesicles
RECEPTOR-MEDIATED
ENDOCYTOSIS
• 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
Exocytosis – moves substance from the cell
interior to the extracellular space