Download Cell Biology Part II Notes

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CELL BIOLOGY
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Part II: Cell Membrane and Cellular Transport
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CELLULAR MEMBRANE
Cell
membrane
called plasma membrane
 Separates cell from its surroundings
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
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 Also
8 nm thick

Need 8,000 to equal thickness of a piece of paper
 Supports
the cell
 Protects the cell
 Controls what enters and exits the cell

Selectively permeable

Different membranes with different functions
differ in their chemical composition and
structure
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CELL MEMBRANE
Mostly
made of proteins and lipids
are used for transport
 Lipids are mostly phospholipids
Creates a bilayer

Molecular arrangement shelters the hydrophobic tails
from water while exposing the hydrophilic heads to
water
 Some

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
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 Proteins
lipids are cholesterol molecules
Help strengthen the cell membrane
Also
contains carbohydrates
 Used

for cell to cell recognition
Carbohydrates can attach to lipids creating
glycolipids or attach to proteins creating
glycoproteins
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FLUID MOSAIC MODEL
mosaic model refers to the
nature of cell membranes
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Fluid
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 Fluid
structure with a ‘mosaic’ of
various molecules embedded in or
attached to the bilayer of
phospholipids.
Mosaic - a pattern made of numerous
small pieces fitted together

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MEMBRANES ARE FLUID
are not molecules
locked rigidly in place
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Membranes
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 The
lipids and proteins can shift
laterally

This happens frequently and rapidly
Phospholipids switch places 107 times per
second
 Proteins are larger and move more slowly
but some do shift their positions

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TEMPERATURE AND MEMBRANES
Membranes
must be fluid to work
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properly
 They
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are usually about as fluid as
salad oil
 Temperature can affect fluidity
Fluidity will decrease as temperature
decreases until phospholipids become a
closely packed solid
Cholesterol molecules buffer the effect of
temperature changes


Restrains the movement of phospholipids but
also prevents them from packing together too
tightly.
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MEMBRANE PROTEINS AND
THEIR FUNCTIONS
Proteins
determine the membrane’s function
Different types of cells contain different
types of membrane proteins.

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membrane is a collage of different
proteins embedded in the fluid
matrix of the lipid bilayer
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A
More than 50 different types of proteins in just
RBC!
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TWO TYPES OF
MEMBRANE PROTEINS
proteins are
associated with a cell membrane.
 Integral

proteins
Embedded in hydrophobic core of bilayer

Many are transmembrane proteins


Span entire membrane
Others are only partially embedded in core
 Peripheral

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They are very specific to what they do with
which molecules they react.

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Membrane
proteins
Not embedded in lipid bilayer at all

Loosely bound to surface of membrane
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FUNCTIONS OF MEMBRANE PROTEINS

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
Enzymatic activity – a protein built into the
membrane that is an enzyme with its active site
exposed to substances outside the cell. Several
enzymes in a membrane may be grouped together to
take part in a metabolic pathway.
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
Transport – a protein that spans the membrane may
provide a channel across the membrane that is
selective for the transport of certain molecules.
Channel proteins are involved in passive transport.
Carrier proteins may be involved in passive or
active transport; some require energy, some do not.
Signal transduction – a membrane protein may act as
a receptor and have a binding site that fits a
chemical messenger (ligand) like a hormone. The
protein can relay the message to the inside of the cell 12
without the chemical messenger actually having to
enter the cell.
FUNCTIONS OF MEMBRANE PROTEINS

Attachment to the cytoskeleton and extracellular
matrix – elements of the cytoskeleton may be bound
to membrane proteins to help maintain cell shape
and stabilize the location of certain membrane
proteins.
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
Intercellular joining – membrane proteins of
adjacent cells may hook together in various kinds of
junctions, such as gap junctions or tight junctions.
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
Cell to cell recognition – some glycoproteins serve
as identification tags that are specifically recognized
by membrane proteins of other cells
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CELL – CELL RECOGNITION
 Needed
for organism to function
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ability to distinguish one
type of neighboring cell from
another
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Cell’s
Ex: sorting of cells into tissues and
organs in an animal embryo
Basis for rejection of foreign cells by
immune system (including those of
transplanted organs)


A, B, AB, and O blood types are designated by
the carbohydrates on the surface of the RBC
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SELECTIVE PERMEABILITY
Cells


Nutrients enter the cell
Wastes leave the cell
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Cells regulate transport of materials across
the membrane in both directions

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maintain homeostasis by having
selectively permeable cell membranes
Large
molecules require vesicles
Small, hydrophobic (nonpolar) molecules can
pass through the membrane rapidly.

Ex: hydrocarbons
Small,
polar molecules require help to cross
the membrane - transport proteins

Water, sugar
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TRANSPORT ACROSS THE
CELL MEMBRANE
Two

Simple Diffusion


Transport
Dialysis and Osmosis
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 Passive
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types of movement occur across
a cell membrane:
Facilitated Diffusion
Channel proteins
 Carrier proteins

 Active

Protein pumps


Transport
Type of carrier protein
Endocytosis and exocytosis
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PASSIVE TRANSPORT
Transport
 Does
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not require energy
 Moves molecules down their
concentration gradient from an area of
high concentration to an area of low
concentration
 Includes:
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Passive
(Simple) Diffusion
Facilitated Diffusion

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PASSIVE TRANSPORT
 Substance
Concentration gradient – a region along which
the density of a chemical substance increases or
decreases
 Dynamic equilibrium - occurs when both
solutions have equal concentration and as many
molecules are crossing the membrane in one
direction as in the other
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will diffuse down its
concentration gradient until
dynamic equilibrium is reached.
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Diffusion

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PASSIVE TRANSPORT
Dialysis – the passive movement of particles
across a semi-permeable membrane from an
area of high concentration to an area of low
concentration

Osmosis – the passive movement of water
across a semi-permeable membrane from an
area of high concentration to an area of low
concentration


Water can diffuse through the cell membrane but
since it is polar this will happen very slowly – too
slowly to support life.

Facilitated diffusion through aquaporins allows water to
move quickly across a cell membrane.
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This occurs with nonpolar and very small uncharged polar
molecules.
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
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PASSIVE TRANSPORT
Facilitated
diffusion
Still being studied
Most transport proteins are very specific;
transport some substances but not others.
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used by many polar molecules
and ions to cross a membrane.
 Uses transport proteins
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 Process
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PASSIVE TRANSPORT
Facilitated
channels
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 Protein
diffusion involves:
Provide a corridor that allows a specific
molecule or ion to cross the membrane
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Aquaporins
 Ion channels



Move ions such as Na+,
Gated channels


Type of ion channel
Open/close in response to electrical or chemical stimulus
 Carrier

proteins
Change shape in process of transporting molecules
across membrane

Example: glucose
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ACTIVE TRANSPORT
Active
Transport
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 Requires energy supplied by ATP
 Moves molecules against their

Protein pumps (type of carrier proteins)

Example: sodium-potassium pump


Pumps sodium out of the cell and potassium into the cell
Example: proton pump used during chemiosmosis
Endocytosis
Exocytosis

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concentration gradient from an area of
low concentration to an area of high
concentration
 Include:
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TRANSPORT PROTEINS
Transport
proteins (2 types)
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Allows polar substances to pass
through the membrane quickly
Specific for the substance it moves

Example: glucose enters RBC by specific carrier proteins
 Channel
proteins (protein channels)
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
Have hydrophilic channel that can be used as a
tunnel through the membrane
 Example: Aquaporins


Facilitate the passage of water
 Carrier

proteins (passive or active)
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Hold onto substances and change shape in a way
that shuttles them across the membrane
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TRANSPORT ACROSS THE
CELL MEMBRANE
types of movement occur across
a cell membrane:

Simple Diffusion


Transport
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 Passive
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Two
Dialysis and Osmosis
Facilitated Diffusion
Channel proteins
 Carrier proteins

 Active
Transport
Protein pumps
Endocytosis and exocytosis

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PASSIVE TRANSPORT
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Dialysis
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PASSIVE TRANSPORT
Osmosis
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
Water diffuses across
the membrane from
the region of higher
water concentration to
that of lower water
concentration until
the solute
concentrations on
both sides of the
membrane are equal.
Water moving into a
cell by osmosis creates
pressure which is
called osmotic
pressure
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
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SOLUTIONS
 Solids
that are dissolved in a liquid
 Liquids
that dissolve the solids
 Usually present in greater amounts
 Water is considered the universal
solvent
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Solvents
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Solutes
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TYPES OF SOLUTIONS
The
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concentration of all molecules
dissolved in a solution (the solutes) is
called the osmotic concentration of
the solution.
Three terms are used to compare the
osmotic concentrations of two
solutions:
 Isotonic
 Hypertonic
 Hypotonic
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TYPES OF SOLUTIONS
Isotonic

Equal osmotic concentration
 If
a cell is immersed in an isotonic
environment there will be no net
movement of water across the plasma
membrane.
Water continues to flow across the
membrane, but at the same rate in both
directions.
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with equal solute
concentrations
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 Solutions


Dynamic equilibrium
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TYPES OF SOLUTIONS
cells need osmotic
pressure to help
maintain structure so
they become flaccid
(limp) in isotonic
conditions.
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animal cell fares best
in an isotonic
environment
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 An
 Plant
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TYPES OF SOLUTIONS
Hypertonic
Greater osmotic concentration
 If
a cell is placed in a hypertonic
solution the net movement of water
will be out of the cell and the cell will
shrink because osmosis moves water
from an area where there is more
water to an area where there is less
water.

Example: increase in salinity
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
with more solutes, less water
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 Solution
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TYPES OF SOLUTIONS
cells, with cell walls,
will become plasmolyzed
(shrunken/shriveled, cell
membrane pulls away from
the cell wall) due to lack of
osmotic pressure.
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cells placed in a
hypertonic solution will
crenate (shrink/shrivel) as
water moves out of the cell
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 Animal
 Plant
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TYPES OF SOLUTIONS
 Solution
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with less solutes, more water
 If a cell is placed in a hypotonic
solution the net movement of water
will be into the cell and the cell will
swell and burst because osmosis moves
water from an area where there is
more water to an area where there is
less water
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Hypotonic
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TYPES OF SOLUTIONS
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cells can by
lysed (or burst) due to
osmotic pressure.
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 Animal
 Plant
cells can be
turgid, or full of water
but not lysed due to
their cell walls.

This is healthy for plants
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TYPES OF SOLUTIONS
• ANIMAL CELL – fares
best in an isotonic
environment… will lyse
in hypotonic solution or
crenate in hypertonic
solution.
• PLANT CELL – turgid
and generally fares best
in a hypotonic
environment….tendency
for water uptake
balanced by the elastic
cell wall pushing back on
the cell. Become
plasmolyzed if placed in
hypertonic solution.
• ARROW INDICATES
WATER MOVEMENT
WHEN CELL IS FIRST
PLACED IN THE
SOLUTIONS!!!
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OSMOREGULATION
Organisms
control of water balance.
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without cell walls living in
hypertonic or hypotonic environments
must have adaptations for
osmoregulation.
 Example:
Paramecium live in pond water
which is hypotonic to the cell
Paramecium doesn’t burst because it is
equipped with a contractile vacuole, an
organelle that functions as a bilge pump to
force water out of the cell as fast as it enters 37
by osmosis.

OSMOREGULATION
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TRANSPORT ACROSS THE
CELL MEMBRANE
types of movement occur across
a cell membrane:

Simple Diffusion


Transport
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 Passive
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Two
Dialysis and Osmosis
Facilitated Diffusion
Channel proteins
 Carrier proteins

 Active
Transport
Protein pumps
Endocytosis and exocytosis

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ACTIVE TRANSPORT
Active
Transport
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 Requires energy supplied by ATP
 Moves molecules against their

Protein pumps (type of carrier proteins)

Example: sodium-potassium pump


Pumps sodium out of the cell and potassium into the cell
Example: proton pump used during chemiosmosis
Endocytosis
Exocytosis

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concentration gradient from an area of
low concentration to an area of high
concentration
 Include:
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ACTIVE TRANSPORT
Sodium-potassium
pump
in animal cells
 Uses energy from ATP

This causes transport protein to change shape in
a manner that moves a solute across the
membrane.
pump oscillates between two
conformational states in a pumping
cycle that translocates three Na+ ions
out of the cell for every two K+ ions
pumped into the cell.
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Phosphate group is transferred directly to
the transport protein

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 Found
 The
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SODIUM – POTASSIUM PUMP
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SODIUM – POTASSIUM PUMP
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ACTIVE TRANSPORT
Moving

Examples: proteins, polysaccharides, viruses, – too large to
pass through membrane using channels or carrier proteins

exocytosis and endocytosis
Exocytosis


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 Includes
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LARGE molecules across the
membrane
Vesicle from cytoplasm fuses with membrane and
materials are pushed out.
Endocytosis
Vesicle from extracellular area fuses with
membrane and materials are brought in
 Three types of endocytosis




Phagocytosis
Pinocytosis
Receptor-mediated endocytosis
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ACTIVE TRANSPORT
Exocytosis
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of active transport
 Vesicles fuse with membrane to secrete
materials out of the cell
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 Type
A transport vesicle that has budded from the
Golgi apparatus moves along microtubules of the
cytoskeleton to the cell membrane
 When the vesicle membrane and the cell
membrane come into contact, the lipid molecules
of the two bilayers rearrange themselves so that
the two membranes fuse.
 The contents of the vesicle than spill to the
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outside of the cell and the vesicle membrane
becomes part of the cell membrane

EXOCYTOSIS
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ACTIVE TRANSPORT
Endocytosis
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of active transport
 Vesicle from extracellular area fuses
with membrane and materials are
brought in
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 Type
A small area of the cell membrane sinks
inward to form a pocket
As the pocket deepens, it pinches in,
forming a vesicle containing material that
had been outside the cell

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ENDOCYTOSIS
Phagocytosis

eating” - particles taken in
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In phagocytosis, a cell engulfs a particle by wrapping
pseudopodia around it and packaging it within a
membrane-enclosed sac that can be large enough to be
considered a vacuole. The particle is digested after the
vacuole fuses with a lysosome containing hydrolytic
enzymes.
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 “cellular
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ENDOCYTOSIS
Pinocytosis

drinking” – fluid taken in
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In pinocytosis, the cell “gulps” droplets of extracellular
fluid into tiny vesicles. It is not the fluid itself that is
needed by the cell, but the molecules dissolved in the
droplets. Because any and all included solutes are taken
into the cell, pinocytosis is nonspecific in the substances
it transports.
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 “cellular
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RECEPTOR-MEDIATED ENDOCYTOSIS

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This enables the cell to acquire
bulk quantities of specific
substance, even though those
substances may not be very
concentrated in the extracellular
fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The
receptor proteins are usually
clustered in regions of the
membrane called coated pits,
which are lined on their
cytoplasmic side by a fuzzy layer
of coat proteins. The specific
substances (ligands) bind to these
receptors. When binding occurs,
the coated pit forms a vesicle
containing the ligand molecules.
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Example: cholesterol
TRANSPORT REVIEW
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