Download Cell Transport B

6/7/16
Collin County Community College
BIOL. 2401
(Chapter 3)
Membrane Transport
.
Facilitated Diffusion via Channels
•  Most often occurs for small ions
•  Ions will move through the water filled channels
•  Each channel is very specific for the kind of ion that
can go through, but there is no direct “binding”
between ion and channel protein.
•  Movement of the ions is from high concentration to
low concentration ( down the concentration gradient)
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Facilitated diffusion via carriers
•  Refers to the movement of molecules down their
concentration gradient into the cell by means of an
integral membrane protein or transporter (carrier)
•  This is selective and highly specific as the molecule
has to recognize and bind to the carrier molecule.
•  This kind of diffusion occurs for those molecules that
are too large and /or too lipid insoluble. ( such as
amino acids, carbohydrates)
Facilitated diffusion
Low
High
Low
High
Concentration gradient
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Facilitated diffusion via carriers
The magnitude of flux resulting from carrier mediated
transport is depending on
•  Extent of saturation of the carriers by the ligand (molecule)
•  The number of specific carriers in a membrane
•  The rate at which the carrier transports the ligand from
one side to the other side of the membrane
Facilitated diffusion via carriers
Let’s look at Ficks equation for diffusion again
A. D. ( C2 - C1)
Flux =
X
•  Assume we are dealing with simple diffusion from
outside to inside which is being measured in cells with
a constant shape.
•  Also assume the inside concentration is zero to start
with.
•  In this equation, this means that A = constant, x is
constant and C1 = 0 and C2 represents the extracellular
concentration
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Facilitated diffusion via carriers
In the equation, the circled parameters are now constant
A. D. ( C2 - C1)
Flux =
X
•  The equation then becomes much easier since C1=0
Flux = K . C2
Which mathematically represent a linear line through the origin.
In other words, if we are dealing with simple diffusion, no carriers are
used and rate of diffusion is purely and linearly dependant on
(extracellular) concentration gradient !
Simple versus Facilitated diffusion
Linearly dependency of simple diffusion on (extracellular) concentration gradient !
In facilitated diffusion, transport is dependant on protein
carriers and concentration gradient.
It is now not a linear relationship
because when all carriers are
“busy”, a maximal flux is reached.
We cannot transport more than the
available carriers allow.
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Simple versus Facilitated diffusion
The fact that many
transported molecules
become metabolized
(ex. glucose) keeps the
internal concentrations
low.
This assures that diffusion will not be limited
by a decreasing concentration gradient.
The flux of Facilitated diffusion can be
increased by increasing the numbers of
transporters (insulin action results in upregulation of glucose transporters).
Some Definitions
If a transporter only carries
one molecule or ion, it is called
a uni-porter
If it transport two molecules it
is called a coupled transporter.
If both molecules are moved in
the same direction is a cotransporter or symporter.
If the molecules are moved in
opposite directions it is
referred to as an antitransporter
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ACTIVE Transport
Some substances need to enter the cell but cannot cross
the membrane passively due to the fact that they have to
move against a concentration gradient
In such a case, integral proteins and energy is needed
to overcome those obstacles.
The energy used is cellular energy in the form of ATP.
Since the transport proteins are now moving
molecules uphill ( against the thermodynamically
favored way), they are usually referred to as PUMPS.
Active Transport
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Active Transport
High
Low concentration
These pumps exhibit similar characteristics as facilitated
diffusion such as saturation, specificity, competition,…
Types of Active Transport
Types of Active Transport
Primary Active Transport
•  uses ATP directly as a driving force to pump ions and
small molecules across the membrane.
Secondary Active Transport
•  uses the energy in an ion concentration gradient to move
another molecule against a concentration gradient
•  the energy to create the ion concentration gradient came
from ATP ; thus ATP is used indirectly
Vesicular Transport
•  Large particles are ferried across the plasma
membrane via membrane vesicles.
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Primary Active Transport
Remember the opposite values for Na+ and K+ .
•  Na+ high outside, and K+ high inside
•  All cells have leakage channels for these ions
•  This means that Na+ “leaks” into the cell and K+ “leaks” out
through their respective channels due to respective
concentration gradients
The Na/K pumps re-establish and maintain these important
concentration gradients or the cell will die.
It requires input of cellular ATP to drive these pumps !
Primary Active Transport
Most primary active transporters are recognizable by the fact that
are called pumps or ATPases.
For example, if you know that intracelular concentration of Ca2+
is very low compared to the outside of a cell, what is the
function of a Calcium pump ( aka Ca2+ ATPase) ?
Some cuboidal nephrons have a Proton pump at their apical
side . What does that imply ?
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Primary Active Transport
Example : Na+ / K+ pump or Na+ / K+ ATPase
Na and K “leak” in or out
down their respective
concentration gradients.
The Na/K pumps maintain
the concentration
gradients by pumping
these ions against their
concentration gradients.
Secondary Active Transport
Secondary Active Transport
•  Ion concentration gradients are like potential energy reservoirs
•  Moving ions down a concentration gradient releases energy
The unequal distribution of Na+ generated by the Na-K pump
across the membrane can be used as an energy source.
Secondary active transport does not require ATP input directly ;
it uses the energy input by another system , e.g. the Na/K pump.
WHY ? Because these pumps create the ion gradients for Na+ .
Most coupled transporters that involve sodium are secondary
active transport system.
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Secondary Active Transport
The energetically favored downhill flow of sodium into the cell
can now be coupled via a carrier to
•  drag a substance inward against its concentration gradient (via a symport )
•  or it can be used to move a molecule out of the cell against its
concentration gradient (via an antiport)
Secondary Active Transport
K+
Na+
Na+
out
in
ATP
Ca++
In this diagram, the Na/K pumps creates and maintains a
gradient for Na+ .
The energy in Na+ gradient can now be used to drag
another molecule against its concentration gradient !
( Calcium is always high outside compared to inside).
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Secondary Active Transport
EXAMPLE :
•  Na+ /glucose co-transporter
•  Moves glucose against a
concentration gradient
EXAMPLE :
•  Na+ /Ca2+ anti-transporter
•  Moves calcium against a
concentration gradient
Vesicular Transport
ENDOCYTOSIS
•  RECEPTOR MEDIATED
•  PHAGOCYTOSIS
•  PINOCYTOSIS
EXOCYTOSIS
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EXOCYTOSIS
SNARE proteins in vesicles and plasma
Membrane act as docking sites. They
interact and promote the fusion of the
vesicle with the cell membrane
TYPES OF ENDOCYTOSIS
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Clathrin ENDOCYTOSIS
Clathrin-coated pits provide
the main route for
endocytosis of bulk solids
and macro -molecules !
Clathrins are proteins that
are important in cargo
selection and deforming
the membrane to produce
an internal vesicle.
The system is used in
transcytosis for example,
to move particles in on
one side of the cell, and
out the other side !
Clathrin ENDOCYTOSIS
Clathrin-coated pits are
also used in the typical
phagocytosis process,
where large chinks of
material are internalized !
Phagocytosis is used by
many white blood cells
called macro-phages. The
vesicles fuse with
lysosomes where the
content is digested.
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Clathrin ENDOCYTOSIS
In receptor mediated
endocytosis, the vesicle
formation is triggered by
the binding of a molecule
to receptors in the
Clathrin-coated pits!
This process is thus quite
selective in that it requires
binding to a receptor.
The process of Pinocytosis is also referred to as cell drinking. It is not
very selective and allows the cell to take in dissolved substances from the
outside.
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