Chapter 12 - Membrane Transport

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Transcript Chapter 12 - Membrane Transport

Lec-4 Membrane Transport 2
Lecturer: Dr. Twana A. Mustafa
OSMOSIS
OSMOSIS
Definition:
The diffusion of water down its concentration
gradient (that is, an area of higher water
concentration to an area of lower water
concentration) thru a semi-permeable membrane is
called Osmosis.
Concept: Because solutions are always referred to in
terms of concentration of solute, water moves by
osmosis to the area of higher solute concentration.
Despite the impression that the solutes are “pulling,”
or attracting, water, osmosis is nothing more than
diffusion of water down its own concentration
gradient across the membrane.
Osmotic pressure: is the pressure that is required to stop osmosis. It is the pressure
necessary to prevent osmosis into a given solution when the solution is separated
from the pure solvent by a semipermeable membrane. The greater the solute conc.
of a solution, the greater its osmotic pressure.
(HYDROSTATIC PRESSURE = OSMOTIC PRESSURE)
An osmole is one mole of dissolved particles in a solution. E.g. glucose when
dissolved in solution does not dissociate, so 1 mole of glucose is also 1 osmole of
glucose. On the other hand, NaCl dissociates into 2 ions (Na and Cl) so is taken as
2 moles.
Osmolarity is the number of osmoles of solute per liter of solution. Simply put,
osmolarity is a measure of total solute conc. given in terms of number of particles
of the solute in 1 liter of solution. The osmolarity of body fluids is usually
expressed in milliosmoles per liter (mOsm/L). (The normal osmolarity of body
fluid is 300 mOsm.) It is usually employed in clinical settings.
Osmolality is the number of milliosmoles of solute per kg of solvent. It is usually
calculated in laboratories using an osmometer.
• Hyper = above
• Iso = same
• Hypo = below
tonic refers to the
shape of the cell
Isotonic Solution
NO NET
MOVEMENT OF
H2O (equal amounts
entering & leaving)
Hypotonic
Solution
CYTOLYSIS
copyright cmassengale
Hypertonic
Solution
PLASMOLYSIS
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Active transport
• Primary active transport:the transporter itself
is an ATPase that cause the breakdown of ATP
and phosphorylate itself. Therefore, change the
affinity of the transporters solute binding site.
• Secondary active transport:use of an ion
concentration gradient across a membrane as the
energy source. The flow of the ion provides
energy for the uphill movement of the actively
transported solutes.
Na+/K+ Pump
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Ca-ATPase
• In the plasma membrane, the active transport
of Ca is from cytosol into extracellular fluid.
• In the organelle membranes, the active
transport of Ca is from cytosol into organelle.
• The extracellular [Ca] is 1 mM, while the
intracellular [Ca] is 0.1 uM.
H+ Pumps
Diverse examples of carrier-mediated transport.
Figure 4-15
3.2 Secondary Active Transport
Coupled transport.
Energy needed for “uphill” movement obtained
from “downhill” transport of Na+.
Hydrolysis of ATP by Na+/K+ pump required
indirectly to maintain [Na+] gradient.
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Figure
4-13
Secondary active transport uses the energy in
an ion gradient to move a second solute.
Figure 4-14
Cotransport:
the ion and the
second solute
cross the membrane
in the same direction.
Countertransport:
the ion and the
second solute
move in
opposite directions.
Secondary active transport
co-transport
(symport)
out
in
Na+
glucose
Co-transporters will move one
moiety, e.g. glucose, in the same
direction as the Na+.
counter-transport
(antiport)
out
in
Na+
H+
Counter-transporters will move
one moiety, e.g. H+, in the
opposite direction to the Na+.
2 Methods of Glucose Transport
• 2 mechanisms are separate
– Passive transport at the
basal surface
– Active transport at the apical
surface
• Caused by the tight junctions
Na+-Driven Transport
• Na+ driven symport
– Used to move other sugars and amino acids
• Na+ driven antiport
– Also very important in cells
– Na+-H+ exchanger is used to move Na+ into the cell
and then moves the H+ out of the cell
• Regulates the pH of the cytosol
Figure 4-22
Figure 4-24
4. Bulk Transport (Endocytosis and
Excytosis)
 Movement of many large molecules, that cannot be
transported by carriers.
 Exocytosis:
A process in which some large particles move from
inside to outside of the cell by a specialized function of
the cell membrane
 Endocytosis:
Exocytosis in reverse.
Specific molecules can be taken into the cell because of
the interaction of the molecule and protein receptor.
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 Exocytosis
 Vesicle containing the secretory protein fuses with
plasma membrane, to remove contents from cell.

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Endocytosis
Material enters the cell through the plasma membrane
within vesicles.
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Types of Endocytosis



Phagocytosis - (“cellular eating”) cell
engulfs a particle and packages it with a food
vacuole.
Pinocytosis – (“cellular drinking”) cell gulps
droplets of fluid by forming tiny vesicles.
(unspecific)
Receptor-Mediated – binding of external
molecules to specific receptor proteins in the
plasma membrane. (specific)
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Example of Receptor-Mediated Endocytosis
in human cells
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Figure 4-21
Alternative functions
of endocytosis:
1. Transcellular
transport
2. Endosomal
processing
3. Recycling the
membrane
4. Destroying
engulfed materials