Download Transport through plasma membranes

Transport through plasma membrane
Physiology -I
PHL 215
Dr/Gamal Gabr
Pharmacy College
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Membranes
Membranes may be:
1- Impermeable membrane:
Membrane through which nothing can pass.
2- Freely permeable membrane:
Membrane through which any substance can pass.
3- Selectively permeable membrane:
Membrane through which some substance can pass while others can not.
Plasma membrane is selectively permeable
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Mechanisms of transport through plasma membrane
3 basic mechanisms:
I) Passive Transport:
passive means no ATP energy is required
II) Active Transport:
ATP energy is required
III) Vesicular Transport:
NOTE:
 There are 2 types of transport through cells: one requires energy and the other
does not
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I) Passive transport
o Passive transport is the movement of molecules through a cell membrane
without expending any energy.
o In passive transport
1- Energy is not needed to move molecules across the membrane.
2- Molecules move from high to low concentration.
Types:
1: Simple Diffusion.
2: Facilitated Diffusion.
3: Osmosis.
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1: Simple Diffusion
 It is the passage of molecules directly through the lipid
layer of plasma membrane (between phospholipid
molecules) according to the concentration gradient
i.e: from high to low concentration .
 Molecules diffuse from high to low concentration, so NO
ATP energy is needed for this process to occur.
 Diffusion continues until the concentration of the
molecules on both sides of the membrane is equal
(equilibrium state).

What two elements are required for diffusion?
 Concentration gradient and membrane permeability
NOTES:
o A cell that suddenly stopped producing energy could still
carry out diffusion.
o In the process of simple diffusion, No pores or openings are
needed.
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o What does the term Equilibrium mean?
1- The concentration of molecules is equal on both sides of a
membrane.
2- The rate of movement of molecules across a membrane is equal
in both directions.
o What does the term Concentration gradient mean?
It is the difference in the concentration of molecules on both
sides of the membrane.
Factors affecting the rate of diffusion:
1) Concentration gradient:
 Increased concentration gradient increases the rate of
diffusion
2) Molecular size:
 Molecules of small size diffuse more easily through lipid
layer of the membrane.
3) Lipid solubility:
 Lipid soluble molecules (Non-polar) diffuse more easily
than water soluble molecules (polar)
 because Non-polar molecules are easily soluble in the lipid
layer of the membrane.
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Molecules that transport through plasma membranes by simple diffusion:
1) Hydrophobic (Non-polar) molecules as O2, CO2, Nitrogen and steroids
 diffuses very quickly.
2) Polar molecules of very small size as H2O, glycerol, urea, ethanol  diffuses freely, but
more slowly than non-polar molecules.
NOTES:
 Water molecules are polar, but are small enough to pass through cell membranes.
 Because O2 and CO2 are soluble in lipids, they can diffuse directly through the
phospholipid bilayer of the cell membrane.
 Because the size and polarity of Glucose and sucrose, they cannot diffuse directly through
the lipid bilayer of the cell membrane
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2: Facilitated diffusion
 Facilitated diffusion is a type of passive transport.
 It is the passage of large molecules or ions through the plasma membrane with the
aid of integral proteins (Channel proteins), according to the concentration
gradient.
 Because the process occurs from high to low concentration, NO energy is
required.
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Molecules that transport through plasma membrane by facilitated diffusion:
 Glucose molecules diffuse through the cell membrane by simple diffusion very
slowly because glucose is not easily soluble in the phospholipid bilayer.
However, glucose diffuses very quickly across a cell membrane by facilitated
diffusion because the Channel proteins help the glucose molecule to cross into
the cell.
 Ions such as Na+ or Cl-.
 Water.
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3: Osmosis
It is the movement of water through the plasma membrane
from high to low concentration with the aid of specific
type of channel proteins called aquaporins.
 Osmosis continue until the concentration of water on both
sides of the membrane are equal.
 Osmosis is a completely passive process, so ATP energy is
not required.
NOTES:
1- Water moves from area of high water concentration to area
of low water concentration.
OR
2- Water moves from area of low solute concentration to area
of high solute concentration.
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o The pressure caused by the movement of water
through the membrane is called osmotic pressure.
o A solution of high osmotic pressure is the solution
that contain excess solute and less water.
SO,
o
Water moves from area of low osmotic pressure to
area of high osmotic pressure.
o Water flows until the osmotic pressures on both sides
of the membrane are equal
NOTE:
 The greater the concentration of solutes in a solution,
the greater the osmotic pressure of that solution.
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Tonicity
It is the ability of a solution to affect the volume of fluid and
pressure inside the cell
 cause the cell to shrink or swell
 According to tonicity, there are 3 types of solution:
1- Hypertonic Solution (= More solute concentration)
(+ Low water concentration)
2- Hypotonic Solution (= Less solute concentration)
(+ High water concentration)
3- Isotonic Solution.
 Tonicity and direction of water flow
Water moves from area of hypotonic solution to the area of
hypertonic solution.
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1- Hypertonic Solution:
It is the solution that has higher concentration of solutes
than that of the cytosol.
 In this case, concentration of solutes in the ECF is higher
than in the ICF.
► So, water moves from inside the cell to the outside.
►The cell lose water & shrinks (crenation).
2- Hypotonic Solution:
It is the solution that has lower concentration of solutes
than that of the cytosol.
 In this case, concentration of solutes in the ECF is lower
than in the ICF.
► So, water moves from outside the cell to the inside.
► The cell absorb water, swells and may damaged, so
release its contents (lysis).
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3- Isotonic Solution:
It is the solution with the same solute concentration as that of
the cytosol.
 In this case, concentration of solutes in the ECF and ICF
are equal.
► So, no net water movement
► No change in cell volume or shape .
NOTES:
 Solution of NaCL 0.9% and glucose 5% are isotonic
solutions, so they are important in clinical medicine because
the cell neither swell nor shrink.
 NaCL solution (0.9%) is called Normal saline.
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II) Active transport
 It is the pumping of specific molecules through the
plasma membrane against the concentration gradient,
with the aid of carrier proteins (pumps).
 i.e: from low to high concentration, therefore Energy
is needed to move molecules across the membrane
 The carrier proteins obtain the energy from ATP to
pump the molecules.
 Active transport stops if cellular respiration stops,
since there is no energy.
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Examples:
1- Na+ - K+ Pump:
 3 Na+ moves out of the cell and 2 K+ moves in.
 Both ions move from low to high concentration.
 This maintain the concentration of sodium and potassium
ions on either side of the nerve cell membrane at a
certain level.
2- Reabsorption of valuable molecules from the urine such
as glucose, amino acids and sodium ions.
o Which energy molecule is needed to activate the
sodium-potassium pump so it will work?
 ATP
o How many Na+ ions must be pumped out of the cell
during each cycle of the sodium-potassium pump?
 3 Na+ ions
o How many K+ ions must be pumped into the cell
during each cycle of the sodium-potassium pump?
 2 K+ ions
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III) Vesicular Transport
It is the passage of large molecules
(macromolecules) such as proteins or fluid
droplets through the plasma membrane in
vesicles.
Types:
1: Exocytosis: Transport out of cell
2: Endocytosis: Transport into cell
a) Phagocytosis: Engulfing solid particles
b) Pinocytosis: Engulfing fluid droplets
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