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Membrane Transport
Suzanne D'Anna
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Membrane Transport
method by which substances move
through the cell membrane
 cells bathed by interstitial fluid
 cells must take nutrients from this fluid
to remain healthy
 reject unnecessary substances

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Selective Permeability

the ability of the plasma membrane to
regulate entry and exit of materials

Movement of materials through
plasma membrane can be:
- passive
- active
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Passive Processes
simple diffusion
 osmosis
 filtration
 facilitated diffusion

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Passive Processes Simple Diffusion
random movement of particles (ions and
molecules)
 particles move along a concentration
gradient
- a region of higher concentration to a
region of lower concentration.

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Passive Processes Osmosis
diffusion of solvent
- water diffuses from a region of higher
solvent concentration toward a region of
lower solvent concentration through a
selectively permeable membrane
 Example:
- the small intestines absorb water from
digested food by osmosis

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Osmotic Pressure
the pressure required to prevent the net
movement of water from one solution to
another when the solutions are
separated by a membrane permeable
only to water
 the pressure needed to stop the flow of
water across the membrane

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Tonicity
normal firmness or functional readiness
in body tissues or organs
 maintain the shape of red blood cells;
they must be bathed in an isotonic
solution.
- normal saline solution is isotonic for
red blood cells

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Isotonic Solution
(iso = same)
same concentration of water and
particles (solute and solutes) as in the
cells
 when RBC’s are in 0.9% NaCl solution
water molecules enter and exit the cells
at equal rates (5% glucose)
 RBC’s maintain their normal shape and
volume

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Hypotonic Solution
(hypo = less than)
lower concentration of solutes than in
cells
 higher concentration of water
 when RBC’s are placed in distilled H2O,
water molecules enter the cell faster
than they leave
 RBC’s will swell and burst (hemolysis)

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Hypertonic Solution
(hyper = greater than)
higher concentration of solutes
 lower concentration of water than in the
cells
 if RBC’s are placed in hypertonic
solution more water leaves the cells
than enters
- cells will shrivel and die (crenation)

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Passive Processes Filtration
solvent and solute move across a
membrane due to gravity or water
pressure (hydrostatic)
 Example:
- in kidneys blood pressure forces water
and small creatinine (a waste product)
molecules through a plasma membrane

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Filtration
(cont.)
filtered liquid enters kidneys
 large molecules such as proteins
remain in blood
 smaller harmful molecules can then be
eliminated in the urine

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Passive Processes Facilitated Diffusion
lipid-insoluble molecules and molecules
too large to diffuse through membrane
or ion channels are carried through
membrane by transporter (carrier)
proteins
 Example:
- glucose

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Facilitated Diffusion
(cont.)
after glucose bonds to transporter on
the outside of the membrane, the
transporter changes shape
 glucose passes through membrane and
is released inside the cell

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Active Transport
requires the use of energy from ATP to
move substances across the membrane
 substances move from areas of lesser
concentration to areas of greater
concentration
 opposite direction of diffusion
 movement occurs against a
concentration gradient

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Actively Transported
Substances:
Na+
 K+
 H+
 Ca + 2
 I Cl
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(Sodium)
(Potassium)
(Hydrogen)
(Calcium)
(Iodine)
(Chlorine)
amino acids
 monosaccharides

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Active Transport

(cont.)
Two Types:
- primary active transport
- secondary active transport
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Primary Active Transport:
The Sodium Pump
ATP energy directly moves substances
across membrane
 sodium pump - most prevalent
- maintains a low concentration of (Na+)
ions in the cytosol
- also moves (K+) ions into cells
 in nerve and muscle cells

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The Sodium Pump
Na+ ions are more abundant outside the
cell
 they are constantly diffusing into the
cell, (their area of lesser concentration)
 not needed in such abundance
 sodium pumps must return Na+ ions to
outside (their presence will bring about
unwanted nerve impulses or muscle
contractions)

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Secondary Active Transport
energy stored in ion gradients moves
substances across the membrane
 indirectly uses energy from splitting
ATP

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Endocytosis

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Processes that bring large substances
into cells:
- phagocytosis
- pinocytosis
- receptor-mediated endocytosis
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Phagocytosis
projections on outside of plasma
membrane, called pseudopods, flow
around large particles (bacteria, debris)
engulfing and bringing them into cell
 once surrounded, membrane sac is
formed (phagocytic vesicle)
 vesicle enters cytoplasm
 material in vesicle is digested by
enzyme produced by lysosome

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Pinocytosis
membrane folds or sinks beneath liquid
to be digested forming pinocytic vesicle
 pinocytic vesicle allows liquid to flow
inward trapping the liquid
 vesicle pinches off or detaches from
membrane into cell interior

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Receptor-Mediated
Endocytosis
similar process to pinocytosis
 substances bind to receptors on the
plasma membrane causing a folding of
the membrane and the formation of an
endocytic vesicle
 highly selective
 cells take up specific molecules or
particles

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Exocytosis
membrane enclosed sacs (secretory
vesicles) form inside cell
- fuse with plasma membrane
- release contents into extracellular fluid
- important in nerve cells
- secrete digestive enzymes or protein
hormones such as insulin
 requires ATP

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