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Cell Physiology: Membrane Transport
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings
Selective Permeability
 Plasma membrane allows some materials
to pass while excluding others
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SOLUTION CONCENTRATIONS
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I. Passive Transport Processes
1. Diffusion = Particles distribute evenly
within a solution

Movement =
high to low
concentration

NO Energy
required !
Figure 3.9
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
Simple diffusion:
CAN pass through
membrane !
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b. Osmosis—water
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2. Facilitated diffusion
 Require a protein carrier !
 Lipid-insoluble & LARGE substances
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3. Filtration
 Hydrostatic pressure  pressure gradient
 Solute-containing fluid is pushed from high
to lower pressure area
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<BREAK>
 LAB #6 – Part A:
Transport Mechanisms
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II. Active Transport Processes
 Substances UNABLE to pass by diffusion
 too LARGE !
 HYROPHILIC --insoluble in lipid membrane
 move AGAINST a concentration gradient
 ATP is used for transport
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ex. “Na+ - K+ Pump”
1. Solute pumping
Na+
Extracellular fluid
K+
P
Na+
P
K+
Cytoplasm
ADP
Binding of cytoplasmic Na+
to the pump protein
stimulates phosphorylation
by ATP, which causes the
pump protein to change its
shape.
The shape change expels
Na+ to the outside.
Extracellular K+ binds,
causing release of the
phosphate group.
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Loss of phosphate restores
the original conformation of
the pump protein. K+ is
released to the cytoplasm and
Na+ sites are ready to bind Na+
again; the cycle repeats.
2. Exocytosis
 Moves materials OUT of the cell
 Vesicle combines with plasma membrane
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3. Endocytosis
 Extracellular substances are engulfed by
being enclosed in a membranous vesicle
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Endocytosis
Exocytosis
Extracellular
fluid
Cytoplasm
Pit
Extracellular
fluid
Plasma
membrane
Recycling of membrane
and receptors (if present)
to plasma membrane
Ingested
substance
Transport to plasma
membrane and
exocytosis of
vesicle contents
Vesicle
Lysosome
Detachment
of vesicle
Plasma
membrane
Vesicle containing
ingested material
Vesicle fusing
with lysosome
for digestion
Release of
contents to
cytoplasm
(a)
Figure 3.13a, step 6
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings