Chapter 5 Membrane Transport
Body Fluid Compartments
1
Fluid in the body
• Extracellular fluid
– Plasma
– Interstitial fluid
– Some layers of cells are “leaky” to fluid e.g.
fluid can pass through capillary walls.
• Intracellular fluid
2
How can substances move into a
cell?
• Some small molecules can move directly
across the phospholipid bilayer.
• Very small
• Lipid soluble
• diffusion
3
Diffusion
4
Diffusion
• Movement of molecules from an area of
high concentration to a region of low
concentration.
• Diffusion uses only the kinetic energy
possessed by all molecules.
• There will be a net movement of molecules
until the concentration is in equilibrium.
5
Diffusion
• Diffusion is fast for short distances and slower for
long distances (e.g. travel Canada to U.S.;
Windsor to Detroit vs Vancouver to Boston).
Blood vessels help to bring materials to cells.
• Diffusion rates depend on temperature
6
Diffusion
• Larger molecules diffuse slower than
smaller molecules. (more friction on a
larger surface)
• Diffusion can occur in a space or it can
permeate across a structure (eg. Spray on
perfume, the perfume can permeate your
clothes, and into the air)
7
Lipophilic molecules can diffuse
through the phospholipid bilayer.
• Simple diffusion
• A molecules diffuses across the cell membrane if
it is very soluble in the phospholipid bilayer. E.g.
steroids, lipids, small lipophilic molecules.
• water molecules can slip between the lipid tails.
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9
Diffusion Rate
• More diffusion with a large surface area
(e.g. decreased alveolar surface area in lung
diseases)
• Faster diffusion across a thin cell
membrane. ( e.g. if lung alveoli contain
scar tissue)
10
What molecules do not diffuse
across the cell membrane?
• Lipophobic molecules
– Ions
– Large lipophobic molecules, glucose, amino
acids etc.
11
Carrier Mediate Transport across
the cell membrane
• Mediated transport, with a carrier protein
• Uniport
• Cotransport
– Symport
– antiport
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Uniport carriers
13
Symport carriers
14
Antiport carriers
15
Mediated transport
• Can be passive, along a diffusion gradient,
facilitated diffusion.
16
Facilitated Diffusion
17
Carrier Mediated Transport
• Can move against a concentration gradient,
this requires energy. (E.g. Canada imports
coffee.)
– Active transport
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Facilitated Transport
and Active Transport
• Depend on cell membrane proteins
– Carrier proteins are specific for substrates.
Some transporters are more promiscuous than
others.
– GLUT transporters are specific for naturally occuring 6
carbon sugars, glucose, mannose, galactose, fructose.
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Facilitated Transport
and Active Transport
• A tranporter will bind similar substances
• Competition experiments
• Higher binding affinity for some substrates
– E.g GLUT has highest affinity for glucose.
Glucose competes with galactose etc.
– Graph of a competition experiment, measure
galactose transport before and after glucose
added.
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Facilitated Transport
and Active Transport
• Competitive inhibitor binds to a transporter
but does not move.
– E.g. disaccharide maltose is a competitive
inhibitor for the GLUT.
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Facilitated Transport
and Active Transport
• The transport rate can saturate.
• Graph of rate of tranport of substrate X
compared to extracellular concentration of
stubstrate X.
• E.g. Evacuation of a city depended on the
availability of buses.
• Transport maximum.
22
Facilitated Transport
and Active Transport
• How can a cell alter transport rate, if the
rate is saturated?
– Make more carrier proteins
– Make fewer carrier proteins (type 2 diabetes)
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