Chapter 6
Interactions
Between Cells
and the
Extracellular
Environment
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6-1
Extracellular Environment
6-3
Extracellular Environment
Includes
all constituents of body outside cells
67% of total body H2O is inside cells (=intracellular
compartment); 33% is outside cells (=extracellular
compartment-ECF)
20% of ECF is blood plasma
80% of ECF is interstitial fluid contained in gel-like
matrix- where we want H2O to go.
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6-4
Body-Fluid Compartments
 Body
has intracellular and extracellular compartments
 Intracellular is inside cells
 Extracellular is outside cells
 Separated by cell’s outer membrane
 Extracellular is composed of blood plasma and interstitial fluid
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1-45
Extracellular Matrix
Is
a meshwork of collagen and elastin fibers linked to
molecules of gel-like ground substance and to plasma
membrane integrins
= glycoprotein adhesion molecules that link intracellular
and extracellular compartments
Interstitial fluid resides in hydrated gel of ground substance
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6-5
Movement Across Plasma Membrane
6-6
Transport Across Plasma Membrane
Plasma
membrane is selectively permeable--allows only
certain kinds of molecules to pass
Many important molecules have transporters and channels
Carrier-mediated transport involves specific protein
transporters
Non-carrier mediated transport occurs by diffusion
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6-7
Transport Across Plasma Membrane continued
Passive
transport moves compounds down concentration
gradient; requires no energy
Active transport moves compounds against a concentration
gradient; requires energy and transporters
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6-8
Diffusion
Is
random motion of molecules
Net movement is from region of high to low
concentration
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6-9
Diffusion continued
Non-polar
compounds readily diffuse thru cell membrane
Also some small molecules such as CO2 and H2O
Gas exchange occurs this way
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6-10
Diffusion continued
Cell
membranes are
impermeable to
charged and most
polar compounds
Charged molecules
must have an ion
channel or
transporter to move
across membrane
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6-11
Diffusion continued
Rate
of diffusion of a compound depends on:
Magnitude of its concentration gradient
Permeability of membrane to it
Temperature
Surface area of membrane
H2O levels
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6-12
Osmosis
6-13
Osmosis
 Is
net diffusion of H2O across a
selectively permeable membrane
 H2O diffuses down its
concentration gradient
 H2O is less concentrated where
there are more solutes
 Solutes have to be
osmotically active
 i.e., cannot freely move
across membrane
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6-14
Osmosis continued
H2O
diffuses down its
concentration gradient until
its concentration is equal on
both sides of a membrane
Some cells have water
channels (aquaporins) to
facilitate osmosis
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6-15
Osmotic Pressure
Is
the force that would have to be exerted to stop osmosis
Indicates how strongly H2O wants to diffuse
Is proportional to solute concentration
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6-16
Tonicity
Is
the effect of a solution on osmotic movement of H2O
Isotonic solutions have same osmotic pressure
Hypertonic solutions have higher osmotic pressure and are
osmotically active
Hypotonics have lower osmotic pressure
Isosmotic solutions have same osmolality as plasma
Hypo-osmotic solutions have lower osmotic pressure than
plasma
 Hyperosmotics have higher pressure than plasma
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6-19
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6-20
Regulation of Blood Osmolality
Blood
osmolality is
maintained in narrow range
around 300mOsm
If dehydration occurs,
osmoreceptors in
hypothalamus stimulate:
ADH release
Which causes kidney
to conserve H2O and
thirst
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6-21
Membrane Transport Systems
6-22
Facilitated Diffusion
Is
passive transport down concentration gradient by carrier
proteins
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6-25
Active Transport
Is
transport of molecules
against a concentration
gradient
ATP is required
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6-26
Na+/K+ Pump
Uses
ATP to move 3
Na+ out and 2 K+ in
Against their
gradients
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6-27
Membrane Potential
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Membrane Potential
 Is
difference in charge
across membranes
 Results in part from
presence of large anions
being trapped inside cell
 Diffusable cations such
as K+ are attracted into
cell by anions
 Na+ is not permeable and is
actively transported out
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6-36
Equilibrium Potential
 Describes
voltage across cell membrane if only 1 ion could diffuse
 If
membrane permeable only to
K+, it would diffuse until it
reaches its equilibrium potential
(Ek)
 K+ is attracted inside by
trapped anions but also
driven out by its
concentration gradient
 At K+ equilibrium, electrical
and diffusion forces are =
and opposite
 Inside of cell has a negative
charge of about -90mV
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6-37
Resting Membrane Potential (RMP)
Is
membrane voltage of cell not producing impulses
RMP of most cells is –65 to –85 mV
RMP depends on concentrations of ions inside and out
And on permeability of each ion
Affected most by K+ because it is most permeable
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6-41
Resting Membrane Potential (RMP) continued
Na+
diffuses in so
RMP is less
negative than EK+
Some
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6-42
Role of Na+/K+ Pumps in RMP
Because
3 Na+ are
pumped out for
every 2 K+ taken in,
pump is electrogenic
It adds about
–3mV to RMP
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6-43
Summary of Processes that Affect the
Resting Membrane Potential
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6-44