Extracellular
Environment
Movement Across Plasma Membrane
Osmosis
Membrane Transport Systems
Membrane Potential
Cell Signaling
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6-2
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
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6-4
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
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
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6-12
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
Molarity and Molality
1
molar solution (1.0M) = 1 mole of solute dissolved in
1L of solution
Doesn't specify exact amount of H2O
1 molal solution (1.0m) = 1 mole of solute dissolved in
1 kg H2O
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6-17
Molarity and Molality continued
Osmolality
(Osm) is
total molality of a
solution
E.g., 1.0m of NaCl
yields a 2 Osm
solution
Because NaCl
dissociates into
Na+ and Cl-
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6-18
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
Carrier-Mediated Transport
Molecules
too large and polar to diffuse are
transported across membrane by protein carriers
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6-23
Carrier-Mediated Transport continued
Protein
carriers exhibit:
Specificity for single
molecule
Competition among
substrates for
transport
Saturation when all
carriers are occupied
This is called Tm
(transport
maximum)
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6-24
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
Secondary Active Transport
Requires
ATP to first move Na+ uphill to create a
gradient
Secondary active transport then uses energy from
“downhill” movement of Na+ to drive “uphill” transport
of another molecule
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6-28
Secondary Active Transport continued
Cotransport
(symport) is secondary transport in same
direction as Na+
Countertransport (antiport) moves molecule in
opposite direction to Na+
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6-29
Transport Across Epithelial Membranes
Absorption
is
transport of
digestion products
across intestinal
epithelium into
blood
Reabsorption
transports
compounds out of
urinary filtrate back
into blood
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6-30
Transport Across Epithelial Membranes
continued
Transcellular
transport moves material from 1 side to
other of epithelial cells
Paracellular transport moves material through tiny
spaces between epithelial cells
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6-31
Transport Across Epithelial Membranes
continued
Transport
between cells is limited by junctional
complexes that connect adjacent epithelial cells
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6-32
Transport Across Epithelial Membranes
continued
Plasma
membranes can
join together to form tight
junctions
In adherens junctions
membranes are “glued”
together by proteins that
pass through both
membranes and attach
to cytoskeletons
In desmosomes proteins
“button” two membranes
together
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6-33
Bulk Transport
Moves
large molecules and particles across plasma
membrane
Occurs by endocytosis and exocytosis
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6-34
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
Summary of Processes that Affect the
Resting Membrane Potential
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6-44
Cell Signaling
Is
how cells communicate with each other
Some use gap junctions thru which signals pass
directly from 1 cell to next
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6-46
Cell Signaling continued
To
respond to a chemical signal, a target cell must
have a receptor protein for it
In paracrine signaling, cells secrete regulatory
molecules that diffuse to nearby target cells
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6-47
Cell Signaling continued
In
synaptic signaling 1 neuron sends neurotransmitter
messages to another cell via synapses
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6-48
Cell Signaling continued
In
endocrine signaling, cells secrete chemical
regulators that move thru blood stream to distant target
cells
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6-49
How Regulatory Molecules Influence
Target Cells
Nonpolar
regulatory molecules pass through plasma
membrane, bind to receptors in nucleus, and affect
transcription
Examples include steroid and thryroid hormones
and nitric oxide
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6-50
How Regulatory Molecules Influence
Target Cells
Polar
regulatory molecules bind to cell surface
receptors
Activated receptors send 2nd messengers into
cytoplasm to mediate actions of regulatory molecule
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6-51
Second Messengers
be ions (e.g. Ca++) or other molecules such as
cyclic AMP (cAMP) or G-proteins
May
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6-52
G-proteins
Are
part of 2nd messenger pathway in many cells
Contain 3 subunits whose components dissociate
when a cell surface receptor is activated
A subunit binds to an ion channel or enzyme,
changing their activity
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6-53
G-proteins continued
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6-54