Interactions Between Cells & the
Extracellular Environment
Chapter 6
C
H
E
M
I
C
A
L
extracellular environment
represents all constituents
outside the cell
impact/interact neighboring (1) cells,
(2) different tissues &/or
(3) different organs
R
E
G
U
L
A
T
O
R
S
obtain
nourishment,
release secretions,
eliminate wastes
1
Body Fluids
• Divided into compartments:
– extracellular
• 33%
• bld plasma & interstitial fluid
– intracellular
• 67%
• fluids serve as communication link – cells  tissues  organs
2
Extracellular Matrix
ECM
• complex network of proteins
– specific for any given tissue
• EC fluid interspersed within
• functions:
– scaffolding for cellular
attachment
– transmits information to
regulate activity, migration,
growth & differentiation
connective tissue of a tissue/organ
3
ECM composed of fibrous proteins but
also ground substance, analogous to a
hydrated gel & location of IF
An Example
comprised of glycoproteins & proteoglycans
 gel
binds water
highly functional, complex
organization of molecules chemically
linked to EC protein fibers &
glycoproteins of glycocalyx
block integrin - binding
site on platelets, slows
bld clotting
integrins - adhesion molecule between cell &
ECM, physically joining EC & IC compartments
serve to relay signals or integrate them
4
Transport Across the
Plasma Membrane
• plasma membrane
– serves as a “barrier” to
movement - EC & IC compartments
• selectively permeable
– membrane transport processes
• E requirements
– passive transport
– active transport
• carrier-mediated transport versus
transport without a carrier
5
Diffusion & Osmosis
molecules of a solution are
in constant motion
solvent
solute
mean diffusion time
conc gradient exists, motion tends to eliminate the difference, with
the random motion of molecules is diffusion
will move in both directions but a net movement from higher to lower
until equilibrium is reached
increases with distance; distances kept within 100 μm for effective exchange
6
Diffusion through a PM
nonpolar molecules  O2 & steroids, or small polar covalent molecules
without charge  CO2, ethanol & urea, can easily cross the PM
will follow concentration gradient between compartments
7
Membrane Channels
charged inorganic ions, Na+ & K+,
utilize channels
channels may be OPEN or GATED
particular physiological stimuli opens/closes gate
for large, polar molecules, carrier proteins are needed in the PM for movement
8
Rate of Diffusion
speed of diffusion per unit time
J = PA (Co – Ci)
• magnitude of conc gradient
• diffusing substance’s
permeability to PM
• temperature
• SA available
• distance
magnitude of conc gradient driving
force for diffusion BUT will not move
if PM not permeable to that molecule
net flow (J) is directly
proportional to
the conc gradient (Co-Ci),
the surface area (A) and
the membrane permeability
coefficient (P)
greater P, larger the J across the PM for
any given conc difference & A
9
Osmosis
• net diffusion of water
– follows waters conc gradient
• requirements:
– conc difference of solute between
sides of membrane
– membrane selectively
impermeable to solute
• nonpenetrating solute
• osmotically active
– creates osmotic pressure
» “pull” of water
• aquaporins – water channels
– facilitate movement of water
– present in some cell types or can
be inserted in response to
regulatory molecules
10
Osmotic Pressure (OP)
water drawn more rapidly with
greater solute conc
OP of a sol’n represents the pressure
that must be applied to a solution to
prevent the net flow of water
indicates how strongly a sol’n
draws water
11
ratio of solute to solvent not completely specified
amt of water changes due to MW of substance
MOLARITY
Molarity vs Molality
better measurement of concentration
when discussing osmosis
MOLALITY
weigh one mole of substance and place
in 1Kg water thus can compare between
solutes since both in same amt of water
12
Osmolality
• OP depends on ratio of
solute to solvent, not
chemical characteristics of
solute
– osmolality – Osm
• total molality of a solution
• what about electrolytes?
– ionize
• plasma & other biological
fluids have a complex
osmolality due to the
presence of organic
molecules & electrolytes
– cell activity leads to
constant change
13
Tonicity
• describes effect of
solution on osmotic
movement of water, thus
cell shape & volume
– solute load – 300 mOsm
• solutions described by
how they change cell
volume (& thus shape) by
causing water movement
• take into account both
solute conc & solute
permeability for each
solute crossing the PM
14
Osmolality vs Tonicity
Can a solution be iso-osmotic
but not isotonic?
YES, if solute is permeable to
membrane, ie is a penetrating
molecule, thus can freely
cross the PM
15
Question
When a cell comes in contact with a sol’n,
hypertonic or hypotonic, the initial conc of
solutes determines the degree of change
• - impermeant
If a cell were placed in a
solution containing 100 mOsm
impermeant solutes, how
would its final volume
compare to its initial volume?
answer on notecard
16
Homeostasis of Plasma Concentration
variety of mechanisms exist
to keep blood plasma
osmolality maintained
within very narrow limits
17
Carrier-Mediated Transport
cellular metabolism relies on the cell’s ability to
uptake molecules it needs from the EC fluid
many of these molecules cannot be attained
by simple diffusion, require protein carriers
specificity, saturation
& competition
Comparison
carrier proteins display the
characteristic of saturation
if carrier can transport more one
molecule type, then they will
compete for transport
becomes saturated
18
Facilitated
Diffusion
passive transport
cell stimulated, insert
carriers into PM to
meet cell needs
display specificity,
competition & saturation
19
Active Transport
Primary
• energy required for carrier
function
• typically, molecules/ions moved
against their conc gradient
• process:
– binding of molecule to be
transported to “recognition site”
– binding stimulates ATP hydrolysis
– phosphorylation causes carrier
protein to undergo conformational
change
– hingelike motion of carrier protein
releases transported molecule to
other side
• often referred to as pumps
20
Na-K Pump
• creates steep ion gradient
• functions:
– provides E for coupled transport of other molecules
– used to generate electrochemical events (impulse) in
nervous & muscle tissue
– Na movement impt for osmotic reasons
• stops, observe Nai, cause osmotic influx of water (damage cell)
21
Active Transport
sets up
gradient
Secondary
X
driven indirectly by passive ion gradients
created by operation of primary active pump
symport
what happens if the Na-K pump is poisoned?
22
Movement of solutes across
a typical PM involving
membrane proteins
Many of these membrane
proteins can be modulated by
various signals, resulting in
controlled rise or fall in specific
solute fluxes across PM
Specialized cells may contain
additional transporters and channels
23
Transport Across Epithelial Membranes
♦
Since epithelial cells line the
body’s surface as well as cavities
of hollow organs, molecules
entering the body must pass
through an epithelial cell layer
♦
absorption
reabsorption
transcellular transport, transepithelial
transport, transcytosis
paracellular transport
Movement of
Glucose
NOTICE polarity, or definite direction
of transport in epithelial cells
apical - basolateral
24
Junctional Complex
physically join
presence & number
dependent on
location
“glued” together
“velcroed” together
25
Bulk
Transport
endocytotic events
for secretion, use exocytosis
movement of molecules too large to be transport through PM
26