Intracellular vs. extracellular concentrations

1
Intracellular vs. extracellular concentrations
Note Na, K, Cl-, phosphate,- protein-
2
IC vs. EC important points
Intracellular cations Intracellular anions
(mEq/L) Extracelluar cations Extracellular
anions (mEq/L) miniscule, unmeasurable
differences Intracellular particles
Extracellular particles i.e. IC osmolality EC
osmolality
3
Membrane transport overview
No carrier simple diffusion (lipid soluble
substances) diffusion through ion
channels diffusion through water
channels Carrier mediated transport facilitated
diffusion (passive) primary active transport
(active, uses ATP) secondary active transport
(active, uses ion gradient) Endocytosis
exocytosis
4
Simple diffusion
Through phospholipid bilayer Lipid soluble
substances e.g. O2, CO2, NH3, N2, fatty acids,
steroids, ethanol, Passive (down concentration
gradient) No carrier (? no saturation,
competition)
5
Simple diffusion
fig 4-2
6
Simple diffusion (flux)
fig 4-3
At equilibrium compartment 1 concentration
compartment 2 concentration one-way flux (left ?
right) one-way flux (right ? left) net flux 0
7
Simple diffusion (graph of Ci vs. time)
fig 4-4
Graph shows that transport is passive i.e. over
time Ci will reach, but never exceed Co
8
Simple diffusion (graph of rate vs. concentration)
Graph shows that transport is not carrier
mediated because no saturation of transport rate
9
Transport through ion channels
fig 4-7
10
Properties of ion channels
Usually (not always) highly specific for the
ion Ion transport is passive ions are
charged therefore, gradient depends on
concentration charge combination is
electrochemical gradient Channels open and
close spontaneously Percentage of open time can
be regulated (gating) Open time regulated
by binding of ligands to the channels (ligand
gating) voltage difference across membrane
(voltage gating) stretch of membrane (mechanical
gating) covalent alteration of channel protein
11
Facilitated diffusion
fig 4-8
12
Facilitated diffusion (properties)
Passive, carrier mediated Examples glucose into
most cells (not luminal membrane of kidney or
intestine), urea, some amino acids Kinetics
shows passive
shows carrier mediated
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Non-mediated vs. mediated transport
fig 4-9
14
Primary active transport (Na/K ATPase pump)
3 Nas out, 2 Ks in, 1 ATP hydrolyzed
fig 4-11
15
Primary active transport properties
Active (energy from direct hydrolysis of
ATP) Carrier mediated Used when many ions
moved (e.g. 5 for Na/K ATPase pump) ions moved
against steep gradient (Ca ATPase in muscle,
H/K ATPase in stomach, H ATPase in kidney)
16
Primary active transport kinetics
shows active transport
shows carrier mediated
17
Effect of Na/K ATPase pump
fig 4-12
18
Secondary active transport
fig 4-13
19
Secondary active transport properties
Active (energy from ion gradient, usually
Na) Carrier mediated Can be cotransport
(symport) or countertransport (antiport) Examples
(many) Na/amino acids, Na/glucose (luminal
membrane kidney, GI tract), Na/H kidney,
Ca/3Na muscle, Cl-/HCO3- red cell. (
countertransport) Kinetics see primary active
transport graphs
20
Transport, the big picture
fig 4-15
21
Table 4-2
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Water transport (aka osmosis)
Water moves through aquaporin channels Water
moves passively down its own concentration
gradient Dissolving solute in water reduces the
water concentration Water therefore moves from a
dilute solution to a more concentrated
solution The solute concentration depends on
the number of particles The number of particles
is called osmolarity (?osmolality?) The units
of osmolarity are milliosmoles/L (mOsm/L)
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Calculation of osmolarity

• The osmolarity of a 100 mM glucose solution is
  100 mOsm/L
• A 100 mM NaCl solution dissociates into 100 mM
  Na and 100 mM Cl- its osmolarity is therefore
  200 mOsm/L
• Assuming complete dissociation, calculate the
  osmolarity of the following solutions
• 100 mM NaCl, 50 mM urea
• 2. 200 mM glucose, 30 mM CaCl2

Answer 250 mOsm/L Answer 290 mOsm/L
24
Red cells in solution
Notes nonpenetrating solutes, cell osmolarity
300 mOsm/L
fig 4-19
25
Crenated red cells
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Osmolarity and tonicity
Osmolarity is a measure of the total number of
particles Tonicity is a measure of the solute
particles which do not cross the cell membrane
non-penetrating solutes Tonicity therefore
depends on the properties of the solute and the
cell membrane For example, urea crosses most cell
membranes, and will enter the cell down its
concentration gradient A solution of 300 mM urea
is isosmotic to red cells but is hypotonic
27
Osmolarity and tonicity problems

• Consider a solution of 100 mM NaCl and 200 mM
  urea. How does its osmolarity and tonicity
  compare with red cells having an osmolarity of
  300 mOsm/L?
• Answer hyperosmolar and hypotonic
• 2. Consider a solution of 125 mM NaCl and 50 mM
  urea. How does its osmolarity and tonicity
  compare with red cells having an osmolarity of
  300 mOsm/L?
• Answer isosmolar and hypotonic

28
Osmolarity (important concept)
Because cells contain abundant aquaporin
channels, water rapidly equilibrates across the
cell membrane Therefore, the osmolarity of
virtually all body cells is equal, and equal to
the osmolality of extracellular fluid
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Drinking water
30
Endocytosis and exocytosis
fig 4-20
31
Endocytosis and exocytosis properties
Endocytosis pinocytosis, phagocytosis specifici
ty conferred by receptor mediated
endocytosis route see next slide Exocytosis re
lease of neurotransmitters, hormones, digestive
enzymes route rough er ? Golgi ? secretory
vesicles release usually triggered by ?
cytosolic Ca insertion of glucose
transporters (insulin), insertion of water
channels (ADH)
32
Endocytosis route
fig 4-21
33
Epithelial transport (Na)
fig 4-22
34
Epithelial transport (water)
fig 4-24
35
Epithelial transport (glucose in kidney, GI tract)
fig 4-23