Chapter 3a Membrane Transport
Membrane Transport
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= moving things across the cell membrane
= changing concentration gradients
Why?
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out of cell
secretions
into cell
absorptions
through the cell
transepithelial transport
what’s a membrane ?
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separates cells , compartments
allows some substances through
semi-permeable
using what we know
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nature wants equilibrium of concentration
nature wants equilibrium of pressures
nature wants equilibrium of electrical charge
increasing concentration gradient requires Energy
Membrane Transport
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2 properties determine transport
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is energy required?
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passive
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active transport
transport
molecular size
lipid solubility
toward equilibrium
no E required
toward disequilibrium
E required
means of transport :
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diffusion
through membrane
protein mediated
through protein channels
vesicular
via vesicles
passive transport
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diffusion
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simple diffusion
particles move thru membrane
facilitated diffusion
protein mediated
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osmosis
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limited by:
water through membrane
concentration gradient
size of molecules
opposing forces
simple diffusion
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molecules move easily through membrane
down concentration gradient
eg
lipids
O2
CO2
fat soluble vitamins
alcohol
permeable membranes
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both H2O and solute move through membrane
passive diffusion of water and solute
semi-permeable membrane
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membrane impermeable to solute -
only H2O moves
H2O moves toward high solute concentration
H2O moves toward low water concentration
osmosis
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= the movement of water in response to a solute concentration gradient
water moves toward high solute concentration
until concentration equilibrium
“water follows Na”
“water follows glucose”
“water follows protein”
Molarity
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mole = molecular weight of molecule in grams
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H
1g
C
12 g
NaCl
58 g
glucose
180 g
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1 mole = 6.02 x 1023 molecules
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molarity =
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concentration of solute in solution
# moles / L
1M glucose
1M NaCl
180 g / L
58 g/L
Osmolarity
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osmolarity (OsM) = concentration of particles in solution
some molecules dissociate into ions
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NaCl  Na+ + Cl-
1 mole NaCl = 1 mole Na + 1 mole Cl = 2 OsM
1 M glucose = 1 OsM glucose
1 M NaCl
= 2 OsM NaCl
H2O moves toward solution with > Osmolarity
Osmosis
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H2O moves toward high solute concentration
H2O moves toward high Osmolarity
Osmolality
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osmolality = concentration of particles in 1L solvent
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osmolarity = concentration of particles in 1L solution
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osmolality is more clinically important
• gm in 1 kg H2O
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depends on patients body wt. and water content
Osmotic pressure
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the pull of H2O toward area of more particles (solute)
force needed to resist osmosis
** NaCl has greater osmotic pressure than Glucose
tonicity
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= ability of solution to change movement of H2O
= ability of solution to change the shape (tone) of a cell
tonicity
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plasma and most body fluids
0.3 Osm
terms refer to the extracellular solution :
isotonic
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same concentration IN / OUT of cell
H2O moves ?
hypertonic
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H2O moves ?
hypotonic
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cell shape ?
greater concentration OUT
cell ?
lesser concentration OUT
H2O moves ?
cell ?
isotonic solutions
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isotonic
same concentration IN / OUT of cell
any 0.3 Osm solution
iso-osmotic
dextrose
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5.0% glucose
1M glucose
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180 g/L
.3 OsM
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180 x .3 = 54g/L
normal saline
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5%
0.9% saline (NaCl)
1M NaCl =
58 g/L
1M
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2 OsM
1 OsM
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29 g/L
.3 OsM
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29 x .3 = 8.7g/L =
.9%
let’s play Doctor
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IV
intravenous fluids
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adding fluid to blood
what kind of solution would you give?
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blood loss
dehydration
(cells lack water)
edema
(cells are filing with water)
filtration
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pressure = stuff / volume
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more stuff
less volume
pressure gradient
more pressure
more pressure
nature wants ?
fluid moves toward lower pressure
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fluid pressure = hydrostatic pressure
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eg. capillaries, kidney
net H2O movement
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osmotic pressure
pulls H2O toward high solute
hydrostatic pressure
pushes H2O toward lower pressure
toward less fluid
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net movement = hydrostatic pressure – osmotic pressure
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= helped by membrane proteins
protein mediated transport
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transporters
channel proteins
continuous channel
carrier proteins
not continuous channel
protein changes shape
facilitated diffusion
toward equilibrium
active transport
increases concentration gradient
facilitated diffusion
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molecule can’t move through membrane
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requires special protein channels
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eg
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• too big
• too polar
(usually cations)
• molecule specific
cations :
Na+ Ca++K+
glucose
AA
GLUT transporter
passive transport
channel mediated facilitated diffusion
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leakage channel
always open
gated channel
open or closed
• chemical gated
• voltage gated
• mechanical gated
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carrier mediated facilitated diffusion
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no continuous channel
change 3D of transport protein
receptors for molecule
depends on:
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presence of molecule
other control molecule
active processes
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require input of energy
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active transport
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vesicular transport
protein mediated transport
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helped by membrane proteins
transporters
facilitated diffusion
toward equilibrium
active transport
increases concentration gradient
requires input of Energy
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primary active transport
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secondary active transport
– E from ATP
– E from concentration gradient of another molecule
active transport
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movement against gradient (uphill)
requires energy
requires transporter (carrier enzyme)
uniport
one molecule
co-transport
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two molecules
symport
two molecules , same direction
antiport
two molecules, opposite directions
primary active transport
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carrier protein moves molecule thru membrane
ATP changes 3D of carrier protein
ATPase
uniport :
• Ca pumps into organelles
• proton pump
Ca ATPase
H ATPase
co-transport:
• Na – K pump
• H – K pump
Na – K ATPase
H – K ATPase
primary active transport uses
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maintain resting membrane potential
move water
nerve, muscle
nutrient absorption
secondary active transport
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downhill movt of one molecule provides E for uphill movt of a second molecule
Na diffuses down its gradient
- creates energy
energy used to move glucose into cell against its gradient = secondary active transport
Na pumped out of cell
(primary active transport)
creates Na concentration gradient
uses for secondary active transport
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glucose into epithelial cells
AA into epithelial cells
Ca++ out of cells
H+ in/out
(pH regulation)
most rely on Na – K primary AT
SGLT tranporters
vesicular transport
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movement of large molecules, small organisms
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contained in vesicles made from cell membrane
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require energy – ATP – to move membrane
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into cell
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phagocytosis
membrane engulfs
endocytosis
membrane indents
out of cell
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exocytosis
phagocytosis
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cell membrane engulfs bacteria or cell debris
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defense ; tissue repair
endocytosis
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cell membrane indents to form vesicle
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pinocytosis
large molecules (proteins)
receptor mediated
molecules bind to receptors
exocytosis
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secretions : hormone , neurotransmitter , wastes
usually stim by membrane receptors or Ca++
ions and membranes
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nature wants: equilibrium of concentration and charge
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noncharged molecules
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charged molecules (ions)
• move per concentration gradient only
• move per concentration and electric gradients
• = electrochemical gradient
electrical stuff
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opposite charges attract
if opposite charges are separated - it takes energy
potential energy
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measurement
Volt
millivolt
V
mV
cells lack electrochemical equilibrium at the cell membrane
negative charge inside cell
= resting membrane potential
resting membrane potential
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Anions ( A )
negative charged molecule (proteins)
• inside cell
can’t move thru membrane
+
K inside cell
• K+ channels open
• concentration gradient pulls K+
• electric gradient pulls K+ ?
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slight movement of K+ out , leaves inside negative
equilibrium at
-90 mV
Na+ leaks in
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Na+ leaks in a little
electrochemical grad.
Na+ - K+ ATPase
maintains resting potential
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pumps 3 Na+ out
pumps 2 K+ in
overall picture
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A inside
K+ inside
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K+ channels open
K+ @ equilibrium
Na+ outside
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Na+ channels closed
Na+ pumped out
resting potential
huge Na+ gradient
-90 to -70 mV
you call this resting ?
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membrane potential requires energy.
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maintains osmotic balance
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if Na moved in, where would H2O go?
ready to “act” instantly
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change ion permeability
• eg. open Na+ or Ca++
• eg. close K+ channel
changes potential
channel
this is how nerves and muscles act