Chapter 5a
Membrane
Dynamics
About this Chapter
• Mass balance and homeostasis
• Diffusion
• Protein-mediated, vesicular, and
transepithelial transport
• Osmosis and tonicity
• The resting membrane potential
• Insulin secretion
Mass Balance in the Body
Intake
Excretion
(through intestine,
lungs, skin)
(by kidneys, liver,
lungs, skin)
BODY
LOAD
Metabolic
production
Metabolism to
a new substance
Law of Mass Balance
Mass balance = Existing +
body load
Intake or
metabolic
production
–
Excretion or
metabolic
removal
Figure 5-2
Mass Balance and Homeostasis
• Clearance
• Rate at which a molecule
disappears from the body
• Mass flow = concentration 
volume flow
• Homeostasis  equilibrium
• Living things not EQ across
membranes
• Osmotic equilibrium
• Where?
• Chemical disequilibrium
• Electrical disequilibrium
• Where?
Homeostasis
Figure 5-3a
Homeostasis vs Equlibrium
Compare:
ECF vs ICF
I vs P
Figure 5-3b
Diffusion
• Map of membrane transport
• Active vs Passive
ENERGY REQUIREMENTS
Uses energy of
molecular motion.
Does not require ATP
MEMBRANE TRANSPORT
Requires energy
from ATP
Diffusion
Endocytosis
Simple
diffusion
Molecule
goes through
lipid bilayer
Facilitated
diffusion
Secondary
active
transport
creates
concentration
gradient
for
Mediated transport
requires a
membrane protein
Primary
active
transport
Exocytosis
Phagocytosis
Uses a
membrane-bound
vesicle
PHYSICAL REQUIREMENTS
Figure 5-4
Diffusion: Seven Properties
1. Passive process
2. High concentration to low
concentration
3. Net movement until
concentration is equal
4. Rapid over short distances
5. Directly related to
temperature
•
How?
6. Inversely related to molecular
size
7. In open system or across a
partition
•
Membrane – composition
related to function
Simple Diffusion
Figure 5-5
Simple Diffusion
Extracellular fluid
• Fick’s law
of diffusion
Membrane
surface area
Concentration
outside cell
Molecular
size
Lipid
solubility
Membrane
thickness
Concentration
gradient
Composition
of lipid layer
Concentration
inside cell
Intracellular fluid
Fick's Law of Diffusion says:
Rate of diffusion
surface area • concentration gradient • membrane permeability
membrane thickness
Membrane permeability
Membrane permeability
lipid solubility
molecular size
Changing the composition of the lipid layer can
increase or decrease membrane permeability.
Figure 5-6
Simple Diffusion
Table 5-1
Functions of Membrane Proteins
•
•
•
•
Structural proteins
Enzymes
Membrane receptor proteins
Transporters
• Channel proteins
• Carrier proteins
Membrane Transport Proteins
MEMBRANE
PROTEINS
can be categorized according to
Structure
Lipidanchored
proteins
Integral
proteins
Function
Peripheral
proteins
Membrane
transporters
Structural
proteins
Membrane
enzymes
activate
are active in
are found in
Carrier
proteins
Channel
proteins
change
conformation
Open channels
Mechanically
gated
channel
Membrane
receptors
are active in
Cell junctions
Receptormediated
endocytosis
Cytoskeleton
form
Gated channels
Voltage-gated
channel
Metabolism
Chemically
gated
channel
Signal
transfer
open and
close
Figure 5-7
Membrane Transport Proteins
Ligand binds to
a cell membrane
receptor protein.
Ligand-receptor complex
triggers intracellular response.
Extracellular
fluid
Receptor
Intracellular fluid
Events in the cell
Figure 5-8
Membrane Transport Proteins
MEMBRANE TRANSPORTERS
Channel proteins create a water-filled pore
Carrier proteins never form an open channel between
the two sides of the membrane
ECF
Cell
membrane
Carrier open
to ICF
ICF
Same carrier
open to ECF
can be classified
can be classified
Cotransporters
Gated channels
Open
Open channels
Uniport carriers
Symport carriers
Antiport carriers
Closed
Figure 5-9
Membrane Channel Proteins
Channel
One protein
subunit
of channel
Channel
Figure 5-10
Gating of Channel Proteins
Chemically, voltage or mechanically controlled
Closed gate
Extracellular fluid
Pacific
Ocean
Atlantic
Ocean
Passage
open to
one side
Molecule
to be
transported
Intracellular fluid
Gate closed
Carrier
Membrane
Pacific
Ocean
Atlantic
Ocean
Transition
state with
both gates
closed
Pacific
Ocean
Atlantic
Ocean
Passage
open to
other side
(a)
Gate closed
(b)
Figure 5-11
Facilitated Diffusion of Glucose
Figure 5-12
Primary Active Transport
Primary Active Transporters
•ATPases
• Na/K pump
• Ca
Secondary Active
Use potential energy
•Na+ glucose
•SGLT
Primary Active Transport
• Mechanism of the Na+-K+-ATPase
1
ECF
ATP
ADP
5
2
3 Na+ from
ICF bind
2
ICF
ATPase is
phosphorylated
with Pi from ATP.
K+
released
into ICF
Protein changes
conformation.
4
2 K+ from
ECF bind
Protein changes
conformation.
3
3 Na+ released
into ECF
Figure 5-14
Primary Active Transport
1
ECF
3 Na+ from
ICF bind
ICF
Figure 5-14, step 1
Primary Active Transport
1
ECF
ATP
ADP
2
3 Na+ from
ICF bind
ICF
ATPase is
phosphorylated
with Pi from ATP.
Figure 5-14, steps 1–2
Primary Active Transport
1
ECF
ATP
ADP
2
3 Na+ from
ICF bind
ICF
ATPase is
phosphorylated
with Pi from ATP.
Protein changes
conformation.
3
3 Na+ released
into ECF
Figure 5-14, steps 1–3
Primary Active Transport
1
ECF
ATP
ADP
2
3 Na+ from
ICF bind
ICF
ATPase is
phosphorylated
with Pi from ATP.
Protein changes
conformation.
4
2 K+ from
ECF bind
3
3 Na+ released
into ECF
Figure 5-14, steps 1–4
Primary Active Transport
1
ECF
ATP
ADP
5
2
3 Na+ from
ICF bind
ICF
ATPase is
phosphorylated
with Pi from ATP.
2 K+ released
into ICF
Protein changes
conformation.
4
2 K+ from
ECF bind
Protein changes
conformation.
3
3 Na+ released
into ECF
Figure 5-14, steps 1–5
Secondary Active Transport
• Mechanism of the SGLT Transporter
1 Na+ binds to carrier.
Lumen of intestine
or kidney
Intracellular fluid
3 Glucose binding changes
carrier conformation.
SGLT protein
[Na+] high
[glucose] low
[Na+] low
[glucose] high
4 Na+ released into cytosol.
Glucose follows.
2 Na+ binding creates
a site for glucose.
Figure 5-15
Secondary Active Transport
1 Na+ binds to carrier.
Lumen of intestine
or kidney
Intracellular fluid
SGLT protein
[Na+] high
[glucose] low
[Na+] low
[glucose] high
Figure 5-15, step 1
Secondary Active Transport
1 Na+ binds to carrier.
Lumen of intestine
or kidney
Intracellular fluid
SGLT protein
[Na+] high
[glucose] low
[Na+] low
[glucose] high
2 Na+ binding creates
a site for glucose.
Figure 5-15, steps 1–2
Secondary Active Transport
1 Na+ binds to carrier.
Lumen of intestine
or kidney
Intracellular fluid
3 Glucose binding changes
carrier conformation.
SGLT protein
[Na+] high
[glucose] low
[Na+] low
[glucose] high
2 Na+ binding creates
a site for glucose.
Figure 5-15, steps 1–3
Secondary Active Transport
1 Na+ binds to carrier.
Lumen of intestine
or kidney
Intracellular fluid
3 Glucose binding changes
carrier conformation.
SGLT protein
[Na+] high
[glucose] low
[Na+] low
[glucose] high
4 Na+ released into cytosol.
Glucose follows.
2 Na+ binding creates
a site for glucose.
Figure 5-15, steps 1–4