1. Transport of Substances Through the Cell Membrane
Chapter 4:
Transport of Substances Through the Cell Membrane
Slides by Thomas H. Adair, PhD

2. Lipid Bilayer: barrier to water and water-soluble substances CO2 O2 N2
halothane
H2O
urea
ions
glucose

3. Permeability coefficients (cm/sec)
(** across an artificial lipid bilayer)
10-2
10-4
10-8
10-10
10-12
water
urea
glycerol
glucose
Cl- K+
Na+
10-6
high permeability
low permeability

4. Molecular Gradients inside outside 14 142 140 4 0.5 1-2 Na+ 10-4 K+
(in mM) 14
140
0.5
10-4
(pH 7.2) 10
5-15 2 75 40
outside
(in mM)
142 4
1-2
(pH 7.4) 28
110 1 5
Na+ K+
Mg2+
Ca2+ H+
HCO3-
Cl-
SO42-
PO3-
protein

5. Proteins: provide “specificity” to a membrane provide “function”
ion channels
carrier proteins K+

6. Diffusion Active Transport
occurs against a concn.
gradient
involves a “carrier”
requires ENERGY
occurs down a concn.
gradient
no mediator or involves
a “channel” or “carrier”
no additional energy
Figure 4-2; Guyton & Hall

7. Simple Diffusion (a) (b)
(a) lipid-soluble molecules move readily across the membrane
(rate depends on lipid solubility)
(b) water-soluble molecules cross via channels or pores
(a)
(b)

8. Ion Channels Characteristics: ungated
determined by size, shape, distribution of charge, etc.
gated
voltage (e.g. voltage-dependent Na+ channels)
chemically (e.g. nicotinic ACh receptor channels) in
out
Na+ and other ions
Na+

9. How to Study? Extracellular Inside Cell “Patch Clamp”
Nobel Prize in Physiology & Medicine -1991
Extracellular
Inside Cell

10. Ion Channels in
out
Two separate recordings of electrical current in picoamps flowing through a single sodium channel when a 25 mV current was applied across membrane. The sodium channel conducts current all or none (opened or closed).
Recorded using patch clamp technique.
Na+
Figure 4-5; Guyton & Hall

11. Ionophores 1. Mobile ion carriers (e.g. valinomycin, A23187)
- hydrophobic molecules that dissolve in lipid bilayers and increase permeability to specific inorganic ions. Ionophores mediate passive transport.
1. Mobile ion carriers (e.g. valinomycin, A23187)
“pick up” ion from one side of the membrane and deposit it on the other
2. Channel formers (e.g. gramicidin A)
form ion-permeable pores in the membrane
transport 1000x more ions per unit time than mobile ion carriers
Ionophores. A molecule that allows ions to cross lipid bilayers. There are two classes: carriers and channels. Carriers, like valinomycin, form cage like structures around specific ions, diffusing freely through the hydrophobic regions of the bilayer. Channels, like gramicidin, form continuous aqueous pores through the bilayer, allowing ions to diffuse through.
Valinomycin A cyclododecadepsipeptide ionophore antibiotic produced by Streptomyces fulvissimus and related to the enniatins. It is composed of 3 moles each of L-valine, D-alpha-hydroxyisovaleric acid, D-valine, and L-lactic acid linked alternately to form a 36-membered ring. (From Merck Index, 11th ed) Valinomycin is a potassium selective ionophore and is commonly used as a tool in biochemical studies.
Gramicidin A group of antibiotics (ANTIBIOTICS, PEPTIDE) from Bacillus brevis. Gramicidin C or S is a cyclic, ten-amino acid polypeptide and gramicidins A, B, D are linear. Gramicidin is one of the two principal components of TYROTHRICIN which is used topically for gram-positive organisms. It is toxic to blood, liver, kidneys, meninges, and the olfactory apparatus.
Valinomycin and gramicidin A are made by certain bacteria and have been used as antibiotics.

12. Simple vs. Facilitated Vmax Tm
simple diffusion
rate of diffusion  (Co-Ci)
rate of
diffusion
Vmax Tm
facilitated diffusion
Concn of substance
What limits maximum rate of facilitated diffusion?

13. Facilitated Diffusion
(also called carrier mediated diffusion)
Rate of diffusion is limited by
Vmax of the carrier
protein
the density of carrier
proteins in the membrane
(i.e., number per unit area)
Figure 4-7; Guyton & Hall

14. Factors that affect the net rate of diffusion:
1. Concentration difference (Co-Ci)
net diffusion  D (Co-Ci)
Figure 4-8; Guyton & Hall

15. Net Diffusion
A B
Can a molecule diffuse from side B to side A?

16. + + 2. Electrical potential (EMF) - - - - - - - - - - - - - - - - - -
When will the negatively charged molecules stop entering the cell? - - - - - - - - - - - - - - - - - - - - - - - - - -
In this case we’re changing the electrical potential across the membrane and see what happens to the concn gradient of the ion. In reality, the concentraction gradient is changed various cellular processes which determines the Nernst potential. - -
The Nernst potential (equilibrium potential) is the theoretical intracellular electrical potential that would be equal in magnitude but opposite in direction to the concentration force.
EMF (mV) = ±61 log (Co / Ci)

17. 3. Pressure difference
Higher pressure results in increased energy available to cause net movement from high to low pressure.
Occurs at capillary membrane. Higher pressure in the capillary facilitates the diffusion of molecules into the tissues.
Pressure actually means the sum of all the forces of the different molecules striking a unit area of membrane at a given instant.
Figure 4-8; Guyton & Hall

18. Osmosis: - Net diffusion of water -
Osmosis occurs from pure water toward a water/salt solution. Water moves down its concn gradient.
Osmosis is the net diffusion of water across a selectively permeable membrane.
Figure 4-9; Guyton & Hall

19. Osmotic Pressure: the amount of pressure required to counter osmosis
Osmotic pressure is
attributed to the
osmolarity
of a soln
Figure 4-10; Guyton & Hall

20. Major determinant of osmotic pressureB
100 g
in 1 L
1000 g
in 1L
Solute A
Mw = 100
Solute B
Mw = 1000
Which solution has the greatest osmolarity?
Which has the greatest molar concn?
Which has the greatest number of molecules?
(6.02 x 1023 particles)

21. Relation between osmolarity and molarity
mOsm (millisomolar) = index of the concn
or mOsm/L of particles per liter soln
mM (millimolar) = index of concn of
or mM/L molecules per liter soln
150 mM NaCl =
300 mM glucose =
300 mOsm
300 mOsm

22. Estimating Plasma Osmolarity
Plasma is clinically accessible.
Dominated by [Na+] and the associated anions
Under normal conditions, ECF osmolarity can be roughly estimated as: POSM = 2 [Na+]p mOSM 54 54

23. Isotonic and Isosmotic
Isotonic Isosmotic
150 mM NaCl Yes Yes
300 mOsm NaCl Yes Yes
0.9% NaCl Yes Yes
300 mM glucose Yes Yes
300 mOsm glucose Yes Yes
5% glucose Yes Yes
300 mM urea
300 mOsm urea No
Yes No
Yes 54 54

24. higher permeability = more transient the change
Steady-state cell volume is dependent upon the concentration of impermeant particles in the extracellular fluid (e.g. Na+, K +, protein-)
Permeant particles cause only transient changes in cell volume (e.g. urea, glycerol)
Time course of the change in cell volume is dependent on the permeability of the particle
higher permeability = more transient the change
urea > glycerol

25. Example: Shrink then swell Swell Shrink Time course??
300 mOsm
NaCl
Swell
Shrink
Time course??
200 mOsm glycerol
200 mOsm NaCl
Shrink then swell

26. Example:
300 mOsm
NaCl
Swell
Shrink
No change??
200 mOsm
Urea

27. Clinical Abnormalities of Fluid Volume Regulation
Hypernatremia (increased plasma Na):
increased water loss
excessive sweat loss
central or nephrogenic diabetes insipidus
**decreased ADH secretion or responsiveness to ADH
Hyponatremia (decreased plasma Na):
large water ingestion
Syndrome of Inappropriate ADH Secretion (SIADH) **too much ADH leads to water retention, hyponatremia, and excretion of concentrated urine. 66 66

28. Active Transport Primary Active Transport Secondary Active Transport
molecules are “pumped” against a concentration
gradient at the expense of energy (ATP)
– direct use of energy
Secondary Active Transport
transport is driven by the energy stored in the
concentration gradient of another molecule (Na+)
– indirect use of energy

29. Primary Active Transport
1. Na+/K+ ATPase
carrier protein located on the plasma membrane of
all cells
plays an important role in regulating osmotic balance
by maintaining Na+ and K+ balance (inhibition by
ouabain causes cells to swell and burst!)
requires one to two thirds of cells energy!

30. Transport is electrogenic but contributes
 subunit
100,000 MW
binds ATP, 3 Na+, and 2 K+
 subunit
55,000 MW
function ???
Figure 4-11; Guyton & Hall
Transport is electrogenic but contributes
less than 10% to the membrane potential

31. 2. Ca2+ ATPase
present on the cell membrane and the sarcoplasmic
reticulum
maintains a low cytosolic Ca2+ concentration
3. H+ ATPase
found in parietal cells of gastric glands (HCl secretion)
and intercalated cells of renal tubules (controls blood
pH)
concentrates H+ ions up to 1 million-fold

32. Saturation Energetics similar to facilitated diffusion
rate limited by Vmax of the transporters
Energetics
up to 90% of cell energy expended for active
transport!

33. Secondary Active Transport
- co-transport and counter-transport -
Co-transport (co-porters): substance is transported in the same direction as the “driver” ion (Na+)
Examples:
outside
Na+ AA
gluc
2 HCO3-
inside

34. 2. Counter-transport (anti-porters): substance is transported in the opposite direction as the “driver” ion (Na+)
Examples:
outside
Na+
Ca2+ H+
Cl-/H+
Na+/HCO3-
inside

35. Q: How do cardiac glycosides increase cardiac contractility?
Glycosides (eg. digoxin) inhibit the Na/K ATPase…
increase intracellular Na+
decrease Na+ gradient
decrease Na+/Ca2+ counter-transport
increase intracellular Ca2+ 45

36. Q: How do cardiac glycosides increase cardiac contractility?
Na+
Na+ K+
Na+
Digoxin has been a cornerstone for the treatment of heart failure for decades and is the only oral inotropic support agent currently used in clinical practice.
Ca++ 45

37. Transcellular Transport of Glucose / AA
extracellular
fluid
lumen
epithelium
low
high
low AA AA AA
Na+
Na+ K+
glucose
glucose
glucose
Na+
Na+ K+