1. The plasma membrane is selectively permeable, allowing passage of some substances and restricting others based on size, charge, shape, and solubility. 2. Transport across the membrane can be passive via diffusion or filtration, requiring no ATP, or active via carrier-mediated transport or vesicular transport, requiring ATP to move against gradients. 3. Diffusion is the passive movement of particles from high to low concentration down a gradient, and can occur directly through the lipid bilayer or through channel proteins.

1. Transport Processes
AP 151

2. Membrane Transport
• Plasma membrane is selectively permeable
– Impermeable membrane - membrane though which nothing can
pass
– Freely permeable membrane - any substance can pass through it
– Selectively permeable membrane - permits free passage of some
materials and restricts passage of others
• Distinction may be based on size, electrical charge, molecular shape,
lipid solubility
• Cells differ in their permeabilities; depending on
– what lipids and proteins are present in the membrane and
– how these components are arranged.

3. Membrane Permeability
• Passage across the membrane is either passive or active
– Passive transport requires no ATP
• movement down concentration gradient
• filtration and simple diffusion
–Active transport requires ATP
• movement against concentration gradient
• carrier mediated
• vesicular transport

4. Types of Transport Processes
• Diffusion
– results from random motion of particles (ions, molec.)
– is a passive process
• Carrier-mediated transport
– Requires the presence of specialized integral proteins
– Can be passive or active
• Vesicular transport
– Movment of materials with small membranous sacs, or
vesicles
– Always an active process

5. Membrane Permeability
• Diffusion through lipid bilayer
– Nonpolar, hydrophobic substances diffuse through
lipid layer; these are “lipid soluble” or lipophilic
(fat-loving) substances
• Diffusion through channel proteins
– water and charged hydrophilic solutes diffuse
through channel proteins; these are lipid insoluble
or lipophobic (fat fearing) substances
• Cells control permeability by regulating number of
channel proteins

6. Simple Diffusion
• Net movement of particles from area of high
concentration to area of low concentration
– due to their constant, random motion
– Difference between the high and low
concentrations is a concentration gradient
– Diffusion tends to eliminate the gradient
– Also known as movement “down the concentra-
tion gradient”

7. Diffusion
• Examples:
– Scent of fresh flowers, drop of ink coloring
a glass of water, movement of oxygen and
CO2 through cell membranes
• Simple diffusion – nonpolar and lipid-
soluble substances
– Diffuse directly through the lipid bilayer
– Diffuse through channel proteins

8. Factors that Influence Diffusion Rates
• Distance -
– The shorter the distance, the more quickly [ ] gradients are
eliminated
– Few cells are father than 125 microns from a blood vessel
• Molecular Size
– Ions and small molecules diffuse more rapidly
• Temperature -
↑ temp., ↑ motion of particles
• Steepness of concentrated gradient -
– The larger the [ ] gradient, the faster diffusion proceeds
• Membrane surface area -
– The larger the area, the faster diffusion proceed

9. Diffusion Across Membranes
• Simple Diffusion
– Lipophilic substances can enter cells easily because
they diffuse through the lipid portion of the
membrane
• Examples are fatty acids, steroids, alcohol, oxygen, carbon
dioxide, and urea,
• Channel-Mediated Diffusion
– Membrane channels are transmembrane proteins
• Only 0.8 nm in diameter
– Used by ions, very small water-soluble compounds
– Much more complex than simple diffusion
• Are there enough channels available?
• Size and charge of the ion affects which channels it can
pass through

10. Diffusion Through the Plasma
Membrane
Figure 3.7

11. Effect of Membrane Permeability on
Diffusion
Figure 3.8a

12. Osmosis: A Special Case of Diffusion
• Each solute in the intra- and extracellular fluids
diffuses as if it were the only material in solution.
– From more to less, i.e., down the [ ] gradient
– Some into the cytosol, others out of the cytosol
– Yet, total concentration of ions and molecules on either
side of the membrane stays the same
– This equilibrium persists because a typical cell membrane
is freely permeable to water.
• Whenever a solute concentration gradient exist, a
concentration gradient for water also exists.
– Thus, the higher the solute concentration, the lower the
water concentration.

13. Osmosis - By Definition
• Movement of water
• Across a selectively permeable membrane
• Down its concentration gradient (from high to low
concentration)
• Toward the solution containing the higher solute
concentration
– This solution has a lower water concentration
– Continues until water concentrations and solute concen-
trations are the same on either side of the membrane

14. Effect of Membrane Permeability on
Diffusion and Osmosis
Figure 3.8b

15. Osmolarity and Tonicity
• Mole - the gram molecular weight of a substance
– 1 mole of Glucose =180; 1 mole of NaCl = 58.5
• Molarity - the number of moles of solute per liter of solution
– 1.0 M glucose contains 180 g/L; 1.0 M NaCl contains 58.5 g/L
– Most body fluids are less concentrated than 1 M; use mM
(millimolar) or µM (micromolar) concentrations --10-3 and 10-6,
respectively.
• Osmolarity = the total solute concentration in an aqueous solution
– Osmolarity = molarity (mol/L) x # of particles in solutions
• A 1 M Glucose solution = 1 Osmolar (Osm)
• But a 1 M NaCl soln = 2 Osmolar because NaCl dissociates
into 2 particles (Na and Cl) whereas Glucose does not
• A 1 M MgCl2 solution = what osmolarity???? __________
• Physiological solutions are expressed in milliosmoles per liter
(mOsm/L)
– blood plasma = 300 mOsm/L or 0.3 Osm/L

16. Tonicity
• Tonicity - ability of a solution to affect fluid volume and
pressure within a cell
– depends on concentration and permeability of solute
• Isotonic solution
– solution with the same solute concentration as that of the cytosol;
normal saline
• Hypotonic solution
– lower concentration of nonpermeating solutes than that of the cytosol
(high water concentration)
– cells absorb water, swell and may burst (lyse)
• Hypertonic solution
– has higher concentration of nonpermeating solutes than that of the
cytosol (low water concentration)
– cells lose water + shrivel (crenate)

17. Osmosis and Cells
• Important because large volume changes caused by
water movement disrupt normal cell function
• Cell shrinkage or swelling
– Isotonic: cell neither shrinks nor swells
– Hypertonic: cell shrinks (crenation)
– Hypotonic: cell swells (lysis)

18. Effects of Tonicity on RBCs
Hypotonic, isotonic and hypertonic solutions affect the fluid
volume of a red blood cell. Notice the crenated and swollen
cells.

19. Filtration
• Cell membrane works like a sieve
• Depends on pressure difference on either side of
a partition
• Moves from side of greater pressure to lower
• Water and small molecules move through the
pores of the membrane while large molecules
don’t.
• Example: urine formation in the kidneys.

20. Carrier Mediated Transport
• Many molecules cannot enter or leave cell by
diffusion
• CMT utilizes proteins to carry solutes across cell
membrane
• Characteristics of mediated transport:
1. Specificity - each transport protein binds to and
transports only a single type of molecule or ion
2. Competition - results from similar molecules binding
to the same protein.
3. Saturation - rate of movement of molecules is limited
by the number of available transport proteins

21. Membrane Carriers
• Uniporter
– carries only one solute at a time
• Symport
– carries 2 or more solutes simultaneously in same
direction (cotransport)
• Antiport
– carries 2 or more solutes in opposite directions
(countertransport)
• sodium-potassium pump brings in K+ and removes Na+ from
cell
• Any carrier type can use either facilitated
diffusion or active transport

22. 1. When the concentration of x
molecules outside the cell is low, the
Saturation of a transport rate is low because it is
limited by the number of molecules
available to be transported.
Carrier Protein 2. When more molecules are present
outside the cell, as long as enough
carrier proteins are available, more
molecules can be transported; thus,
the transport rate increases.
3. The transport rate is limited by the
number of carrier proteins and the
rate at which each carrier protein
can transport solutes. When the
number of molecules outside the cell
is so large that the carrier proteins
are all occupied, the system is
saturated and the transport rate
cannot increase.

23. CMT: Facilitated Diffusion
• Glucose and amino acids are insoluble in lipids and too
large to fit through membrane channels
• Passive process, i.e. no ATP used
• Solute binds to receptor on carrier protein
– Latter changes shape then releases solute on other side of
membrane
– Substance moved down its concentration gradient

24. CMT: Active Transport
• Uses ATP to move solutes across a membrane
• It is not dependent on a [ ] gradient
– Can move substances against their [ ] gradients -
i.e. from lower to higher concentrations! Wow!
– Allows for greater accumulation of a substance on
one side of the membrane than on the other.
• Carrier proteins utilized called ion or
exchange pumps.
– Ion pumps: actively transport Na+, K+, Ca++, Cl-
– Exchange pumps: Na+-K+ pump

25. Types of Active Transport
Figure 3.11

26. + 6
Sodium-Potassium Pump
K is released and Na+ Extracellular Binding1of cytoplasmic Na+ to the pump
sites are ready to bind fluid protein stimulates phosphorylation by ATP.
Na+ again; the cycle
repeats.
Cytoplasm
2
Phosphorylation causes the
protein to change its shape.
Concentration gradients
of K+ and Na+
3
5
The shape change expels Na+ to the
Loss of phosphate restores the outside, and extracellular K+ binds.
original conformation of the 4
pump protein.
K+ binding triggers release of the
phosphate group.
Figure 3.10

27. Functions of Na -K Pump + +
• Regulation of cell volume
– “fixed anions” attract cations causing osmosis
– cell swelling stimulates the Na+- K+ pump to
↓ ion concentration, ↓ osmolarity and cell swelling
• Heat production (thyroid hormone increase # of
pumps; heat a by-product)
• Maintenance of a membrane potential in all cells
– pump keeps inside negative, outside positive
• Secondary active transport (No ATP used)
– steep concentration gradient of Na+ and K+ maintained
across the cell membrane
– carriers move Na+ with 2nd solute easily into cell
• SGLT saves glucose in kidney

28. • Ions or molecules move in
same (symport) or different
Secondary •
(antiport) direction.
Is the movement of glucose a
symporter example or an
Active Transport •
antiporter example?
This example shows
cotransport of Na+ and
glucose.
– A sodium-potassium
exchange pump maintains
a concentration of Na that
is higher outside the cell
than inside. Active
transport.
– Na moves back into the cell
by a carrier protein that
also moves glucose. The
concentration gradient for
Na provides the energy
required to move glucose
against its concentration
gradient.

29. Vesicular Transport
• Transport large particles or fluid droplets through
membrane in vesicles
– uses ATP
• Exocytosis –transport out of cell
• Endocytosis –transport into cell
– phagocytosis – engulfing large particles
– pinocytosis – taking in fluid droplets
– receptor mediated endocytosis – taking in specific
molecules bound to receptors

30. Vesicular Transport
Endocytosis
• Packaging of extracellular materials in vesicles
at the cell surface
• Involves relatively large volumes of extracellular
material
• Requires energy in the form of ATP
• Three major types
1. Receptor-mediated endocytosis
2. Pinocytosis
3. Phagocytosis

31. Receptor Mediated Endocytosis
• A selective process
• Involves formation of vesicles at surface of
membrane
– Vesicles contain receptors on their membrane
– Vesicles contain specific target molecule in
high concentration
• Clathrin-coated vesicle in cytoplasm
– uptake of LDL from bloodstream
– If receptors are lacking, LDL’s accumulate and
hypercholesterolemia develops

32. Receptor Mediated Endocytosis

33. Vesicular Transport
Pinocytosis or “Cell-Drinking”
• Taking in droplets of ECF
– occurs in all human cells
• Not as selective as ‘receptor-mediated
endocytosis’
• Membrane caves in, then pinches off
into the cytoplasm as pinocytotic vesicle

34. Vesicular Transport
Phagocytosis or “Cell-Eating”
Keeps tissues free of debris and infectious microorganisms.

35. Vesicular Transport: Exocytosis
• Secreting material or replacement of plasma
membrane

36. Passive Membrane Transport
– Review -
Process Energy Source Example
Movement of O2 through
Simple diffusion Kinetic energy
membrane
Facilitated diffusion Kinetic energy Movement of glucose into cells
Osmosis Kinetic energy Movement of H2O in & out of cells
Filtration Hydrostatic pressure Formation of kidney filtrate

37. Active Membrane Transport –
Review
Process Energy Source Example
Movement of ions across
Active transport of solutes ATP
membranes
Exocytosis ATP Neurotransmitter secretion
Endocytosis ATP White blood cell phagocytosis
Fluid-phase endocytosis ATP Absorption by intestinal cells
Receptor-mediated Hormone and cholesterol
ATP
endocytosis uptake
Endocytosis via caveoli ATP Cholesterol regulation
Endocytosis via coatomer Intracellular trafficking of
ATP
vesicles molecules