1. Membrane Transport
Chapter 6

2. Cells Need to Exchange Materials with the Extracellular Fluid
Take in nutrients O2
energy substrates
building materials
cofactors
Dispose of wastes
CO2
Urea

3. Cells Must Control Movements of Materials
Need to maintain complexity inside the cell
Must regulate type and amount of material entering and leaving the cell

4. Plasma Membrane Fig. 3.2 Selectively Permeable
some materials can pass readily, others cannot
Fig. 3.2

5. Membrane Permeability
Size
the smaller the particle, the more permeable
small molecules (O2, CO2, H2O) can
large molecules (protein, DNA) cannot
Lipid Solubility
YES: non-polar molecules (O2, cholesterol),
NO: charged atoms/molecules (Na+, Cl-, HCO3-), large polar molecules (glucose)

6. Membrane Transport Requires: Passive Transport Active Transport
Permeability of the membrane
A driving force
Passive Transport
movement of particles along a gradient
does not require energy expenditure
Active Transport
movement of particles against a gradient
requires energy expenditure

7. Some Important Terms Solution Solute Solvent Concentration
mixture of two(+) substances that is uniform at the molecular level
Solute
particles (molecules or ions) present in a solution
Solvent
phase (generally a liquid) in which particles are dissolved (H2O)
Concentration
amt. solute dissolved in a given volume of solution or solvent

8. Passive Membrane Transport
Simple Diffusion
movement of particles along a concentration gradient
Osmosis
diffusion of water across a semi-permeable membrane
Facilitated Diffusion
movement of particles along a concentration gradient through a carrier protein

9. Diffusion
Molecules and ions in a solution are in a constant state of motion
Tend to diffuse - become evenly dispersed throughout the solution
Diffusion = movement of particles in a solution due to random thermal motion
Fig. 6.2

10. Diffusion and Concentration
Solute particles diffuse from regions of high concentration to regions of low concentration
“Down” a concentration gradient (high  low)
Continues until equilibrium is reached
Fig. 6.2

11. Gas Diffusion in Cells
Fig. 6.3

12. Diffusion and Ions Ions = charged particles
Like charges repel, opposites attract
Differences in charge between two areas = electrical gradient
Ions move along an electrical gradient until charges are balanced

13. Diffusion and Ions

14. Diffusion and Ions
Membrane impermeable to (-)
NOTE: Electrical equilibrium may require movement against the concentration gradient

15. Electrochemical Gradient
Net movement of ions due to the combined effects of the electrical gradient and the concentration gradient
Equilibrium may be achieved across a membrane at a point of unequal concentrations and charges

16. Diffusion and Membrane Transport
Lipid bilayer determines what substances can readily pass through the membrane
if bilayer is permeable, substance can diffuse through
if bilayer is impermeable, no diffusion even if gradient exists

17. Diffusion and Membrane Transport
Substances to which the membrane is impermeable must pass via alternate means
Facilitated Diffusion - movement across the cell membrane through a carrier protein
Channel Proteins - allow flow of ions across the cell membrane
Both allow regulation of flow
Fig. 6.14
Fig. 6.4

18. Factors Affecting Rate of Diffusion
magnitude of the gradient
 gradient,  rate
permeability of the membrane to the substance
 permeability,  rate
temperature of the solution
 temperature,  rate
the surface area of the membrane through which diffusion is taking place
 SA,  rate

19. Osmosis
Net diffusion of water across a semi-permeable membrane
diffusion of the solvent, not the solute
Figs. 6.5, 6.6

20. Osmosis For osmosis to occur:
the membrane must be permeable to water and impermeable to at least one of the solutes in the solution
there must be a difference in solute concentration between the two sides of the membrane

21. Osmotic Pressure
Osmosis results in changes in volume on either side of the membrane
Changes in volume could be stopped by applying an equal and opposite force
would effectively stop osmosis
Figs. 6.6, 6.7

22. Osmotic Pressure
Osmotic pressure = amount of pressure that would have to be exerted in order to prevent osmosis
measure of how strongly a solution “draws water into itself”
 [solute] ,  osmotic pressure of the solution

23. Facilitated Diffusion
Many molecules large and/or polar molecules are needed for metabolism
cannot pass through lipid bilayer
Shuttled across membrane by carrier proteins
Facilitated diffusion – carrier-mediated transport along the conc. gradient
no energy expended by the cell
Fig. 6.14

24. Properties of Carrier Proteins in Facilitated Diffusion
Specificity – transport only one or a few different substances
possess special bind sites
Saturation – limited rate of transport
at high concentrations no further increase in transport rate will accompany increases in the conc. gradient
Reversible - direction of movement across membrane is influenced by solute concentration
If [Solute]out > [Solute]in mvmt is from out in
If [Solute]in > [Solute]out mvmt is from in  out
If [Solute]out = [Solute]in net diffusion = 0
Fig. 6.13

25. Active Membrane Transport
Requires energy expenditure by the cell (use of ATP)
Active Carrier Mediated Transport - use membrane proteins to move materials against a gradient
Vesicular Transport - move large amounts of material into and out of the cell

26. Active Carrier-Mediated Transport
A carrier-mediated transport system that moves a substance against its EC gradient across a cell membrane
requires ATP usage
pumps substances from low to high concentrations

27. Example: Ca2+pump Fig. 6.16 Ca2+ binds to protein
ATP breakdown causes protein to change shape AND affinity for Ca2+
Ca+ ejected on opposite side of the membrane
Fig. 6.16

28. Example: Na+/ K+ pump Fig. 6.17 Pumps Na+ out and K+ in
3 Na+ out per 2 K+ in
Generates concentration gradients
Generates electrical gradient
Fig. 6.17

29. ACMT vs. Facilitated Diffusion
Similarities
Carrier Protein Mediated
Exhibit Chemical Specificity
Differences
ACMT requires energy (ATP)
Binding affinity of carrier changes in ACMT
does not change for facilitated diffusion - gradient determines net movement

30. Types of Active Carrier-Mediated Transport
Primary Active Transport
hydrolysis (breakdown) of ATP directly required for the function of the carrier
e.g. Ca2+ pump, Na+/K+ pump
Figs & 6.17

31. Types of Active Carrier-Mediated Transport
Secondary Active Transport (Coupled Transport)
energy needed for movement of a substance against gradient is provided by the movement of another substance along its gradient
Example: Na+-glucose cotransport
indirectly requires ATP via Na+/K+ pump (establishes gradient)
Figs & 6.19

32. Vesicular Transport Fig. 6.20
Transport of vesicle contents across cell membranes
“bulk transport” - move large amounts of material
very large molecules can be moved this way
Two types of movement
exocytosis - movement of material out of the cell
hormones, neurotransmitters, etc.
endocytosis - movement of material into the cell
cellular debris, bacteria, etc.
Fig. 6.20