1. Chapter 3 Membrane Physiology Sections 3.1-3.3

2. 3.1 Membrane Structure and Composition
Plasma membrane
Encloses the intracellular contents
Selectively permits specific substances to enter or leave the cell
Responds to changes in cell’s environment
Trilaminar structure under electron microscopy 2

3. Cell 1 Plasma membranes Intercellular space Cell 2
FIGURE 3-1 Trilaminar appearance of a plasma membrane in an electron micrograph. Depicted are the plasma membranes of two adjacent cells. Note that each membrane appears as two dark layers separated by a light middle layer.
Intercellular
space
Cell 2
Figure 3-1 p71

4. 3.1 Membrane Structure and Composition
The plasma membrane is a fluid lipid bilayer embedded with proteins.
Phospholipids
Most abundant membrane component
Head contains charged phosphate group (hydrophilic)
Two nonpolar fatty acid tails (hydrophobic)
Assemble into lipid bilayer with hydrophobic tails in the center and hydrophilic heads in contact with water
Fluid structure as phospholipids are not held together by chemical bonds 4

5. 3.1 Membrane Structure and Composition5

6. Head (negatively charged, polar, hydrophilic)
Choline
Head (negatively charged, polar, hydrophilic)
Phosphate
Glycerol
Tails (uncharged, nonpolar, hydrophobic)
Fatty acids
FIGURE 3-2 Structure and organization of phospholipid molecules in a lipid bilayer. (a) Phospholipid molecule. (b) In water, phospholipid molecules organize themselves into a lipid bilayer with the polar heads interacting with the polar water molecules at each surface and the nonpolar tails all facing the interior of the bilayer. (c) An exaggerated view of the plasma membrane enclosing a cell, separating the ICF from the ECF.
(a) Phospholipid molecule
Figure 3-2a p71 6

7. (b) Organization of phospholipids into a bilayer in water
ECF (water)
Polar heads
(hydrophilic)
Nonpolar tails
(hydrophobic)
Lipid bilayer
Polar heads
(hydrophilic)
FIGURE 3-2 Structure and organization of phospholipid molecules in a lipid bilayer. (a) Phospholipid molecule. (b) In water, phospholipid molecules organize themselves into a lipid bilayer with the polar heads interacting with the polar water molecules at each surface and the nonpolar tails all facing the interior of the bilayer. (c) An exaggerated view of the plasma membrane enclosing a cell, separating the ICF from the ECF.
ICF (water)
(b) Organization of phospholipids
into a bilayer in water
Figure 3-2b p71

8. (c) Separation of ECF and ICF by the lipid bilayer
Intracellular
fluid
FIGURE 3-2 Structure and organization of phospholipid molecules in a lipid bilayer. (a) Phospholipid molecule. (b) In water, phospholipid molecules organize themselves into a lipid bilayer with the polar heads interacting with the polar water molecules at each surface and the nonpolar tails all facing the interior of the bilayer. (c) An exaggerated view of the plasma membrane enclosing a cell, separating the ICF from the ECF.
Extracellular
fluid
(c) Separation of ECF and ICF by the lipid bilayer
Figure 3-2c p71

9. 3.1 Membrane Structure and Composition
The plasma membrane is a fluid lipid bilayer embedded with proteins.
Cholesterol
Placed between phospholipids to prevent crystallization of fatty acid chains
Helps stabilize phospholipids’ position
Provides rigidity, especially in cold temperatures
Cold-induced rigidity is countered in some poikilotherms by enriching membrane lipids with polyunsaturated fatty acids 9

10. 3.1 Membrane Structure and Composition
Membrane proteins
Integral proteins are embedded in the lipid bilayer
Have hydrophilic and hydrophobic regions
Transmembrane proteins extend through the entire thickness of the membrane
Peripheral proteins are found on inner or outer surface of membrane
Polar molecules
Anchored by weak chemical bonds to polar parts of integral proteins or phospholipids 10

11. 3.1 Membrane Structure and Composition
Two models of membrane structure
Fluid mosaic model
Membrane proteins float freely in a “sea” of lipids
Membrane-skeleton fence model
Mobility of membrane proteins is restricted by the cytoskeleton 11

12. 3.1 Membrane Structure and Composition
Specialized functions of membrane proteins
Channels
Carriers
Receptors
Docking-marker acceptors
Enzymes
Cell-adhesion molecules (CAMs)
Self-identity markers 12

13. 3.1 Membrane Structure and Composition
Membrane carbohydrates
Located only on outer surface of membrane
Short-chain carbohydrates bound to membrane proteins (glycoproteins) or lipids (glycolipids)
Important roles in self-recognition and cell-to-cell interactions 13

14. 3.1 Membrane Structure and Composition14

15. Extracellular fluid
Dark line
Integral proteins
Carbohydrate chain
Appearance using an electron microscope
Light space
Phospholipid molecule
Dark line
Glycolipid
Glycoprotein
Receptor protein
FIGURE 3-3 Fluid mosaic model of plasma membrane structure. The plasma membrane is composed of a lipid bilayer embedded with proteins. Integral proteins extend through the thickness of the membrane or are partially submerged in the membrane, and peripheral proteins are loosely à ached to the surface of the membrane. Short carbohydrate chains à ach to proteins or lipids on the outer surface only.
Lipid bilayer
Cholesterol molecule
Leak channel protein
Gated channel protein
Peripheral proteins
Cell adhesion molecule (linking microtubule to membrane)
Carrier protein
Microfilament
of cytoskeleton
lntracellular fluid
Figure 3-3 p72 15

16. ANIMATION: Cell membranes
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17. 3.2 Unassisted Membrane Transport
The plasma membrane is selectively permeable
Permeability across the lipid bilayer depends on:
High lipid solubility
Small size
Force is needed to produce the movement of particles across the membrane
Passive forces do not require the cell to expend energy
Active forces require cellular energy (ATP) 17

18. 3.2 Unassisted Membrane Transport
Diffusion
Random collisions and intermingling of molecules as a result of their continuous, thermally induced random motion
Net movement of molecules from an area of higher concentration to an area of lower concentration
Equilibrium is reached when there is no concentration gradient and no net diffusion 18

19. 3.2 Unassisted Membrane Transport19

20. Diffusion from area A to area B
FIGURE 3-4 Diffusion. (a) Diffusion down a concentration gradient. (b) Dynamic equilibrium, with no net diffusion occurring.
Diffusion from area B to area A
Net diffusion
(a) Diffusion
Figure 3-4a p75 20

21. Diffusion from area A to area B
FIGURE 3-4 Diffusion. (a) Diffusion down a concentration gradient. (b) Dynamic equilibrium, with no net diffusion occurring.
Diffusion from area A to area B
Diffusion from area B to area A
No net diffusion
(b) Dynamic equilibrium
Figure 3-4b p75 21

22. 3.2 Unassisted Membrane Transport
Fick’s law of diffusion
The rate at which diffusion occurs depends on:
Concentration gradient
Permeability
Surface area
Molecular weight
Distance
Temperature 22

23. 3.2 Unassisted Membrane Transport
The movement of ions across the membrane is affected by their electrical charge.
A difference in charge between two adjacent areas produces an electrical gradient.
An electrical gradient passively induces ion movement -- conduction
Only ions that can permeate the plasma membrane can conduct down this gradient.
The simultaneous existence of an electrical gradient and concentration gradient is called an electrochemical gradient. 23

24. 3.2 Unassisted Membrane Transport24

25. Positively charged area Negatively charged area– –
Cations (positively charged ions) attracted toward negative area
Anions (negatively charged ions) attracted toward positive area
FIGURE 3-6 Movement along an electrical gradient.
Figure 3-6 p77 25

26. 3.2 Unassisted Membrane Transport
Osmosis
Water moves across a membrane by osmosis, from an area of lower solute concentration to an area of higher solute concentration.
Driving force is the water concentration gradient
Hydrostatic pressure opposes osmosis
Osmotic pressure is the pressure required to stop the osmotic flow
Osmotic pressure is proportional to the concentration of nonpenetrating solute 26

27. 3.2 Unassisted Membrane Transport27

28. Membrane (permeable to H2O but impermeable to solute)
Side 1
Side 2
Osmosis
H2O
Solute
Pure water
Lower H2O concentration,
higher solute concentration
H2O moves from side 1 to side 2
down its concentration gradient
Solute unable to move from side 2 to
side 1 down its concentration gradient
Side 1
Side 2
Hydrostatic
(fluid)
pressure
difference
Original
level of
solutions
FIGURE 3-8 Osmosis when pure water is separated from a solution containing a nonpenetrating solute.
Osmosis
Hydrostatic
pressure
• Water concentrations not equal
• Solute concentrations not equal
• Tendency for water to diffuse by osmosis into side 2 is exactly balanced by opposing tendency for hydrostatic pressure difference to push water into side 1
• Osmosis ceases; equilibrium exists
Figure 3-8 p79

29. 3.2 Unassisted Membrane Transport
Colligative properties of solutes depend solely on the number of dissolved particles in a given volume of solution
Osmotic pressure
Elevation of boiling point
Depression of freezing point
Reduction of vapor pressure 29

30. 3.2 Unassisted Membrane Transport
Tonicity refers to the effect of solute concentration on cell volume
Isotonic solution
Same concentration of nonpenetrating solutes as in normal cells
Cell volume remains constant
Hypotonic solution
Lower solute concentration than in normal cells
Cell volume increases, perhaps to the point of lysis
Hypertonic solution
Higher solute concentration than in normal cells
Cell volume decreases, causing crenation 30

31. 3.2 Unassisted Membrane Transport31

32. Intracellular fluid: 300 mOsm nonpenetrating solutes
Normal cell volume
Intracellular fluid: 300 mOsm nonpenetrating solutes
H2O
H2O
FIGURE 3-9 Tonicity and osmotic water movement.
300 mOsm
nonpenetrating solutes
200 mOsm
nonpenetrating solutes
400 m Osm
nonpenetrating solutes
No net movement of water; no change in cell volume.
Water diffuses into cells; cells swell.
Water diffuses out of cells; cells shrink.
(a) Isotonic conditions
(b) Hypotonic conditions
(c) Hypertonic conditions
Figure 3-9 p80 32

33. 3D ANIMATION: Osmosis 33

34. 3.3 Assisted Membrane Transport
Phospholipid bilayer is impermeable to:
Large, poorly lipid-soluble molecules (proteins, glucose, and amino acids)
Small, charged molecules (ions)
Mechanisms for transporting these molecules into or out of the cell
Channel transport
Carrier-mediated transport
Vesicular transport 34

35. 3.3 Assisted Membrane Transport
Channel transport
Transmembrane proteins form narrow channels
Highly selective
Permit passage of ions or water (aquaporins)
Gated channels can be open or closed
Leak channels are open at all times
Movement through channels is faster than carrier-mediated transport 35

36. 3.3 Assisted Membrane Transport36

37. Outside cell Water molecule Aquaporin Cytosol Lipid bilayer membrane
FIGURE 3-10 Aquaporins. (a) Structure of an aquaporin forming a channel in a membrane. The channel is lined with hydrophilic amino acids that attract water molecules in single file. (b) One of the Nobel Prize–winning experiments from Peter Agre’s laboratory. Incubation in hypotonic buffer fails to cause swelling of a control frog oocyte (leȀ ), due to low water permeability. In contrast, an oocyte injected with mammalian aquaporin RNA (right) exhibits high water permeability due to production of aquaporin proteins, and the cell has exploded.
Cytosol
Figure 3-10 p82 37

38. FIGURE 3-10 Aquaporins. (a) Structure of an aquaporin forming a channel in a membrane. The channel is lined with hydrophilic amino acids that attract water molecules in single file. (b) One of the Nobel Prize–winning experiments from Peter Agre’s laboratory. Incubation in hypotonic buffer fails to cause swelling of a control frog oocyte (leȀ ), due to low water permeability. In contrast, an oocyte injected with mammalian aquaporin RNA (right) exhibits high water permeability due to production of aquaporin proteins, and the cell has exploded.
Figure 3-10 p82 38

39. 3.3 Assisted Membrane Transport
Carrier-mediated transport
Transmembrane proteins that can undergo reversible changes in shape
Binding sites can be exposed to either side of membrane
Transport small water-soluble substances
Facilitated diffusion or active transport 39

40. 3.3 Assisted Membrane Transport
Characteristics of carrier-mediated transport systems
Specificity -- each carrier protein is specialized to transport a specific substance
Saturation -- limit to the amount of a substance that a carrier can transport in a given time (transport maximum or Tm)
Competition -- closely related compounds may compete for the same carrier 40

41. 3.3 Assisted Membrane Transport41

42. concentration gradient (facilitated diffusion)
Simple diffusion
down concentration
gradient
Carrier-mediated
transport down
concentration gradient
(facilitated diffusion)
Rate of transport of molecule into cell
FIGURE 3-12 Comparison of carrier-mediated transport and simple diffusion down a concentration gradient. With simple diffusion of a molecule down its concentration gradient, the rate of transport of the molecule into the cell is directly proportional to the extracellular concentration of the molecule. With carrier-mediated transport of a molecule down its concentration gradient, the rate of transport of the molecule into the cell is directly proportional to the extracellular concentration of the molecule until the carrier is saturated, at which time the rate of transport reaches the transport maximum (Tm). After Tm is reached, the rate of transport levels off despite further increases in the ECF concentration of the molecule.
Low
High
Concentration of transported molecules in ECF
Figure 3-12 p83

43. 3.3 Assisted Membrane Transport
Facilitated diffusion
Passive carrier-mediated transport from high to low concentration
Does not require energy
Example: Glucose transport into cells 43

44. 3.3 Assisted Membrane Transport
Facilitated diffusion
Molecule to be transported attaches on binding site on protein carrier
Carrier protein changes conformation, exposing bound molecule to the other side of the membrane (lower concentration side)
Bound molecule detaches from the carrier
Carrier returns to its original conformation (binding site on higher concentration side) 44

45. 3.3 Assisted Membrane Transport45

46. 1 4 2 3 Carrier protein takes conformation in which solute
binding site is exposed to
region of higher concentration. 1
Concentration
gradient
ECF
Solute molecule
to be transported
(High)
Carrier protein
Plasma
membrane
Binding site
(Low)
ICF
Direction of
transport
Transported
solute is released
and carrier protein
returns to
conformation in
step 1. 4
Solute
molecule binds to carrier protein. 2
FIGURE 3-11 Model for facilitated diffusion, a passive form of carrier-mediated transport.
Carrier protein changes
conformation so that binding
site is exposed to region of
lower concentration. 3
Figure 3-11 p82

47. ANIMATION: Active and Facilitated Diffusion
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48. ANIMATION: Passive transport
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49. 3.3 Assisted Membrane Transport
Active transport
Carrier-mediated transport that moves a substance against its concentration gradient
Requires energy
Primary active transport
Energy is directly required
ATP is split to power the transport process
Secondary active transport
ATP is not used directly
Carrier uses energy stored in the form of an ion concentration gradient built by primary active transport 49

50. 3.3 Assisted Membrane Transport50

51. 1 5 2 4 3 Concentration gradient ECF Carrier protein splits ATP
into ADP plus phosphate.
Phosphate group binds to
carrier, increasing affinity
of its binding site for ion. 1
Plasma
membrane
Binding
site
Carrier
protein
ICF
Ion to be
transported
When binding
site is free,
carrier reverts to
its original shape. 5
Ion to be
transported binds to carrier on
low-concentration
side. 2
FIGURE 3-13 Model for active transport. The energy of ATP is required in the phosphorylation–dephosphorylation cycle of the carrier to transport the molecule uphill from a region of low concentration to a region of high concentration.
Carrier releases
ion to side of higher
concentration.
Phosphate group is
also released. 4
In response to ion binding, carrier changes conformation so that binding site is exposed
to opposite side of membrane.The change in shape also reduces affinity of site for ion. 3
Direction
of transport
Figure 3-13 p85

52. ANIMATION: Active Transport
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53. 3.3 Assisted Membrane Transport
Na+-K+ ATPase pump
Pumps 3 Na+ out of cell for every 2 K+ in
Splits ATP for energy
Phosphorylation induces change in shape of transport protein
Maintains Na+ and K+ concentration gradients across the plasma membrane
Helps regulate cell volume 53

54. 3.3 Assisted Membrane Transport54

55. 3.3 Assisted Membrane Transport
Secondary active transport
Simultaneous transport of a nutrient molecule and an ion across the plasma membrane by a cotransport protein
Nutrient molecule is transported against its concentration gradient
Driven by simultaneous transport of an ion along its concentration gradient
Example: Cotransport of glucose and Na+ across the luminal membrane of intestinal epithelial cells 55

56. 3.3 Assisted Membrane Transport56

57. 1 2 3 Lumen of intestine Na+ high Glucose low Na+ high Glucose high
Na+ low
Na+ low
Glucose
high
Na-
high
Epithelial cell
lining small
intestine
Glucose low
Na+ low
Glucose high
FIGURE 3-15 Symport of glucose. Glucose is transported across intestinal and kidney cells against its concentration gradient by means of secondary active transport mediated by the sodium and glucose cotransporter (SGLT) at the cells’ luminal membrane.
Na+–K+ pump
(active)
GLUT
(passive)
Na-
high
Glucose low
Blood vessel
Primary Active Transport
establishes Na+
concentration
gradient from
lumen to cell,
which drives
Secondary Active Transport
creating glucose
concentration
gradient from cell
to blood used for
Facilitated Diffusion
Na+–K+ pump uses energy
to drive Na+ uphill out of cell. 1
SGLT uses Na+ concentration
gradient to simultaneously move
Na+ downhill and glucose uphill
from lumen into cell. 2
GLUT passively moves
glucose downhill out of cell
into blood. 3
Figure 3-15 p88

58. 3.3 Assisted Membrane Transport
Vesicular transport
Transport between ICF and ECF of large particles wrapped in membrane-bound vesicles
Endocytosis -- incorporates outside substances into cell
Exocytosis -- releases substances into the ECF
The rate of endocytosis and exocytosis must be balanced to maintain a constant membrane surface area and cell volume
Caveolae may play a role in transport of substances and cell signaling 58