CHAPTER 7

MEMBRANE STRUCTURE AND FUNCTION

OVERVIEW

FLUID MOSAICS

CONCEPT 7.1

Phospholipids are amphipathic:Two regions.

Fluid mosaic model: phospholipids + various proteins embedded

Freeze-fracture supported theory.

FLUIDITY OF MEMBRANES

Movement

Drifting

Flip-flop

Temperature and type of fatty acids affect fluidity.

Cholesterol molecules help maintain fluidity.

MEMBRANE PROTEINS

Variety of proteins.

Determine most of membrane’s specific functions.

SIX MAJOR FUNCTIONS (1-3)

SIX MAJOR FUNCTIONS (4-6)

SIDEDNESS OF MEMBRANESMembranes have distinct inside and outside determined when build by ER and Golgi

SELECTIVE PERMEABILITY

CONCEPT 7.2

PERMEABILITY

Material exchange controlled by membrane.

Hydrophobic (nonpolar) dissolve in lipid bilayer and pass through rapidly.

Polar molecules (sugars) do not cross easily.

TRANSPORT PROTEINS

Allow passage of hydrophilic substances.

Channel proteins – hydrophilic channel used as a tunnel.

Aquaporins – facilitate passage of water.

Carrier proteins – change shape to move (specific for substance it moves.)

PASSIVE TRANSPORT

CONCEPT 7.2

Down concentration gradient.

OSMOSIS

Diffusion of water.

Moves from area of lower solute concentration to an area of higher solute concentration.

TONICITY

The ability of a solution to cause a cell to gain or lose water.

TONICITY

Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Osmoregulation, the control of water balance

Facilitated diffusion.

ACTIVE TRANSPORT

CONCEPT 7.4

ACTIVE TRANSPORT

Needs energy.

Sodium-potassium pump is an example.

Fig. 7-16-1

EXTRACELLULAR

FLUID [Na+] high [K+] low

Na+

Na+

Na+ [Na+] low[K+] high CYTOPLASM

Cytoplasmic Na+ binds to

the sodium-potassium pump. 1

Na+ binding stimulatesphosphorylation by ATP.

Fig. 7-16-2

Na+

Na+

Na+

ATP P

ADP

2

Fig. 7-16-3

Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.

Na+

P

Na+ Na+

3

Fig. 7-16-4

K+ binds on theextracellular side andtriggers release of thephosphate group.

P P

K+

K+

4

Fig. 7-16-5

Loss of the phosphaterestores the protein’s originalshape.

K+

K+

5

Fig. 7-16-6

K+ is released, and thecycle repeats.

K+

K+

6

MEMBRANE POTENTIAL

Voltage difference of positive and negative ions.

Electrochemical gradient.

Electrogenic pump: generates voltage across membrane.

Cotransport

BULK TRANSPORT

CONCEPT 7.5

EXOCYTOSIS

Large release of materials in a transport vesicle.

ENDOCYTOSIS

Taking in large molecules by forming vesicles.

Three Types:

• Phagocytosis• Pinocytosis• Receptor-mediated endocytosis.