EPOB 1210 Lecture 15, 1996
Membranes, part 2

This has been the 1208th visit to this site since Oct. 1, 1996.

Word roots:
hypo (G) = under as in hypodermic
hyper (G) = over as in hyperactive
osmo (G) = pushing as in osmosis

MEMBRANES (cont.)
TRAFFIC OF SMALL MOLECULES across membranes.

Selective permeability of membranes:
1) Nonpolar (hydrophobic) molecules dissolve in lipid membrane and pass through relatively easily. The smaller they are the faster. (e.g. O2)

2) Polar (hydrophilic) molecules will pass through a lipid bilayer if they are small enough and not charged (e.g. H2O and CO2).

Ions and large polar molecules will NOT pass.

DIFFUSION

(Fig. 8.8)

CONCENTRATION GRADIENT = Concentration change in one direction.
NET MOVEMENT = Overall movement down the gradient even though molecules are moving randomly.

DIFFUSION = The NET MOVEMENT of a substance down a CONC. GRADIENT

= movement of molecules from an area of higher concentration to an area of lower conc. (remember kinetic energy)
Continues until a dynamic equilibrium is reached.

Many molecules move across membranes via diffusion.
This is one type of PASSIVE TRANSPORT
PASSIVE TRANSPORT = Cell does not expend energy,
Energy is from potential energy of conc. gradient.
OSMOSIS = diffusion of H2O across a semi-permeable membrane.
Water diffuses down its concentration gradient.
FIG. 8.9

a HYPOTONIC solution has a lower conc. of solutes than a HYPERTONIC solution.

HOW DO CELLS REACT TO SOLUTE CONCENTRATION?

FIG. 8.10

HOW DO SOLUTES GET ACROSS MEMBRANES?

IN biological membranes Ions and large polar molecules are transported across the membrane via TRANSPORT PROTEINS
FACILITATED DIFFUSION = Diffusion with the help of a transport protein.

Passive process----important for polar molecules
FIG. 8.12

ACTIVE TRANSPORT = Requires energy to move molecules AGAINST the conc. gradient.
Is energetically "uphill" (+ delta G)
Uses ATP
e.g.
The SODIUM-POTASSIUM PUMP (an example of active transport)
(Fig. 8.13)

One other big player in membrane transport (and many other things) is membrane potential:
MEMBRANE POTENTIAL = Voltage across membrane
Overall inside of cell is - charged (more anions),
outside is positive (more cations).
Therefore we actually have 2 types of gradients across membranes: a chemical one and an electrical one = ELECTROCHEMICAL GRADIENT. Set up by an electrogenic pump.
(remember potential energy)

COTRANSPORT

Fig. 8.15 (Electrogenic pump)

Fig. 8.16 (Symport of sucrose)

(Compare active and passive transport with Fig. 8.14)
Transport of Large Molecules and particles.

ENDOCYTOSIS

(Fig. 8.17)
Phagocytosis (already discussed)
Pinocytosis (slurping)
Receptor-mediated Endocytosis (more selective than pinocytosis)
EXOCYTOSIS (talked about already, see Fig. 8.7)