Biology Reference
In-Depth Information
membrane phospholipids in opposite directions. Statistical pores have only a fleeting
existence and cannot be isolated or imaged.
3. Passage down water chains.
4. Water can be carried down kinks in acyl chains that result from acyl chain
melting (see lipid melting in Chapter 9).
5. Water may rapidly cross membranes through non-lamellar membrane patches (e.g.
micelle, cubic or H
II
phase).
6. High water permeability will occur at locations of packing defect (e.g. surface of integral
proteins, boundary between membrane domains).
7. Through pores or channels used to conduct ions.
8. Through specific water channels known as aquaporins.
The only molecules that can cross a membrane by simple passive diffusion are water, small
non-charged solutes, and gasses. Charged or large solutes are virtually excluded from
membranes and so require more than just simple passive diffusion to cross a membrane.
C. FACILITATED DIFFUSION
Facilitated diffusion (also known as carrier-mediated diffusion) is, like simple passive
diffusion, dependent on the inherent energy in a solute gradient. No additional energy is
required to transport the solute and the final solute distribution reaches equilibrium
across the membrane. Facilitated diffusion, unlike simple diffusion, usually requires
a highly specific trans-membrane integral protein to assist in the solute's membrane
passage. Facilitators come in two basic types, carriers and gated channels. Facilitated
diffusion exhibits Michaelis-Menton saturation kinetics (
Figure 14.5
), indicating the
carrier has an enzyme-like active site. Like enzymes, facilitated diffusion carriers recog-
nize their solute with exquisite precision, easily distinguishing chemically similar
isomers like D-glucose from L-glucose and exhibit saturation kinetics.
Figure 14.5
compares simple passive diffusion to facilitated diffusion. The figure is not to scale,
however, as facilitated diffusion is orders of magnitude faster than simple passive
diffusion.
Glucose Transporter
Awell-studied example of a facilitated diffusion carrier is the glucose transporter or GLUT
[9]
. From the activation energy for trans-membrane simple passive diffusion of glycol, glyc-
erol and erythritol presented in
Table 14.2
, it can be estimated that the activation energy for
glucose should be well over 100 KJ/mol, but instead it is only 16 KJ/mol. This large discrep-
ancy is attributed to the presence of a glucose facilitated diffusion carrier.
Figure 14.6
demon-
strates the mode of action of one of these transporters, GLUT-1, from the erythrocyte
[10]
.
GLUTs occur in nearly all cells and are particularly abundant in cells lining the small intes-
tine. GLUTs are but one example in a superfamily of transport facilitators. GLUTs are integral
membrane proteins whose membrane-spanning region is composed of 12
-helices. GLUTs
function through a typical membrane transport mechanism
[10]
. Glucose binds to the
a
