Biology Reference
In-Depth Information
FIGURE 14.11
Peter Agre (1949
e
). Courtesy of The Johns Hopkins University, The Johns Hopkins Hospital, and Johns
Hopkins Health System
the potassium channel. Aquaporins are specific for water permeability, excluding the passage
of other solutes. A type of aquaporin known as aqua-glyceroporins can also conduct some
very small uncharged solutes such as glycerol, CO
2
, ammonia, and urea across the
membrane. However, all aquaporins are impermeable to charged solutes. Water molecules
traverse the aquaporin channel in single file (
Figure 14.12
)
[17]
.
D. ACTIVE TRANSPORT
A characteristic of all living membranes is the formation and maintenance of trans-
membrane gradients of all solutes including salts, biochemicals, macromolecules, and even
water. In living cells large gradients of Na
þ
and K
þ
are particularly important. Typical cell
concentrations are:
Cell Interior: 400 mM K
þ
,50mMNa
þ
Cell Exterior: 20 mM K
þ
, 440 mM Na
þ
DJ
200mV (negative interior) resulting from the
uneven distribution of all ionic solutes including Na
þ
and K
þ
. The chemical and electrical
gradients are maintained far from equilibrium by a multitude of active transport systems.
Active transport requires a formof energy (often ATP) to drive the movement of solutes against
their electrochemical gradient, resulting in a non-equilibrium distribution of the solute across
the membrane. A number of non-exclusive and overlapping terms are commonly used to
describe the different types of active transport. Some of these are depicted in
Figure 14.13 [18]
.
Living cells will also have a
from
30 to
