Biology Reference
In-Depth Information
FIGURE 14.10
Saxitoxin.
Where:
D
G is the free energy change
[s
o
'] is the solute concentration on the right side of a membrane
[s
o
] is the solute concentration on the left side of a membrane
R is the gas constant
T is the temperature in
K
Z is the charge of the solute
F is the Faraday
DJ
is the trans-membrane electrical potential
Solute movement will continue until
D
G
¼
0. If
D
G is negative, solute movement is left to
right (it is favorable as written). If
G is positive, solute movement is right to left (it is unfavor-
able in the left to right direction) or energy must be added for the solute to go from left to right
The equation has two parts: a trans-membrane chemical gradient ([s
o
'] / [s
o
]); and a trans-
membrane electrical gradient (
D
). The net movement of solute is therefore determined by
a combination of the solute's chemical gradient and an electrical gradient inherent to the
cell. If the solute has no charge, Z
DJ
¼
0 (as is the case for glucose) and the right hand part of
the equation (ZF
) drops out. Therefore, the final equilibrium distribution of glucose across
the membrane will have the internal glucose concentration equal to the external glucose
concentration and is independent of
DJ
DJ
, the electrical potential. At equilibrium for a non-
charged solute,
D
G
¼
RT ln [s
o
'] / [s
o
]and
D
G can only be equal to zero if [s
o
']
¼
[s
o
].
The situation for a charged solute like K
þ
is more complicated. The net
D
G is determined
by both the chemical gradient ([s
o
'] / [s
o
]) and electrical gradient (
results from
the sum of all charged solutes on both sides of the membrane, not just K
þ
. Therefore, even if
the K
þ
concentration is higher inside the cell than outside (the chemical gradient is unfavor-
able for K
þ
uptake), the
DJ
). The
DJ
may be in the correct direction (negative interior) and of suffi-
cient magnitude to drive K
þ
uptake against its chemical gradient.
DJ
Aquaporins
Aquaporins are also known as water channels and are considered to be 'the plumbing
system for cells'
[15,16]
. For decades it was assumed that water simply leaked through bio-
logical membranes by numerous processes described above. However, these methods of
water permeability could not come close to explaining the rapid movement of water across
some cells. Although it had been predicted that water pores must exist in very leaky cells, it
wasn't until 1992 that Peter Agre (
Figure 14.11
) at Johns Hopkins University identified
a specific trans-membrane water pore that was later called aquaporin-1. For this accomplish-
ment Agre shared the 2003 Nobel Prize in Chemistry with Rod MacKinnon for his work on
