Biology Reference
In-Depth Information
opposite direction (Antiport, see
Figure 14.13 [18]
). Movement of the pumped ion down
its electrochemical gradient is by facilitated diffusion. The purpose of both types of co-
transport is to use the energy in an electrochemical gradient to drive the movement of
another solute against its gradient. An example of Symport is the SGLT1 (sodium glucose
transport protein-1) in the intestinal epithelium
[20]
. SGLT1 uses the energy in a downhill
trans-membrane movement of Na
þ
to transport glucose across the apical membrane
against an uphill glucose gradient so that the sugar can be transported into the blood-
stream on the opposite side of the cell.
Bacterial Lactose Transport
The secondary active symport system for lactose uptake in E. coli is shown in
Figure 14.16 [21]
. Lactose uptake is driven through a channel by an H
þ
gradient generated
by the bacterial electron transport system
[22]
. The free energy equation for transport
described above (
D
G
¼
RT ln [s
o
'] / [s
o
]
þ
ZF
DJ
) can be rearranged for cases employing
H
þ
gradients to:
Dm
H
þ
¼ DJ
RT
=
nF
D
pH
Where:
Dm
H
þ
is the proton motive force
DJ
is the trans-membrane electrical potential
FIGURE 14.16
Lactose transport system in Escheria coli
[21]
. Uptake of lactose is coupled to the movement
of H
þ
down its electrochemical gradient. This is an example of active transport, co-transport, and active
symport.
