56 Chapter 3 Microscopy and Cell Structure
Table 3.4 A Summary of Transport Mechanisms Used by Prokaryotic Cells
Major facilitator superfamily
ABC transporters Group Translocation
Rarely used by bacteria. Exploits a concentration gradient to move molecules; can only eliminate a gradient, not create one. No energy is expended.
Energy is expended to accumulate molecules against a concentration gradient.
In bacteria, the proton motive force drives these transporters. As a proton is allowed into the cell another substance is either brought along or expelled.
ATP is used as an energy source. Extracellular binding proteins deliver a molecule to the transporter.
The transported molecule is chemically altered as it passes into the cell. Consequently, uptake of that molecule does not affect its concentration gradient.
Cells do not produce all of the transport proteins all of the time. Instead, regulatory mechanisms sense the presence of specific compounds and, based on the concentration of those compounds and availability of other molecules, determine which transport proteins are synthesized. The mechanisms of transport are summarized in table 3.4. ■ regulation of gene expression, p. 183
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