© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

4.2.7 Temporal Concentration Gradients

Koshland and Macnab⁴³⁵ have shown that bacteria possess a mechanism for sensing small temporal concentration gradients. A bacterium detects a spatial gradient of attractant not by comparing the concentration at its head and tail, but rather by traveling through space and comparing its observations over time. E. coli can measure a ~0.01% gradient⁴³⁶ during the time it travels one body length (~2 microns) at top speed (20-40 microns/sec), a minimum temporal gradient of 0.1% per second. This appears very efficient, since the counting rotor sensor proposed in Section 4.2.3 is not much better, achieving ~0.01% per second. As for peak gradients, the 10-probe receptor array sensor described in Section 4.2.1 can detect a maximum concentration change (e.g., from an already-known rare concentration) in ~10^-4 sec, a peak temporal gradient of ~10¹⁵ % per second.

Last updated on 17 February 2003