4.5 WHAT INFORMATION CAN BE OBTAINED FROM CELLS?
The state of a living cell can be monitored by sensing different physical parameters at or near the cell
surface [3]. Some examples are given below categorized by the nature of the stimulus.
(i) Chemical. Examples of cell chemical signals are ions, such as H
D
or Ca
2D
, smaller organic
molecules, such as sugar, lactones, and larger biomolecules, such as proteins, DNA, etc. One
important chemical indicat or of the physiological state is pH, which is related to the
concentration of hydrogen ions or protons H
D
(see Box 4.2 ). For example, studies of the
mammalian cell’s acidification rate (the rate of proton extrusion into extracellular medium)
have shown that under steady-state conditions, one cell produces on average ~10
7
–10
8
protons
(H
D
) per second [4]. If a particular function of the cell is activated, the number of extruded
protons can increase manyfold. For example, if the antimicrobial function phagocytosis is
stimulated in a type of white blood cell, neutrophils, the acidification rate is increased more
than fourfold from 1.3 10
7
H
þ
/s in the resting state to 5.5 10
7
H
þ
/s [4].
(ii) Electrical. Electrical signaling has a primary importance for the nervous system. Cells that have
the ability to generate electrical signals are called electrogenic cells (see Box 4.3), such as brain
cells (neurons) and heart cells. For example, neurons are characterized by ‘action potential’
BOX 4.2 PH OF A SOLUTION
pH (from Latin potentia hydrogeni – power of hydrogen) is a measure of the acidity of a solution. The property of
acidity is directly related to the concentration of hydrogen ions H
D
or protons. Quantitatively, it can be approxi-
mately expressed as
pHz log
H
þ
where [H
þ
] is the molar concentration of dissolved hydrogen ions.
In living systems, pH is tightly regulated and is an important measure of the physiological state and, in many
cases, an indicator of health. Table B4.1 contains examples of pH of some important liquid constituents of both the
exterior and interior of living organisms.
Table B4.1 Examples of pH of some important liquid constituents of both the exterior and interior of
living organisms
Medium pH
Distilled water
7
Seawater
7.5–8.4
Human blood
7.36–7.44
Human skin
4.5–6.0
Human saliva
5.6–7.6
Gastric juice (acid)
1–2
100 CHAPTER 4 Sensors at the micro-scale
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