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3 Fundamentals of the Formation of Biosignals
cell
cell membrane
phospholipid
Fig. 3.4: Schematic representation of a cell (top), a cell membrane as a lipid bilayer with embedded
membrane proteins (bottom left) and a phospholipid (bottom right).
In electrical analogy, the membrane wall itself is a well insulating layer with electrical
resistance, RLDS ∼109Ω, which, due to its impermeability for ions, acts like an energy
storage of a capacitance CLDS. Figure 3.5 shows a lipid bilayer (left) with corresponding
Na+, K+ and Cl−ion distribution between intra- and extracellular space. The corres-
ponding ion concentrations for one ion species A±, in the intracellular space ci
A± and
ce
A± in the extracellular space, are shown in the equivalent circuit on the right (con-
centrations found in Figure 3.9).
However, the functional properties of the biomembrane result from the behaviour
of the embedded proteins, which, for example, act as receptors for certain substances
or as transporter proteins for the transport of certain ion types or metabolic products
across the cell membrane. A distinction is made between diffusion-driven passive
channel proteins in the form of a simple pore and so-called active transport proteins
in the form of a molecular pump (cf. Figure 3.6). Passive transport through an open
channel (pore) occurs through diffusion along a gradient of the electric field or the
substance concentration of certain ions. Active metabolic transport requires energy
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intracellular space
CLDS
RLDS
extracellular space
ce
Na+
ce
K+
ce
Cl−
ci
Na+
ci
K+
ci
Cl−
Fig. 3.5: Schematic representation of a nearly impermeable cell membrane as a lipid bilayer
(phosphate-head group – red, hydrocarbon-lipids – yellow): in the intra- and extracellular space
there are positive (red) and negative (blue) ions. The corresponding equivalent circuit of the mem-
brane wall built by an electrical capacitance CLDS and the insulation resistance RLDS.