3.1 Physiology and Electrical Activity of Muscle and Nerve Cells

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micelle

inverse micelle

lipid bilayer

vesicle

Fig. 3.3: Behaviour of the self-assembly of lipids in aqueous solution: depending on the concentra-

tion and temperature of the lipid-water-mixture, different liquid crystalline structures are formed. At

a low lipid concentration, micelles (top left) are energetically most favourable. If the concentration

is increased, lipid bilayers (bottom left) and closed vesicles (right) form. Inverse micelles are usually

found only in organic solvents (top centre).

are formed. During phase separation, the hydrophilic head groups align with the ad-

jacent water molecules, whereas the hydrophobic ends cluster together to form their

own internal lipid phase. Inverse micelles with reversed arrangement, on the other

hand, only form in organic solvents, because there the interactions behave in exactly

the opposite way compared to water.

If the lipid concentration is further increased, so-called vesicles are formed.¹³

These are about one micrometre in size and are responsible for the transport of many

substances into and between cells. Substances stored in the vesicles are released, for

example, by fusion of the vesicles with the cell membrane. These synaptic vesicles

are involved in the transmission and processing of nerve impulses through the release

of neurotransmitters in the synaptic cleft. The liposomal effect is also used for target-

oriented drug release, corresponding migration processes are observed by so-called

tracers.¹⁴

Above stated properties site phospholipids a central role in biological systems

such as the cell and their signal interactions. In the cell membrane they form a lipid-

double layer in which numerous functional molecules such as proteins or glycolipids¹⁵

are embedded (cf. Figure 3.4).

13 vesicula = vesicle; roundish small intracellular (located in the cell) vesicles surrounded by a single

or double membrane.

14 A labelled substance (radioactive or fluorescent) being introduced into the living body to particip-

ate in the metabolism.

15 Glycolipids (Greek glykys = sweet, lipos = fat) are phosphorus-free membrane lipids on the outside

of the lipid double layer.