With the help of the technique of Electron Spin Resonance (ESR) spectroscopy, the fluidity of the membrane has been established. Harden MeConnell and O Hayes Griffith made an experiment using ESR spectrum after labelling the fatty acid tail of lipid layer of the membrane with a nitroxide group, having an unpaired electron.
The presence of this spin-label, i.e., nitroxide group emits energy when exposed to an external magnetic field of suitable intensity. The ESR spectrum is then noted taking a phospholipid compound with spin-label as control.
The spectrum of biological membrane is found to be intermediate between completely mobile and immobilised molecule (Fig. 2.9). Thus, the ESR spectra showed that lipid molecules of the biological membranes are neither in a fixed state as in crystal, nor like completely mobile molecule (fluid).
Hence the intermediate state of the membrane is referred to as liquid crystal state. The lipid molecules can move laterally within the bilayer keeping the orientation intact (i.e., hydrophilic head groups pointed towards the membrane surface and hydrophobic tails towards the membrane interior).
It is known that during the transition of one physical state to another—i.e., from solid to liquid or liquid to gas—heat is generated. Hence, the transition of the physical state in the biological membrane (i.e., from solid to liquid) has been observed by Differential Scanning Calorimetry (Fig. 2.10).
It has been observed that at low temperatures, lipid layers are changed to solid or gel state .and at higher temperatures it melts to fluid or liquid crystalline state. The transition temperature, at which the change of state occurs, is below the temperature at which most physiological functions occur.
There are different factors which control this membrane fluidity. The increase in membrane fluidity is related with the increase of unsaturated fatty acids and decrease of fatty acid chain length and cholesterol content.
Again, the increase in membrane fluidity is inversely proportional to the transition temperature. This alteration in the physical state of the membrane has an important role in the function of the membrane. The mobility of the membrane proteins has also been established using fluorescein-labelled antibodies. The membrane proteins are capable of rapid lateral migration in fluid lipid layer.
Thus, the random distribution of membrane proteins has been observed and the behaviour of membrane proteins depends on the membrane fluidity. Thus, this model offers a dynamic picture of the membrane.