<P> Phase transitions play many important roles in biological systems . Examples include the lipid bilayer formation, the coil - globule transition in the process of protein folding and DNA melting, liquid crystal - like transitions in the process of DNA condensation, and cooperative ligand binding to DNA and proteins with the character of phase transition . </P> <P> In biological membranes, gel to liquid crystalline phase transitions play a critical role in physiological functioning of biomembranes . In gel phase, due to low fluidity of membrane lipid fatty - acyl chains, membrane proteins have restricted movement and thus are restrained in exercise of their physiological role . Plants depend critically on photosynthesis by chloroplast thylakoid membranes which are exposed cold environmental temperatures . Thylakoid membranes retain innate fluidity even at relatively low temperatures because of high degree of fatty - acyl disorder allowed by their high content of linolenic acid, 18 - carbon chain with 3 - double bonds . Gel - to - liquid crystalline phase transition temperature of biological membranes can be determined by many techniques including calorimetry, flouorescence, spin label electron paramagnetic resonance and NMR by recording measurements of the concerned parameter by at series of sample temperatures . A simple method for its determination from 13 - C NMR line intensities has also been proposed . </P> <P> It has been proposed that some biological systems might lie near critical points . Examples include neural networks in the salamander retina, bird flocks gene expression networks in Drosophila, and protein folding . However, it is not clear whether or not alternative reasons could explain some of the phenomena supporting arguments for criticality . It has also been suggested that biological organisms share two key properties of phase transitions: the change of macroscopic behavior and the coherence of a system at a critical point . </P>

At what temperature will water change from a solid to a liquid