Editing Phase transition (section)

===Phase transitions in biology===
Phase transitions play many important roles in biological systems. Examples include the [[lipid bilayer]] formation, the [[Coil–globule transition|coil-globule transition]] in the process of [[protein folding]] and [[DNA melting]], liquid crystal-like transitions in the process of [[DNA condensation]], cooperative ligand binding to DNA and proteins with the character of phase transition<ref>{{cite journal |author=D.Y. Lando and V.B. Teif |year=2000 |title=Long-range interactions between ligands bound to a DNA molecule give rise to adsorption with the character of phase transition of the first kind |journal=J. Biomol. Struct. Dyn. |volume=17 |issue=5 |pages=903–911 |doi=10.1080/07391102.2000.10506578 |pmid=10798534 |s2cid=23837885}}</ref> or the change in the process of genetic expression at the onset of eukaryotes, marked by an algorithmic phase transition.<ref name="Muro et al. 2025">{{cite journal |last1=Muro |first1=Enrique M. |last2=Ballesteros |first2=Fernando J. |last3=Luque |first3=Bartolo |last4=Bascompte |first4=Jordi |title=The emergence of eukaryotes as an evolutionary algorithmic phase transition |journal=PNAS |volume=122 |issue=13 |year=2025 |doi=10.1073/pnas.2422968122 |pages=e2422968122 |pmid=40146859 |doi-access=free |pmc=12002324 }}</ref>

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 membrane]]s 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.<ref name="YashRoy_1987">{{cite journal |vauthors=Yashroy RC |year=1987 |title=<sup>13</sup>C NMR studies of lipid fatty acyl chains of chloroplast membranes |url=https://www.researchgate.net/publication/230822408 |journal=Indian Journal of Biochemistry and Biophysics |volume=24 |issue=6 |pages=177–178 |doi=10.1016/0165-022X(91)90019-S |pmid=3428918}}</ref> Gel-to-liquid crystalline phase transition temperature of biological membranes can be determined by many techniques including calorimetry, fluorescence, [[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.<ref>{{cite journal |last1=YashRoy |first1=R C |year=1990 |title=Determination of membrane lipid phase transition temperature from 13-C NMR intensities |url=https://www.researchgate.net/publication/20790042 |journal=Journal of Biochemical and Biophysical Methods |volume=20 |issue=4 |pages=353–356 |doi=10.1016/0165-022X(90)90097-V |pmid=2365951}}</ref>

It has been proposed that some biological systems might lie near critical points. Examples include [[neural network (biology)|neural networks]] in the salamander retina,<ref>{{cite arXiv |eprint=1407.5946 |class=q-bio.NC |first1=Gasper |last1=Tkacik |first2=Thierry |last2=Mora |title=Thermodynamics for a network of neurons: Signatures of criticality |last3=Marre |first3=Olivier |last4=Amodei |first4=Dario |last5=Berry II |first5=Michael J. |last6=Bialek |first6=William |year=2014}}</ref> bird flocks<ref>{{cite journal |last1=Bialek |first1=W |last2=Cavagna |first2=A |last3=Giardina |first3=I |year=2014 |title=Social interactions dominate speed control in poising natural flocks near criticality |journal=PNAS |volume=111 |issue=20 |pages=7212–7217 |arxiv=1307.5563 |bibcode=2014PNAS..111.7212B |doi=10.1073/pnas.1324045111 |pmc=4034227 |pmid=24785504 |doi-access=free}}</ref>
gene expression networks in Drosophila,<ref>{{cite journal |last1=Krotov |first1=D |last2=Dubuis |first2=J O |last3=Gregor |first3=T |last4=Bialek |first4=W |year=2014 |title=Morphogenesis at criticality |journal=PNAS |volume=111 |issue=10 |pages=3683–3688 |arxiv=1309.2614 |bibcode=2014PNAS..111.3683K |doi=10.1073/pnas.1324186111 |pmc=3956198 |pmid=24516161 |doi-access=free}}</ref> and protein folding.<ref>{{cite journal |last1=Mora |first1=Thierry |last2=Bialek |first2=William |year=2011 |title=Are biological systems poised at criticality? |journal=Journal of Statistical Physics |volume=144 |issue=2 |pages=268–302 |arxiv=1012.2242 |bibcode=2011JSP...144..268M |doi=10.1007/s10955-011-0229-4 |s2cid=703231}}</ref> However, it is not clear whether or not alternative reasons could explain some of the phenomena supporting arguments for criticality.<ref>{{cite journal |last1=Schwab |first1=David J |last2=Nemenman |first2=Ilya |last3=Mehta |first3=Pankaj |year=2014 |title=Zipf's law and criticality in multivariate data without fine-tuning |journal=Physical Review Letters |volume=113 |issue=6 |page=068102 |arxiv=1310.0448 |bibcode=2014PhRvL.113f8102S |doi=10.1103/PhysRevLett.113.068102 |pmc=5142845 |pmid=25148352}}</ref> 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.<ref>{{Cite journal |last1=Longo |first1=G. |last2=Montévil |first2=M. |date=2011-08-01 |title=From physics to biology by extending criticality and symmetry breakings |url=https://www.academia.edu/23155991 |journal=Progress in Biophysics and Molecular Biology |series=Systems Biology and Cancer |volume=106 |issue=2 |pages=340–347 |arxiv=1103.1833 |doi=10.1016/j.pbiomolbio.2011.03.005 |pmid=21419157 |s2cid=723820}}</ref> Phase transitions are prominent feature of motor behavior in biological systems.<ref>{{cite book |author=Kelso, J. A. Scott |title=Dynamic Patterns: The Self-Organization of Brain and Behavior (Complex Adaptive Systems) |publisher=MIT Press |year=1995 |isbn=978-0-262-61131-2}}</ref> Spontaneous gait transitions,<ref>{{cite journal |last1=Diedrich |first1=F. J. |last2=Warren |first2=W. H. Jr. |year=1995 |title=Why change gaits? Dynamics of the walk-run transition |journal=Journal of Experimental Psychology. Human Perception and Performance |volume=21 |issue=1 |pages=183–202 |doi=10.1037/0096-1523.21.1.183 |pmid=7707029}}</ref> as well as fatigue-induced motor task disengagements,<ref>{{cite journal |last1=Hristovski |first1=R. |last2=Balagué |first2=N. |year=2010 |title=Fatigue-induced spontaneous termination point--nonequilibrium phase transitions and critical behavior in quasi-isometric exertion |journal=Human Movement Science |volume=29 |issue=4 |pages=483–493 |doi=10.1016/j.humov.2010.05.004 |pmid=20619908}}</ref> show typical critical behavior as an intimation of the sudden qualitative change of the previously stable motor behavioral pattern.

The characteristic feature of second order phase transitions is the appearance of [[fractal]]s in some [[Scale-free network|scale-free]] properties.  It has long been known that protein globules are shaped by interactions with water.  There are 20 amino acids that form side groups on protein peptide chains range from [[hydrophilic]] to hydrophobic, causing the former to lie near the globular surface, while the latter lie closer to the globular center.  Twenty fractals were discovered in solvent associated surface areas of > 5000 protein segments.<ref>{{cite journal |last1=Moret |first1=Marcelo |last2=Zebende |first2=Gilney |date=January 2007 |title=Amino acid hydrophobicity and accessible surface area |journal=Physical Review E |volume=75 |issue=1 |page=011920 |bibcode=2007PhRvE..75a1920M |doi=10.1103/PhysRevE.75.011920 |pmid=17358197}}</ref>  The existence of these fractals proves that proteins function near critical points of second-order phase transitions. 
  
In groups of organisms in stress (when approaching critical transitions), correlations tend to increase, while at the same time, fluctuations also increase. This effect is supported by many experiments and observations of groups of people, mice, trees, and grassy plants.<ref>{{cite journal |last1=Gorban |first1=A.N. |last2=Smirnova |first2=E.V. |last3=Tyukina |first3=T.A. |date=August 2010 |title=Correlations, risk and crisis: From physiology to finance |url=https://www.researchgate.net/publication/222687003 |journal=Physica A: Statistical Mechanics and Its Applications |volume=389 |issue=16 |pages=3193–3217 |arxiv=0905.0129 |bibcode=2010PhyA..389.3193G |doi=10.1016/j.physa.2010.03.035 |s2cid=276956}}</ref>