The phase transitions of life

Abstract

Cells, Gels and the Engines of Life: A New Unifying Approach to Cell Functionby G.H. Pollack, Ebner & Sons, 2001. US$27.95 (paperback) (xiv + 303 pages) ISBN 0 962 689 5 21Cells, Gels and the Engines of Life is an attempt to explain, in popular terms, an alternative paradigm of cell function. This paradigm is not new, but has its roots as far back as the discovery of ‘protoplasm’ almost two centuries ago, before the discovery of the cell membrane. In the 1960s and 1970s, while scientists were proposing and isolating ion pumps, Gilbert Ling and other researchers rejected the idea of an ion pump embedded in the cell membrane. In particular, Ling developed the ‘association–induction hypothesis’ to explain the ion partitioning between the inside and outside of the cell without relying on membranes or pumps. In a series of three books (published in 1962, 1984 and 1992) 1xMalabar, F.L. and Ling, G.N. See all References, 2xLing, G.N. CrossRefSee all References, 3xLing, G.N. See all References Ling extended his ideas to account for other cellular phenomena. In this new book, Pollack further extends Ling's ideas and explains, in a very engaging and readable fashion, cellular behavior in light of these ideas.The book opens with several questions about the cell membrane. How can ion channels be selective for large molecules over small? Can the cell produce enough energy to run all of its pumps? How do certain cells survive when holes are poked in their membranes? More generally, why does intercellular water have different properties (e.g. ion diffusivity and freezing point) from bulk water?The solution to these difficulties lies in the viewpoint of the cytoplasm as a gel rather than a simple bag of salt water with larger molecules floating around. The water inside the cell is more ‘structured’ than bulk water and is able to selectively exclude Na+ over K+ ions and to cause volume-changing phase transitions among the cytoskeletal proteins. Several chapters of the book explain the basic physical chemistry of water, ions and proteins, with an emphasis on the properties of the structured water and cation binding to proteins. This exposition gradually builds up to the climax of the book – the hypothesis that phase transitions in the cytoplasm are the engines that drive cell dynamics. These phase transitions are triggered by small changes in ion concentrations and pH, and can result in a volume change of an order of magnitude or more.The second half of the book is devoted to specific examples of cellular function. Rapidly expanding gel helps expel the contents of secretory vesicles, whereas intracellular transport is driven by a propagating transition involving the unstructuring and restructuring of water. The neuronal action potential is mediated not by ion pumps in the cell membrane, but by a Ca2+-mediated phase transition in the thin cytoskeletal layer next to the membrane. Finally, Pollack rejects the cross-bridge model of muscle contraction in favor of a helix-to-coil transition of the myosin rod; his book Muscle and Molecules: Uncovering the Principles of Biological Motion 4xPollack, G.H. See all References4 (published in 1990) describes this in more detail.The final question remains: how is the chemical energy of ATP converted into movement by means of phase transitions? In addressing this issue in the penultimate chapter, Pollack emphasizes not only the ability of ATP to induce conformational changes in proteins, but also the supposed water-structuring ability of ATP. Unlike the earlier chapters, the exposition at this point is rather unclear, and proposed interplay between the enthalpy of ATP hydrolysis and the entropy of structured water seems rather vague. A diagram of a thermodynamic cycle, showing changes in enthalpy and entropy, might have clarified his ideas.Like his 1990 book, this book is very well written and is filled with clear, informative and sometimes humorous illustrations. This book can easily be understood by an intelligent college freshman, whereas university professors will find it enjoyable and thought provoking. Unfortunately, much of the material within the book is not accepted by mainstream science. During the 1960s and 1970s, a bitter debate raged between the proponents of structured water and those of ion pumps. For reasons too lengthy to discuss here, the structured water faction eventually fell into obscurity and Ling's work has been largely ignored since. Although Pollack performs a valuable service by raising valid questions regarding the mainstream theories, the true test of his theories will be the ability to move forward by suggesting new experiments and predicting their outcomes. Regardless of the eventual fate of the ideas in this book, cell biologists should continue to be alert for evidence of sudden, collective transitions among the complex assemblage of molecules in the cytoskeleton.