An artist's depiction of a cell membrane with phospholipids of different sizes.

Physicists Show How Cell Membranes Take Shape

By Kirsten Heuring Email Kirsten Heuring

Heidi Opdyke

Cells constantly move and squish as their outer membranes shift. 麻豆村 physicists have confirmed some of the mechanics behind these motions through a mix of experiments and mathematical modeling.

鈥淲e focused very directly on the physical properties of the membrane and how they emerge,鈥� said Lucy Knox, a rising senior studying physics.

Cell membranes are made of phospholipids, molecules made of fatty acids. These molecules have two distinct parts: a water-attracting 鈥渉ead鈥� and water-repelling 鈥渢ails.鈥� These molecules naturally arrange themselves into a double layer with the heads facing outward and the tails inside. Within this structure, lipids are constantly moving, giving cell membranes their rippling behavior.

But not all phospholipids are the same. Some have larger heads than others, and that difference matters. Knox, along with recent physics graduate Peter Winstel, investigated how variations in head sizes affect membranes.

鈥淟ipid membranes are a unique system that exhibits shape fluctuations, and when you have more than one type of lipid, you also get all the fun statistical mechanics that come into play,鈥� Winstel said.

Using X-ray diffuse scattering, Knox captured detailed images of how the cell membranes are softened when mixed together. Winstel then compared those results to theoretical models to see how well the data matched predictions.

They found that lipids with similar-sized heads tend to cluster together. Larger heads push outward, while smaller heads pull inward, bending the membrane. This effect is known as diffusional softening, and it affects the membrane鈥檚 structure.

鈥淭his makes larger undulations within the membrane more likely,鈥� Knox said. 鈥淲hen you go to measure the softness of the membrane, these larger undulations will read as the membrane being softer.鈥�

Knox and Winstel鈥檚 work lays the groundwork for future breakthroughs. Understanding these mechanics could help researchers better study how cells divide and how membranes respond to stress.

The research also contributes to a long-running debate in biophysics about how cholesterol affects membranes. John Nagle, emeritus professor in the Department of Physics, was one of the research advisors on the project. He said the work is a step toward understanding how the length scale affects membrane mechanics.

Knox, Winstel and Nagle published their findings in Biophysical Journal in a paper titled They were joined by Professor Markus Deserno and Research Professor Emerita Stephanie Tristram-Nagle. The work was supported by the National Science Foundation and the National Institutes of Health. Additional papers are expected later this year.

鈥淪cience thrives when experiment and theory work seamlessly together,鈥� Deserno said. 鈥淭hrough constant discussions with our experimental colleagues we were able to cleanly identify diffusional softening in the data. We hope this will help us to better understand the intricate biophysics of lipid mixtures.鈥�