Clay Structures Form Part of Cell

A team of researchers at the Harvard School of Engineering and Applied Sciences, Princeton University, and Brandeis University recently demonstrated the ability of clay to assemble itself into semi-permeable membranes, the first time that such membranes—which are an important building block in cells—have been formed out of an inorganic material.

According to lead author Anand B. Subramaniam, a graduate student at SEAS, researchers have been wondering for years how the first cells were able to separate their internal compartments. Initially, he said, researchers believed that life may have originated from a “primordial soup” of organic molecules.

However, Subramaniam continued, this theory does not provide an adequate explanation because reactions can only occur at a reasonable pace if the molecules are in high concentrations. One way to create these pockets of highly concentrated organic material necessary for life is to create semi-permeable containers that will preferentially allow organic material to enter the compartment—in other words, to create “protocells.”

Subramaniam and his fellow researchers found that such cell-like “clay-armored bubbles” form naturally when particles of a particular clay accumulate on the surface of air bubbles under water.

When these bubbles are exposed to organic liquids, the air bubbles dissipate. The result is a strong, spherical shell of clay physically separating molecules on the inside from those on the outside. The researchers also demonstrated that microscopic holes in the clay shell form a semi-permeable membrane allowing certain chemicals to enter the cells while preventing others from leaving.

Subramaniam tested the semipermeability a number of different ways. In one test, he used a fatty acid that assembles into vesicles (bubble-like structures) when in a highly basic environment.

“We put this fatty acid in the solution and saw that fatty acid liposomes [vesicles] were formed inside the cell. We figured that the [fats] could go through the membrane, but then when they formed liposomes, they were too big to exit again,” Subramaniam explained.

Subramaniam said he is particularly excited about this work because the clay material used, montmorillonite, is known to have catalytic properties—aiding in the biologically important tasks of cell membrane formation and RNA production. He and his fellow researchers hope to explore this property of the clay in conjunction with its ability to form cell-like structures. The researchers also hope to find examples of such clay structures in today’s natural environment.

“The big question is whether you can find these sorts of structures in the natural world,” says Subramaniam.

— Staff writer Nitish Lakhanpal can be reached at nitishlakhanpal@college.harvard.edu.