Artificial cells - a simple model for a complex structure

A simple model made up of chemical substances could lead to a better understanding of the structure and organization of a cell, according to Constitution of Pennsylvania State University

A simple model made up of chemical substances could lead to a better understanding of the structure and organization of a cell, according to Constitution of Penn State University. "Biological cells are interesting because they exhibit organization even at the level of the cytoplasm (the cell fluid outside the nucleus), and while researchers appreciate that this organization is important to the functions of the cell, it is not always clear how this organization is achieved," said Christine Keating, professor of chemistry. "We employ a materials chemistry approach in developing simple experimental models for the organization of cytoplasm," she explains.

The cytoplasm is the substance that fills the biological cell and is densely filled with very large compounds. It "wraps" the organelles - tiny organs such as mitochondria and the cell nucleus. Unlike the types of organelles, the cytoplasm is a basic characteristic of all cells. Many important biochemical processes take place in the cytoplasm and therefore it is interesting as a main component of cell function.

Creating an artificial cell with organelles would be a formidable task, but creating a cell that exhibits diversity and molecular density – in other words, a non-uniform composition – is possible through the use of large polymeric compounds and a lipid membrane.

The researcher used fats to produce vesicles - tiny bubbles the size of a fat membrane cell - in an aqueous solution of two large polymers. In one case, she used polyethylene glycol (PEG) - a common polymer - and dextran - a polymerized sugar to create the cell.

"None of these compounds is important for the cell's activity, but they illustrate the possibility of separating large compounds inside the cell without having an inner membrane," explained the researcher. Normally, the cytoplasm is full of large compounds such as: proteins, nucleic acids and sugars.

Mixing a small amount of PEG, a small amount of dextran, water and dried fat allowed the fat to absorb water and form a blister. Both types of polymers filled the vesicles in the same concentrations that were present in the fluid surrounding the vesicles. Although the fat membrane enveloped the polymer mixture in water, the substance was divided into two separate complexes, one of which has a higher concentration of PEG, and the other has a higher concentration of dextran.

"The materials did not go through a complete separation in which one type of polymer is only found in one area and the other in another, completely distant area," notes the lead researcher. "However, the two aqueous layers are different enough so that additional compounds, such as: proteins or nucleic acids, will prefer one layer over the other and its concentration there will increase."

In other materials, researchers have been able to create aqueous layer separation of up to 15 different compounds. Similarly, it is possible that multiple separations also exist in the cytoplasm of the cell. Biologists know that enzymes and other proteins tend to clump together in certain complexes in the cell at certain times. This crowding will help the chemical reactions in the metabolic pathways to occur faster because the reactants needed for the next step will be located nearby. Applying heat or changes in osmotic pressure can cause the separate materials to mix together. However, upon cooling or returning to the original pressure, the materials will separate again.

"Grouping materials in an artificial cell and the reversibility of this process should be possible," said the researcher. "Then we can group together certain compounds that we want at a certain point in time and separate them afterwards, and this is to control their activity."

The researcher believes that some of the chemical reactions that occur in the cell may be controlled by this grouping and scattering process. If she can produce this type of grouping in her artificial cells, this system can be studied more easily.

"The answer to the question of whether a kibbutz could be used in this role is probably positive," said the researcher. "However, the proof of this is not easy at all. Using a reversible and simple model system we can test this idea and learn what the nature and level of this factor is."

These primitive cells also demonstrate macromolecular crowding in the cytoplasm, which occurs due to the fact that large particles, such as proteins, compete with each other for the same given volume. Macromolecules have long chains of atoms, sometimes branched, and within their structure there is a great deal of free volume. When they stand in front of another large compound, these macromolecules cannot fill the same volume as their original and therefore they shrink inward, filling part of the free volume and eventually becoming less voluminous (more compressed), without any loss of mass.

Compressed macromolecules behave differently from non-compressed macromolecules. The reaction rates of dense macromolecules can vary significantly relative to the rates of the same molecules in an unlimited volume. These artificial cells with artificial cytoplasms will allow scientists to study the effects of molecular crowding in a controlled manner. The researchers are also examining the effects that polarization and the addition of other types of compounds have on these heterogeneous cells.

"One of the things we are looking at is the question of how pigeons behave in this type of model cell," said the researcher. "We are interested in taking advantage of the effect that common ions such as potassium and magnesium have on the structures and activity of proteins and nucleic acids."

These artificial cells, which contain polymers, may offer a very simple experimental system for examining the processes that occur in the much more complex environment of biological cells.

The original news on the university website