A discovery by scientists in Germany shows how water is the driving force of specific reactions taking place within biological cells.

The human body is composed of trillions of cells that provide structure for the body, take in nutrients from food, generate energy, and carry out specialised functions. Cells also contain the body's hereditary material.

Within the cytoplasm - or main cell body - are organelles, compartments within the cell that are necessary for them to perform a specific function. These compartments are usually isolated from the rest of the cytoplasm through intracellular membranes, but some exist in in spatially distinct units without a membrane.

Biomolecular condensates are a class of membrane-less organelle, they lack the lipid bilayer that keeps certain molecules where to a particular compartment. It is only recently that studies have proved the existence of the condensates as spaces where reaction take place within cells.

Now the reason condensates form, and under what circumstances, is being researched by scientists at Ruhr University Bochum, in Germany, and have identified an often overlooked player - water.

Using computer simulations, Professor Lars Schäfer and Dr Saumyak Mukherjee have found that the sheer number of small water molecules makes them just as important as larger biomolecules in the molecular tug-of-war that underlies the formation of condensates. Writing in the journal Nature Communications, the researchers say understanding the nature of these molecular interactions is important in understanding diseases and complications like Alzheimer’s, Parkinson’s and cataracts.

“These condensates are incredibly densely packed," explains Schäfer, "which means that there is a molecular scrum of biological macromolecules such as proteins and nucleic acids.”

Since only certain macromolecules form such condensates with each other, they can act as micro-reactors for very specific biochemical reactions taking place in the cell.

“It’s therefore not surprising that disruptions in these processes are associated with various diseases,” says Schäfer.

Mukherjee continues, “The underlying driving forces are ultimately hidden in the chemical interactions between the different molecules in the cell. Computer simulations can help shed light on this phenomenon, even in atomic detail.”

As it turns out, water is a frequently overlooked player in the molecular interaction. The properties of the water molecules found within the dense scrum inside condensates, differ from those of the water molecules on the outside.

“The confinement of the water molecules inside the condensate is an unfavourable driving force, while the freedom of the water molecules outside is favourable," explains Schäfer. "The latter win this molecular tug-of-war – if only by a narrow margin.”

In addition to the well known interactions between macromolecules such as proteins within cells, water molecules also play an important role in the formation of biomolecular condensates.

“It’s a bit like David and Goliath,” says Mukherjee. “Here are the small water molecules and there are the big protein molecules. However, there are a lot of water molecules, and together they add as much to the driving force as the large proteins.”