How cells build compartments without walls
Some cellular structures form not by assembling a membrane around a space, but when certain molecules gather into a concentrated droplet within the cell’s watery interior. This process, called liquid–liquid phase separation, resembles oil separating from water: under the right conditions, molecules become more attracted to one another than to their surroundings and collect into a distinct phase.
Proteins often participate through many weak, reversible interactions. A protein may have several regions that can briefly bind other proteins or RNA; together, those contacts can make a loose network. When enough molecules are present—or when chemical signals change how they interact—the network separates from the surrounding fluid and forms a dense compartment. RNA can help build or organize some of these structures, too.
A temporary workspace, not a sealed organelle
The resulting condensate can enrich particular proteins and RNA while leaving other molecules outside. That brings the right biochemical partners into closer proximity, making a useful workspace for processes such as processing RNA or responding to stress. The cell can alter the condensate’s composition and behavior by changing which molecules are available and how strongly they bind.
Unlike a membrane-bound organelle, a condensate has no enclosing lipid wall. Its molecules can exchange with the surrounding cell, and many condensates can form, change, or dissolve as conditions shift. The nucleolus, where cells make and process components of ribosomes, is a well-known example of a cellular compartment without a surrounding membrane.
Why the droplets are reversible
The same weak interactions that let molecules gather also let them separate again. If a signal changes their binding, or the cell’s conditions no longer favor condensation, the droplet can disperse and its molecules return to the surrounding fluid. This gives cells a way to reorganize biochemical activity without building and dismantling a membrane each time.
The lava-lamp image captures the striking appearance, but the key idea is molecular: a cell can create a concentrated reaction zone simply by changing how its molecules interact. Not every cellular droplet is a perfectly free-flowing liquid, yet this reversible clustering helps explain how cells organize work on short timescales.
