open system

Meaning: In thermodynamics, the system is the part of the universe under study, and everything else is the surroundings. A system is classified by what can cross its boundary. An open system allows both matter and energy to cross. A closed system allows energy (heat, work) but not matter. An isolated system allows neither, so its total energy stays constant.

On the MCAT: Passages often describe a setup, such as an uncovered beaker of water, a living cell, or a gas in a container with a leak, and ask you to classify it. An uncovered beaker of hot water is open because water vapor escapes and heat flows out. A sealed flask on a hot plate is closed. A perfectly insulated, sealed thermos is approximately isolated. Living organisms and cells are open systems because they take in nutrients and release wastes and heat. This links to biology and biochemistry, where a cell can maintain a steady state far from equilibrium only because it exchanges matter and energy. The first law, ΔU = Q − W, applies cleanly to closed systems. In an open system you must also account for energy carried by matter entering or leaving.

Common confusion: Students mix up closed and isolated. Closed does not mean nothing gets in or out. It means no matter crosses, but heat and work still can. A sealed, uninsulated can of soup still cools to room temperature, so it is closed, not isolated.