by which process do the molecules move across the plasma membrane?

It depends entirely on the type of molecule.

Water moves across via osmosis, which does not use any energy.

However, other molecules move across via passive or active transport.

Passive transport or diffusion does not use any energy. Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. Examples are Water, O2, and CO2.


Active transport does take energy. Active transport requires the cell to spend energy, usually in the form of ATP. Examples include transport of large molecules (non-lipid soluble) and the sodium-potassium pump. Common examples of this are Na+, Ca+, or K+