What term describes the ease with which substances can cross the cell membrane?

The term that describes the ease with which substances can cross the cell membrane is permeability. Permeability refers to the ability of a membrane to allow certain substances to pass through it while restricting others. This property is crucial for maintaining cellular homeostasis, as it influences how materials like nutrients, ions, and waste products enter and exit the cell.

In the context of cell membranes, permeability can vary significantly based on the structure and composition of the membrane itself, including factors such as lipid composition and the presence of proteins. A membrane with high permeability allows many substances to cross easily, while a membrane with low permeability restricts movement.

Other terms listed have specific meanings but do not accurately describe the concept in question. Diffusion refers to the passive movement of molecules from an area of higher concentration to an area of lower concentration. Osmosis is a specific type of diffusion that involves the movement of water across a semipermeable membrane. Active transport involves the movement of substances across a cell membrane against their concentration gradient, which requires energy. Thus, while these concepts are related to membrane dynamics, permeability is the most accurate answer in this context.