Final answer:
The cell membrane is selectively permeable, allowing it to regulate the movement of molecules in and out of the cell; passive and active transport are the methods by which substances are moved across the membrane.
Step-by-step explanation:
The cell membrane is selectively permeable, which means it allows only certain molecules to pass through while blocking others. This functionality is crucial for maintaining the cell's internal environment.
Passive transport is one process that drives the movement of substances across the cell membrane without the need for cellular energy. It includes methods such as diffusion, osmosis, and facilitated diffusion. Small, nonpolar substances can move through the lipid bilayer because they are compatible with the hydrophobic interior of the membrane. Hydrophilic substances, however, require assistance due to their inability to pass through the hydrophobic lipid tails. This assistance comes in various forms, such as protein channels or carriers, which facilitate their movement across the membrane.
Some substances are critical to a cell's function, and active transport mechanisms must be used to move these substances against their concentration gradient, often requiring energy in the form of ATP. Hence, the cell membrane's selective permeability is essential for the cell's survival and function.