Final answer:
Protein molecules embedded in phospholipid bilayers form channels and pumps that enable selective passage of ions and molecules through the cell membrane, maintaining the internal balance of the cell.
Step-by-step explanation:
Molecules embedded in phospholipid bilayers that form channels allowing molecules to pass into and out of the cell are made of protein molecules. The plasma membrane, which contains the phospholipid bilayer, is selectively permeable. This permeability is mainly because of the embedded proteins, which provide pathways for certain molecules to cross the membrane. These transport proteins facilitate the movement of substances that otherwise could not traverse the hydrophobic interior of the membrane, such as ions and polar molecules.
The proteins are arranged within the membrane such that they create channels, acting as gatekeepers, which enable the selective passage of specific molecules. They thus maintain the necessary balance within the cellular environment by regulating the entry and exit of ions, nutrients, and waste products. Moreover, some of these proteins can act as pumps, using energy to actively transport substances across the cell membrane against a concentration gradient.