Protein channels are specialized structures embedded in cell membranes that allow specific molecules to pass in and out of the cell. These channels are made up of proteins that create openings, enabling substances like ions, water, and nutrients to move across the membrane without using energy. This process is essential for maintaining the cell's internal environment and supporting various cellular functions. Each protein channel is selective, meaning it only permits certain molecules to enter or exit. For example, ion channels allow specific ions, such as sodium or potassium, to flow through, while aquaporins facilitate the transport of water. This selectivity is crucial for processes like nerve signaling and maintaining cellular homeostasis.