Homeostasis is defined as the maintenance of a stable internal environment. In order to maintain homeostasis, cells continually transport substances in and out across the cell (plasma) membrane.
The cell membrane is the cellular structure that regulates the transport of materials into and out of the cell. The lipid bilayer architecture of the cell membrane allows certain molecules to pass through while keeping others out. The cell membrane is selectively permeable.
Enter cell: Ions, Nucleotides, Sugars, Oxygen, Amino acids, Water, Hormones (some), Vitamins
Leave cell: Ions, Urea, Water, Carbon dioxide, Secreted proteins