The cell membrane is responsible for the transport of molecules and ions into and out of a cell and by doing so, acts like a crossing guard directing traffic. It is called selectively permeable which means that only specific substances are allowed to cross the membrane. Water, oxygen, carbon dioxide and glucose are some substances that are transported across a cell membrane. In terms of its composition, the membrane is a phospholipid bilayer with protein molecules sandwiched between. Proteins embedded in the membrane perform various functions including transport across the membrane.
Because of the nature of the membrane, some substances pass through easily and others need assistance. The interior of the cell membrane is hydrophobic and non-polar substances like oxygen can easily dissolve in the lipid bilayer and cross the barrier and gain entry into the cell. However, substances that are polar can slowly cross the barrier--including water and glucose. Therefore, to facilitate their passage, transport proteins help by providing a channel into the cell from the outside. These transport proteins work like a passageway or tunnel to speed up the transport of polar molecules.
For example, channel proteins facilitate the swift passage of water across the membrane by allowing the molecules to avoid contact with the lipid bilayer molecules. This type of transport is called facilitated diffusion which relies on no additional expenditure of energy on the cell's part and is based on a concentration gradient from high to low. When water is being transported, the process is also referred to as osmosis. This difference in concentration on one side of the cell membrane compared to the other side, along with the transport proteins providing a way to easily get through the membrane allows polar water molecules to be transported across an animal's cell. Since water is vital to the survival of an animal, this function of transport proteins is critical.