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Animal cells have a selectively permeable membrane surrounding them that separates the interior contents of the cell from the exterior environment. The process by which ions and small soluble molecules, or solutes, pass through the cell membrane is known as membrane transport. These molecules are usually substances vital to the function and maintenance of the cell, such as glucose and amino acids. There are four main types of membrane transport: passive diffusion, or simply diffusion; facilitated diffusion; primary active transport; and secondary active transport. Many of these transport mechanisms involve the use of specialized protein molecules located in the cell membrane called membrane transport proteins.
Passive diffusion occurs spontaneously, and is driven by the random activity of molecules in a solution. Molecules move from an area of high concentration, where there are many of them densely packed together, to an area of low concentration, where there are fewer molecules spaced further apart. Small molecules may achieve membrane transport by diffusing through the cell membrane. Rate of diffusion can be affected by many things, including the composition of the cell membrane and the size and charge of the molecule. The most well-known kind of passive diffusion is osmosis, a process involving the movement of water molecules from an area of high concentration to an area of lower concentration.
Facilitated diffusion involves the use of membrane transport proteins within the cell membrane called channel proteins. These proteins act like pores in the cell membrane, allowing water soluble particles to pass through, but barring the passage of lipophilic, or "fat-loving", molecules. Diffusion follows the same mechanism of action, with molecules moving from areas of high concentration to areas of low concentration.
Primary active transport uses energy to move ions and molecules from areas of high concentration to areas of low concentration. The energy required for primary active transport to take place is usually in the form of a nucleotide called adenosine triphosphate (ATP). One of the most commonly occurring forms of active transport is the sodium-potassium pump, which helps cells to maintain an electrical charge known as the resting potential, and also controls cell volume. The sodium-potassium pump moves sodium ions to the exterior of the cell and releases potassium ions into the cell's cytoplasm.
Secondary active transport uses membrane transport proteins called antiporters and symporters. Antiporters move ions and molecules by transporting one type of particle against its usual concentration gradient, from low to high concentration, while transporting the other type of particle in the normal way, from high to low concentration. Symporters transport two different types of molecule or ion across the cell membrane at the same time and in the same direction.