Carrier proteins are proteins involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. Carrier proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion

Facilitated diffusion

..Facilitated diffusion is a process of passive transport, facilitated by integral proteins. Facilitated diffusion is the spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins...

 or active transport

Active transport

Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...

. These mechanisms of movement are known as carrier mediated transport. Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.

Active Transport

Active transport

Active transport

Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...

 is the movement of a substance across a membrane against its concentration gradient. This is usually to accumulate high concentrations of molecules that a cell needs, such as glucose or amino acids. When the lipid bilayer is impermeable to the molecule needing transport, active transport is also necessary. If the process uses chemical energy, such as adenosine triphosphate (ATP), it is called primary active transport

Primary active transport

Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.Most of the enzymes that perform this type of active transport are transmembrane ATPases. A primary ATPase universal to all cellular life is the sodium-potassium pump, which...

. Secondary active transport

Secondary active transport

In secondary active transport or co-transport, uses energy to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used...

 involves the use of an electrochemical gradient

Electrochemical gradient

An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...

, involves channel proteins as opposed to carrier proteins, and does not use energy produced in the cell.

A carrier protein is required to move particles from areas of low concentration to areas of high concentration. These carrier proteins have receptors that bind to a specific molecule (substrate) needing transport. The molecule or ion to be transported (the substrate) must first bind at a binding site at the carrier molecule, with a certain binding affinity. Following binding, and while the binding site is facing the same way, the carrier will capture or occlude (take in and retain) the substrate

Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...

 within its molecular structure and cause an internal translocation so that the opening in the protein now faces the other side of the plasma membrane. The carrier protein substrate is released at that site, according to its binding affinity there.

Facilitated diffusion

Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific carrier proteins and requires no energy. Facilitated diffusion is used especially in the case of large polar molecules and charged ions; once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the bilayers.
The type of carrier proteins used in facilitated diffusion is slightly different than in active transport. They are still transmembrane carrier proteins, but these are gated transmembrane channels, meaning they do not internally translocate, nor require ATP to function. The substrate is taken in one side of the gated carrier, and without using ATP the substrate is released into the cell.

Examples

Each carrier protein, even within the same cell membrane, is specific to one type or family of molecules. For example, GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer. Other specific carrier proteins also help the body function in important ways. Cytochromes operate in the electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

as carrier proteins for electrons.

Pathology

A number of inherited diseases involve defects in carrier proteins in a particular substance or group of cells. Cysteinuria (cysteine in the urine) is such a disease involving defective cysteine carrier proteins in the kidney cell membranes. This transport system normally removes cysteine from the fluid destined to become urine and returns this essential amino acid to the blood. When this carrier malfunctions, large quantities of cysteine remain in the urine, where it is relatively insoluble and tends to precipitate. This is one cause of urinary stones. Some vitamin carrier proteins have been shown to be overexpressed in patients with malignant disease. For example, levels of riboflavin carrier protein (RCP) have been shown to be significantly elevated in people with breast cancer.