<unk> s are proteins that transport a specific substance or group of substances across intracellular compartments or in extracellular fluids (e.g. in the blood) or else across the cell membrane. Some of the<unk> s are water-soluble proteins that may or may not interact with biological membranes, such as some transporters of small hydrophobic molecules, whereas others are integral transmembrane proteins.<unk> s of the cell membrane transport substances down their concentration gradient out of or into the cell by facilitated diffusion and active transport.
Each<unk>  is designed to recognize only one substance or one group of very similar substances. The molecule or ion to be transported (the substrate) must first bind at a binding site at the<unk>  molecule, with a certain binding affinity. Following binding, and while the binding site is facing, say, outwards, the<unk>  will capture or occlude (take in and retain) the substrate within its molecular structure and cause an internal translocation, so that it now faces the other side of the membrane. The substrate is finally released at that site, according to its binding affinity there. All steps are reversible.
For example:
Carrier protein
Apolipoprotein
Lipoprotein
Glycoprotein
Hemoglobin
Gamma globulin
Cardiolipin
Beta globulins