Transport proteins are proteins within the membranes of cells that transport substances such as molecules and ions across the membrane or within the cell, or can be involved in vesicular transport.

Transport proteins in blood plasma bind and carry specific molecules or ions from one organ to another.
Hemoglobin of erythrocytes binds oxygen as the blood passes through the lungs, carries it to the peripheral tissues, and there releases it to participate in the energy-yielding oxidation of nutrients. The blood plasma contains lipoproteins, which carry lipids from the liver to other organs.

Other kinds of transport proteins are present in the plasma membranes and intracellular membranes of all organisms; these are adapted to bind glucose, amino acids, or other substances and transport them across the membrane.