Proteins are the molecular workhorses of the cell, and among their most critical functions is the controlled transport of ions, nutrients, and metabolites across biological membranes. Due to the hydrophobic barrier of the lipid bilayer, most polar molecules and ions cannot diffuse passively. This paper reviews the two primary classes of transport proteins: (facilitating passive diffusion) and carriers/transporters (enabling passive or active transport). We explore the mechanisms of uniport, symport, and antiport, the role of ATP-powered pumps (such as the Na⁺/K⁺-ATPase), and the physiological consequences of transport protein dysfunction, including cystic fibrosis and glucose-galactose malabsorption. Understanding these mechanisms is fundamental to comprehending cellular homeostasis, signal transduction, and pharmacological drug targeting.
: The most abundant protein in human plasma, it transports hormones, fatty acids, and even certain medications through the bloodstream.