What Are Transport Proteins? Transport proteins are integral membrane proteins responsible for moving ions, small molecules, and macromolecules across biological membranes. Because the lipid bilayer is inherently impermeable to most polar or charged substances (e.g., glucose, amino acids, ions), these proteins are essential for cellular nutrition, signaling, waste removal, and maintaining homeostasis. Core Functions 1. Selective Permeability & Regulation Transport proteins act as gatekeepers, ensuring that only specific molecules enter or leave the cell or its organelles. They prevent toxic accumulations, maintain pH, and control ion gradients (e.g., Na⁺, K⁺, Ca²⁺) necessary for electrical excitability in neurons and muscle cells. 2. Two Primary Transport Mechanisms Transport proteins function via two main strategies: channels and carriers (or transporters).