生物膜的脂质双层结构本身是离子和极性分子的通透障碍。但 是要维持正常的细胞功能，生物膜必须允许一些离子和极性分子通 过。 The lipid bilayer of biological membranes is intrinsically impermeable to ions and polar molecules,yet certain such species must be able to cross these membranes for normal cell function 三类膜蛋白一泵，载体和通道 (pumps,carriers and channels)一使生物膜具有这样的功能。 Transported molecule Channel Carrier Pump protein protein Plasma High membrane Energy Low Electrochemical Simple diffusion potential gradient Passive transport Primary active transport (in the direction of (against the direction electrochemical gradient) of electrochemical gradient)生物膜的脂质双层结构本身是离子和极性分子的通透障碍。但 是要维持正常的细胞功能，生物膜必须允许一些离子和极性分子通 过。 The lipid bilayer of biological membranes is intrinsically impermeable to ions and polar molecules, yet certain such species must be able to cross these membranes for normal cell function 三类膜蛋白 － 泵，载体和通道 （pumps, carriers and channels）－使生物膜具有这样的功能