B. The protein-free lipid bilayers are highly impermeable to ions HYDROPHOBIC COz o If uncharged solutes are small enough, MOLECULES N2 they can move down their concentration benzene gradients directly across the lipid UNCHARGED H20 bilayer by simple diffusion. POLAR urea MOLECULES lye erol Most solutes can cross the membrane LARGE only if there is a membrane transport UNCHARGED glucose POLAR sucrose protein to transfer them. MOLECULES H Na x Passive transport, in the same IONS HCO, K Ca C direction as a concentration gradient. M Active transport, is mediated by Diffusion of small molecules across carrier proteins, against a concentration synthetic phospholipid lipid gradient, require an input of energy. bilayer bilayersFigure 11-1 The relative permeability of a synthetic lipid bilayer to different classes of molecules. The smaller the molecule and, more important, the fewer hydrogen bonds it makes with water, the more rapidly the molecule diffuses across the bilayer. B. The protein-free lipid bilayers are highly impermeable to ions. ❖If uncharged solutes are small enough, they can move down their concentration gradients directly across the lipid bilayer by simple diffusion. ❖Most solutes can cross the membrane only if there is a membrane transport protein to transfer them. ❖Passive transport, in the same direction as a concentration gradient. ❖ Active transport, is mediated by carrier proteins, against a concentration gradient, require an input of energy. Diffusion of small molecules across phospholipid bilayers