The Hydrophobic Core Makes the Bilayer a BarrierPhospholipids are amphipathic: their heads are attracted to water and their tails avoid it.In water they form a bilayer, with heads facing the water and tails clustered inward, away from it.Lipid bilayer in plasma membraneDefinitionSee other references1AmphiphaticA molecule with both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions, such as a phospholipid.The Hydrophobic Core Blocks Ions, Polar Molecules, and Large MoleculesThe hydrophobic core formed by the tails limits what can cross the membrane.Ions and polar molecules, such as glucose, cannot cross easily because they are attracted to water.Large molecules, such as proteins, are too big to pass without help.Only small, nonpolar molecules like oxygen and carbon dioxide pass through freely.AnalogyThe hydrophobic core works like a security gate.Small, nonpolar molecules such as oxygen and carbon dioxide walk straight through.Larger or polar molecules need a "key", which is a transport protein.Aqueous Solutions Sit on Both Sides of the MembraneCell membranes are surrounded by aqueous solutions on both sides.External solution: water with dissolved solutes outside the cell.Internal solution (cytoplasm): also a watery environment with dissolved solutes.Water and small solutes move about randomly in these solutions.Solutes near the surface may reach the hydrophilic phosphate heads.When they meet the hydrophobic core, they are repelled back into the solution.ExampleWater molecules are polar but small enough to cross the membrane slowly.Larger molecules like glucose need transport proteins to cross.Active recallWhy can water pass through the membrane more easily than glucose?Which part of the phospholipid forms the barrier, and why?Name two types of substance blocked by the hydrophobic core.What kind of molecule can diffuse freely across the bilayer?
