3.3 cell membranes

Cell membrane Also known as plasma membraneAlso known as plasma membrane Makes boundary between cell and outside worldMakes boundary between cell and outside world Selective channels, or pores, allow passage of certain materials in and out of cellSelective channels, or pores, allow passage of certain materials in and out of cell Like a bouncer or gate keeperLike a bouncer or gate keeper

Membrane structure Two layers of phospholipids ( bi layer)Two layers of phospholipids ( bi layer) Various other molecules are dispersed through membrane as wellVarious other molecules are dispersed through membrane as well

Phospholipid structure Charged phosphate groupCharged phosphate group GlycerolGlycerol Two fatty acid chainsTwo fatty acid chains Glycerol + phosphate group = headGlycerol + phosphate group = head 2 fatty acid chains = tail2 fatty acid chains = tail

Head of phospholipid Charged (polar) Charged (polar) Can form hydrogen bonds with water Can form hydrogen bonds with water Hydrophilic (hydro=water philic=loving) Hydrophilic (hydro=water philic=loving)

Tail of phospholipid NonpolarNonpolar Hydrophobic (water fearing)Hydrophobic (water fearing) Because the heads are exposed to the cytoplasm inside the cell and fluids on the outside of the cell, the tails are always pointing INWARD (away from water)Because the heads are exposed to the cytoplasm inside the cell and fluids on the outside of the cell, the tails are always pointing INWARD (away from water) This is why a bilayer formsThis is why a bilayer forms Like a sandwichLike a sandwich

Some Other Molecules Embedded in Membrane CholesterolCholesterol Strengthens cell membraneStrengthens cell membrane ProteinsProteins Extend through one or both layers and help different materials cross membraneExtend through one or both layers and help different materials cross membrane Different cells have different membrane bound proteinsDifferent cells have different membrane bound proteins CarbohydratesCarbohydrates Attached to membrane proteinsAttached to membrane proteins Act as ID tag so cells can distinguish between one anotherAct as ID tag so cells can distinguish between one another

Membrane Fluidity Membranes are flexible, not rigidMembranes are flexible, not rigid Phospholipids in each layer can slide past each other and move aroundPhospholipids in each layer can slide past each other and move around Very “fluid” motionVery “fluid” motion

Fluid Mosaic Model *video clip**video clip*

Selective permeability Membrane allows SOME but not all materials to pass throughMembrane allows SOME but not all materials to pass through Allows cell to maintain homeostasis despite unpredictable outside surroundingsAllows cell to maintain homeostasis despite unpredictable outside surroundings Cell must control import and export of molecules and ionsCell must control import and export of molecules and ions

Crossing the membrane Some methods of crossing require energy, some do not.Some methods of crossing require energy, some do not.  Depends on many factors Molecule sizeMolecule size PolarityPolarity Concentration inside and outside cellConcentration inside and outside cell Generally…Generally…  Small nonpolar molecules easily cross  Small polar molecules are transported by proteins  Large molecules moved in vesicles

Chemical signals are transmitted across membrane Receptor - protein that detects a signaling molecule, binds to it and performs an action in response Receptor - protein that detects a signaling molecule, binds to it and performs an action in response Molecule it binds to is called a ligandMolecule it binds to is called a ligand When a receptor and ligand bind, a conformational (shape) change occursWhen a receptor and ligand bind, a conformational (shape) change occurs Allows interaction with other moleculesAllows interaction with other molecules Intracellular receptor - found inside cell and bind with molecules that have crossed the membrane already Intracellular receptor - found inside cell and bind with molecules that have crossed the membrane already Membrane receptor - stuck in membrane and bind to molecules that cannot enter cell (changes shape and sends message) Membrane receptor - stuck in membrane and bind to molecules that cannot enter cell (changes shape and sends message) *inside the cell video clip!**inside the cell video clip!*