Transcript Chapter 7
CHAPTER 7 MEMBRANE STRUCTURE AND FUNCTION CELL MEMBRANE • Selectively Permeable – some substances cross more easily than others • Made of: 1. Lipids 2. Proteins 3. Carbs PHOSPHOLIPIDS • Amphipathic – has a hydrophobic (nonpolar) and hydrophilic (polar) side • Forms a bilayer FLUID MOSAIC MODEL • Plasma membrane is fluid • Proteins are embedded and attached like a mosaic DAVSON AND DANIELLI - 1935 • Scientists knew the membrane has proteins and lipids • D & D sandwich model • Phospholipid bilayer between two layers of protein SINGER AND NICOLSON • Used freeze-fracture to split the membrane • Saw that the proteins are embedded • Like chunky peanut butter! MEMBRANE FLUIDITY • Membrane held together by hydrophobic interactions • Weaker than covalent bonds • Phospholipids move rapidly within membrane • About the consistency of salad oil BARRIERS TO FLUIDITY • Temperature • Unsaturated can stay liquid longer • Kinks in lipid tails prevent packing BARRIERS TO FLUIDITY • Cholesterol • Steroid that packs in between • high temps restricts phospholipid movement • low temps keeps it from solidifying MEMBRANE PROTEIN MOVEMENT • Some drift • Some move in directed manner • Others are attached to cytoskeleton MEMBRANE PROTEINS • Integral Proteins • Within the hydrophobic core • Nonpolar amino acids make up middle region • Peripheral Proteins • Not embedded in bilayer • Loosely bound to surface Fig. 7-7 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE TRANSPORT PROTEINS • Span the membrane like a tunnel • Change shape to move things in and out • Some use ATP ENZYMATIC PROTEINS • Active site exposed to inside or outside • Sometimes several in a pathway SIGNAL TRANSDUCTION • Receptor for a chemical messenger • Ex: hormone • Usually changes shape and relays message CELL-CELL RECOGNITION • ID tags that are recognized by proteins on other cells ECM ATTACHMENT • Protein noncovalently bonded to parts of ECM • Microfilaments etc. • Helps to: • stabilize ECM • maintain cell shape • coordinate changes INTERCELLULAR JOINING • Membrane proteins of adjacent cells hook together • Ex: tight or gap junctions