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Transcript
Membrane Structure and Function Chapter 7 Objectives Describe the fluid mosaic model of cell membranes and the roles of proteins in the membranes Define the terms diffusion, osmosis, facilitated diffusion, active transport, exocytosis, endocytosis, isotonic, hypotonic and hypertonic Membrane Function Organizes chemical activities of cell separates cells from outside environment controls passage of molecules across membranes partitions organelle function in eukaryotes provides reaction surfaces and organizes enzymes and their substrates Membrane Structure Phospholipids of membranes form bilayers phospholipids have polar “head” and nonpolar “tail” form stable bilayer in water with heads out and tails in hydrophobic interior forms barrier to hydrophilic molecules Membrane Structure Membrane is fluid mosaic of lipids and proteins proteins embedded in phospholipid bilayer individual some molecules free to move laterally proteins extend through both sides of bilayer cholesterol helps stabilize animal cell membranes at different temperatures lipids in membrane are not fixed lipids can move in membrane - semi-fluid nature of membrane two major classes of proteins in membrane integral – transmembrane peripheral - loosely associated with membrane surface Amphipathic: ___________________ membrane also shows “sidedness” interior - attachment to cytoskeleton exterior - carbohydrates, extracellular matrix An integral protein could be a transporter A. B. True False Membrane Function Proteins make membrane a mosaic of function identification tags-glycoproteins enzymes receptors-trigger binds cell junctions transporters cell activity when molecular messenger Membrane Function Diffusion across a membrane diffusion is tendency of molecules to spread out spontaneously from area of high concentration to area of low concentration passive diffusion across membranes occurs when molecules diffuse down concentration gradient at equilibrium molecules diffuse back and forth-no net gain or loss Different molecules diffuse independently of each other Ex. Water, Sodium, Chloride Membrane Function Osmosis is diffusion of water (passive) if cell membrane permeable to water but not solute separates area of high solute concentration (hypertonic) from area of low concentration (hypotonic), water diffuses from hypotonic area to hypertonic area until concentrations are equal direction of osmosis is determined by differences in relative concentrations Gradient = concentration difference Isotonic Isotonic = solutions of equal solute concentration (no osmosis) are isotonic Hypotonic = more solvent relative to solute Hypertonic = more solute relative to solvent Membrane Function Water balance between cells and surroundings critical cell membranes semi-permeable cells in isotonic solution do not change size -no osmosis cells in hypotonic solution gain water cells in hypertonic solutions lose water The cell contains 80% water, the beaker 10% salt. What will happen to the cell? A cell that contains 22% solute is in a beaker that contains 22% solvent. Which of the following is correct? A. B. C. The cell is hypotonic to the solution in the beaker The cell is hypertonic to the cell in the beaker The cell is isotonic to the solution in the beaker Membrane Function Specific proteins facilitate diffusion across membranes facilitated diffusion occurs when protein pore in membrane allows solute to diffuse down concentration gradient no energy required rate depends on number of transport proteins and strength of gradient Membrane Function Cells expend energy for active transport transport protein involved in moving solute against concentration gradient energy from ATP-mediated phosphorylation changes protein shape and moves solute molecule across membrane active transport of two solutes in opposite directions often coupled, but not always Transporters have complex subcategories Uniports- One Substance in One Direction Symport- Two Substances in Same Direction Antiports- Two Substances in Opposite Directions Aquaporins – Rapid water transport channels Electrogenic Pumps Sometimes the transporters pump ions that cause an electrical gradient to from across the membrane An example: This occurs in your nerve cells Electrogenic pumps, like the Na+-K+ pump, generate voltage across membranes. The resulting voltage, or membrane potential, is energy that can be used to drive the transport of ions against a chemical gradient Co-transport Membrane proteins co-transport two solutes by coupling the downhill diffusion of one solute with the uphill diffusion of the other Membrane Function Exocytosis and endocytosis transport large molecules exocytosis: membrane-bound vesicles containing large molecules fuse with plasma membrane and release contents outside cell endocytosis: plasma membrane surrounds materials outside cell, closes around materials, and forms membrane-bound vesicles Types of Endocytosis Three important types of endocytosis are: phagocytosis pinocytosis receptor-mediated endocytosis Signal Transduction Across Membranes Proteins embedded in the cell membrane can carry messages and cause a chain reaction of signals that “tell” you cells its time to do an activity or make a chemical in short supply You will learn more about this in other classes