Membrane Structure and Function Chapter 7

n Objectives F Describe the fluid mosaic model of cell membranes and the roles of proteins in the membranes F Define the terms diffusion, osmosis, facilitated diffusion, active transport, exocytosis, endocytosis, isotonic, hypotonic and hypertonic

Membrane Function n Organizes chemical activities of cell u separates cells from outside environment u controls passage of molecules across membranes u partitions organelle function in eukaryotes u provides reaction surfaces and organizes enzymes and their substrates

Membrane Structure n Phospholipids of membranes form bilayers u phospholipids have polar “head” and nonpolar “tail” u form stable bilayer in water with heads out and tails in u hydrophobic interior forms barrier to hydrophilic molecules

Membrane Structure n Membrane is fluid mosaic of lipids and proteins u proteins embedded in phospholipid bilayer u individual molecules free to move laterally u some proteins extend through both sides of bilayer u cholesterol helps stabilize animal cell membranes at different temperatures

u lipids in membrane are not fixed F lipids can move in membrane - semi-fluid nature of membrane u two major classes of proteins in membrane F integral – transmembrane peripheral - loosely associated with membrane surface Amphipathic: ___________________ u membrane also shows “sidedness” F interior - attachment to cytoskeleton F exterior - carbohydrates, extracellular matrix

An integral protein could be a transporter A. True B. False

Membrane Function n Proteins make membrane a mosaic of function u identification tags-glycoproteins u enzymes u receptors-trigger cell activity when molecular messenger binds u cell junctions u transporters

Membrane Function n Diffusion across a membrane u diffusion is tendency of molecules to spread out spontaneously from area of high concentration to area of low concentration u passive diffusion across membranes occurs when molecules diffuse down concentration gradient F at equilibrium molecules diffuse back and forth-no net gain or loss

n Different molecules diffuse independently of each other n Ex. Water, Sodium, Chloride

Membrane Function n Osmosis is diffusion of water (passive) u 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 u direction of osmosis is determined by differences in relative concentrations u Gradient = concentration difference

Isotonic u Isotonic = solutions of equal solute concentration (no osmosis) are isotonic u Hypotonic = more solvent relative to solute u Hypertonic = more solute relative to solvent

Membrane Function n Water balance between cells and surroundings critical u cell membranes semi-permeable u cells in isotonic solution do not change size -no osmosis u cells in hypotonic solution gain water u 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. The cell is hypotonic to the solution in the beaker B. The cell is hypertonic to the cell in the beaker C. The cell is isotonic to the solution in the beaker

Membrane Function n Specific proteins facilitate diffusion across membranes u facilitated diffusion occurs when protein pore in membrane allows solute to diffuse down concentration gradient u no energy required u rate depends on number of transport proteins and strength of gradient

Membrane Function n Cells expend energy for active transport u transport protein involved in moving solute against concentration gradient u energy from ATP-mediated phosphorylation changes protein shape and moves solute molecule across membrane u active transport of two solutes in opposite directions often coupled, but not always

Transporters have complex subcategories n Uniports- One Substance in One Direction n Symport- Two Substances in Same Direction n Antiports- Two Substances in Opposite Directions n Aquaporins – Rapid water transport channels

Electrogenic Pumps n Sometimes the transporters pump ions that cause an electrical gradient to from across the membrane n An example: This occurs in your nerve cells n 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 n Membrane proteins co-transport two solutes by coupling the downhill diffusion of one solute with the uphill diffusion of the other

Membrane Function n Exocytosis and endocytosis transport large molecules u exocytosis: membrane-bound vesicles containing large molecules fuse with plasma membrane and release contents outside cell u endocytosis: plasma membrane surrounds materials outside cell, closes around materials, and forms membrane-bound vesicles

Types of Endocytosis u Three important types of endocytosis are: F phagocytosis F pinocytosis F receptor-mediated endocytosis

Signal Transduction Across Membranes n 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 n You will learn more about this in other classes