Kip McGilliard Eastern Illinois University Lauralee Sherwood Hillar Klandorf Paul Yancey Chapter 3 Membrane Physiology Sections

3.1 Membrane Structure and Composition  Plasma membrane Encloses the intracellular contents Selectively permits specific substances to enter or leave the cell Responds to changes in cell’s environment Trilaminar structure under electron microscopy

Cell 1Plasma membranes Intercellular space Cell 2 Figure 3-1 p71

3.1 Membrane Structure and Composition  The plasma membrane is a fluid lipid bilayer embedded with proteins. Phospholipids Most abundant membrane component Head contains charged phosphate group (hydrophilic) Two nonpolar fatty acid tails (hydrophobic) Assemble into lipid bilayer with hydrophobic tails in the center and hydrophilic heads in contact with water Fluid structure as phospholipids are not held together by chemical bonds

3.1 Membrane Structure and Composition

Figure 3-2a p71 Choline Head (negatively charged, polar, hydrophilic) Phosphate Glycerol Tails (uncharged, nonpolar, hydrophobic) Fatty acids (a) Phospholipid molecule

(b) Organization of phospholipids into a bilayer in water Polar heads (hydrophilic) Nonpolar tails (hydrophobic) Polar heads (hydrophilic) ECF (water) ICF (water) Lipid bilayer Figure 3-2b p71

Intracellular fluid Extracellular fluid Lipid bilayer (c) Separation of ECF and ICF by the lipid bilayer Figure 3-2c p71

3.1 Membrane Structure and Composition  The plasma membrane is a fluid lipid bilayer embedded with proteins. Cholesterol Placed between phospholipids to prevent crystallization of fatty acid chains Helps stabilize phospholipids’ position Provides rigidity, especially in cold temperatures Cold-induced rigidity is countered in some poikilotherms by enriching membrane lipids with polyunsaturated fatty acids

3.1 Membrane Structure and Composition  Membrane proteins Integral proteins are embedded in the lipid bilayer Have hydrophilic and hydrophobic regions Transmembrane proteins extend through the entire thickness of the membrane Peripheral proteins are found on inner or outer surface of membrane Polar molecules Anchored by weak chemical bonds to polar parts of integral proteins or phospholipids

3.1 Membrane Structure and Composition  Two models of membrane structure Fluid mosaic model Membrane proteins float freely in a “sea” of lipids Membrane-skeleton fence model Mobility of membrane proteins is restricted by the cytoskeleton

3.1 Membrane Structure and Composition  Specialized functions of membrane proteins Channels Carriers Receptors Docking-marker acceptors Enzymes Cell-adhesion molecules (CAMs) Self-identity markers

3.1 Membrane Structure and Composition  Membrane carbohydrates Located only on outer surface of membrane Short-chain carbohydrates bound to membrane proteins (glycoproteins) or lipids (glycolipids) Important roles in self-recognition and cell-to-cell interactions

Extracellular fluid Dark line Integral proteins Carbohydrate chain Appearance using an electron microscope Light space Phospholipid molecule Dark line Glycolipid Glycoprotein Receptor protein Lipid bilayer Cholesterol molecule Leak channel protein Gated channel protein Peripheral proteins Cell adhesion molecule (linking microtubule to membrane) Carrier protein Microfilament of cytoskeleton lntracellular fluid Figure 3-3 p72

ANIMATION: Cell membranes To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

3.2 Unassisted Membrane Transport  The plasma membrane is selectively permeable Permeability across the lipid bilayer depends on: High lipid solubility Small size Force is needed to produce the movement of particles across the membrane Passive forces do not require the cell to expend energy Active forces require cellular energy (ATP)

3.2 Unassisted Membrane Transport  Diffusion Random collisions and intermingling of molecules as a result of their continuous, thermally induced random motion Net movement of molecules from an area of higher concentration to an area of lower concentration Equilibrium is reached when there is no concentration gradient and no net diffusion

3.2 Unassisted Membrane Transport

 Fick’s law of diffusion The rate at which diffusion occurs depends on: Concentration gradient Permeability Surface area Molecular weight Distance Temperature

3.2 Unassisted Membrane Transport  The movement of ions across the membrane is affected by their electrical charge. A difference in charge between two adjacent areas produces an electrical gradient. An electrical gradient passively induces ion movement -- conduction Only ions that can permeate the plasma membrane can conduct down this gradient. The simultaneous existence of an electrical gradient and concentration gradient is called an electrochemical gradient.

3.2 Unassisted Membrane Transport

 Osmosis Water moves across a membrane by osmosis, from an area of lower solute concentration to an area of higher solute concentration. Driving force is the water concentration gradient Hydrostatic pressure opposes osmosis Osmotic pressure is the pressure required to stop the osmotic flow Osmotic pressure is proportional to the concentration of nonpenetrating solute

3.2 Unassisted Membrane Transport

 Colligative properties of solutes depend solely on the number of dissolved particles in a given volume of solution Osmotic pressure Elevation of boiling point Depression of freezing point Reduction of vapor pressure

3.2 Unassisted Membrane Transport  Tonicity refers to the effect of solute concentration on cell volume Isotonic solution Same concentration of nonpenetrating solutes as in normal cells Cell volume remains constant Hypotonic solution Lower solute concentration than in normal cells Cell volume increases, perhaps to the point of lysis Hypertonic solution Higher solute concentration than in normal cells Cell volume decreases, causing crenation

3.2 Unassisted Membrane Transport

3D ANIMATION: Osmosis

3.3 Assisted Membrane Transport  Phospholipid bilayer is impermeable to: Large, poorly lipid-soluble molecules (proteins, glucose, and amino acids) Small, charged molecules (ions)  Mechanisms for transporting these molecules into or out of the cell Channel transport Carrier-mediated transport Vesicular transport

3.3 Assisted Membrane Transport  Channel transport Transmembrane proteins form narrow channels Highly selective Permit passage of ions or water (aquaporins) Gated channels can be open or closed Leak channels are open at all times Movement through channels is faster than carrier-mediated transport

3.3 Assisted Membrane Transport

Figure 3-10 p82 Outside cell Water molecule Lipid bilayer membrane Aquaporin Cytosol

Figure 3-10 p82

3.3 Assisted Membrane Transport  Carrier-mediated transport Transmembrane proteins that can undergo reversible changes in shape Binding sites can be exposed to either side of membrane Transport small water-soluble substances Facilitated diffusion or active transport

3.3 Assisted Membrane Transport  Characteristics of carrier-mediated transport systems Specificity -- each carrier protein is specialized to transport a specific substance Saturation -- limit to the amount of a substance that a carrier can transport in a given time (transport maximum or T m ) Competition -- closely related compounds may compete for the same carrier

3.3 Assisted Membrane Transport

 Facilitated diffusion Passive carrier-mediated transport from high to low concentration Does not require energy Example: Glucose transport into cells

3.3 Assisted Membrane Transport  Facilitated diffusion Molecule to be transported attaches on binding site on protein carrier Carrier protein changes conformation, exposing bound molecule to the other side of the membrane (lower concentration side) Bound molecule detaches from the carrier Carrier returns to its original conformation (binding site on higher concentration side)

3.3 Assisted Membrane Transport

ANIMATION: Active and Facilitated Diffusion To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

ANIMATION: Passive transport To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

3.3 Assisted Membrane Transport  Active transport Carrier-mediated transport that moves a substance against its concentration gradient Requires energy Primary active transport Energy is directly required ATP is split to power the transport process Secondary active transport ATP is not used directly Carrier uses energy stored in the form of an ion concentration gradient built by primary active transport

3.3 Assisted Membrane Transport

ANIMATION: Active Transport To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

3.3 Assisted Membrane Transport  Na + -K + ATPase pump Pumps 3 Na + out of cell for every 2 K + in Splits ATP for energy Phosphorylation induces change in shape of transport protein Maintains Na + and K + concentration gradients across the plasma membrane Helps regulate cell volume

3.3 Assisted Membrane Transport

 Secondary active transport Simultaneous transport of a nutrient molecule and an ion across the plasma membrane by a cotransport protein Nutrient molecule is transported against its concentration gradient Driven by simultaneous transport of an ion along its concentration gradient Example: Cotransport of glucose and Na + across the luminal membrane of intestinal epithelial cells

3.3 Assisted Membrane Transport

 Vesicular transport Transport between ICF and ECF of large particles wrapped in membrane-bound vesicles Endocytosis -- incorporates outside substances into cell Exocytosis -- releases substances into the ECF The rate of endocytosis and exocytosis must be balanced to maintain a constant membrane surface area and cell volume Caveolae may play a role in transport of substances and cell signaling