I. Chemical Properties of Phospholipids. Figure 1: Phospholipid Molecule. Amphiphatic:

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1 I. Chemical Properties of Phospholipids Figure 1: Phospholipid Molecule Amphiphatic: a) The amphiphatic nature & cylindrical shape of phospholipids contributes to their ability to assume bilayers in an aqueous environment (each layer is a mirror-image of the other, with fatty acid tails oriented inward to form a hydrophobic core -see figure 1.1). Figure 1.1: Phospholipid Bilayer 1

2 II. Plasma Membrane Models Figure 2: Davson-Danielli Sandwich Model (1935) Davson-Danielli (1935): a) When isolated from the other membrane components, membrane proteins were found to vary widely in both their shape & size. b) Later, it was discovered that the plasma membrane maintained a uniform thickness of 10 nm s & the membrane protein exhibited amphiphatic properties. These observations called into question the placement of the membrane proteins in the sandwich since: The irregular size & shape of the membrane proteins was incompatible with the observation that the membrane maintained a uniform thickness of 10nm s. The amphiphatic nature of the membrane proteins prohibited them from sitting atop the phospholipid bilayer (some of their hydrophobic regions would be in contact with the aqueous surroundings). c) Since it could not be reconciled with new evidence, the Davson & Danielli s model was ultimately abandoned. Figure 2.1: Fluid Mosaic Model (1970 s) Fluid Mosaic Model: *The protein-phospholipid ratio of the plasma membrane is about 1.14 to 1 (ratio varies among organelles). 2

3 Integral Proteins: a) Transmembrane Proteins are integral proteins that span the entire bilayer (e.g. transport proteins). Peripheral Proteins: Maintaining Membrane Bilayer Fluidity In order to prevent solidification at low temperatures, cells can alter their chemical makeup in response to changing temperature. In animal cells, cholesterol is added to the membrane in order to maintain fluidity. In plant cells, more unsaturated phospholipids are incorporated. Figure 3: Methods of Maintaining Fluidity Animal Cell Membranes Plant Cell Membranes Cell-Cell (Intercellular) Junctions Figure 4: Tight Junctions Tight Junctions: a) Found between the cells lining a body cavity (endothelium) to create a tight seal & prevent fluid leakage into underlying tissue. b) Prevents migration of membrane proteins between the top & bottom surfaces to maintain surface specialization. 3

4 Figure 4.1: Desmosomes Desmosomes: a) Found between cells subjected to tension or stretching forces (e.g. skin, gums, cervix). Figure 4.2: Gap Junctions Gap Junctions: a) Allows for the rapid transmission of substances between cells (e.g. ions, hormones, etc) for rapid intercellular communication. 4

5 Figure 4.3: Plasmodesmata Plasmodesmata: III. Transport through Membrane Bilayers Figure 5: Solutes & Hydration Shells Hydration Shells can increase the size of polar solutes & ions to make their movement through the bilayer portion of the plasma membrane more difficult than that of nonpolar substance that lack hydration shells. 5

6 Table 1: Relative Membrane Permeability of Various Particles Particle Type Example Membrane Migration Small Uncharged Polar Molecules Water, Glycerol, Ethanol Yes Small Hydrophobic Molecules Carbon Dioxide, Oxygen, Nitrogen Yes Nonpolar Molecules Hydrocarbons Yes Ions & Polar Molecules H +, K +, Na +, etc Not Directly Large Uncharged Polar Molecules Glucose, Amino Acids, Nucleotides Not Directly Forms of Membrane Transport Figure 6: Passive Transport Passive Transport: 6

7 Figure 7: Passive Transport: Filtration Filtration to Produce Coffee Filtration of Solutes to form ICF *The Hydrostatic Pressure of the fluid in the capillary is enough to force it & dissolved substances out of the vessel & into the surrounding tissue. This is similar to hot water flowing through a coffee filter, to carry solutes from the grounds into the pot. Filtration: a) Hydrostatic Pressure: force exerted on a membrane or other barrier by water, leading to its outward migration from a space (assuming the barrier is permeable). 7

8 Figure 7.1: Passive Transport: Simple Diffusion Simple Diffusion: Factors affecting simple diffusion rates a) Particle Size & Charge: the smaller, more hydrophobic (nonpolar) the molecule, the more readily it diffuses through the phospholipid bilayer. b) Concentration Gradient: particles diffuse more rapidly with a greater concentration difference between 2 regions. c) Distance: time required for diffusion is directly proportional to the distance traveled. d) Temperature: an increase in temperature indicates an increase in KE (kinetic energy) & thus an increase in diffusion rate (molecules vibrate more vigorously). e) Membrane Permeability: membrane fluidity promotes higher diffusion rates. Osmosis & Water Potential Osmosis is a special case of diffusion involving the passive migration of free (unbound) water molecules. Figure 7.2: Passive Transport: Osmosis Water Potential ( ) is the tendency of water to flow away from a region via osmosis & is determined by 2 factors: = s + p s (Solute Potential): is the tendency of water to flow from an area based on its solute concentration relative to surrounding areas. When considering solute potential, remember the following: a) Pure water is assigned a solute potential of 0. b) Adding solutes lowers solute potential to a negative value, making water LESS LIKELY to flow from a region. Solutions are described according to their solute concentrations relative to other solutions using the following terms: a) Hypertonic Solution: exhibits a lower ψs compared to another solution. b) Hypotonic Solution: exhibits a higher ψs compared to another solution. c) Isotonic Solution: same ψs as another solution. d) In the absence of an external force (ψp), water always flows from a region of high ψs (less solutes, more free water) to low ψs (more solutes, less free water). 8

9 p (Pressure Potential): is the tendency of water to flow from an area based on the application of an outside force. When considering solute potential, remember the following: a) Regions open to their surroundings exhibit pressure potentials of 0. b) Compressional forces raise the pressure potential to positive values, increasing the likelihood of water flowing from an area. c) Tensional forces lower pressure potential to negative values, making water less likely to flow from an area. Figure 7.3: Predicting Osmotic Flow *Water will flow toward the region of lower water potential (side B [ = -.3]) until equilibrium is achieved between osmotic pressure & hydrostatic pressure. Figure 7.4: Illustrating Water Potential: Physical Model (Glass U-Tube) Time 0: Initial Conditions 9

10 Time 1: Final Conditions Figure 7.5: Illustrating Effects of Pressure Potential *As can be seen in the final diagram, a positive (+) pressure potential & negative (-) solute potential have antagonistic effects on one another (the larger of the two values will determine the overall water potential & direction of flow). **A negative (-) solute potential & negative pressure potential have synergistic (additive) effects on the direction (and rate) the osmotic migration of water. 10

11 Figure 7.6: Effects of Water Potential on Animal Cells Figure 7.7: Effects of Water Potential on Plant Cells Calculating Water Potential Problem 1: If a plant cell s p is 2 bars & its s is -3.5 bars, what is the cell s overall? Problem 2: This plant cell is then placed in an open beaker of sugar water with a s of -4.0 bars. Towards which direction will the net flow of water be? 11

12 Problem 3: A plant cell with a of s of -.85MPa maintains a constant volume when placed in an open container having a solution with a s of -.64MPa. Determine the p of the cell. Problem 4: Determine the solute potential of a solution in an open container at 20C having a molar concentration of.5m. i = ionization constant (assumed to be 1 for non-electrolytes). C = molar concentration. R = pressure constant (.0831L bars/mole K OR.00831LMPa/moleK). T = temperature in Kelvin (273 + o C). Figure 8: Aquaporin Structure Due to the relatively high water permeability of some cells, it was long suspected that an additional means for transmembrane water transport must exist. Aquaporins: a) Are abundant in certain cells, such as those associated with the tubules of the kidneys (nephrons) to regulate the reabsorption of water back into the bloodstream during urine production. 12

13 Carrier Mediated Transport To passively transport essential ions & polar molecules through the plasma membrane, transmembrane Carrier Proteins are required. This method of transport is called Facilitated Diffusion, for the existence of carrier proteins facilitate ( make easier ) the migration of specific solutes. Carrier proteins that participate in facilitated diffusion include: Figure 10: Carrier Mediated Passive Transport: Facilitated Ion Diffusion Ion Channels Electrochemical Gradient Ion Channels: are lined with charged regions of amino acids that attracted ions of the opposite charge. Upon entering the channel, the ion sheds its hydration shell & is pushed to the opposite side of the membrane by the like-charged ions behind it. Electrochemical Gradient a) The concentration gradient of an ion & the orientation of electric charge across the membrane may combine to promote the migration of an ion across a membrane. In some cases, these factors may oppose each other to prevent the migration of an ion. Figure 10.1: Carrier Mediated Passive Transport: Carrier Proteins Carrier Proteins: bind a specific solute & undergo a series of conformational (shape) changes that carries the solute to the other side of the membrane. The carrier releases the solute &, through another conformational change, reorients to its original shape. 13

14 Figure 10.2: Active Transport: Sodium-Potassium Pump The Sodium-Potassium Pump is an ATP-powered carrier protein that pumps 3Na + out of the cell for every 2K + pumped into the cell. The Na-K pump performs vital functions: Carrier Mediated Active Transport Figure 10.3: Secondary Active Transport / Cotransport 14

15 Cotransport (Secondary Active Transport): Vesicle Mediated Transport (VMT): Figure 10.4: VMT: Endocytosis Endocytosis: Figure 10.5: Forms of Endocytosis Phagocytosis Pinocytosis Endocytosis: Phagocytosis -localized region of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches into the cytoplasm from the plasma membrane as a vesicle containing material that had been outside the cell. Practiced mainly by unicellular eukaryotes (protists) & certain white blood cells. Endocytosis: Pinocytosis -tiny drops of fluid containing dissolved materials are trapped by folds in the plasma membrane which pinch off into the cytosol to form tiny vesicles. The liquid contents are then transferred into the cytosol. Practiced by ALL eukaryotic cells. 15

16 Figure 10.6: Receptor-Mediated Endocytosis Receptor Mediated Endocytosis (RME): enables the cell to select for & transport large quantities of a specific solute into the cell at the exclusion of all others. Examples: insulin & most protein hormones are absorbed via RME. LDL cholesterols in transit are taken into cells via RME; LDL metabolized & cholesterol aspect incorporated into membranes. Figure 10.7: VMT: Exocytosis Exocytosis: 16