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I. Chemical Properties Plasma Membrane Figure 1: Phospholipid Molecule

• Amphiphatic molecules are those that exhibit both hydrophilic & hydrophobic regions. 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

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II. Plasma Membrane Models: 1930’s-Present

Figure 2: Davson-Danielli “Sandwich” Model (1935)

• Davson-Danielli (1935): proposed a plasma membrane model in which the phospholipid bilayer was “sandwiched” between 2 layers of globular (spherical) proteins. In the 1960’s, the validity of this model was questioned, for upon isolating the proteins of the plasma membrane, they were determined to exhibit the following properties … 1)

2)

• These observations regarding the membrane proteins contradicted the sandwich model proposed by Davson & Danielli. Consider the observation that the membrane proteins are of varying sizes & shapes …

• Contradiction to Sandwich Model:

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• The revelation the membrane proteins had both hydrophilic & hydrophobic properties called into question the placement of these molecules as proposed by the sandwich model …

• Contradiction to Sandwich Model:

a) Since it could not be reconciled with new evidence, the Davson & Danielli model was ultimately abandoned. • Later in the 1960’s new evidence was uncovered that provided the basis for formulating a new, more accurate model of the plasma membrane … Figure 3: Further Membrane Research: Freeze-Fracture (1960’s)

• The Freeze-Fracture of the plasma membrane revealed that the proteins were not organized on the surface of the bilayer, but were rather inserted into it (like mosaic tiles inserted into a plaster matrix). This explained why, despite the fact the proteins were irregular in size & shape, the membrane exhibited a constant thickness of 10nm.

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Figure 3.1: Further Membrane Research: Fluidity Studies (1970’s)

• Upon radioactively tagging membrane proteins (w/different radioisotopes) of mouse & human cells, they were fused to form a single, hybrid cell. It was found that the proteins did not remain fixed within the membrane, but rather migrated throughout the bilayer. This observation suggested that the bilayer was fluid in nature. • In the early 1970’s, Singer & Nicolson used the data generated from freeze-fracture & fluidity studies to synthesize a new model that agreed with these new observations … Figure 3.2: Fluid Mosaic Model (1970’s)

*Within the bilayer, phospholipids can bob up & down, rotate, & move laterally. They are unlikely, however, to migrate from one side of the bilayer to the other. **The protein-phospholipid ratio of the plasma membrane is about 1.14 to 1 (ratio varies among organelles).

• The Singer-Nicolson Fluid-Mosaic Model postulates that the plasma membrane is composed of a fluid phospholipid bilayer. Within this bilayer, amphiphatic proteins are inserted. Both phospholipids & proteins are free to migrate within the membrane’s bilayer.

• Integral Proteins:

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a) Transmembrane Proteins are integral proteins that span the entire bilayer (e.g. transport proteins).

• Peripheral Proteins: Figure 4: Adaptations for Maintaining Membrane 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…

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II. Intercellular Junctions Figure 5: Intercellular Junctions: Tight Junctions

• Tight Junctions: a) Are usually found between the cells lining a body cavity (endothelium) to create a tight seal & prevent fluid

leakage into the underlying tissue …

*Within the lining of the small intestine, tight junctions prevent digested nutrients from seeping in between the endothelial cells. Instead, nutrients must pass across the endothelial cell membranes, upon which they will ultimately pass into the bloodstream.

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b) Tight junctions also prevent the migration of membrane proteins between the top & bottom surfaces of the cell. This ensures that each aspect of the cell membrane possesses a specialized assemblage of proteins…

Figure 5.1: Intercellular Junctions: Desmosomes

• Desmosomes:

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Figure 5.2: Intercellular Junctions: Gap Junctions

• Gap Junctions: • Gap junctions can serve to promote the transmission of nutrients & other vital solutes between cells during early embryonic development prior to the establishment of circulatory vessels …

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• Gap junctions also allow for the rapid transmission of substances between cells (e.g. ions, hormones) for rapid intercellular communication. In the case of heart muscle cells, the rapid exchange of sodium ions promotes a simultaneous response, the coordinated contraction of the heart …

• Gap junctions between endocrine cells can allow for the rapid transport of hormones that stimulate the simultaneous response of the tissue. For example, if the hormone in question is adrenaline from the adrenal gland, then it will be received by cells of the liver, causing them to simultaneously produce & release glucose into the bloodstream …

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Figure 5.3: Intercellular Junctions: Plasmodesmata

• Plasmodesmata: III. Transport Through Membrane Bilayers Figure 6: Factors Affecting Transport Rates

• The bilayer of the plasma membrane regulates the passage of solutes into & out of the cell based on their following physical & chemical properties: 1) 2)

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Figure 6.1: Factors Affecting Transport Rates: Effect of Charge

• 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…

Figure 6.2: Combined Effects of Size & Charge on Solute Migration Rates

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Figure 6.3: Effects of Gradients & Transport Proteins on Solute Migration Rates

• Forms of membrane transport can be either Active or Passive. Passive transport involves the flow of solutes from regions of high to low concentration until a dynamic equilibrium is reached. This form of transport exhibits the following additional properties… 1)

2)

3)

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• In contrast, forms of active transport exhibit the following properties… 1) 2) • Filtration involves the migration of solutes across a membrane as a result of Hydrostatic Pressure, or the force that water exerts on a barrier… Figure 7: Forms of Passive Transport: Filtration

Filtration to Produce Coffee Filtration of Plasma 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.

Figure 7.1: Passive Transport: Simple Diffusion

• The following factors can influence the rate at which solutes migrate as a result of simple diffusion: 1) Concentration Gradient Increase:

2) System Temperature Increase:

3) Membrane Surface Area Increase:

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• Osmosis is a special case of diffusion involving the passive migration of free (unbound) water molecules. Water

Potential () is the tendency of water to flow away from a region via osmosis & is determined by 2 factors:

= _____ + ______

• The water potential formula is used to predict the following:

and • General Rule:

Figure 7.3: Predicting Osmotic Flow as a Function of Solute Potential

• 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.

• When considering Pressure Potential, remember the following: a) Regions open to their surroundings exhibit pressure potentials of 0. b) Compressional forces raise pressure potential to positive values, increasing the likelihood of water flowing from an

area. Tensional forces lower pressure potential to negative values, making water less likely to flow from an area.

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Figure 7.4: Predicting Osmotic Flow as a Function of Solute Potential (Glass U-Tube) Time 0: Initial Conditions

Time 1: Final Conditions

• Osmotic Pressure (OP) is a form of hydrostatic pressure that can inhibit the osmotic flow of water into a region. The magnitude of OP is directly proportional to the difference in column height between sides (A) & (B). Thus, OP will result in less water flowing into side (B) & more water flowing into side (A) …

• The water column on side (B) will fall as more water flows toward side (A). The final height of the water column on each side of the tube is achieved once an equilibrium is established between hydrostatic & osmotic pressure…

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Figure 7.5: Illustrating Effects of Pressure Potential

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*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.

Figure 7.6: Effects of Water Potential Gradients on Animal Cells

• In animal cells, direction of osmosis is dictated mainly by differences in Ψs between the cell its surroundings. In contrast to animal cells, plant cell osmosis is dictated by BOTH solute & pressure potential …

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Figure 7.7: Effects of Water Potential Gradients on Plant Cells

• In a hypotonic environment, water flows into plant cells & is absorbed into the central vacuole. As cells absorb H2O the central vacuole exerts hydrostatic pressure (outwardly-directed) on the cell wall = Turgor Pressure. Consequently, the cell swells to become Turgid & the plant body assumes an upright position ...

• As the cells swell, the cell wall stretches & ultimately recoils to generate an inward directed Wall Pressure. This results in a POSITIVE PRESSURE POTENTIAL to push some H2O out of the cell back into its surroundings ...

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• As more water is pushed out of the cell, wall pressure decreases & ultimately matches turgor pressure. At this point the plant cell is Flaccid (neither plasmolyzed nor turgid) & net H2O flow ceases. Calculating Water Potential: Practice Problems

• 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?

• 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 with a 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 + oC).

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• 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 the migration of specific solutes. Figure 8: Facilitated Diffusion

*Facilitated Diffusion is a passive form of carrier transport in which solutes migrate down their concentration gradient via specific carrier proteins w/o a net ATP expenditure …

Figure 8.1: Facilitated Diffusion: Aquaporins

• Aquaporins (+) are transmembrane proteins specific to water molecules that act to exclude other solutes based on charge & size. These channels provide an additional route by which water molecules can cross the plasma membrane (direct AND via aquaporins). They 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. Figure 8.2: Facilitated Ion Diffusion

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• 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. When considering the direction that an ion will pass across a channel, both its charge AND its concentration must be considered, a property known as the Electrochemical Gradient …

*In the above example, the charge & concentration gradient associated with sodium are favorable to promote its migration into the cell. If, however, positive charge existed on both sides of the membrane, OR the concentration of sodium was equal on both sides, the ion will not likely migrate through its specific channel. Figure 8.3: Facilitated Diffusion: 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 within the membrane.

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Figure 9: 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: 1)

2) Figure 10: Secondary Active Transport / Cotransport

• Cotransport is a mechanism that involves two membrane-bound proteins, a proton pump & cotransporter. The H+ gradient established by the pump provides energy to the cotransporter as H+ flows through it back into the cell. This enables the cotransporter to transport a solute (like sucrose) against its concentration gradient across the membrane.

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Figure 11: Vesicle Transport: Endocytosis

• Endocytosis: a) Endocytosis is a form of active transport, for the resulting vesicle attaches to microtubules of the cytoskeleton & is

directed to appropriate cellular destination via ATP-consuming motor proteins. Figure 11.1: 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 away from the plasma membrane as a vesicle containing the material that had been outside the cell. Practiced mainly by unicellular eukaryotes (protists) & certain white blood cells (phagocytes). • 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.

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Figure 11.2: Receptor-Mediated Endocytosis

• Receptor Mediated Endocytosis (RME): enables the cell to select for & transport large quantities of specific extracellular solids at the exclusion of all others. Examples: insulin & most other protein hormones are absorbed via RME. Figure 11.3: Vesicle Transport: Exocytosis

• Exocytosis:

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b) Exocytosis is a form of active transport, for the resulting vesicle attaches to microtubules of the cytoskeleton & is directed to appropriate cellular destination via ATP-consuming motor proteins.