I. Membrane Structure Figure 1: Phospholipid Figure 1.1: Plasma Membrane Plasma Membrane: 1
II. Early Plasma Membrane Models Figure 2: Davson-Danielli Sandwich Model In the 1960 s new evidence suggested that the proteins associated with the plasma membrane were irregular in size & shape, as well as being amphiphatic. This resulted in a modification of Davson & Danielli s original model The modified model, with its irregularly shaped proteins, was not able to explain the membrane s uniform thickness of 10nm. In addition, the placement of the proteins atop both surfaces of the membrane would bring their hydrophobic regions in contact with water. Thus, the Davson & Danielli model was eventually abandoned. Davson-Danielli/Sandwich Model: 2
Figure 2.1: Singer-Nicholson Fluid Mosaic Model The Singer-Nicolson Fluid-Mosaic Model states 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. This represents the currently accepted model of the plasma membrane. Figure 3: Membrane Fluidity: Animal Cells In order to prevent solidification at low temperatures, cells can alter the makeup of their membranes. In animal cells, cholesterol is added to prevent phospholipids from packing too tightly & causing the membrane to solidify at low temperatures. As a result of the incorporation of cholesterols, the membrane is able to maintain fluidity & remain permeable. 3
Figure 3.1: Membrane Fluidity: Plant Cells In plant cells, instead of cholesterol, more unsaturated phospholipids are added to the membrane s bilayer. The kinked tails of these molecules prevent them from packing too tightly at low temperature to keep the membrane fluid & permeable. III. Transport across Membranes Figure 4: Membrane Permeability & Hydration Shells Hydration Shells consist of water molecules that are electrically attracted to solutes that are polar or ionic. These shells increase the size of solutes to make their movement through the bilayer portion of the plasma membrane more difficult than that of nonpolar substance that lack hydration shells.. Table 1: Bilayer Permeability to Solutes Particle Type Example Pass through Membrane?? Small Uncharged Polar Molecules Water, Glycerol, Ethanol YES Small Nonpolar Molecules Carbon Dioxide, Oxygen, Nitrogen YES Nonpolar Molecules Hydrocarbons YES Ions & Polar Molecules H+, K+, Na+, etc NO Large Uncharged Polar Molecules Glucose, Amino Acids, Nucleotides NO 4
Figure 4.1: Methods of Membrane Transport: Passive Transport Passive Transport: Figure 4: Passive Transport: Simple Diffusion Simple Diffusion: Factors affecting the rate of simple diffusion within a cell include the following: (1) membrane permeability & surface area (2) magnitude of concentration gradient (3) temperature (4) distance 5
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Figure 5: Passive Transport: Osmosis Osmosis describes the diffusion of free water molecules (unbound) across a membrane. Figure 5.1: Osmosis: Predicting the Direction of Water Migration Describing Concentration of Solutions Hypertonic Solution: high solute concentration (low free water) relative to other solutions. Hypotonic Solution: low solute concentration (high free water) relative to other solutions. Isotonic Solution: same solute concentration (= amounts of free water) as another solution. Since more free water molecules exist in a hypotonic solution relative to a hyperosmotic solution, water will always flow from a hypotonic solution (high free water) to a hypertonic solution (low free water). Osmosis: 7
Figure 6: Osmosis & Water Balance in Animal vs Plant Cells Solution Cell Type Result Cell Type Result Hypertonic Plant Plasmolysis Animal Hypotonic Plant Turgid (swollen) Animal Isotonic Plant Equilibrium Animal Figure 7: Passive Transport: Facilitated Diffusion Ion Channel Carrier Protein Provide a channel through which specific ions may passively cross the membrane unimpeded. Bind to & escort specific molecules across bilayer. Facilitated Diffusion: 8
Figure 8: Active Transport All forms of active transport involve Protein Pumps that consume energy (ATP) in order to move substances across a membrane against the concentration gradient (low to high concentrations). Figure 8.1: Active Transport: Sodium-Potassium Pump In the case of the Na-K pump, it is activated to make the cell less hypertonic to its environment to prevent it from taking on too much water & lysing (bursting). Active Transport: 9
Figure 9: Vesicle Transport (Active): Endocytosis Figure 9.1: Endocytosis: Phagocytosis & Pinocytosis Phagocytosis Pinocytosis Endocytosis: a) Phagocytosis is the process by which food particles are taken into the cell by endocytosis. A localized region of the plasma membrane folds 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. This activity is common among unicellular eukaryotes, but is only performed by some white blood cells in the human body! b) Pinocytosis is the process by which essential solutes may be taken into the cell by endocytosis. All eukaryotic cells carry out this form of endocytosis. Figure 9: Vesicle Transport (Active): Exocytosis Exocytosis: 10
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