Cell Structure & Function. Membrane Models Structure / Function Permeability Modification of...

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Transcript of Cell Structure & Function. Membrane Models Structure / Function Permeability Modification of...

  • Slide 1
  • Cell Structure & Function
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  • Membrane Models Structure / Function Permeability Modification of Cell Surface
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  • Sandwich Model Concept: phospholipids form a double bilayer that is a filling between two layers of protein. Evidence: electron microscopy images. Model was revised and added to Unit Membrane Model Concept: all membranes are the same, just like all DNA is the same, and all amino acids are the same. Evidence: electron microscopy Problem: some kinds of membranes looked different under the electron microscope Not contested strongly until early 1970s. Fluid-Mosaic Model Concept: the cell surface is a viscous fluid, with proteins embedded in it that can freely diffuse, producing a mosaic pattern. Evidence: mouse-human hybrid membranes Problem: none, holds true to this day
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  • Surface area of a sphere: 4*pi*r 2 1925 There are enough phospholipids to make a bilayer. Gorder & Grendel proposed hydrophobic tails on the inside and polar heads on the outside. 1940s Proteins are part of the membrane. Danielli and Davson proposed the sandwhich model. 1950s electron microscopes were good enough to look directly at the membrane, confirming the sandwhich model. Robertson suggests the membrane is standardized for all cells, and proposes the unit membrane
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  • 1972 Singer & Nicolson propose fluid-mosaic model, which fits observations of cells that the unit membrane model did not. So who is right? Do the experiment to find out. Hypothesis 1: Unit Membrane Hypothesis 2: Fluid-Mosaic. Experiment: cut open a plasma membrane and compare the inside and outside of a phospholipid layer. Expected Results: If the Unit Membrane is correctly, only the outside of the membrane will show proteins. If the Fluid-Mosaic model is correct, the inside and outside will show proteins.
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  • Figure 5.1d
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  • Experiment 2 Hypothesis: The lipid membrane is fluid in nature. Prediction: If the membrane is fluid, then embedded proteins should move freely in the fluid. Test: merge cells of 2 species (having different proteins in their membrane), and determine whether the proteins mix with each other after fusion. Results:
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  • A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation. Science. 1972 Feb 18;175(23):720-31
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  • Brownian Motion Diffusion Osmosis
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  • Figure 5.5
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  • Diffusion Watch some videos on diffusion to get the idea. Simulation of a lateral diffusion in a lipid bilayer (1)Simulation of a lateral diffusion in a lipid bilayer (1) Simulation of lateral diffusion in a lipid bilayer (2)Simulation of lateral diffusion in a lipid bilayer Same thing, with obstaclesSame thing, with obstacles Trippy diffusionTrippy diffusion Example of diffusion of a hydrogen ion through waterExample of diffusion of a hydrogen ion through water Brownian Motion Cellular Movements Treadmilling Macrophage
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  • (my original teaching demonstration)
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  • When we think about diffusion, we dont concern ourselves about the liquid (the solvent, e.g. water) in which something is dissolved. A solute particle moving randomly in a solution of water (Brownian motion) (Check out www.dhmo.org)www.dhmo.org O HH A water molecule Brownian Motion of liquid water Molecules But water is just a molecule, bouncing around like any other.
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  • To distinguish between diffusion of dissolved particles (the solute), and the surrounding liquid (the solvent), we use the term osmosis Why do we care about osmosis in biology? $.02 Answer: Because it is essential for bringing water in and out of cells.
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  • Fill a divided chamber with water. The constantly moving water molecules can pass freely through the barrier.
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  • Add a solute to the Red side (such as salt) The solute molecules act as a sort of molecular sponge, strongly binding up the water molecules, thereby removing them from the surrounding volume. This results in an effective decrease in water concentration in the Red chamber. The solute molecules are too big to pass through the barrier The constantly moving water molecules, just like any other molecule, will diffuse from the area of HIGH concentration to the area of LOW concentration (RED BLUE) There will be some movement of water the other way, too, but the average will be a net increase in the red side. The result is a net movement of water from the Blue side, where there are more free water molecules to the Red side. Movement will continue until a Dynamic Equilibrium is achieved. Net Movement of Water
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  • Who Pulls, Who Pushes?
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  • The solution with the higher concentration of solute is said to be hypertonic with respect to the solution of lower concentration. The solution with lower concentration, in turn, is referred to as hypotonic Hypertonic More Concentrated Hypotonic Less Concentrated
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  • Key Concept: Water always towards the hypertonic (more concentrated) solution (assuming theres a semi-permeable membrane) Hypertonic More Concentrated Hypotonic Less Concentrated
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  • Isotonic Equal Concentration When two solutions have the exact same concentration, they are said to be isotonic. (iso- the same)
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  • Hypertonic, hypotonic, and isotonic, describe relationships between solutions. They are relative terms. If you ever see the word describing a solution itself, you can usually figure out the context. An isotonic saline solution for eyes, would be referring to its concentration relative to the liquid of your eye. It answers the question, which solution will pull the water away, and which will have water taken from it? A solution is never just hypertonic. It is always hypertonic with respect to another solution. Hyper: more or higher (hyperactive) Hypo: less or lower (hypothermia)
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  • Osmotic Pressure Revealed
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  • Lets give the solutions some breathing room. What happens to the volume of each side?
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  • Since