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1.4 Cell Membranes and Diffusion Principles Essential Human Anatomy and Physiology I with Lab- Bowma

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1.4: Cell Membranes and DiffusionPrinciples15:40In 1972, thefluid-mosaic model (Figure 1.52)of membrane structure was introduced, whichproposes that the membrane is a phospholipid bi-layer in which proteins are either partially orentirely embedded, like tiles in a mosaic picture. This mosaic pattern is not static because thepositions of the proteins and phospholipids are "fluid" and can move around within the membrane.The figure below shows the structure of the cell membrane.1.4: Cell Membranes and Diffusion Principles: Essential Human Anatom......1 of 811/28/2020, 3:33 PM
Figure 1.52Fluid-mosaic model of a cell membraneA membrane has, primarily, two components, lipids and proteins. Most of the lipids in themembrane areamphipathic phospholipidsand arrange themselves into a bi-layer with theirpolar (hydrophilic) heads comprising the inner and outer faces of the membrane and their non-polar (hydrophobic) tails facing each other to form the core of the membrane.Hydrophilicmeanswater loving, andhydrophobicmeanswater hating. Remember that polar substances areattracted to water and non-polar substances repel water. The heads of amphipathic phospholipidsface out because of their attraction to water, while the tails face the inner portion because of howthey repel water. Note inFigure 1.52the two layers of polar heads. One faces outside the cell,and the other layer of polar heads faces inside the cell. The fatty acid tails which are hydrophobicline the center of the two layers of outer polar heads. While the heads are arranged in an orderlyfashion, the tails are in constant fluid motion. The movement of the tails allows the lipids to rotateand to even move laterally within their layer, but always with the head facing outward and the tailinward. The fluidity of the lipids also allows for protein movement within the membrane structure.Some membranes contain cholesterol as a stabilizer against heat and cold.Cholesterol(a steroidlipid) is largely hydrophobic (repels water) with a single polar hydroxyl group. The hydroxyl groupis attracted to the polar heads of the phospholipids, while the rest of the cholesterol molecule linesup with the phospholipid tails. The cholesterol molecules act as spacers between the tails toreduce solidification during low temperatures. Cholesterol also stabilizes the head during elevatedtemperatures by restricting motion and preventing movement as energy is added to the system.Proteins are the other major constituent of membranes and includeintegral proteins(Figure1.4: Cell Membranes and Diffusion Principles: Essential Human Anatom......2 of 811/28/2020, 3:33 PM
1.53), which typically span across the lipid bi-layer.Peripheral proteinsare located only on theinner or outer surfaces of the membrane. Integral proteins are firmly bound within the membrane.

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