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1.4_ Cell Membranes and Diffusion Principles_ Essential Human Anatomy and Physiology I with Lab- Kec

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8/20/20201.4: Cell Membranes and Diffusion Principles: Essential Human Anatomy and Physiology I with Lab- Keck2/9Figure 1.52Fluid-mosaic model of a cell membraneA membrane has, primarily, two components, lipids and proteins. Most of the lipids inthe membrane are amphipathic phospholipidsand arrange themselves into a bi-layer with their polar (hydrophilic) heads comprising the inner and outer faces of themembrane and their non-polar (hydrophobic) tails facing each other to form the coreof the membrane. Hydrophilicmeans water loving, and hydrophobicmeans waterhating. Remember that polar substances are attracted to water and non-polarsubstances repel water. The heads of amphipathic phospholipids face out becauseof their attraction to water, while the tails face the inner portion because of how theyrepel water. Note in Figure 1.52the two layers of polar heads. One faces outsidethe cell, and the other layer of polar heads faces inside the cell. The fatty acid tailswhich are hydrophobic line the center of the two layers of outer polar heads. Whilethe heads are arranged in an orderly fashion, the tails are in constant fluid motion.The movement of the tails allows the lipids to rotate and to even move laterallywithin their layer, but always with the head facing outward and the tail inward. Thefluidity 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 steroid lipid) is largely hydrophobic (repels water) with a single polar8/20/20201.4: Cell Membranes and Diffusion Principles: Essential Human Anatomy and Physiology I with Lab- Keck3/9hydroxyl group. The hydroxyl group is attracted to the polar heads of thephospholipids, while the rest of the cholesterol molecule lines up with thephospholipid tails. The cholesterol molecules act as spacers between the tails toreduce solidification during low temperatures. Cholesterol also stabilizes the headduring elevated temperatures by restricting motion and preventing movement asenergy is added to the system.Proteins are the other major constituent of membranes and include integralproteins (Figure 1.53), which typically span across the lipid bi-layer. Peripheralproteinsare located only on the inner or outer surfaces of the membrane. Integralproteins are firmly bound within the membrane. They usually have one or morehydrophobic regions embedded in the core of the lipid bi-layer with hydrophilicregions that extend out of each face. Some integral proteins are large enough toweave back and forth through the membrane several times. Peripheral proteins canalso attach to exposed regions of integral proteins.

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