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3.1 The Cell Membrane - Anatomy and Physiology _ OpenStax.pdf

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1/26/22, 12:51 PM3.1 The Cell Membrane - Anatomy and Physiology | OpenStax1/14Learning ObjectivesBy the end of this section, you will be able to:Describe the molecular components that make up the cell membraneExplain the major features and properties of the cell membraneDifferentiate between materials that can and cannot diffuse through the lipid bilayerCompare and contrast different types of passive transport with active transport,providing examples of eachDespite differences in structure and function, all living cells in multicellular organisms have asurrounding cell membrane. As the outer layer of your skin separates your body from itsenvironment, the cell membrane (also known as the plasma membrane) separates the innercontents of a cell from its exterior environment. This cell membrane provides a protectivebarrier around the cell and regulates which materials can pass in or out.Structure and Composition of the Cell MembraneThecell membraneis an extremely pliable structure composed primarily of back-to-backphospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of themembrane, and there are various proteins embedded within the membrane that have a varietyof functions.A single phospholipid molecule has a phosphate group on one end, called the “head,” andtwo side-by-side chains of fatty acids that make up the lipid tails (Figure 3.2). The phosphategroup is negatively charged, making the head polar and hydrophilic—or “water loving.” Ahydrophilicmolecule (or region of a molecule) is one that is attracted to water. The phosphateheads are thus attracted to the water molecules of both the extracellular and intracellularenvironments. The lipid tails, on the other hand, are uncharged, or nonpolar, and arehydrophobic—or “water fearing.” Ahydrophobicmolecule (or region of a molecule) repels andis repelled by water. Some lipid tails consist of saturated fatty acids and some containunsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly inmotion. Phospholipids are thus amphipathic molecules. Anamphipathicmolecule is one thatcontains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil andgrease stains because it has amphipathic properties. The hydrophilic portion can dissolve inwater while the hydrophobic portion can trap grease in micelles that then can be washedaway.
1/26/22, 12:51 PM3.1 The Cell Membrane - Anatomy and Physiology | OpenStax2/14Figure 3.2Phospholipid StructureA phospholipidmolecule consists of a polar phosphate “head,”which is hydrophilic and a non-polar lipid “tail,”which is hydrophobic. Unsaturated fatty acids resultin kinks in the hydrophobic tails.

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