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Chapter 5 - Chapter Outline - Biological Membranes

Chapter 5 - Chapter Outline - Biological Membranes - Purav...

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Purav Patel Chapter 5 AP Biology Biological Membranes Introduction The plasma membrane, made of lipids and proteins, regulates the passage of materials, participates in chemical reactions, transmits signals between the environment and the cell, and is involved in energy transfer and storage. Many membrane-associated proteins are enzymes; cell adhesion molecules connect cells to form tissues. Cadherins are the principal cell adhesion molecules; they form junctions important in the structure of the epithelium. Without these proteins, tumors may occur. The Structure of Biological Membranes The core of plasma membranes consists mostly of lipids like phospholipids. Phospholipids Form Bilayers in Water Phospholipid molecules consist of two fatty acids linked to a glycerol molecule. The fatty acid chain is nonpolar and hydrophilic. The phosphate group on the glycerol molecule is polar and hydrophilic. Amphipathic Molecules - molecules (e.g. lipids) with hydrophilic and hydrophobic regions Due to the amphipathic properties of phospholipids, the bilayer structure is formed. This allows the hydrophilic phosphate heads to touch water, while the hydrophobic fatty acids point away from the water in the middle region of the plasma membrane. The shape of phospholipids, their uniform width, and their cylindrical structure, combined with their amphipathic properties creates a bilayer. The hydrocarbon chains of amphipathic molecules in detergents, when placed in water, form spherical membranes useful in solubilizing oil molecules. Current Data Support a Fluid Mosaic Model of Membrane Structure Process of Science The membrane of red blood cells was calculated to be about two phospholipid molecules thick by comparing the surface area to the lipid molecules of the cell. The Davson-Danielli “sandwich” model of two membrane proteins surrounding the lipid bilayer was the first model of the plasma membrane. The later use of electron microscopes found that plasma membranes are uniform, thin, three-layered structures. Years later, when cells were purified by cell fractionation, it was discovered that the membrane proteins (some flat, others globular) varied in size and shape. The fluid mosaic model formed, with the fluid lipid bilayer intact, but with proteins embedded in the lipid bilayer surfacing slightly at both ends. These proteins are constantly changing position in the lipid bilayer like icebergs in water. 1
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Purav Patel Chapter 5 AP Biology Biological Membranes are Two-Dimensional Fluids Phospholipid bilayers behave like liquid crystals. They are associated with the term liquid crystals because the phospholipid heads on the outside are ordered, as are the fatty chains on the inside. However, the constantly moving hydrocarbons move laterally and rotate freely, giving the bilayer the properties of water.
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