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Chapter 1

Membrane Structure and
Membranous Organelles

 
Cell membranes are inherited structures that serve as barriers to the diffusion of most water-soluble molecules and enable cells to create compartments in which the chemical composition differs from the surroundings. They are composed of polar lipids that form a bilayer continuum and proteins that are responsible for most membrane functions, including transport, receptor-based signaling, and the generation of ATP. Membrane proteins are classified as integral, peripheral, or lipid-anchored based on the nature of their noncovalent association with the lipid bilayer. Both lipids and proteins can diffuse laterally in the plane of the bilayer.
      The plasma membrane serves as the diffusional boundary of individual cells and controls the transport of molecules into and out of cells. Plasma membranes contain receptors that participate in the interactions of cells with their surroundings. The process of endocytosis participates in the recycling of molecules from the plasma membrane.
      The endomembrane system comprises membranes that are continuous with the ER (e.g., nuclear envelope) or derived from the ER (e.g., Golgi apparatus, trans-Golgi network, plasma membrane, vacuole, transport vesicles). The ER forms a dynamic network that permeates all regions of the cytosol and is differentiated into numerous functional domains. The sheet-like rough ER domains are defined by the presence of bound polysomes that produce membrane and secretory/ vacuolar proteins, whereas the tubular smooth ER membranes are mostly involved in the synthesis of lipidic compounds. The Golgi apparatus consists of flattened cisternae organized into stacks. Its principle function is to serve as a carbohydrate factory, adding sugars to glycoproteins and assembling complex polysaccharides such as hemicelluloses and pectins. The trans-Golgi network sorts secretory and vacuolar molecules and packages them into separate vesicles. Vacuoles enable plant cells to grow rapidly with expenditure of minimal amounts of energy and proteins. They perform a multitude of functions including, storage, digestion, ionic homeostasis, defense against pathogens and herbivores, and sequestration of toxic compounds.
      The nuclear genome-containing nucleus is surrounded by an envelope with pore complexes that regulate trafficking between the nuclear matrix and the cytosol. The envelope breaks down during mitosis and reassembles around the daughter nuclei. Ribosomes are assembled in the nucleolus and are then exported into the cytosol. Peroxisomes and glyoxysomes are small vesicle-like organelles that contain many enzymes in their lumen.
      Peroxisomes work together with chloroplasts and mitochondria in the glycolate pathway. In certain tissues, peroxisomes can be transiently converted to glyoxysomes, which participate in the mobilization of fatty acids and in conversion of fixed N2 to ureides.
      The term plastid refers to a family of organelles that are developmentally related to proplastids, reproduce by fission, and are semiautonomous. The principal types of plastids are the photosynthetic chloroplasts, the starch-storing amyloplasts, the carotenoid-forming and colorful chromoplasts, and the monoterpene-producing leucoplasts. The aqueous stroma contains the plastid’s genetic machinery as well as enzymes and variable amounts of internal membranes. The photosynthetic apparatus of chloroplasts is located in thylakoid membranes, which are organized into three-dimensional networks with granal (stacked) and stroma-expose domains.
      Mitochondria, like chloroplasts, are semiautonomous. organelles that divide by fission. Mitochondria consist of two membranes and an internal matrix. Their principal function is to generate ATP, which is produced by enzymes in the inner membrane and the matrix.

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