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Introduction

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High-Yield Terms

  • Lipid bilayer: a thin polar membrane made of 2 layers of lipid molecules

  • Integral membrane protein: a protein molecule that is permanently attached to the biological membrane

  • Peripheral membrane protein: a protein that adheres only temporarily to the biological membrane with which it is associated

  • Endoplasmic reticulum (ER): the ER functions as a packaging system working in concert with the Golgi apparatus, to create a network of membranes found throughout the whole cell

  • Golgi apparatus: the Golgi apparatus packages proteins inside the cell before they are sent to their destination; it is particularly important in the processing of proteins for secretion

  • Apical membrane: the layer of plasma membrane on the side toward the lumen of the epithelial cells

  • Basolateral membrane: the fraction of the plasma membrane at the basolateral side of the cell which faces adjacent cells and the underlying connective tissue

  • Gap junctions: intercellular channels designed for intercellular communication

  • Tight junctions: barriers that regulate the movement of solutes and water between various epithelial layers

  • Antiporter: protein or complex that couples the transport of 2 compounds across a membrane in opposite directions

  • Uniporter: protein or complex that transports 1 molecule of solute at a time down the solute gradient

  • Symporter: protein or complex that couples the transport of 2 compounds across a membrane in the same direction

  • Aquaporins: membrane water channels that facilitate rapid, highly selective water transport

  • Na+/K+-ATPases: membrane transporters that utilize the energy of ATP hydrolysis to transport sodium into and potassium out of a cell

  • Cardiotonic steroids: endogenous digitalis-like factors that inhibit the Na+/K+-ATPases

  • ABC family transporters: ATP-binding cassette transporters that couple the energy of ATP binding and hydrolysis to substrate transport

  • SLC family transporters: solute carrier transporters that include transporters that function by secondary active transport and facilitative diffusion

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High-Yield Concept

The lipids that make up the components of membranes are of three major classes: glycerophospholipids, sphingolipids, and cholesterol.

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Composition and Structure of Biological Membranes

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Biological membranes are composed of lipid, protein, and carbohydrate that exist in a fluid state. Biological membranes are the structures that define and control the composition of the space that they enclose. All membranes exist as dynamic structures whose composition changes throughout the life of a cell. In addition to the outer membrane that results in the formation of a typical cell (this membrane is often referred to as the plasma membrane), cells contain intracellular membranes that serve distinct functions in the formation of the various intracellular organelles, for example, the nucleus and the mitochondria.

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Sphingolipids and glycerophospholipids constitute the largest percentage of the lipid weight of biological membranes. The hydrocarbon tails of these two classes of lipid result in steric limitations to their packing such that they will form disk-like micelles. The structure of these micelles results from the interactions of the hydrophobic tails of the lipids and the ...

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