Chapter 11 - Membrane Structure

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Glycerol-Containing Phospholipid Structure

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Phospholipids have a glycerol backbone linked to two fatty acid tails (hydrophobic) attached via ester bonds, one phosphate group (hydrophilic), and a polar head group (e.g., choline, serine, ethanolamine) attached to the phosphate. This amphipathic structure is key for membrane formation and function.

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Amphipathic

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An amphipathic molecule contains both a hydrophilic (polar) region (e.g., phosphate head) and a hydrophobic (nonpolar) region (e.g., fatty acid tails). This dual nature allows them to spontaneously arrange into bilayers in aqueous environments.

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Liposome Formation

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When placed in water, amphipathic phospholipids self-assemble into bilayers that close into spherical vesicles called liposomes. This structure minimizes exposure of hydrophobic tails to water, reducing free energy. Liposomes are used as models of biological membranes and drug delivery systems.

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Water and Polar Solutes

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Water can form hydrogen bonds with polar molecules, stabilizing them in solution. Polar solutes (e.g., sugars, amino acids) dissolve easily due to favorable interactions with water molecules, forming hydration shells.

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Hydrophobic Solutes in Water

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Nonpolar molecules (e.g., oils, hydrocarbons) disrupt water’s hydrogen bond network, forcing water molecules to form ordered shells around them. This ordering decreases entropy. The system reduces this energy cost by aggregating hydrophobic molecules, a principle driving membrane formation and protein folding.

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Fats vs. Membrane Lipids

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Fats (triacylglycerols) have three fatty acids esterified to glycerol and are entirely hydrophobic, forming oil droplets in cells for energy storage. Phospholipids have two fatty acids and one phosphate group with a polar head, are amphipathic, and form bilayers in membranes.

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Saturated vs. Unsaturated Tails

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Saturated fatty acids have no double bonds, resulting in straight tails that pack tightly and are less fluid. Unsaturated fatty acids have one or more double bonds, resulting in kinked tails that prevent tight packing and create more fluid membranes.

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Lateral Diffusion

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Lipids and many proteins can move side-to-side (laterally) within a leaflet of the bilayer. Lateral mobility allows dynamic changes in membrane shape, function, and protein clustering. Movements can be restricted by cytoskeletal or protein interactions.

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Rare Spontaneous Flip-Flop

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Flip-flop is the movement of lipids from one leaflet to the other, which is energetically unfavorable due to the need to move a polar head group through the hydrophobic core. It occurs slowly without enzymes and only rapidly with flippases, floppases, or scramblases.

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Tail Length and Saturation Influence Fluidity

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Shorter tails and unsaturated tails reduce Van der Waals interactions, leading to a more fluid membrane. Longer saturated tails increase packing, resulting in a less fluid membrane. Membranes exist on a fluid–gel continuum, and cells regulate tail properties to adjust fluidity.