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The cell membrane is selectively permeable due to its phospholipid bilayer structure and embedded proteins. Small nonpolar molecules (O₂, CO₂) can pass freely, while ions and large polar molecules require transport proteins.
Semi-permeable: Based on physical/chemical properties (like a coffee filter). Selectively permeable: Involves protein-mediated selection of molecules.
Describes the cell membrane as a dynamic structure with proteins embedded in a phospholipid bilayer. Selective permeability is a direct consequence of this model.
Provides structural support and acts as a permeability barrier for some substances. Composed of complex carbohydrates: cellulose (plants), chitin (fungi/arthropods), peptidoglycan (bacteria).
Small nonpolar molecules diffuse freely. Hydrophilic substances move through channel/transport proteins. ATP synthase uses proton gradients to produce ATP.
Cells need selectively permeable membranes to control water movement (osmosis) and maintain internal conditions (homeostasis). Some cells use contractile vacuoles to expel excess water.
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