Cell Membrane Function Protects cell Allows for passage

Cell Membrane

Function • Protects cell • Allows for passage of certain materials in and out of cell (helps maintain homeostasis)

Structure • Phospholipid bilayer • Embedded proteins

Phospholipids • Heads contain glycerol and phosphate and are hydrophilic (attract water) • Tails are made of fatty acids and are hydrophobic (repel water) • Make up a bilayer where tails point inward toward each other • Can move laterally to allow small molecules to enter (O 2, CO 2, H 2 O)

Cell Membrane Proteins • Proteins help move large molecules or aid in cell recognition • Peripheral proteins are attached on the surface (inner or outer) • Integral proteins are embedded completely through the membrane

Glycoproteins

Fluid Mosaic Model • Describes the arrangement of molecules making up the cell membrane • Based on two characteristics: o Flexibility- phospholipids move and slide past each other (fluid) o Appearance- variety of molecules scattered throughout membrane (mosaic)

Selective Permeability • Allows some (but not all) materials to cross • Important to maintain homeostasis within the cell • Passage through membrane varies, but in general: o Small nonpolar molecules easily pass through o Small polar molecules transported via proteins o Large molecules moved through in vesicles

Transport of Materials • Passive transport- passage of materials does not require energy o Diffusion o Osmosis o Facilitated diffusion • Active transport- requires energy o Endocytosis o Exocytosis

Diffusion • Movement of molecules from high to low concentration • Concentration gradient- difference in concentration of a substance throughout the solution • Dynamic equilibrium- concentration is uniform throughout solution (equilibrium), but molecules are always moving (dynamic) • Small lipids and other nonpolar molecules diffuse across

Osmosis • Movement of water molecules from high to low concentration • Isotonic solution- same concentration of dissolved particles as cell • Hypertonic solution- has a higher concentration of dissolved particles than cell; cell shrivels • Hypotonic solution- lower concentration of dissolved particles than cell; cell expands and bursts

Facilitated Diffusion • Diffusion of molecules across the membrane through transport proteins • Still no energy required… passive transport

Active Transport • Requires energy to move molecules through cell membrane • Molecules move against concentration gradient (from low to high concentration) • Uses transport proteins o Span membrane o Use chemical energy (typically ATP) to move substance

Endocytosis • Process of taking in liquids/other large molecules by engulfing them in the cell membrane • The resulting vesicle then fuses with a lysosome to be broken down

Endocytosis • Phagocytosis o “cell eating” • Pinocytosis o “cell drinking” • Exocytosis- opposite of endocytosis o Release of substance out of cell