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Week_3 - Chapter 7 Membrane Structure and Function Plasma...

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Chapter 7- Membrane Structure and Function: C C Plasma membranes are the boundaries, which separate living cells from their nonliving surroundings. The plasma membrane exhibits selective permeability. This means that the membrane allows certain substances to pass into/ out of the cell more easily than others. m The fluid Mosaic Model , envisions the membrane as a mosaic of individual amphipathic protein molecules drifting laterally in a fluid bilayer of phospholipids. Phospholipids and to a lesser extent, proteins move as stated, laterally, in this membrane. Cholesterol and unsaturated hydrocarbon tails in the phospholipids affect membrane fluidity. h Unsaturated hydrocarbon tails of phospholipids have kinks that keep the molecule from packing tightly together, which enhances membrane fluidity. f Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipids movement, but at low temps. It hinders solidification by disrupting the regular packing of phospholipids. p Membrane Proteins: Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane’s specific functions. r Integral proteins penetrate the hydrophobic core of the lipid bilayer. Many are transmembrane proteins, which completely span the membrane. r Peripheral proteins are not embedded in the lipid bilayer at all: they are appendages loosely bound to the surface of the membrane, often to the exposed parts of integral proteins l Membrane Protein Functions: Transport r Enzymatic activity r Signal transduction r Cell-cell recognition r Intercellular joining r Attachment to the cytoskeleton and extracellular matrix A Selective Permeability: A cell must exchange small molecules and ions with its surroundings, a process that is controlled by the plasma membrane. r Hydrophobic substances are soluble in lipid and pass through membranes rapidly. r Hydrophilic substances can avoid contact with the lipid bilayer by passing though transport proteins that span the membrane. r A transport protein is a transmembrane protein that helps a certain substance or classes of closely related substances cross the membrane. Transport proteins are specific for the substance it translocates. i Passive Transport: Passive transport is diffusion of a substance across a membrane with no energy
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