Membrane Structure and Function Chapter 7 Unit 2

Cell Membrane Controls traffic into and out of the cell  Selectively permeable Fluid Mosaic Model  Made of phospholipids, proteins, carbohydrates, and cholesterol  Amphipathic: hydrophobic and hydrophilic regions  Held together by hydrophobic interactions

Fluid Mosaic Model  Singer and Nicolson

Membrane fluidity Low temps: phospholipids w/unsaturated tails  kinks prevent close packing Cholesterol resists changes by:  limit fluidity at high temps  hinder close packing at low temps

Evolutionary Adaptations Fish that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid Bacteria and Archaea that live in extreme hot conditions include unusual lipids that prevent excessive fluidity

Membrane Proteins Integral Proteins Peripheral Proteins Embedded in membrane Determined by freeze fracture Transmembrane with hydrophilic heads/tails and hydrophobic middles Extracellular or cytoplasmic sides of membrane NOT embedded Held in place by the cytoskeleton or ECM Provides stronger framework

Membranes provide a variety of cell functions

Membrane Carbohydrates and cell-cell recognition Cell-cell recognition  Sorting of an animals embryo’s cells into tissues and organs  Rejection of foreign cells by the immune system Two types of cell markers  Glycolipids: covalently bonded to lipids  Glycoproteins: covalently bonded to protein

Selectively Permeability  Nonpolar molecules (hydrophobic) Dissolve and cross with ease Hydrocarbons, O, and CO 2  Polar molecules (hydrophilic) Will pass if small enough (H 2 O, ethanol) Larger will not easily pass through (glucose) All ions, even small ones have difficulty passing (Na+, H+)

Transport Proteins Channel Proteins: have hydrophilic channels  Aquaporins: channel water Carrier Proteins: hold on to molecules and change shape to shuttle them through the membrane Specific to what they transport  EX: glucose transporter only transports glucose

Passive Transport Diffusion  Movement from area of high concentration to low concentration  Random molecular motion  Continues until equilibrium is reached

Osmosis  Osmotic Pressure: Measure of the tendency for a solution to take up water  Hypertonic: more solute, less water Plasmolysis plant  Hypotonic: more water, less solute Turgid plant cell  Isotonic: equal solute and water Flaccid plant

Cell survival depends on balancing water uptake and loss

Facilitated Diffusion Diffusion of many polar molecules and ions Transport Proteins or Gated Channels Cystinuria: kidney disease caused by missing carriers for cystine

Active Transport Movement from low area of concentration to area of high concentration Energy-requiring Transport proteins involved

Cotransport Single ATP-powered pump actively transports one solute and indirectly drives the transport of other solutes against their concentration gradients

Exocytosis Endocytosis exporting macromolecules Vesicle usually budded from the ER or Golgi Used by secretory cells Importing macromolecules pinches off into the cytoplasm Incorporate extracellular substances

Three types of endocytosis Phagocytosis: cell eating Pinocytosis: cell drinking Receptor mediated endocytosis: process of importing specific macromolecules into the cell by the inward budding of vesicles formed from coated pits. Occurs in response to the binding of specific ligands to receptors on the cell’s surface