Membrane Structure and Function Selectively permeable membranes are key to the cell's ability to function
Amphipathic Molecules Have both hydrophilic and hydrophobic regions Phospholipids have hydrophilic heads and hydrophobic tails
Phospholipid Bilayer Phospholipids naturally arrange themselves to keep hydrophobic portions away from water
Fluid Mosaic Model Proteins are embedded in the membrane These proteins have many different functions
Membrane Fluidity Membranes get more rigid as the temperature gets lower Unsaturated phospholipids and cholesterol keep the membrane from becoming completely solid
Proteins in the Membrane Integral proteins found on the inside A transmembrane protein spans the entire membrane Peripheral proteins loosely bound to surface Often bound to extracellular matrix or cytoskeleton
Membrane Protein Functions Transport Enzymatic Activity Binding of messengers Intercellular connections Cell recognition Attachment
Membrane Carbohydrates Important in cell-cell recognition Short polysaccharides Often bonded to proteins (glycoproteins) Distinguish cells i.e. blood types
Selective Permeability Membrane is selective in what molecules can pass through and how quickly they pass Energy must sometimes be used to transport molecules
Simple Permeability Small non-polar molecules rapidly pass through membrane Very small polar molecules can slip through Larger polar molecules cannot Very impermeable to ions
Transport Proteins Transmembrane proteins can provide channels for hydrophilic ions and molecules Other proteins physically move molecules (using energy) Proteins are specific to a particular molecule
Diffusion Natural process of a substance spreading into available space A substance diffuses from where it is more concentrated to where it is less concentrated Down its concentration gradient
*Unless there is pressure Passive Transport Diffusion of a substance across a membrane Net flow of substance occurs until concentration is equal on both sides of membrane (equilibrium)
Hypertonic/Hypotonic/Isotonic A solution is: Hypertonic if it has a higher concentration of solutes Hypotonic if it has a lower concentration of solutes Isotonic if it has an equal concentration
Osmosis If solutes cannot pass through the membrane, water will to create equilibrium Water moves from hypotonic (High Water Potential) to hypertonic solution (Low Water Potential) Water moves to the more sugary side
Water Follows Sugar If you eat something very sugary, usually you will be thirsty Water flows to the side with more sugar This is the hypertonic side (sugar makes you hyper) But not really, this is not actually true
Cells depend on water balance (osmoregulation)
Plant Cell Walls Hold Water Plant cells normally exist in hypotonic solutions In Rigid cell walls keep plant cell from bursting Wall is instead turgid Plants wilt in isotonic solutions
Facilitated Diffusion Transport proteins allow polar molecules and ions to travel down concentration gradient No usage of energy! Aquaporins special protein channels for water Peter Agre won the 2003 Nobel Prize for the discovery of aquaporins!
Active Transport Some transport proteins pump molecules against their concentration gradient Requires energy Necessary for cell to control cellular environment
Sodium-Potassium Pump Pumps 3 molecules of sodium out of the cell Takes ATP to move them 2 potassium molecules are transported back into the cell
Sodium Potassium Pump
Ion Pumps Generate Voltage Voltage= the separation of charge If one side has more + ions than ions, it will have a + charge - ions will flow towards sides with a net positive charge
Electrochemical Gradient Two different forces create a gradient across the membrane Electric potential (voltage) Chemical potential (concentration gradient) These two effects combine to form an electrochemical gradient
Cotransport Cotransporter proteins use the energy of one molecule diffusing down its gradient to move another molecule against its gradient
Bulk Transport Many molecules are too large to pass through the membrane i.e. proteins They enter the cell through endocytosis and exocytosis
Endocytosis The cell intakes large molecules by forming new vesicles Small area of membrane sinks inward, forming a pocket which eventually pinches off 3 types Phagocytosis Pinocytosis Receptor-mediated
Exocytosis Transport vesicle fuses with plasma membrane, expelling contents from the cell Vesicle membrane becomes part of plasma membrane