Membrane Structure and Function Selectively permeable membranes are key to the cell's ability to function Membrane Structure and 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”

Passive Transport Diffusion of a substance across a membrane Net flow of substance occurs until concentration is equal on both sides of membrane (equilibrium) *Unless there is pressure

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 In Plant cells normally exist in hypotonic solutions 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