Chapter 7

Chapter 7Membrane Structure and Function

Boundary that separates the living cell from its non-living surrounding Exhibits selective (semi) permeability, allowing some substances to cross more easily than others

Plasma MembraneCellular membranes are fluid mosaics of lipids and proteins.Fluid Mosaic Model

Phospholipids (fluid part):Most abundant lipid in the plasma membraneAmphipathic: Containing both hydrophobic and hydrophilic regionsProteins (mosaic part):Integral (transmembrane if they cross the whole membrane): Embedded into the phospholipids ORPeripheral: Attached to just one side or otherFluid Mosaic ModelThe type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane.

FluidViscousUnsaturated hydrocarbontails with kinksSaturated hydro-Carbon tailsUnsaturated vs. Saturated HydrocarbonsThe steroid cholesterol has different effects on animal membrane fluidity at different temperatures.

Cholesterol within the animal cell membraneCholesterolRole of CholesterolMembrane Proteins

Membrane proteins (mosaic part) are dispersed and individually inserted into the phospholipid bilayer (fluid part).Integral proteins have hydrophobic AND hydrophilic stretches created by the identity of the amino acids. Membrane CarbohydratesShort carbohydrates bound to lipids (glycolipids) or proteins (glycoproteins) cover the outer surface of cellsThese carbohydrates mediate cell-cell recognition.

Example: ABO Blood Types

Crossing the MembraneCells need to control the exchange of material with their environment.

Crossing the MembranePolarity determines ease of passage Hydrophobic molecules (non-polar)Lipid soluble and can pass through the membrane rapidlyHydrophilic molecules (polar, ionic)Do NOT cross the membrane rapidlyNeed help of transport proteinsPassive vs. Active TransportPassive transport: Movement of a substance across a membrane with NO energy investmentCan be simple or facilitated diffusionActive transport: Energy required for transport, typically supplied by ATP Passive Transport: Simple Diffusion

Simple Diffusion: Tendency for molecules to move from areas of high concentration to areas of low concentration.

Passive Transport: Simple DiffusionSmall, hydrophobic molecules can move across the membrane this way.Note: Diffusion stops at equilibrium but movement continues.

Facilitated DiffusionTransport proteins speed the movement of molecules (e.g. polar or ions) that can not easily pass across the plasma membrane.Can be called channel or carrier proteinsStill passive so NO energy required

Facilitated DiffusionChannel proteins:Provide corridors that allow a specific molecule or ion to cross the membraneIon channels: If transporting ions

EXTRACELLULARFLUIDChannel protein Polar or ionic SoluteCYTOPLASMFacilitated DiffusionCarrier proteins:Undergo slight shape changes when solutes bindChange allows movement of solute binding site across the membrane

Special Case of Facilitated DiffusionOsmosis:Diffusion of water across a semipermeable membrane

Lower concentrationof solute = Higherconcentration of waterHigher concentrationof solute = Lower concentration of waterWater moves from an area of higher water concentration to an area of lower water concentrationsTonicity:Ability of a solutions solute concentration to cause a cell to gain or lose waterIn general, water moves down ITS concentration gradient from high to low.

solute in a solution means water solute in a solution means waterTonicityIf a solution is ISOtonic:Concentration of solutes outside is EQUAL to the concentration inside the cellThere is no NET movement of water.20% NaCl80% H2O20% NaCl80% H2O Solution(Environment)CellIsotonicIf a solution is HYPOtonic:Concentration of solutes outside is LOWER to the concentration inside the cellNet movement of water INTO cell, i.e. Cell gains water10% NaCl90% H2O20% NaCl80% H2O Solution(Environment)CellHypotonicIf a solution is HYPERtonic:Concentration of solutes outside is HIGHER to the concentration inside the cellNet movement of water OUT of the cell, i.e. Cell loses water.20% NaCl80% H2O10% NaCl90% H2O Solution(Environment)CellHypertonic5% NaCl95% H2O5% NaCl95% H2O10% NaCl90% H2O20% NaCl80% H2OIsotonicHypotonicHypertonicHypotonicHypertonicWater movement ABCDTonicity Diagram: Its all relative!Osmosis in Living Cells

Osmosis in Living Cells

Cell wall and central vacuole play important roles.Active TransportMoves substances AGAINST their concentration gradient from LOW to HIGH concentrationRequires energy, usually in the form of ATPUses carrier proteins which act as pumpsSodium Potassium PumpSodium Potassium Pump: Example of active transportCells typically have higher K+ inside and lower Na+Maintaining this concentration gradient requires ATP paymentSodium Potassium Pump

Movement of MacromoleculesTransport of large molecules across the plasma membrane occurs by EXOCYTOSIS and ENDOCYTOSIS.

Exocytosis (ex = exit):Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

ExocytosisEndocytosis (endo = in):Cell takes in macromolecules by forming new vesicles from the plasma membrane.

Endocytosis

PhagocytosisEatingPinocytosisDrinkingReceptor MediatedSelective drinking3 Categories of Endocytosis