Biology 102Biology 102

Lectures 6 & 7: Lectures 6 & 7:

Biological MembranesBiological Membranes

Lecture outlineLecture outline1.1. Relationship of membrane structure and Relationship of membrane structure and

functionfunction FunctionsFunctions Structure: The fluid-mosaic model of membranesStructure: The fluid-mosaic model of membranes

2.2. Movement of substances across Movement of substances across membranesmembranes

Principles of DiffusionPrinciples of Diffusion Passive and active transport of solutesPassive and active transport of solutes OsmosisOsmosis Endocytosis and exocytosisEndocytosis and exocytosis

3.3. Specialization of cell surfacesSpecialization of cell surfaces

1. Membrane structure 1. Membrane structure and functionand function

Biological membranes Biological membranes Thin barrier separating the inside of the cell Thin barrier separating the inside of the cell

(or structure) from the outside environment(or structure) from the outside environment

Functions (focus on plasma membrane)Functions (focus on plasma membrane)Selectively isolate the cell’s contents from Selectively isolate the cell’s contents from

the external environmentthe external environmentRegulate the exchange of substances Regulate the exchange of substances

between the inside and outside of the cellbetween the inside and outside of the cellCommunicate with other cellsCommunicate with other cells

Fluid-mosaic model of Fluid-mosaic model of membrane structuremembrane structure

The phospholipid bilayer is the The phospholipid bilayer is the fluid portion of the membranefluid portion of the membrane

Double layer Double layer Polar head group: hydrophilic exteriorPolar head group: hydrophilic exterior Non-polar hydrocarbon tails: hydrophobic interiorNon-polar hydrocarbon tails: hydrophobic interior Unsaturated hydrocarbon chains: maintains fluidityUnsaturated hydrocarbon chains: maintains fluidity

Phospholipid bilayer as a barrierPhospholipid bilayer as a barrier

HydroHydrophilicphilic molecules molecules cannotcannot pass freely pass freely through the membrane’s hydrothrough the membrane’s hydrophobicphobic interiorinterior Sugars, amino acids, charged ions (i.e. NaSugars, amino acids, charged ions (i.e. Na++ and Cl and Cl--) )

are some examplesare some examples Though polar, HThough polar, H220 is so small it does pass through.0 is so small it does pass through.

Many hydroMany hydrophobicphobic molecules molecules cancan pass freely pass freely through the membrane’s hydrothrough the membrane’s hydrophobicphobic interiorinterior Steroid hormones and other lipids are some examples Steroid hormones and other lipids are some examples

Cholesterol molecules are part of Cholesterol molecules are part of the lipid bilayerthe lipid bilayer

Adds strengthAdds strengthAdds flexibilityAdds flexibilityAffects fluidityAffects fluidity

Decreases fluidity at Decreases fluidity at “moderate” temperatures“moderate” temperatures Harder for phospholipids Harder for phospholipids

to moveto movePrevents solidification at Prevents solidification at

low temperatureslow temperatures Keeps phospholipids from Keeps phospholipids from

binding to each otherbinding to each other

A mosaic of proteins is embeded A mosaic of proteins is embeded in the membranein the membrane

Glycoproteins: proteins with attached carbohydratesGlycoproteins: proteins with attached carbohydrates

Types of membrane proteinsTypes of membrane proteins Transport proteinsTransport proteins

For passage of materials through the For passage of materials through the plasma membraneplasma membrane

Channel vs. carrier proteins Channel vs. carrier proteins

Receptor proteinsReceptor proteins Bind molecules and trigger cellular Bind molecules and trigger cellular

responsesresponses Example: hormonesExample: hormones

Recognition proteinsRecognition proteins Self vs. non-self (glycoprotein-based) Self vs. non-self (glycoprotein-based)

recognitionrecognition Markers during developmentMarkers during development

2. Movement of substances 2. Movement of substances across membranesacross membranes

DefinitionsDefinitionsConcentrationConcentration

Number of molecules in a given volumeNumber of molecules in a given volumeGradientGradient

Differences in concentration between two regions of Differences in concentration between two regions of space. space.

This causes molecules to move from one region to the This causes molecules to move from one region to the other (if no barrier to movement)other (if no barrier to movement)

DiffusionDiffusion Net movement of molecules from regions of high Net movement of molecules from regions of high

concentration to regions of low concentrationconcentration to regions of low concentration Considered as movement “down” its concentration gradientConsidered as movement “down” its concentration gradient

Diffusion of Dye in WaterDiffusion of Dye in Water

Time 0Time 0SteepSteep

ConcentrationConcentrationGradientGradient

Time 1Time 1ReducedReduced

ConcentrationConcentrationGradientGradient

DispersingDispersing

Time 2Time 2NoNo

ConcentrationConcentrationGradientGradient

RandomRandomDispersalDispersal

Passive vs. active transportPassive vs. active transport

Passive transportPassive transportMovement of molecules down their Movement of molecules down their

concentration gradientsconcentration gradientsRequires no net energy expenditureRequires no net energy expenditure

The gradients themselves provide energyThe gradients themselves provide energy

Active transportActive transportMovement of molecules against their Movement of molecules against their

concentration gradientsconcentration gradientsRequires energy!Requires energy!

Focus: Passive transportFocus: Passive transport

1.1. Simple diffusionSimple diffusion

2.2. Facilitated diffusionFacilitated diffusion

3.3. OsmosisOsmosis

Remember that no energy is required, and Remember that no energy is required, and molecules move down their concentration molecules move down their concentration gradientsgradients

Focus: Passive transportFocus: Passive transport

1.1. Simple diffusionSimple diffusion Molecules simply cross cell membrane on their own, Molecules simply cross cell membrane on their own,

down their concentration gradientsdown their concentration gradients

Possible only for molecules that can cross the lipid Possible only for molecules that can cross the lipid bilayer on their ownbilayer on their own

Lipid-soluble molecules Lipid-soluble molecules Examples: ethyl alcohol, vitamin A, steroid hormonesExamples: ethyl alcohol, vitamin A, steroid hormones

Very small moleculesVery small molecules Examples: water, carbon dioxideExamples: water, carbon dioxide

Rate depends uponRate depends upon Concentration gradientConcentration gradient SizeSize Lipid solubilityLipid solubility

Focus: Passive transport (cont.)Focus: Passive transport (cont.)

2.2. Facilitated diffusionFacilitated diffusion Molecules move down their concentration Molecules move down their concentration

gradients (as for simple diffusion), gradients (as for simple diffusion), butbut…… Transport proteins assist these molecules in Transport proteins assist these molecules in

crossing the membranecrossing the membrane No net energy expenditure! (This is a type No net energy expenditure! (This is a type

of diffusion…)of diffusion…)

Focus: Passive transport (cont.): Focus: Passive transport (cont.): Facilitated diffusion via a channelFacilitated diffusion via a channel

Focus: Passive transport (cont.): Focus: Passive transport (cont.): Facilitated diffusion via a carrier proteinFacilitated diffusion via a carrier protein

Carrier proteinhas binding sitefor molecule

Molecule entersbinding site

Carrier protein changesshape, transportingmolecule across membrane

Carrier protein resumesoriginal shape

(Inside Cell)(Inside Cell)(Inside Cell)(Inside Cell)

(Outside(OutsideCell)Cell)

(Outside(OutsideCell)Cell)

DiffusionDiffusionChannelChannelProteinProtein

DiffusionDiffusionChannelChannelProteinProtein

Diffusion

Diffusion

Gradient

Gradient

Molecule inMolecule inTransitTransit

Molecule inMolecule inTransitTransit

Focus: Passive transport (cont.)Focus: Passive transport (cont.)

3.3. OsmosisOsmosis Movement of water from a high [water] to an Movement of water from a high [water] to an

area of low [water] concentration across a semi-area of low [water] concentration across a semi-permeable membrane permeable membrane

Note here that water can pass through, but glucose Note here that water can pass through, but glucose cannotcannot

Think about which way water will move (blackboard demo)

The effects of osmosisThe effects of osmosis

Compare solute and water concentrations Compare solute and water concentrations outside vs. inside the cell (sketches)outside vs. inside the cell (sketches)

Focus: Active TransportFocus: Active Transport

1.1. Movement via active transport proteinsMovement via active transport proteins

2.2. EndocytosisEndocytosis

3.3. ExocytosisExocytosis

Remember that energy Remember that energy isis required, and required, and molecules are moved molecules are moved againstagainst their their concentration gradientsconcentration gradients

Focus: Active transportFocus: Active transport 1.1. Movement via active transport proteinsMovement via active transport proteins

ATP required (has own binding site)ATP required (has own binding site)Note movement of particles (Ca++) Note movement of particles (Ca++)

against their concentration gradientagainst their concentration gradient

Focus: Active transportFocus: Active transport2. Endocytosis2. Endocytosis

Three types of endocytosisThree types of endocytosis PinocytosisPinocytosis

““cell drinking”cell drinking” Extracellular fluid taken inExtracellular fluid taken in

Receptor-mediated endocytosisReceptor-mediated endocytosis Specific for particular moleculesSpecific for particular molecules Molecules bind to receptors.Molecules bind to receptors. Receptor-molecule complex taken inReceptor-molecule complex taken in

PhagocytosisPhagocytosis Large particles engulfedLarge particles engulfed

Focus: Active transportFocus: Active transport3. Exocytosis3. Exocytosis

3. Specialization of cell surfaces3. Specialization of cell surfaces

Connections between cellsConnections between cells DesmosomeDesmosome: Membranes of adjacent cells glued : Membranes of adjacent cells glued

together by proteins and carbohydratestogether by proteins and carbohydrates Tight junctionTight junction: Cells sealed together with proteins: Cells sealed together with proteins

3. Specialization of cell surfaces (cont.)3. Specialization of cell surfaces (cont.)

Communication between cellsCommunication between cells Gap junctionsGap junctions: Channels connect adjacent cells: Channels connect adjacent cells PlasmodesmataPlasmodesmata: Continuous cytoplasm bridges : Continuous cytoplasm bridges

between two cells (plants)between two cells (plants) Note also cell walls. Note also cell walls. Only certain cell types have cell wallsOnly certain cell types have cell walls!!