Biological MembranesBiological Membranes

Biological membranesBiological membranes Complex, dynamic structures Complex, dynamic structures

made of lipid and protein made of lipid and protein moleculesmolecules

Perform many functionsPerform many functions Define cell as a compartmentDefine cell as a compartment Regulate passage of materialsRegulate passage of materials Participate in chemical reactionsParticipate in chemical reactions Transmit signals between cell interior Transmit signals between cell interior

and the environmentand the environment Act as part of energy transfer and Act as part of energy transfer and

storagestorage

Biological membranesBiological membranes Physically separate cell interior Physically separate cell interior

from extracellular environmentfrom extracellular environment Form compartments within Form compartments within

eukaryotic cellseukaryotic cells Plasma membranePlasma membrane

Regulates passage of materialsRegulates passage of materials Participates in biochemical reactionsParticipates in biochemical reactions Receives information about Receives information about

environmentenvironment Communicates with other cellsCommunicates with other cells

Fluid mosaic modelFluid mosaic model Membranes consist of fluid Membranes consist of fluid

phospholipid bilayer with a phospholipid bilayer with a mosaic pattern of associated mosaic pattern of associated proteinsproteins

Phospholipid molecules are Phospholipid molecules are amphipathic and containamphipathic and containHydrophobic regions which are Hydrophobic regions which are repelled by waterrepelled by water

Hydrophilic regions which are Hydrophilic regions which are attracted to waterattracted to water

Phospholipid moleculesPhospholipid molecules

Since the cell is a very watery Since the cell is a very watery environment, the heads of the environment, the heads of the phospholipid molecules face phospholipid molecules face toward the cell.toward the cell.

The fatty acid “tails” of the The fatty acid “tails” of the molecules face inward toward molecules face inward toward the center of the bilayer. the center of the bilayer.

Phospholipids form bilayers in waterPhospholipids form bilayers in water

Phospholipids in water Detergent in water

Plasma membrane of mammalian red blood cellPlasma membrane of mammalian red blood cell

Membrane propertiesMembrane properties Orderly arrangement of Orderly arrangement of

phospholipid molecules make the phospholipid molecules make the cell membrane a liquid crystalcell membrane a liquid crystal Allow molecules to move rapidlyAllow molecules to move rapidly Proteins move within membraneProteins move within membrane

Lipid bilayers areLipid bilayers are FlexibleFlexible Self-sealingSelf-sealing Can fuse with other membranesCan fuse with other membranes

Detailed structure of the plasma membraneDetailed structure of the plasma membrane

Proteins are embedded in the Proteins are embedded in the bilayerbilayer

Integral membrane proteinsIntegral membrane proteins Embedded in the bilayerEmbedded in the bilayer

Transmembrane proteinsTransmembrane proteins Integral proteins that extend Integral proteins that extend

completely through the membranecompletely through the membrane Peripheral member proteinsPeripheral member proteins

Associated with the surface of the Associated with the surface of the bilayerbilayer

Membrane proteins, lipids, and Membrane proteins, lipids, and carbohydratescarbohydrates Asymmetrically positioned to Asymmetrically positioned to

bilayerbilayer Sides have different Sides have different

composition and structurecomposition and structure Function of member proteinsFunction of member proteins

Transport of materialsTransport of materialsActing as enzymes or receptorsActing as enzymes or receptorsCell recognitionCell recognitionStructurally linking cellsStructurally linking cells

Asymmetry of the plasma membraneAsymmetry of the plasma membrane

Functions of membrane proteinsFunctions of membrane proteins

Membranes are selectively Membranes are selectively permeable (only some permeable (only some materials are allowed in and materials are allowed in and out)out) Physical processesPhysical processes

OsmosisOsmosisDiffusionDiffusion

Carrier-mediated processesCarrier-mediated processesChannel proteinsChannel proteinsCarrier proteinsCarrier proteins

Diffusion: the movement of Diffusion: the movement of molecules from a region of higher molecules from a region of higher

concentration to one of lower concentration to one of lower concentration. concentration.

Rate of diffusion depends Rate of diffusion depends on:on:

Temperature (higher temperature, Temperature (higher temperature, more movement of molecules)more movement of molecules)

Size of molecules (smaller molecules Size of molecules (smaller molecules tend to diffuse faster)tend to diffuse faster)

Electrical charge (like charges repel Electrical charge (like charges repel and opposites attract)and opposites attract)

Difference in concentration Difference in concentration (concentration gradient)(concentration gradient)

Some molecules easily diffuse Some molecules easily diffuse through the membranethrough the membrane

WaterWaterGasesGasesSmall polar moleculesSmall polar moleculesLarge, lipid soluble Large, lipid soluble moleculesmolecules

OsmosisOsmosis The diffusion of water across a The diffusion of water across a

selectively permeable membrane selectively permeable membrane from a region where there is a from a region where there is a greater concentration of water to a greater concentration of water to a region where there is less water.region where there is less water.

Osmotic pressure: the tendency of Osmotic pressure: the tendency of water to move into a solution. water to move into a solution.

Osmotic pressureOsmotic pressure Concentration of dissolved Concentration of dissolved

substances in a solutionsubstances in a solution Isotonic: equal solute concentrationIsotonic: equal solute concentration Hypertonic: solution has a greater Hypertonic: solution has a greater

concentration of solutes than the concentration of solutes than the cell, loses water in plasmolysiscell, loses water in plasmolysis

Plasmolysis (collapse of a cell due Plasmolysis (collapse of a cell due to loss of water. to loss of water.

Hypotonic: solution has a lesser Hypotonic: solution has a lesser concentration of solutes than the concentration of solutes than the cell, gains water and swellscell, gains water and swells

Turgor pressureTurgor pressure

The internal hydrostatic The internal hydrostatic pressure usually present in pressure usually present in walled cells.walled cells.

Turgor pressure provides Turgor pressure provides structural support in plants structural support in plants which do not have wood. which do not have wood.

Movement of particles through Movement of particles through proteinsproteins

If substances cannot pass through the If substances cannot pass through the lipid bilayer, they may still move through lipid bilayer, they may still move through the membrane via protein channels.the membrane via protein channels.

This may or may not require energy This may or may not require energy from the cell. from the cell.

Passive transport: no energy Passive transport: no energy expenditureexpenditure

Active transport: energy expended by Active transport: energy expended by the cell. the cell.

Facilitated diffusion does not Facilitated diffusion does not require energyrequire energy Occurs down a concentration Occurs down a concentration

gradientgradient Active transport requires Active transport requires

energyenergy Moves ions or molecules against Moves ions or molecules against

a concentration gradienta concentration gradient Cotransport requires energyCotransport requires energy

ATP-powered pump maintains a ATP-powered pump maintains a concentration gradientconcentration gradient

Sodium-Potassium PumpSodium-Potassium Pump

Sucrose proton cotransportSucrose proton cotransport

Cells expend metabolic energy to Cells expend metabolic energy to carry on physiological processescarry on physiological processesExocytosis: large molecules Exocytosis: large molecules leave the cell through fusion leave the cell through fusion with the membranewith the membrane

Endocytosis: large molecules Endocytosis: large molecules enter the cell through fusion enter the cell through fusion with the membranewith the membranePhagocytosisPhagocytosisPinocytosisPinocytosisReceptor-mediated endocytosisReceptor-mediated endocytosis

ExocytosisExocytosis

PhagocytosisPhagocytosis

Phagocytosis is used by Phagocytosis is used by macrophages to help defend macrophages to help defend

the body from bacteriathe body from bacteria

Amoebae feed using Amoebae feed using phagocytosisphagocytosis

PinocytosisPinocytosis

Receptor-mediated endocytosisReceptor-mediated endocytosis

Cells communicate by Cells communicate by cell signalingcell signaling

Signaling molecules Signaling molecules includeincludeNeurotransmittersNeurotransmittersHormonesHormonesRegulatory moleculesRegulatory molecules

Cell signaling involvesCell signaling involves Synthesis and release of signaling moleculeSynthesis and release of signaling molecule Transport to target cellsTransport to target cells Reception by target cells involving special Reception by target cells involving special

surface receptors where the signaling protein surface receptors where the signaling protein docks.docks.

Signal transduction cells convert an Signal transduction cells convert an extracellular into an intracellular one. extracellular into an intracellular one.

Response by the cell: interior of the membrane Response by the cell: interior of the membrane protein undergoes a change in conformation, protein undergoes a change in conformation, which activates proteins in the cytoplasm—which activates proteins in the cytoplasm—often in a chain reaction. often in a chain reaction.

Termination of signalTermination of signal

SignalSignaltransductiontransduction

Cells in close contact often Cells in close contact often develop intercellular junctionsdevelop intercellular junctions Anchoring junctions connect Anchoring junctions connect

epithelial cellsepithelial cellsDesmosomes composed of filaments Desmosomes composed of filaments which hold cells subject to mechanical which hold cells subject to mechanical stresses together (in animal cells)stresses together (in animal cells)

Adhering junctions cement cells Adhering junctions cement cells together with proteinstogether with proteins

Tight junctions seal off intercellular Tight junctions seal off intercellular spaces in some animal cells. Seals spaces in some animal cells. Seals are protein links. are protein links.

Gap junctions: permit transfer Gap junctions: permit transfer of small molecules and ions. of small molecules and ions. These contain pores that These contain pores that connect cells. Allow rapid connect cells. Allow rapid chemical and electrical chemical and electrical communication between communication between cells. cells. Plasmodesmata: allow Plasmodesmata: allow movement of certain movement of certain molecules and ions between molecules and ions between plant cells.plant cells. Plasma membranes are continuous though the gaps.

DesmosomesDesmosomes

TightTightjunctionsjunctions

Gap junctionsGap junctions

PlasmodesmataPlasmodesmata