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Chapter 4Chapter 4

The Cell: The Cell:

The Fundamental Unit of LifeThe Fundamental Unit of Life

Lecture Lecture 5: Energy5: Energy--Dependent Membrane Dependent Membrane Transport and Membrane PotentialTransport and Membrane Potential

•• We’ve talked about how cells move solutes across We’ve talked about how cells move solutes across membranesmembranes

•• Simple diffusionSimple diffusion

•• ChannelChannel--mediated diffusionmediated diffusion

•• CarrierCarrier--mediated diffusionmediated diffusion

•• OsmosisOsmosis

Transport Across Cell MembranesTransport Across Cell Membranes

•• What happens when these methods don’t work?What happens when these methods don’t work?

•• What if we need to move solutes AGAINST a What if we need to move solutes AGAINST a concentration gradient?concentration gradient?

•• Requires energyRequires energy

Transport Across Cell MembranesTransport Across Cell Membranes

••

•• Moves substances Moves substances

through carrier proteinsthrough carrier proteins

specific to solutespecific to solute

•• Energy provided by ATPEnergy provided by ATP

•• Works AGAINST Works AGAINST

concentration gradientconcentration gradient

Transport of Small SolutesTransport of Small Solutes

Active transportActive transport

•• Active transportActive transport

•• Used to…Used to…

•• Concentrate substances in the cell or in Concentrate substances in the cell or in

organellesorganelles

•• Get rid of unneeded thingsGet rid of unneeded things

•• Create gradients that drive other processesCreate gradients that drive other processes

•• Example: sodiumExample: sodium--potassium pumppotassium pump

Transport of Small SolutesTransport of Small Solutes

••

•• Also called ionAlso called ion--coupled transportcoupled transport

•• Uses energy provided by ion concentration Uses energy provided by ion concentration

gradients to move solutes across membranegradients to move solutes across membrane

•• ATP is not used to drive process, but is later ATP is not used to drive process, but is later

used to restore homeostasisused to restore homeostasis

Transport of Small SolutesTransport of Small Solutes

Secondary active transportSecondary active transport

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•• 2 kinds of secondary active transport2 kinds of secondary active transport

•• Ion and solute move in the same direction = Ion and solute move in the same direction = symportsymport

•• Example: NaExample: Na++ and glucose in the kidneyand glucose in the kidney

Transport of Small SolutesTransport of Small Solutes

•• 2 kinds of secondary active transport2 kinds of secondary active transport

•• Ion and solute move in opposite directions = Ion and solute move in opposite directions = antiportantiport

•• Example: NaExample: Na++/Ca/Ca2+ 2+ exchanger in kidneyexchanger in kidney

Transport of Small SolutesTransport of Small Solutes

•• FiltrationFiltration

•• Hydrostatic pressure forces water and solute Hydrostatic pressure forces water and solute across a porous membraneacross a porous membrane

•• Process based on pore size of membraneProcess based on pore size of membrane

•• Example: cardiovascular systemExample: cardiovascular system

Transport of Small SolutesTransport of Small Solutes

•• This process is later connected to This process is later connected to symportsymport and and diffusion in the kidneydiffusion in the kidney

Transport of Small SolutesTransport of Small Solutes

•• Some substances are too large to pass through a Some substances are too large to pass through a membrane protein or poremembrane protein or pore

•• Examples: whole proteins, bulk transportExamples: whole proteins, bulk transport

•• Must be moved by vesicular transportMust be moved by vesicular transport

•• Membrane reorganizes to move substances Membrane reorganizes to move substances

through the membrane in vesiclesthrough the membrane in vesicles

•• REQUIRES ATPREQUIRES ATP

Transport of Large SolutesTransport of Large Solutes

•• Substances moving into the cell = Substances moving into the cell = endocytosisendocytosis

•• ClathrinClathrin

•• Organizing protein on inside of membraneOrganizing protein on inside of membrane

•• Helps pinch off a portion of the membrane Helps pinch off a portion of the membrane with solutes trapped insidewith solutes trapped inside

•• Involved in sorting vesicles for multiple Involved in sorting vesicles for multiple pathwayspathways

•• Rapidly recycledRapidly recycled

Transport of Large SolutesTransport of Large Solutes

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•• Specific molecules outside cells bind to specific Specific molecules outside cells bind to specific receptors on cell surfacereceptors on cell surface

•• Bound receptors clusterBound receptors cluster

•• Membrane folds inward to form a pocketMembrane folds inward to form a pocket

•• Vesicle pinches off and is released into the cellVesicle pinches off and is released into the cell

•• Fuses with Fuses with endosomeendosome, and eventually , and eventually lysosomelysosome

•• Molecules released from receptorsMolecules released from receptors

•• Vesicle membrane and receptors recycledVesicle membrane and receptors recycled

•• Cholesterol, iron brought into the cell this wayCholesterol, iron brought into the cell this way

ReceptorReceptor--Mediated Mediated EndocytosisEndocytosis

•• “Cell eating”“Cell eating”

•• EndocytosisEndocytosis of very large objectsof very large objects

•• Examples: bacteria, cell debrisExamples: bacteria, cell debris

•• Specialized process for specific cellsSpecialized process for specific cells

•• Example: macrophages, Example: macrophages, neutrophilsneutrophils

PhagocytosisPhagocytosis

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•• Process is very effective… but sometimes it goes Process is very effective… but sometimes it goes wrongwrong

PhagocytosisPhagocytosis

•• “Cell drinking”“Cell drinking”

•• EndocytosisEndocytosis of extracellular fluid containing small of extracellular fluid containing small solutessolutes

•• Small vesiclesSmall vesicles

•• Not very selectiveNot very selective

•• “Random sampling”“Random sampling”

PinocytosisPinocytosis

•• Essentially the opposite of Essentially the opposite of endocytosisendocytosis

•• Substances moved through the membrane to the Substances moved through the membrane to the outside of the celloutside of the cell

ExocytosisExocytosis EnergyEnergy--Dependent Membrane TransportDependent Membrane Transport

•• Recall: all of these types of transport require Recall: all of these types of transport require energyenergy

•• ATPATP

•• Pressure gradientPressure gradient

•• Solute gradientSolute gradient

ReviewReview

•• Ions are charged particles with predictable Ions are charged particles with predictable behaviorbehavior

•• Opposites attractOpposites attract

•• Likes repelLikes repel

•• Opposite charges separated = still attracted to Opposite charges separated = still attracted to

each othereach other

•• Strength of attraction = Strength of attraction = potential potential or or voltagevoltage

Membrane PotentialMembrane Potential

•• Vast majority of particles in the body are Vast majority of particles in the body are unchargeduncharged

•• Cells intentionally concentrate ions inside and Cells intentionally concentrate ions inside and outside cellsoutside cells

•• NaNa++/K/K++ pump moves Napump moves Na++ out and Kout and K++ inin

•• Creates 2 diffusion gradientsCreates 2 diffusion gradients

•• KK++ tries to get outtries to get out

•• NaNa++ tries to get intries to get in

Membrane PotentialMembrane Potential

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•• Cells have open KCells have open K ++ channels, closed Nachannels, closed Na++ channelschannels

•• KK ++ leaks out, leaves a negative charge balance leaks out, leaves a negative charge balance inside the cellinside the cell

•• KK ++ and Naand Na ++ become attracted to the negative become attracted to the negative charge on the other side of the membrane and charge on the other side of the membrane and create potentialcreate potential

•• Cells use potential to…Cells use potential to…

•• Monitor environmentMonitor environment

•• Communicate with other cellsCommunicate with other cells

Membrane PotentialMembrane Potential