HOMEOSTASIS AND HOMEOSTASIS AND TRANSPORTTRANSPORT

CHAPTER 5CHAPTER 5

CELL MEMBRANESCELL MEMBRANES• Cell membranes help organisms Cell membranes help organisms

maintain maintain homeostasishomeostasis by controlling by controlling what substances may enter or leave what substances may enter or leave cells cells

• To stay alive, a cell must exchange To stay alive, a cell must exchange materials such as food, water, & materials such as food, water, & wastes with its environment wastes with its environment

• These materials must cross the These materials must cross the cell or cell or plasma membraneplasma membrane

• Small molecules like Small molecules like water, water, oxygen, & carbon dioxideoxygen, & carbon dioxide can can move in and out freely move in and out freely

• Large molecules like Large molecules like proteins & proteins & carbohydratescarbohydrates cannot move easily cannot move easily across the plasma membrane across the plasma membrane

• The Cell Membrane is The Cell Membrane is semipermeable or selectively semipermeable or selectively permeablepermeable only allowing certain only allowing certain molecules to pass through molecules to pass through

PASSIVE TRANSPORTPASSIVE TRANSPORT• DEF = when substances move across DEF = when substances move across

a cell membrane without the use of a cell membrane without the use of energy by the cellenergy by the cell

• Three types:Three types:– DiffusionDiffusion– OsmosisOsmosis– Facilitated diffusionFacilitated diffusion

DIFFUSIONDIFFUSION• DiffusionDiffusion is the movement of small is the movement of small

molecules from an area of higher molecules from an area of higher concentration to an area of lower concentration to an area of lower concentration concentration

• This difference in the concentration of This difference in the concentration of molecules across a membrane is called a molecules across a membrane is called a concentration gradientconcentration gradient – Diffusion always occurs Diffusion always occurs down a concentration down a concentration

gradient gradient (water moves from an area where it is (water moves from an area where it is more concentrated to an area where it is less more concentrated to an area where it is less concentrated) concentrated)

• Diffusion is driven by the Diffusion is driven by the kinetic energykinetic energy of of the molecules the molecules

• Kinetic energy keeps molecules in constant motion Kinetic energy keeps molecules in constant motion causing the molecules to move randomly away from causing the molecules to move randomly away from each other in a liquid or a gas each other in a liquid or a gas

• The rate of diffusion depends on The rate of diffusion depends on temperature, size of the molecules, temperature, size of the molecules, & type of molecules diffusing& type of molecules diffusing

•Molecules diffuse faster at higher temperatures Molecules diffuse faster at higher temperatures than at lower temperaturesthan at lower temperatures

•Smaller molecules diffuse faster than larger Smaller molecules diffuse faster than larger moleculesmolecules

• Most short-distance transport of Most short-distance transport of materials into & out of cells occurs by materials into & out of cells occurs by diffusion diffusion

• Diffusion continues until the Diffusion continues until the concentration of the molecules is the concentration of the molecules is the same on both sides of a membranesame on both sides of a membrane

• When a concentration gradient no longer When a concentration gradient no longer exists, exists, equilibriumequilibrium has been reached but has been reached but molecules will continue to move molecules will continue to move equally back & forth across a equally back & forth across a membranemembrane

• simulationsimulation

OSMOSISOSMOSIS• The diffusion of The diffusion of waterwater across a semipermeable across a semipermeable

membrane is called membrane is called osmosisosmosis • Diffusion occurs from an area of Diffusion occurs from an area of high water high water

concentration (less solute) to an area of lower concentration (less solute) to an area of lower water concentration (more solute)water concentration (more solute)

• Movement of water is Movement of water is down its concentration down its concentration gradient gradient & doesn’t require extra energy & doesn’t require extra energy

• Cytoplasm is mostly water containing dissolved Cytoplasm is mostly water containing dissolved solutes solutes

• Concentrated solutionsConcentrated solutions have many solute have many solute molecules & fewer water molecules molecules & fewer water molecules

• Water moves from areas of low solute concentration Water moves from areas of low solute concentration to areas of high solute concentration to areas of high solute concentration

• Water molecules will cross membranes Water molecules will cross membranes until the concentrations of water & until the concentrations of water & solutes is equal on both sides of the solutes is equal on both sides of the membrane; called membrane; called equilibriumequilibrium

• At equilibrium, molecules continue At equilibrium, molecules continue to move across membranes evenly to move across membranes evenly so there is no net movementso there is no net movement

• simulationsimulation

HYPERTONIC SOLUTIONHYPERTONIC SOLUTION1. Solute concentration 1. Solute concentration outside the outside the

cell is hcell is higher (less water)igher (less water)2. Water diffuses out of the cell until 2. Water diffuses out of the cell until

equilibrium is reachedequilibrium is reached3. Cells will shrink & die if too much 3. Cells will shrink & die if too much

water is lostwater is lost4. Plant cells become flaccid (wilt); 4. Plant cells become flaccid (wilt);

called called plasmolysisplasmolysis

HYPOTONIC SOLUTIONHYPOTONIC SOLUTION1.1. Solute concentration greaterSolute concentration greater inside the cell inside the cell

(less water)(less water)2.2. Water moves into the cell until equilibrium is Water moves into the cell until equilibrium is

reachedreached3.3. Animal cellsAnimal cells swell & burst swell & burst (lyse(lyse) if they take in ) if they take in

too much watertoo much water4.4. CytolysisCytolysis is the bursting of cells is the bursting of cells5.5. Plant cellsPlant cells become become turgidturgid due to water due to water

pressing outward against cell wallpressing outward against cell wall6.6. Turgor pressureTurgor pressure in plant cells helps them keep in plant cells helps them keep

their shapetheir shape7.7. Plant cells do best in hypotonic solutions Plant cells do best in hypotonic solutions

ISOTONIC SOLUTIONISOTONIC SOLUTION1.1. ConcentrationConcentration of solutes of solutes samesame

inside & outside the cellinside & outside the cell2.2. Water moves into & out of cell at an Water moves into & out of cell at an

equal rate so there is equal rate so there is no net no net movement of watermovement of water

3.3. Animal cells do best in isotonic Animal cells do best in isotonic solutionssolutions

HOW CELLS DEAL HOW CELLS DEAL W/OSMOSISW/OSMOSIS• The cells of animals on The cells of animals on

land are usually in land are usually in isotonic environment isotonic environment (equilibrium) (equilibrium)

• Freshwater organisms Freshwater organisms live in hypotonic live in hypotonic environments so water environments so water constantly moves into constantly moves into their cells their cells

• Unicellular freshwater Unicellular freshwater organisms use energy to organisms use energy to pump out excess water pump out excess water byby contractile vacuoles contractile vacuoles

• Plant cell walls prevent plant cells from Plant cell walls prevent plant cells from bursting in hypotonic environmentsbursting in hypotonic environments– Turgor pressureTurgor pressure = the pressure that water = the pressure that water

molecules exert against the cell wallmolecules exert against the cell wall– PlasmolysisPlasmolysis= when cells shrink away from = when cells shrink away from

the cell wall losing turgor pressure; why the cell wall losing turgor pressure; why plants wiltplants wilt

• Some Some marine organismsmarine organisms can can pump pump out excess saltout excess salt

• Some cells, like red blood cells, Some cells, like red blood cells, cannot compensate for change in cannot compensate for change in solute concentrations because they solute concentrations because they don’t have contractile vacuoles, don’t have contractile vacuoles, solute pumps or cell wallssolute pumps or cell walls

• CytolysisCytolysis= the bursting of a cell in a = the bursting of a cell in a hypotonic solutionhypotonic solution

FACILITATED DIFFUSION FACILITATED DIFFUSION • Faster than simple diffusionFaster than simple diffusion• Considered passive transport because extra Considered passive transport because extra

energy not usedenergy not used• Occurs down a concentration gradientOccurs down a concentration gradient• Involves Involves carrier proteinscarrier proteins embedded in a cell’s embedded in a cell’s

membrane to help move across certain solutes membrane to help move across certain solutes such as such as glucoseglucose

• Carrier molecules Carrier molecules change shape when solute change shape when solute attachesattaches to them to them

• Change in carrier protein shape helps move Change in carrier protein shape helps move solute across the membranesolute across the membrane

CHANNEL PROTEINSCHANNEL PROTEINS• Channel proteinsChannel proteins in the cell membrane in the cell membrane

form tunnels across the membrane to form tunnels across the membrane to move materialsmove materials

• Channel proteins may always be open or Channel proteins may always be open or have have gatesgates that open & close to control that open & close to control the movement of materials; called the movement of materials; called gated gated channelschannels

• Gates open & close in response to Gates open & close in response to concentration inside & outside the cellconcentration inside & outside the cell

ION CHANNELSION CHANNELS• Provide small passage ways across cell Provide small passage ways across cell

membranes which ions can diffusemembranes which ions can diffuse• Each ion channel is specific for an ionEach ion channel is specific for an ion• Some are always open; some may open or Some are always open; some may open or

close in response to stimuliclose in response to stimuli– Stretching of cell membraneStretching of cell membrane– Electrical signalsElectrical signals– ChemicalsChemicals

• simulationsimulation

ACTIVE TRANSPORTACTIVE TRANSPORTSEC. 5.2SEC. 5.2

ACTIVE TRANSPORTACTIVE TRANSPORT• DEF= REQUIRES CELL TO EXPELL DEF= REQUIRES CELL TO EXPELL

ENERGY (ATP); WHEN MATERIALS ENERGY (ATP); WHEN MATERIALS HAVE TO MOVE HAVE TO MOVE UPUP THEIR THEIR CONCENTRATION GRADIENT; FROM CONCENTRATION GRADIENT; FROM AN AREA OF LOWER AN AREA OF LOWER CONCENTRATION TO AN AREA OF CONCENTRATION TO AN AREA OF HIGHER CONCENTRATIONHIGHER CONCENTRATION

SODIUM-POTASSIUM PUMPSSODIUM-POTASSIUM PUMPS• A carrier proteinA carrier protein• Transports NaTransports Na++ ions and K ions and K++ ions up ions up

their concentration gradientstheir concentration gradients• Sodium-Potassium pumpSodium-Potassium pump moves 3 moves 3

Na+ out for every 2 K+ into the cellNa+ out for every 2 K+ into the cell– 1. Causes a difference in charge inside 1. Causes a difference in charge inside

and outside the celland outside the cell– 2. Difference in charge is called 2. Difference in charge is called

membrane potentialmembrane potential

• Ion pumps Ion pumps help muscle & nerve help muscle & nerve cells workcells work

• PlantsPlants use active transport to use active transport to help help roots absorb nutrients from the roots absorb nutrients from the soilsoil (plant nutrients are more (plant nutrients are more concentrated inside the root than concentrated inside the root than outside)outside)

• simulationsimulation

BULK TRANSPORTBULK TRANSPORT• Moves large, complex molecules such Moves large, complex molecules such

as proteins across the cell membraneas proteins across the cell membrane• Large molecules, food, or fluid Large molecules, food, or fluid

droplets are packaged in membrane-droplets are packaged in membrane-bound sacs called bound sacs called vesiclesvesicles

• EndocytosisEndocytosis moves large particles moves large particles into a cellinto a cell

Endocytosis cont.Endocytosis cont.• Phagocytosis Phagocytosis is one type of endocytosisis one type of endocytosis

– 1. Cell membrane extends out forming 1. Cell membrane extends out forming pseudopodspseudopods (fingerlike projections) that surround (fingerlike projections) that surround the particlethe particle

– 2. Membrane pouch encloses the material & 2. Membrane pouch encloses the material & pinches off inside the cell making a pinches off inside the cell making a vesiclevesicle

– 3. Vesicle can fuse with 3. Vesicle can fuse with lysosomeslysosomes (digestive (digestive organelles) or release their contents in the organelles) or release their contents in the cytoplasmcytoplasm

– 4. Used by4. Used by ameba ameba to feed & to feed & white blood cellswhite blood cells to to kill bacteriakill bacteria

– 5. Known as 5. Known as "cell eating""cell eating"

Endocytosis cont.Endocytosis cont.• Pinocytosis Pinocytosis is another type of is another type of

endocytosisendocytosis– 1. Cell membrane surrounds fluid 1. Cell membrane surrounds fluid

dropletsdroplets– 2. Fluids taken into membrane-bound 2. Fluids taken into membrane-bound

vesiclevesicle– 3. Known as 3. Known as "cell drinking""cell drinking"

EXOCYTOSISEXOCYTOSIS• ExocytosisExocytosis is used to remove large is used to remove large

products from the cell such as products from the cell such as wastes, mucus, & cell products wastes, mucus, & cell products

• Proteins Proteins made by ribosomes in a made by ribosomes in a cell are packaged into transport cell are packaged into transport vesicles by the Golgi Apparatusvesicles by the Golgi Apparatus

• Transport vesicles fuse with the cell Transport vesicles fuse with the cell membrane and then the proteins are membrane and then the proteins are secreted out of the cell (e.g. insulin)secreted out of the cell (e.g. insulin)

• Active transport videoActive transport video