Cells and Their Cells and Their EnvironmentEnvironment

Chapter 4 Section 1Chapter 4 Section 1

Cell Cell MembranesMembranes Cell membranes help organisms Cell membranes help organisms

maintain maintain homeostasishomeostasis by by controlling what substances may controlling what substances may enter or leave cells.enter or leave 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 plasma membrane.cell or plasma membrane.

Cell MembraneCell MembraneThe Cell Membrane is The Cell Membrane is semi-permeable or semi-permeable or selectively permeableselectively permeable only allowing certain only allowing certain molecules to pass molecules to pass through.through.

Cell MembraneCell Membrane

Cell MembraneCell Membrane Small molecules Small molecules

like like water, oxygen, water, oxygen, & carbon dioxide& carbon dioxide can move in and can move in and out freely.out freely.

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

ConcentrationConcentrationThe mass of solute in a given The mass of solute in a given

volume of solution or volume of solution or mass/volume.mass/volume.

Concentration GradientConcentration Gradient is a is a difference in the difference in the concentration of a substance concentration of a substance across a distanceacross a distance

EquilibriumEquilibriumIs a condition in Is a condition in which the which the concentration of a concentration of a substance is equal substance is equal throughout a space.throughout a space.

Passive Passive TransportTransport

Some substances can cross Some substances can cross the cell membrane without the cell membrane without any input of energy by the cell.any input of energy by the cell.

The movement of such The movement of such substances across the substances across the membrane is known as membrane is known as passive passive transport.transport.

DiffusionDiffusion DiffusionDiffusion is the movement of is the movement of

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.

Small molecules can pass through Small molecules can pass through the cell membrane by diffusionthe cell membrane by diffusion

Diffusion across a membrane is a Diffusion across a membrane is a type of type of passive transportpassive transport because because it does not require energy.it does not require energy.

DiffusionDiffusion

DiffusionDiffusion Diffusion is driven by the Diffusion is driven by the

kinetic energykinetic energy of the molecules. of the molecules. Kinetic energy keeps molecules Kinetic energy keeps molecules

in constant motion causing the in constant motion causing the molecules to move randomly molecules to move randomly away from each other in a away from each other in a liquid or a gas.liquid or a gas.

The Rate of The Rate of DiffusionDiffusion

The rate of diffusion depends on The rate of diffusion depends on temperaturetemperature, , sizesize of the molecules, of the molecules, & & type of molecules diffusingtype of molecules diffusing

Molecules diffuse faster at higher Molecules diffuse faster at higher temperatures than at lower temperatures than at lower temperaturestemperatures

Smaller molecules diffuse faster Smaller molecules diffuse faster than larger moleculesthan larger molecules

Diffusion & the Diffusion & the Concentration GradientConcentration Gradient

Diffusion always occurs Diffusion always occurs down a down a concentration gradientconcentration gradient (water moves from (water moves from an area where it is more concentrated to an area where it is more concentrated to an area where it is less concentrated)an area where it is less concentrated)

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 equally molecules will continue to move equally back & forth across a membraneback & forth across a membrane

OsmosisOsmosis The diffusion of The diffusion of waterwater across a semi-across a semi-

permeable membrane is called permeable membrane is called osmosis.osmosis.

Diffusion occurs from an area of high Diffusion occurs from an area of high water concentration (less solute) to water concentration (less solute) to an area of lower water concentration an area of lower water concentration (more solute)(more solute)

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

Hypertonic Hypertonic SolutionsSolutions

Solute concentration Solute concentration outsideoutside the cell isthe cell is higher higher (less water)(less water)

Water diffuses Water diffuses outout of of the cell until the cell until equilibrium is reachedequilibrium is reached

Cells will shrinkCells will shrink & die & die if too much water is if too much water is lostlost

Plant cells become Plant cells become flaccid (wilt); called flaccid (wilt); called plasmolysisplasmolysis

Hypotonic Hypotonic SolutionsSolutions

Solute Solute concentrationconcentration greater greater insideinside the cell (less the cell (less water)water)

Water Water moves moves intointo the cell until the cell until equilibrium is equilibrium is reachedreached

Hypotonic Hypotonic SolutionsSolutions

Animal cells swell & Animal cells swell & burst if they take in burst if they take in too much watertoo much water

Plant cells do best in Plant cells do best in hypotonic solutionshypotonic solutions

Isotonic Isotonic SolutionsSolutions Concentration of Concentration of

solutes solutes samesame inside inside & outside the cell.& outside the cell.

Water moves into & Water moves into & out of cell at an out of cell at an equal rate so there equal rate so there is is no net movement no net movement of water.of water.

Animal cells do best Animal cells do best in isotonic solutionsin isotonic solutions..

Facilitated Facilitated DiffusionDiffusion

Faster than simple diffusionFaster than simple diffusion Considered passive transport because Considered passive transport because

extra energy not usedextra energy not used Occurs down a concentration gradientOccurs down a concentration gradient Involves Involves carrier proteinscarrier proteins embedded in a embedded in a

cell’s membrane to help move across cell’s membrane to help move across certain solutes such as glucosecertain solutes such as glucose

Carrier molecules change shape when Carrier molecules change shape when solute attaches to themsolute attaches to them

Change in carrier protein shape helps Change in carrier protein shape helps move solute across the membranemove solute across the membrane

Carrier ProteinsCarrier Proteins Carrier proteinsCarrier proteins in the cell membrane form in the cell membrane form

tunnels across the membrane to move tunnels across the membrane to move materialsmaterials

Channel proteins may always be open or Channel proteins may always be open or have have gatesgates that open & close to control the that open & close to control the movement of materials; called 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 ChannelIon Channel is a transport protein with a is a transport protein with a polar pore through which ions can pass.polar pore through which ions can pass.

Carrier ProteinsCarrier Proteins

Facilitated DiffusionFacilitated Diffusion

Active TransportActive Transport Requires the use of Requires the use of ATPATP or energy. or energy. Moves materials Moves materials againstagainst their their

concentration gradient from an area of concentration gradient from an area of lower to higher concentration.lower to higher concentration.

May also involve membrane proteinsMay also involve membrane proteins Used to move ions such as NaUsed to move ions such as Na++, Ca, Ca++, ,

and Kand K++ across the cell membrane. across the cell membrane.

Sodium-Potassium Sodium-Potassium PumpPump

Sodium-Potassium pumpSodium-Potassium pump moves 3 Namoves 3 Na++ out for every 2 out for every 2 KK++ into the cell into the cellUses ATP as its energy sourceUses ATP as its energy source

Ion pumps help muscle & Ion pumps help muscle & nerve cells worknerve cells work

Active Active TransportTransport

Plants use Plants use active active transporttransport to help roots to help roots absorb nutrients from the absorb nutrients from the soil (plant nutrients are soil (plant nutrients are more concentrated inside more concentrated inside the root than outside)the root than outside)

EndocytosisEndocytosisEndocytosisEndocytosis moves large moves large

particles (proteins & particles (proteins & polysaccharides) into a polysaccharides) into a cell by using vesicles. cell by using vesicles.

ExocytosisExocytosis ExocytosisExocytosis is used to remove is used to remove

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