Cells and Their Cells and Their EnvironmentEnvironment

Chapter 4Chapter 4

4.1 Passive Transport4.1 Passive TransportPart 1: DiffusionPart 1: Diffusion

Remember: Living things need to maintain Remember: Living things need to maintain homeostasis (responds to external homeostasis (responds to external conditions to maintain a stable internal conditions to maintain a stable internal condition)condition)

One way to do this is controlling the One way to do this is controlling the movement of things in and out of a cellmovement of things in and out of a cell

Passive TransportPassive Transport Requires no energyRequires no energy Diffusion is the Diffusion is the

movement of a movement of a substance from an area substance from an area of high concentration to of high concentration to an area of low an area of low concentration by concentration by random movementrandom movement

A difference in A difference in concentration of a concentration of a substance across a substance across a space is a space is a “concentration gradient”“concentration gradient”

Equilibrium is where Equilibrium is where the concentration is the concentration is equalequal

Since the interior of a Since the interior of a cell is different from cell is different from its external its external environment environment substances move substances move back and forth back and forth through the cell through the cell membranemembrane

The membranes lipid The membranes lipid interior repels ions interior repels ions and polar molecules. and polar molecules. WHY!!WHY!!

The diffusion of The diffusion of small or nonpolar small or nonpolar particles across the particles across the cell membrane is cell membrane is the simplest form the simplest form of passive of passive transporttransport

Part 2: OsmosisPart 2: Osmosis The diffusion of water The diffusion of water

through a selectively through a selectively permeable membranepermeable membrane

Water moves down the Water moves down the concentration gradientconcentration gradient

When something is When something is dissolved in water its dissolved in water its molecules are attracted molecules are attracted to the water moleculesto the water molecules

This causes a difference This causes a difference between “free water” and between “free water” and “non-free water”“non-free water”

This causes the This causes the concentration gradientconcentration gradient

Hypertonic Hypertonic SolutionSolution Fluid outside has Fluid outside has

higher higher concentration of concentration of dissolved dissolved substances than substances than fluid inside the cellfluid inside the cell

Causes water to Causes water to diffuse out of celldiffuse out of cell

Cell shrinksCell shrinks

Hypotonic SolutionHypotonic Solution Fluid inside has Fluid inside has

higher higher concentration of concentration of dissolved dissolved substances than substances than fluid outside cellfluid outside cell

Causes water to Causes water to diffuse into celldiffuse into cell

Cell swellsCell swells

Isotonic SolutionIsotonic Solution Fluid inside has Fluid inside has

same concentration same concentration of dissolved of dissolved substances as fluid substances as fluid outside celloutside cell

Causes no net Causes no net water movementwater movement

Water moves in Water moves in and out at equal and out at equal ratesrates

Equilibrium – no Equilibrium – no change in sizechange in size

How does this affect you?How does this affect you?

Cells have ways of Cells have ways of dealing with swelling dealing with swelling or shrinkingor shrinking

Cell walls keep cells Cell walls keep cells from expanding too from expanding too muchmuch

Some unicellular Some unicellular eukaryotes have eukaryotes have contractile vacuoles contractile vacuoles to collect excess to collect excess water and force it outwater and force it out

Animal cells remove Animal cells remove dissolved particles to dissolved particles to stop osmosisstop osmosis

Part 3: Crossing the Cell Part 3: Crossing the Cell MembraneMembrane

Transport Transport Proteins called Proteins called channels help channels help polar substances polar substances cross cell cross cell membranemembrane

Each only allows Each only allows 1 kind of 1 kind of substance substance throughthrough

Diffusion through ion Diffusion through ion channelschannels Many ions are important Many ions are important

for cell functionsfor cell functions• Important in heartbeat Important in heartbeat

and nerve cellsand nerve cells Ion channels are Ion channels are

transport proteins with a transport proteins with a polar pore so ions can polar pore so ions can pass through pass through membranemembrane

Ions pass without Ions pass without touching nonpolar lipid touching nonpolar lipid tailstails

Ion channel gates Ion channel gates allow some to open allow some to open and close in and close in response with response with stimulistimuli Stretching of cell Stretching of cell

membranemembrane Change in electrical Change in electrical

chargecharge Binding of specific Binding of specific

molecules to channelmolecules to channel

No use of energy so No use of energy so Passive TransportPassive Transport

Electrical Charge and Electrical Charge and Ion TransportIon Transport Positive or negative Positive or negative

charge affects movement charge affects movement of charged particle across of charged particle across membranemembrane

Inside of cells are Inside of cells are negatively charged negatively charged compared to outsidecompared to outside

This causes positively This causes positively charged particles to want charged particles to want to move in and the to move in and the negative ones to want to negative ones to want to move out (trying to reach move out (trying to reach equilibrium)equilibrium)

Facilitated DiffusionFacilitated Diffusion Carrier Proteins (a kind of transport protein) Carrier Proteins (a kind of transport protein)

bind to a specific substance and carry it bind to a specific substance and carry it across membrane and release itacross membrane and release it

Still moving down concentration gradientStill moving down concentration gradient• Carrier protein attaches to substance on one sideCarrier protein attaches to substance on one side• Shape of protein changes bringing substance Shape of protein changes bringing substance

throughthrough• Substance protected from lipids and is released Substance protected from lipids and is released

and protein goes back to original shapeand protein goes back to original shape Passive TransportPassive Transport

4.2 Active Transport4.2 Active TransportPart 1: Movement Against a Concentration GradientPart 1: Movement Against a Concentration Gradient

When concentration is higher inside than outside, cells When concentration is higher inside than outside, cells must move things against gradient from low to highmust move things against gradient from low to high

Called Active TransportCalled Active Transport Requires energy, usually ATPRequires energy, usually ATP

Sometimes uses carrier proteinsSometimes uses carrier proteins Since moving against gradient, called membrane Since moving against gradient, called membrane

pumpspumps

Sodium-Potassium PumpSodium-Potassium Pump 3 sodium ions, Na+, out and two potassium ions, K+, 3 sodium ions, Na+, out and two potassium ions, K+,

inin Active transport of bothActive transport of both

• Sodium higher outside and potassium higher insideSodium higher outside and potassium higher inside Energy supplied by ATPEnergy supplied by ATP

How Does ATP Work?How Does ATP Work? ATP is Adenosine TriPhosphateATP is Adenosine TriPhosphate 3 phosphate groups attached to the sugar 3 phosphate groups attached to the sugar

Ribose and the base, AdenineRibose and the base, Adenine

Phosphates are negative so they repel each Phosphates are negative so they repel each other and are unstableother and are unstable

A little energy in causes one phosphate to break A little energy in causes one phosphate to break off and more energy is given offoff and more energy is given off

This causes ATP to become ADP, Adenosine This causes ATP to become ADP, Adenosine DiPhosphateDiPhosphate

Cell uses energy given off from this reactionCell uses energy given off from this reaction

Sodium-Potassium PumpSodium-Potassium Pump 3 Na+ inside bind to 3 Na+ inside bind to

pump and a pump and a phosphate group phosphate group leaves ATP and binds leaves ATP and binds to pumpto pump

Pump changes shape Pump changes shape and 3 Na+ are and 3 Na+ are released outsidereleased outside

2 K+ bind to pump 2 K+ bind to pump and are transported and are transported acrossacross

2 K+ and P from ATP 2 K+ and P from ATP is released inside cellis released inside cell

Pump prevents Na+ from building up in cell since it Pump prevents Na+ from building up in cell since it constantly flows in due to diffusionconstantly flows in due to diffusion This build up would cause water to flow in due to osmosis, causing This build up would cause water to flow in due to osmosis, causing

the cell to swell or burstthe cell to swell or burst

Allows K+ to be brought inAllows K+ to be brought in Many cells use Na+ pump to bring in things they needMany cells use Na+ pump to bring in things they need

Part 2: Movement in VesiclesPart 2: Movement in Vesicles Some things are too large to use carrier proteinsSome things are too large to use carrier proteins Vesicles used insteadVesicles used instead

Endocytosis when moved inEndocytosis when moved in Cell membrane forms a pouch around a Cell membrane forms a pouch around a

substance, closes up and pinches off substance, closes up and pinches off forming vesicleforming vesicle

Vesicle may fuse with lysosomes or Vesicle may fuse with lysosomes or other organellesother organelles

Exocytosis when moved outExocytosis when moved out Vesicles inside fuse with membrane and Vesicles inside fuse with membrane and

release contentsrelease contents Used for proteins modified by Golgi apparatusUsed for proteins modified by Golgi apparatus Nerve cells and gland cells release proteinsNerve cells and gland cells release proteins