Homeostasis and Cell Homeostasis and Cell TransportTransportChapter 5

Passive TransportPassive TransportPassive Transport – substances

cross the cell membrane with NO energy input from the cell

DiffusionDiffusionDiffusion – movement of molecules

from an area of higher concentration to an area of lower concentration◦Concentration Gradient – difference

in the concentration of molecules across a distance

◦Molecules tend to move from where they are more concentrated to where they are less concentrated (“down” their concentration gradient)

DiffusionDiffusionDiffusion is driven entirely by the

molecules’ kinetic energy◦Molecules are in constant motion

Diffusion will eventually cause the molecules to be in equilibrium – the concentration of molecules will be the same throughout the space the molecules occupy◦At equilibrium, molecules continue to move

How Diffusion Works

DiffusionDiffusion

Diffusion Across Diffusion Across MembranesMembranesDiffusion Across Membranes

◦Simple diffusion – diffusion across a cell membrane

◦Depends on: Size of molecule Type of molecule Chemical nature of the membrane

Diffusion Across Membranes

OsmosisOsmosisWater molecules diffuse across a cell

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

Direction of OsmosisDirection of OsmosisHypotonic solution –

concentration of solute outside the cell is lower than the concentration inside the cell◦Water diffuses into the cell

Direction of OsmosisDirection of OsmosisHypertonic solution –

concentration of solute outside the cell is higher than the concentration inside the cell◦Water diffuses out of the cell

Direction of OsmosisDirection of OsmosisIsotonic solution – concentration

of solutes is equal outside and inside of the cell◦Water diffuses into and out of the

cell at equal rates

OsmosisOsmosis

How Osmosis Works

How Cells Deal With How Cells Deal With OsmosisOsmosisContractile

vacuoles – organelles that remove water

Solute pumps – pump solutes out of the cell

How Cells Deal With How Cells Deal With OsmosisOsmosisCell wall in plants resists the pressure

exerted by water inside of a cell◦ Turgor pressure – pressure that water molecules

exert against a cell wall◦ Plasmolysis – cells shrink away from cell walls

and turgor pressure is lost

How Cells Deal With How Cells Deal With OsmosisOsmosisSome cells cannot compensate for

changes in solute concentration◦ Ex: red blood cells can swell and burst

(cytolysis)

Facilitated DiffusionFacilitated DiffusionFor molecules that cannot diffuse

across cell membranes, even when there is a concentration gradient

Movement of molecules is assisted by carrier proteins

Facilitated DiffusionFacilitated Diffusion

How Facilitated Diffusion Works

Important Properties of Important Properties of Facilitated DiffusionFacilitated DiffusionCan help substances move either

into or out of the cell, depending on the concentration gradient

Carrier proteins involved in facilitated diffusion are each specific for one type of molecule

Diffusion Through Ion Diffusion Through Ion ChannelsChannels

◦Ion channels – transport ions such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-)

◦Some ion channels are always open◦Some have “gates” that open and

close in response to: Stretching of the cell membrane Electrical signals Chemicals in the cell or external

environment

ACTIVE TRANSPORT ACTIVE TRANSPORT Section 2

Active TransportActive TransportMovement of materials from

lower concentration to higher concentration (“up” their concentration gradient)

Requires energy from the cell

Cell Membrane PumpsCell Membrane PumpsSodium-Potassium Pump

◦transports Na+ and K+ ions up their concentration gradients

How the sodium-potassium pump works

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump1. Three Na+

ions from the inside of the cell bind to the carrier protein

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump2. A phosphate

group is removed from ATP and bound to the carrier protein

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump3. The carrier

protein changes shape, allowing three Na+ ions to be released to the outside of the cell

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump4. Two K+

ions from the outside of the cell bind to the carrier protein

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump5. The

phosphate group is released and the carrier protein goes back to its original shape

Steps of the Sodium-Steps of the Sodium-Potassium PumpPotassium Pump6. The two K+

ions are released to the inside of the cell and the cycle is ready to repeat

Importance of the Sodium-Importance of the Sodium-Potassium PumpPotassium PumpThe ion exchange creates an

electrical gradient across the cell membrane◦Outside becomes positively charged◦Inside becomes negatively charged

This difference in charge is important for the conduction of electrical impulses along nerve cells

Movement in VesiclesMovement in VesiclesUsed for:

◦Substances that are too large to pass through the cell membrane

◦Transporting large amounts of small molecules into or out of cells at the same time

EndocytosisEndocytosis

EndocytosisEndocytosisProcess in which cells ingest

external fluid, macromolecules, and large particles, including other cells

Two types of endocytosis:◦Pinocytosis – transport of solutes or

fluids◦Phagocytosis – transport of large

particles or whole cells

Endocytosis

ExocytosisExocytosis

ExocytosisExocytosisProcess by which a substance is

released from the cell through a vesicle that transports the substance to the cell surface and then fuses with the membrane to let the substance out of the cell

Exocytosis