Diffusion and OsmosisDiffusion and Osmosis

Chapter 3, Section 4Chapter 3, Section 4

Of your textbookOf your textbook

Passive TransportPassive Transport

Particles are constantly in motion, colliding Particles are constantly in motion, colliding and scattering. and scattering. This motion is random.This motion is random.

Does not require the cell to use ATP / Does not require the cell to use ATP / energyenergy

The membrane is still semi-permeable, so The membrane is still semi-permeable, so only certain substances can cross (in or only certain substances can cross (in or out) by passive transport.out) by passive transport.

Passive TransportPassive Transport

A A concentrationconcentration gradientgradient is is the difference in the the difference in the concentration of a substance concentration of a substance from one location to anotherfrom one location to another

When there is a When there is a concentration gradient, the concentration gradient, the NET movement is DOWN NET movement is DOWN their concentration gradient - their concentration gradient - from areas of higher from areas of higher concentration to areas of concentration to areas of lower concentrationlower concentration

ANIMATION

DiffusionDiffusion

Movement of Movement of molecules in a molecules in a fluid or gas fluid or gas from an area of from an area of higher higher concentration to concentration to an area of an area of lower lower concentrationconcentration

ANIMATION

EquilibriumEquilibrium

Diffusion Diffusion continues until continues until the solution is at the solution is at equilibrium (no equilibrium (no concentration concentration gradient).gradient).

Molecules still Molecules still move, but there move, but there is no NET is no NET movement.movement.

Facilitated DiffusionFacilitated Diffusion

Some small, Some small, polar polar molecules molecules cannot easily cannot easily diffuse across diffuse across a membrane. a membrane.

They need the They need the help of help of transport transport proteins.proteins.

Facilitated DiffusionFacilitated Diffusion

The process of diffusion being “helped” by The process of diffusion being “helped” by transport proteins is called transport proteins is called facilitatedfacilitated diffusiondiffusion..

Still passive transport because molecules Still passive transport because molecules are still moving down their concentration are still moving down their concentration gradientgradient

Requires no ATP / energyRequires no ATP / energy ANIMATIONANIMATION

Transport ProteinsTransport Proteins

In facilitated diffusion, In facilitated diffusion, transport proteinstransport proteins, , pierce the cell pierce the cell membrane and allow membrane and allow openings for openings for molecules to pass.molecules to pass.

These proteins are These proteins are considered integral considered integral proteins because they proteins because they are fully embedded in are fully embedded in the membrane.the membrane.

Transport ProteinsTransport Proteins

There are many There are many types of transport types of transport proteins. proteins.

Most only allow Most only allow certain ions or certain ions or molecules to pass. molecules to pass.

Some are simple Some are simple channels or tunnels channels or tunnels and some are more and some are more complex, shape-complex, shape-changing proteins.changing proteins.

OsmosisOsmosis

The diffusion of water across The diffusion of water across a semipermeable membrane a semipermeable membrane is called is called osmosisosmosis..

In a solution, there are water In a solution, there are water molecules and dissolved molecules and dissolved particles (the solute). particles (the solute).

The more dissolved particles The more dissolved particles there are, the lower the there are, the lower the concentration of water concentration of water molecules.molecules.

ANIMATION

SolutionsSolutions Water is considered the solvent.Water is considered the solvent. The substance(s) dissolved in water is / The substance(s) dissolved in water is /

are the solute(s).are the solute(s). Together, solvent + solute Together, solvent + solute solution. solution.

Comparing SolutionsComparing Solutions

A solution may be desribed as A solution may be desribed as isotonicisotonic, , hypertonichypertonic or or hypotonichypotonic relative to relative to another solutionanother solution

These are comparisons; they require a These are comparisons; they require a point of reference (ie, my hair is shorter… point of reference (ie, my hair is shorter… …than it was last year). …than it was last year).

The comparison in biology is usually to the The comparison in biology is usually to the inside of a cell.inside of a cell.

IsotonicIsotonic A solution is A solution is isotonicisotonic to a to a

cell if it has the cell if it has the samesame concentration of dissolved concentration of dissolved particles as the cell. particles as the cell.

This means the water This means the water concentration is also the concentration is also the same. same.

Water molecules move into Water molecules move into and out of the cell at an and out of the cell at an equal rate in an isotonic equal rate in an isotonic solution.solution.

The cell size remains the The cell size remains the same.same.

HypertonicHypertonic

A A hypertonichypertonic solution has solution has a a higherhigher concentration of concentration of dissolved particles than a dissolved particles than a cell. cell.

This means the water This means the water concentration is lower than concentration is lower than that of the cell.that of the cell.

Thus, water flows out of Thus, water flows out of the cell – so, the cell will the cell – so, the cell will shrivel and eventually die.shrivel and eventually die.

ANIMATION

HypotonicHypotonic

A A hypotonichypotonic solution has a solution has a lowerlower concentration of concentration of dissolved particles than a dissolved particles than a cell. cell.

Therefore the water Therefore the water concentration is higher concentration is higher than that of the cell.than that of the cell.

Thus, water diffuses into Thus, water diffuses into the cell – causing the cell the cell – causing the cell to expand and potentially to expand and potentially burst.burst.

Impact on CellsImpact on Cells

In an In an isotonicisotonic solution (center), water enters / exits solution (center), water enters / exits red blood cells at equal rates.red blood cells at equal rates.

In a In a hypertonichypertonic solution (like salt water – right), solution (like salt water – right), water rushes out and the cell water rushes out and the cell shrivels.shrivels.

In a In a hypotonichypotonic solution (like distilled water – left), solution (like distilled water – left), water rushes in and the cell swells / bursts (water rushes in and the cell swells / bursts ( lysislysis).).

Video clips:

RBC in isotonic solution

RBC in hypertonic solution

RBC in hypotonic solution

Adaptations - PlantsAdaptations - Plants Plant cells use the cell wall to prevent bursting. Plant cells use the cell wall to prevent bursting.

At center, the plant cell is in an isotonic solution. Water moves in / out at equal rates (no NET movement)

At left, the plant cell is in a hypotonic solution.

Water rushes in, filling the vacuole.

This cell is turgid / has high turgor pressure.

At right, the plant cell is in a hypertonic solution.

Water rushes out of the cell, draining the vacuole.

This is called plasmolysis.Video: Elodea in isotonic / hyper / hypo

Adaptations - ProtistsAdaptations - Protists

Paramecia live in Paramecia live in freshwaterfreshwater

This makes paramecia This makes paramecia hypertonichypertonic to their to their surroundingssurroundings

Water is constantly Water is constantly rushing rushing intointo the the parameciumparamecium

So the paramecium uses So the paramecium uses a a contractile vacuolecontractile vacuole to to pump the water back out pump the water back out (and prevent bursting(and prevent bursting))

Video: The contractile vacuole in action