AP Biology

Movement across the Cell Membrane

AP Biology

The diffusion of solutes across a synthetic membrane

Molecules of dye Membrane (cross section)

WATER

(a) Diffusion of one solute

(b) Diffusion of two solutes

Net diffusion Net diffusion

Net diffusionNet diffusion

Net diffusion Net diffusion

Equilibrium

Equilibrium

Equilibrium

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Diffusion:§ 2nd Law of Thermodynamics

governs biological systemsu universe tends towards disorder (entropy)

§ Diffusionu movement from HIGH ® LOW concentration

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Simple Diffusion:§ Move from HIGH to LOW concentration

u “passive transport”u no energy needed

diffusion osmosis

movement of water

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Facilitated Diffusion:§ Diffusion through protein channels

u channels move specific molecules across cell membrane

u no energy needed

“The Bouncer”

open channel = fast transportfacilitated = with help

HIGH

LOW

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Active Transport:§ Cells may need to move molecules against

concentration gradientu conformational shape change transports solute

from one side of membrane to other u protein “pump”u “costs” energy = ATP

“The Doorman”

ATP

HIGH

conformational changeLOW

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Active transport:§ Many models & mechanisms

symportantiport

ATP ATP

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Getting through cell membrane:§ Passive Transport

u Simple diffusion§ diffusion of nonpolar, hydrophobic molecules

w lipidsw HIGH ® LOW concentration gradient

u Facilitated transport§ diffusion of polar, hydrophilic molecules§ through a protein channel

w HIGH ® LOW concentration gradient

§ Active transportu diffusion against concentration gradient

§ LOW ® HIGHu uses a protein pumpu requires ATP

ATP

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Transport summary:simplediffusion

facilitateddiffusion

activetransport

ATP

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How about large molecules?§ Moving large molecules into & out of cell

u through vesicles & vacuolesu endocytosis

§ phagocytosis = “cellular eating”§ pinocytosis = “cellular drinking”

u exocytosis

exocytosis

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Endocytosis: phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specificprocess

triggered bymolecular signal

Cholesterol (carried by LDL’s) is brought in by receptor-mediated endocytosis.

AP Biology 2007-2008

The Special Case of Water:

Movement of water across the cell membrane

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Osmosis is just diffusion of water:§ Water is very important to life,

so we talk about water separately§ Diffusion of water from

HIGH concentration of water to LOW concentration of wateru across a

semi-permeable membrane

AP Biology

Lower concentrationof solute (sugar)

Higher concentrationof solute

More similarconcentrations of solute

Sugarmolecule

H2O

Selectivelypermeablemembrane

Osmosis

Osmosis is just diffusion of water! Lets see more

Detail here!

AP Biology

Selectivelypermeablemembrane

Osmosis

Watermolecules can

pass throughpores, but sugar

moleculescannot.

Water moleculescluster around

sugar molecules.

This side hasmore solutemolecules,fewer free

water molecules.

This side hasfewer solute

molecules,more free

water molecules.

Osmosis: (detail of membrane)

OK, that’s better

!

AP Biology

Water Balance of Cells Without Cell Walls:§ Tonicity is the ability of a surrounding solution to

cause a cell to gain or lose water (see beakers)This is Hypotonic

relative to the bag contentsThis is Hypertonic

relative to the bag contentsThis is ???

relative to the bag contents

AP Biology

Concentration of water:§ Direction of osmosis is determined by

comparing total solute concentrationsu Hypertonic - more solute, less wateru Hypotonic - less solute, more wateru Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water

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Managing water balance: “Osmoregulation”§ Cell survival depends on balancing water

uptake & loss through osmoregulation

freshwater balanced saltwater

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Managing water balance:§ Hypotonic

u a cell in fresh wateru high concentration of water around cell

§ problem: cell gains water, swells & can burst

§ example: Parameciumw ex: water continually enters Paramecium cell

§ solution: contractile vacuolew pumps water out of cellw ATP

u plant cells§ turgid = full§ cell wall protects from bursting

ATP

1

No problem,here

freshwater

KABOOM!

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Pumping water out:§ Contractile vacuole in Paramecium

ATP

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Managing water balance:§ Hypertonic

u a cell in salt wateru low concentration of water

around cell§ problem: cell loses water &

can die § example: shellfish§ solution: take up water or

pump out saltu plant cells

§ plasmolysis = wilt§ can recover saltwater

2

I willsurvive!

I’m shrinking,I’m shrinking!

AP Biology

Managing water balance:§ Isotonic

u animal cell immersed in mild salt solution

u no difference in concentration of water between cell & environment§ problem: none

w no net movement of watern flows across membrane equally, in both directions

w cell in equilibriumw volume of cell is stable

§ example:blood cells in blood plasmaw slightly salty IV solution in hospital balanced

3

I couldbe better…

That’sperfect!

AP Biology

Aquaporins:§ Water moves rapidly into & out of cells

u evidence that there were water channels§ protein channels allowing flow of water

across cell membrane - é rate of osmosis

1991 | 2003

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller

AP Biology

.05 M .03 M

Do you understand Osmosis…

Cell (compared to beaker) ® hypertonic or hypotonicBeaker (compared to cell) ® hypertonic or hypotonicWhich way does the water flow? ® in or out of cell

AP Biology

Short-Distance Transport of Water Across Plasma Membranes:(see chapter 36, pages 782 – 785)

§ To survive, plants must balance water uptake and loss

§ Osmosis is the diffusion of water into or out of a cell that is affected by solute concentration and pressure = water potential

Y = Water’s ability to get up and go!

This is expressed by the water potential equation: Y = YS + YP

§ Water potential is abbreviated as Y and measured in a unit of pressure called the (bars), aka. [megapascal (MPa)]

§ Yp = 0 (bars), aka. [Mpa] for pure water at sea level and at room temperatures

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§ Water potential determines the direction of movement of water§ Water flows from regions of higher water potential to

regions of lower water potential§ Potential refers to water�s capacity to perform work

How Solutes and Pressure Affect Water Potential:

solutemolecules

Side A Side B

Yp = 0 (bars)

Which side of the membrane will increase in

volume?

Which side of the membrane has the largest

water potential?

membranesolute

molecules

Direction of water movement

AP Biology

§ The solute potential (YS), aka. (osmotic potential) of a solution is directly proportional to its molarity (concentration)

§ Solute potential is always expressed as a negative number, eg. a 0.1 M sucrose is -0.23 bars because it has a negative effect on water potential, i.e. reduces water’s ability to move and do work.

This is expressed by the water potential equation: Y = YS + YP

Selectivelypermeablemembrane

Osmosis

Watermolecules can

pass throughpores, but sugar

moleculescannot.

Water moleculescluster around

sugar molecules.

This side hasmore solutemolecules,fewer free

water molecules.

This side hasfewer solute

molecules,more free

water molecules.

�

(see chapter 36, pages 782 – 785)

Solute Potential Formula: Ψs = -iCRT

i = ionization constant (for solute)C = molar solute concentration at equilibriumR = pressure constant (0.0831 liter bar/mole �K )T = temperature �K (273 + �C )

AP Biology

Lower concentrationof solute (sugar)

Higher concentrationof solute

More similarconcentrations of solute

Sugarmolecule

H2O

Selectivelypermeablemembrane

Osmosis

Osmosis is just diffusion of water! Lets see more

Detail here!

AP Biology

Water Movement Across Plant Cell Membranes§ Water potential affects uptake and loss of water by plant cells§ If a flaccid (limp) cell is placed in an environment with a higher solute

concentration, the cell loses water and undergo plasmolysis (wilts)§ Plasmolysis occurs when the membrane shrinks and pulls away from

the cell wall

Initial flaccid cell:

Final plasmolyzed cell at osmoticequilibrium with its surroundings:

(a) Initial conditions: cellular ψ > environmental ψ

ψP = 0ψS = −0.7ψ = −0.7 MPa

ψP = 0ψS = −0.9

ψ = −0.9 MPa

ψP = 0ψS = −0.9

ψ = −0.9 MPa

Environment0.4 M sucrose solution:

AP Biology

Water Movement Across Plant Cell Membranes§ If a flaccid cell is placed in a solution with a lower solute concentration,

the cell gains water and become turgid; swells§ Turgor loss in plants causes wilting, which can be reversed when the

plant is watered

Initial flaccid cell:

Final turgid cell at osmoticequilibrium with its surroundings:

(b) Initial conditions: cellular ψ < environmental ψ

ψP = 0ψS = −0.7ψ = −0.7 MPa

ψP = 0.7ψS = −0.7

ψ = 0 MPa

ψP = 0ψS = 0ψ = 0 MPa

EnvironmentPure water:

AP Biology

Initial flaccid cell:

Final plasmolyzed cell at osmoticequilibrium with its surroundings:

(a) Initial conditions: cellular ψ > environmental ψ

ψP = 0ψS = −0.7

ψ = −0.7 MPa

ψP = 0ψS = −0.9

ψ = −0.9 MPa

ψP = 0ψS = −0.9

ψ = −0.9 MPa

Environment0.4 M sucrose solution:

“Wilted”

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Initial flaccid cell:

Final turgid cell at osmoticequilibrium with its surroundings:

(b) Initial conditions: cellular ψ < environmental ψ

ψP = 0ψS = −0.7ψ = −0.7 MPa

ψP = 0.7ψS = −0.7ψ = 0 MPa

ψP = 0ψS = 0ψ = 0 MPa

EnvironmentPure water:

“Turgid”

AP Biology

Xylemcells

Xylem sapMesophyll cellsStomaWater moleculeAtmosphere

Transpiration

Adhesion byhydrogen bonding

Cellwall

Cohesionby hydrogen

bondingCohesion andadhesion inthe xylem

Water moleculeRoot hairSoil particle

WaterWater uptake from soil

Wat

er p

oten

tial g

radi

ent

Outside air ψ= −100.0 MPa

Leaf ψ (air spaces)= −7.0 MPa

Leaf ψ (cell walls)= −1.0 MPa

Trunk xylem ψ= −0.8 MPa

Trunk xylem ψ= −0.6 MPa

Soil ψ= −0.3 MPa

�

�

AP Biology

Diffusion through phospholipid bilayer:§ What molecules can get through directly?

u fats & other lipidsinside cell

outside cell

lipidsalt

aa H2Osugar

NH3

§ What molecules can NOT get through directly?u polar molecules

§ H2Ou ions (charged)

§ salts, ammoniau large molecules

§ starches, proteins

AP Biology

Diffusion across cell membrane:§ Cell membrane is the boundary between inside &

outside…u separates cell from its environment

INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O

OUTwasteammoniasaltsCO2H2O products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!

AP Biology

A human pancreatic cell obtains O2, and necessary molecules such as glucose, amino acids, and cholesterol, from its environment, and it releases CO2 as a waste product. In response to hormonal signs, the cell secretes digestive enzymes. It also regulates its ion concentrations by exchange with its environment. Based on what you have just learned about the structure and function of cellular membranes, describe how such a cell accomplishes these interactions with its environment.