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
AP Biology
Diffusion:§ 2nd Law of Thermodynamics
governs biological systemsu universe tends towards disorder (entropy)
§ Diffusionu movement from HIGH ® LOW concentration
AP Biology
Simple Diffusion:§ Move from HIGH to LOW concentration
u “passive transport”u no energy needed
diffusion osmosis
movement of water
AP Biology
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
AP Biology
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
AP Biology
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
AP Biology
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
AP Biology
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
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
AP Biology
Managing water balance: “Osmoregulation”§ Cell survival depends on balancing water
uptake & loss through osmoregulation
freshwater balanced saltwater
AP Biology
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!
AP Biology
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
AP Biology
§ 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”
AP Biology
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.