Download pdf - Chapt. 12, Movement Across Membranes Chapt. 12, …lewisj/Cell.04/Lectures/Chap12_6.pdfChapt. 12, Movement Across Membranes •Two ways substances can cross membranes –Passing through

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Chapt. 12, Movement Across Membranes

• Two ways substances can crossmembranes– Passing through the lipid bilayer– Passing through the membrane as a

result of specialized proteins

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Chapt. 12, Movement through lipid bilayer

• Hydrophobic moleculesand small polarmolecules can diffusethrough a syntheticlipid bilayer or the lipidbilayer of a realbiological membrane.(Fig. 12-2)

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• Larger polar molecules,cannot rapidly diffusethrough the bilayer.

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• Larger polar molecules,cannot rapidly diffusethrough the bilayer.

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• Ions or chargedmolecules cannotrapidly diffuse throughthe bilayer. (Fig. 11-20)

• Ions are small. Whycan’t they diffusethrough?

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Chapt. 12, Protein Based Transport

• Many charged or large polar molecules doenter and exit cells. This requiresmembrane proteins. A simple proof:

Fig 12.1

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• The two classes of membrane transportproteins. Similarities and differences.(Fig. 12-2)

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• The cellular concentrations of ions andmetabolites are very different on theinside and the outside of the cell.

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• Ions– The inside has much less Na+ and much more

K+ than outside.– Other ions more common outside include Ca++,

Mg++, and Cl-.– Fixed anions are much more common inside

(but never diffuse out)– Summary table 12-1

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Table 12-1

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• Metabolites or other organic molecules– One of the major functions of the plasma

membrane is to contain metabolites or othermolecules necessary for cellular functioning.

– Some organic substances are rapidly importedinto certain cells.

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Chapt. 12, Carrier Proteins

• Carrier Proteins are largely responsiblefor the differences in concentration ofsubstances inside and outside of cells.

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• Some examples of carrier proteins in cells.(Fig. 12-5)

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• Nomenclature -- types of transportmediated by carrier proteins. (Fig. 12-14)

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Chapt. 12, Carrier Proteins• Mechanism of action. (Fig. 12-7)

– Molecular recognition/binding– Allosteric conformational change– Solute release– Return to original conformation

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• Sound familiar? (I hope)

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• Similarities between enzymes and carrierproteins:– Specificity in binding– Release of products– Can only carry out events with a negative ∆G– Can be coupled to an energy source to carry

out half reactions that otherwise would have apositive ∆G.

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• Further similarities between enzymes andcarrier proteins:– Speeds up a “permissible” (=spontaneous)

reaction.– It does so by lowering the energy of the

transition state.

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• Compare typical reaction: A ----> B withcarrier based transport: Xin ----> Xout

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• Similarities in kinetics:– Vmax– Km

• Design an experiment to determine Vmaxand Km. Be specific.

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• Active and “Passive” Transport(=facilitated diffusion) Fig. 12-4

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• There is something wrong with this figure.What is it?

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• For uncharged molecules the free energygradient is really the same as theconcentration gradient and the diagram isO.K.

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• However, for any charged particle, thefree energy differences is a composite ofthe concentration gradient and the chargegradient. This combined gradient is calledthe electrochemical gradient, and theenergy difference for the particle iscalled the electrochemical potential. (Fig12-7)

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• (Fig 12-8; alternative version)

+ + +

- - -

Extra panel

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• Passive transport thus can be defined astransport in which the transported moleculedrops down the electrochemical gradient(and thus the free energy gradient)

• Active transport can be defined astransport in which the transported moleculeis moved up the electrochemical gradient.

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• Active transport can be powered by:– Co-transport of another substance down its

energy gradient– ATP hydrolysis– Light energy

– Fig 12-9

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Chapt. 12, The Na+/K+ Pump

• A reminder: K+ is much more commoninside cells than outside; Na+ is much morecommon outside cells than inside. How didit get that way?

• Lets us consider what this fact alone cantell us.

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• Lets us consider what this fact alone cantell us.– We have seen that an ion can diffuse up its

concentration gradient in response to anelectrical gradient. Could this explain theseresults?

– No! Both ions are positive. You cannot attractboth ions in different directions with anelectrical gradient.

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• Lets us consider what this fact alone cantell us.– If these ion distributions cannot be brought

about by facilitated diffusion, what is theother alternative?

– A: at least one (and probably both) ions mustbe pumped against their electrochemicalgradients.

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• Lets us consider what this fact alone cantell us.

– If you had to guess, how do you suppose thatthis pump would be powered?

– ATP is a logical choice.

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• Lets us consider what this fact alone cantell us.– Where should the K+ binding site be located?

(On the portion of the pump facing thecytosolic or non-cytosolic side?)

– Where should the Na+ binding site be located?

– Where should the ATP binding site be located? 34

Chapt. 12, A Model for the Na+/K+ Pump

Fig. 12.12

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Chapt. 12, Functions of the Na+/K+ Pump• This pump is very expensive -- it can use

30% to 70% of the ATP used by an animalcell. What are these gradients used for?

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Chapt. 12, Functions of the Na+/K+ Pump• This pump is very expensive -- it can use

30% to 70% of the ATP used by an animalcell. What are these gradients used for?– Powering co-transport. (Fig. 12-14, 12-15)

38Fig. 12.15

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Chapt. 12, Functions of the Na+/K+ Pump

• What are these gradients used for?– The ion gradients are responsible for

electrically active cells (considered inmore detail later).

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Chapt. 12, Functions of the Na+/K+ Pump

• What are these gradients used for?– In many animals, the pump is necessary to

prevent osmotic lysis.• Typically more non-water molecules inside than

outside; water flows down its own concentrationgradient into the cell and the cell bursts.

• Made worse by Na+ and Cl- diffusing in.• Na+/K+ Pump pumps out Na+, also results in negative

membrane charge which repels Cl-.

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Chapt. 12, Other Important Pumps• The H+ pump.

– Importance in some organelles.– Importance in plants, fungi and bacteria. (Fig.

12-17)

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Chapt. 12, Other Important Pumps• The Ca++

pump.– Well

under-stood

– Import-ance

Fig 12-6 44

Chapt. 12, Ion Channels

• Ion channels are like doors– They are often gated.– They can

be gatedindifferentways.

Fig 12-24

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• Ion channelsare like doors– They show ion

selectivity.• Sometimes

pass only 1particular ion.

• Sometimespass multiplesimilar ions. Fig 12-19

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Chapt. 12, Ion Channels• Ion channels can be in either open or

closed states. The evidence (Fig. 12-22)

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Fig. 12.2248

Chapt. 12, Ion Channels• Channels are either all they way open or all

the way closed. (Fig. 12-23)

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Chapt. 12, Ion Channels and MembranePotential

• What is membrane potential?– The difference in total charges on the

opposite sides of a membrane.– Membrane potential can easily be measured (as

we just saw).– Where does the membrane potential come

from?• Cannot find free negative or positive charges on the

shelf of chemicals.50

10,000 Na+

140,000 K+

1 Ca++

10,000 Cl-

145,000 Na+

5,000 K+

1,000 Ca++

110,000 Cl-

139,999 other neg charges


total net charge = 0 total net charge =0

Difference in charges = 0- 0 or none

Cell inside Cell outside

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10,000 Na+

140,000 K+

1 Ca++

10,000 Cl-

145,000 Na+

5,000 K+

1,000 Ca++

110,000 Cl-



total net charge = +1000 total net charge =- 1000

Difference in charges = 1000 minus - 1000 = 2,000

Suppose a Na+ channel opened and 1000 Na+ diffused down their electrochemical gradient....

144,000 Na+11,000 Na+


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10,000 Na+

140,000 K+

1 Ca++

10,000 Cl-

145,000 Na+

5,000 K+

1,000 Ca++

110,000 Cl-



total net charge = -1000 total net charge =+1000

Difference in charges = - 1000 minus + 1000 = - 2,000

Suppose a K+ channel opened and 1000 K+ diffused down their electrochemical gradient....

6,000 K+139,000 K+


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• What have we learned?– The membrane potential is due to differing net

charges on each side of the membrane.– Changes in membrane potential are due to ions

moving across the membrane.– Because ions do not penetrate the hydrophobic

interior of the lipid bilayer, they must passthrough carrier proteins or channel proteins.

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• The equilibrium potential:– Let us consider again this

figure. The inside of thecell is to the left. There isa large difference in Na+

concentrations. Whathappens if we open up if weopen up Na+ channels only?

10,000 Na+

140,000 K+

1 Ca++

10,000 Cl-

145,000 Na+

5,000 K+

1,000 Ca++

110,000 Cl-



total net charge = 0 total net charge =0

Difference in charges = 0- 0 or none


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• What happens if we openup Na+ channels only?– Na+ flows in.– Changes membrane

potential.– Changes Na+ concentration.

• Will Na+ continue todiffuse in until [Na+] in =[Na+] out ?

10,000 Na+

140,000 K+

1 Ca++

10,000 Cl-

145,000 Na+

5,000 K+

1,000 Ca++

110,000 Cl-






144,000 Na+11,000 Na+


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• Will Na+ continue todiffuse in until [Na+] in =[Na+] out ?

• No, before long thepositive interior of thecell will balance out thegreater concentration ofNa+ on the outside.

10,000 Na+

140,000 K+

1 Ca++

10,000 Cl-

145,000 Na+

5,000 K+

1,000 Ca++

110,000 Cl-






144,000 Na+11,000 Na+


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• There are so many ions on the inside andoutsides of cells that this usually does notchanges the ion’s concentration very much.

Fig 12-27

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• So, now we can define the equilibriumpotential:– The membrane charge where the component of

the electric portion of the electrochemicalgradient exactly balances the concentrationportion of the electrochemical gradient.

– Different for every ion. Depends on:• The relative concentrations of the ion on the inside

v.s. the outside of the cell.• The charge on that ion.

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• The “resting potential” of most cells isnegative.– The Na+/K+ pump (a minor contributor)– K+ leak channels

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• The voltage gated Na+ channel is responsiblefor the action potential of electrically activecells including nerve and muscle.

• What is an action potential? Fig. 12-32

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Fig. 12-32

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• The three states of the voltage gated Na+channel.

Fig 12-23

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• Movementof the Na+ion and theactionpotential.

Fig 12-34 64

• The action potential propagates (=regenerates)along the membrane in one direction.

Fig. 12-38

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• The explanation for unidirectional propagation.

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• Other channels participate in nervetransmission.– The voltage gated K+ channel.– The voltage gated Ca++ channel at the axon

terminus. (Fig. 12-40)

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Fig 12-40

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• Other channels participate in nervetransmission (cont.)– The acetlycholine gated cation channel.

Fig. 12-42

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– How does the acetylcholine gated cationchannel initiate a response?

Fig. 12-41 70


• There are synapses that make an actionpotential more likely (excitatory) or lesslikely(inhibitory)

Fig. 12-43