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

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Text of Chapt. 12, Movement Across Membranes Chapt. 12, lewisj/Cell.04/Lectures/Chap12_6.pdf Chapt. 12,...

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    Chapt. 12, Movement Across Membranes

    • Two ways substances can cross membranes – 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 molecules and small polar molecules can diffuse through a synthetic lipid bilayer or the lipid bilayer of a real biological membrane. (Fig. 12-2)

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    Chapt. 12, Movement through lipid bilayer

    • Larger polar molecules, cannot rapidly diffuse through the bilayer.

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    Chapt. 12, Movement through lipid bilayer

    • Larger polar molecules, cannot rapidly diffuse through the bilayer.

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    Chapt. 12, Movement through lipid bilayer

    • Ions or charged molecules cannot rapidly diffuse through the bilayer. (Fig. 11-20)

    • Ions are small. Why can’t they diffuse through?

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    Chapt. 12, Protein Based Transport

    • Many charged or large polar molecules do enter and exit cells. This requires membrane proteins. A simple proof:

    Fig 12.1

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    Chapt. 12, Protein Based Transport

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

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    Chapt. 12, Protein Based Transport

    • The cellular concentrations of ions and metabolites are very different on the inside and the outside of the cell.

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    Chapt. 12, Protein Based Transport

    • 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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    Chapt. 12, Protein Based Transport

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

    membrane is to contain metabolites or other molecules necessary for cellular functioning.

    – Some organic substances are rapidly imported into certain cells.

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    Chapt. 12, Carrier Proteins

    • Carrier Proteins are largely responsible for the differences in concentration of substances inside and outside of cells.

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    Chapt. 12, Carrier Proteins

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

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    Chapt. 12, Carrier Proteins

    • Nomenclature -- types of transport mediated 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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    Chapt. 12, Carrier Proteins

    • Sound familiar? (I hope)

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    Chapt. 12, Carrier Proteins

    • Similarities between enzymes and carrier proteins: – 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 a positive ∆G.

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    Chapt. 12, Carrier Proteins

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

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

    transition state.

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    Chapt. 12, Carrier Proteins

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

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    Chapt. 12, Carrier Proteins

    • Similarities in kinetics: – Vmax – Km

    • Design an experiment to determine Vmax and Km. Be specific.

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    Chapt. 12, Carrier Proteins

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

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    Chapt. 12, Carrier Proteins

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

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    Chapt. 12, Carrier Proteins

    • For uncharged molecules the free energy gradient is really the same as the concentration gradient and the diagram is O.K.

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    Chapt. 12, Carrier Proteins

    • However, for any charged particle, the free energy differences is a composite of the concentration gradient and the charge gradient. This combined gradient is called the electrochemical gradient, and the energy difference for the particle is called the electrochemical potential. (Fig 12-7)

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    Chapt. 12, Carrier Proteins

    • (Fig 12-8; alternative version)

    + + +

    - - -

    Extra panel

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    Chapt. 12, Carrier Proteins

    • Passive transport thus can be defined as transport in which the transported molecule drops down the electrochemical gradient (and thus the free energy gradient)

    • Active transport can be defined as transport in which the transported molecule is moved up the electrochemical gradient.

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    Chapt. 12, Carrier Proteins

    • 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 common inside cells than outside; Na+ is much more common outside cells than inside. How did it get that way?

    • Lets us consider what this fact alone can tell us.

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    Chapt. 12, The Na+/K+ Pump

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

    concentration gradient in response to an electrical gradient. Could this explain these results?

    – No! Both ions are positive. You cannot attract both ions in different directions with an electrical gradient.

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    Chapt. 12, The Na+/K+ Pump

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

    about by facilitated diffusion, what is the other alternative?

    – A: at least one (and probably both) ions must be pumped against their electrochemical gradients.

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    Chapt. 12, The Na+/K+ Pump

    • Lets us consider what this fact alone can tell us.

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

    – ATP is a logical choice.

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    Chapt. 12, The Na+/K+ Pump

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

    (On the portion of the pump facing the cytosolic 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 animal cell. 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 animal cell. What are these gradients used for? – Powering co-transport. (Fig. 12-14, 12-15)

    38Fig. 12.15

    39 40

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

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

    electrically active cells (considered in more 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 concentration gradient 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 gated in different ways.

    Fig 12-24

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    Chapt. 12, Ion Channels

    • Ion channels are like doors – They show ion

    selectivity. • Sometimes

    pass only 1 particular ion.

    • Sometimes pass multiple similar ions. Fig 12-19 46

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

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

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    Fig. 12.22 48

    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 Membrane Potential

    • 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

    42,000 other neg charges

    tot