L312/Spring 2007	Lecture 4	Drummond
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L312/Spring 2007 Lecture 4 Drummond. Housekeeping issues: In-class analogy exercise today (5 poins) Don’t forget about next Tuesday’s drawing assignment. I will post the first half of a study guide this week (1-4) Review key points:

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L312/Spring 2007 Lecture 4 Drummond

Housekeeping issues: In-class analogy exercise today (5 poins)

Don’t forget about next Tuesday’s drawing assignment.

I will post the first half of a study guide this week (1-4)

Review key points:

a. Phospholipids spontaneously form a small spherical bilayer in aqueous solution

impermeable to proteins, ions, most small molecules (hydrophobics pass) even water

b. Lipid bilayer itself is highly flexible; anchored by protein networks (cytoskeleton)

c. 25 nm liposome bilayer is much smaller, different shape compared with cells

d. Phospholipids and cholesterol have an amphipathic molecule structure

e. Membranes are asymmetric; fed from inside bilayer into inner sheath

  • flippases maintain distribution (ATP dependent)

  • vesicular transport: same membrane face always faces cytoplasm

  • supports extracellularization of carbohydrate face, among other features

  • Today: consider transmembrane structures and membrane roles.

  • FRAP (fluorescence recovery after photobleaching) can be used to

  • characterize difffusion of fluorescently labeled molecules in membrane

  • describe movement of materials across membranes

  • Movement of glucose across membranes


What are the essential roles of membranes within cells?

(no physical transport)

Flexibility

(are all membranes

basically the same?)

(highly selective physical transport)

Show NEW FRAP movie

Think about movement within the membrane


Next wave: transporting molecules across cell membranes

  • Focus on three general strategies (others exist)

    • Passive diffusion: transporter is simply a channel that allows

      flow of a single component down a concentration gradient (favorable);

      may still be gated (open/closed). Example: K+ channel or H20 channel).

    • Symport; still passive diffusion down a concentration gradient, but

      the process is coupled with an unfavorable transport. Example:

      the glucose/Na+ system for glucose uptake.

      3. Active transport. An energy source such as ATP is used to drive uptake

      (or export) of a molecule against a concentration gradient.

      (Movement is unfavorable without ATP hydrolysis, which is favorable).

      What kinds of transmembrane structures support selective transport?



Review of membrane-spanning helical structure proteins?

(what are the essential side chain properties?)


Multiple membrane-spanning helices can form a pore/transporter

Carefully note inside/outside relationships here.



A passive diffusion pore (can be gated to open and close) pore/transporter

Examples: K+, water,

Ammonia, glycerol, others

Why not bigger molecules?

What are the key mechanistic

Features? Which way does K+ flow?

Why?


Glucose uptake as a model system to study small molecule transport

Why the shape

Of the villi?

Note cellular

Structure here.

Pay REALLY close attention to the three transporters

and the driving force for each directional movement


The three classes of transporters used here transport

Glucose deposition

In the extracellular fluid

Na+/K+ exchange

On the ‘inside’

Glucose uptake from

the intestinal lumen


Another example of a passive movement across a membrane transport

What are the two key features

Conferred by the transporter?


Scooby Doo and the Mummy transport

(or the magic fridge commercial)

Scooby and Shaggy are wandering around in a room with a bookcase

Together they lean up against the bookcase

Suddenly the bookcase whirls around

Scooby and Shaggy disappear from room

Scooby and Shaggy are presumed to be deposited in the adjacent room


Coupled transport: a sodium gradient is used to concentrate glucose

What are the key elements of successful transport?

Are transporters enzymes? What is an enzyme?

What makes this process sustainable?


Glucose uptake as a model system to study small molecule transport

Why the shape

Of the villi?

Note cellular

Structure here.

Pay REALLY close attention to the three transporters

and the driving force for each directional movement


What happens to glucose that builds up in the cell? transport

What maintains a low

Glucose concentration in the

Extracellular fluid here?

Uniport!


What drives sodium back outside the cell? transport

(subtext: against a gradient)

Why is ATP essential?

What does it really do?

Why is the advantage to being an exchanger

(Na+ for K+ exchange)

What happens to the K+ (Figs 12.19,20)?

Antiport! (mousetrap?)



What structures might restrict lateral diffusion in a membrane?

How does this relate to cellular or organellar

structure? How might restriction of protein

or lipid movement support cell physiology?


The extracellular surface is coated with carbohydrates membrane?

Note that carbohydrates are linked to both lipids and proteins.

How did they get outside the cell?


What is one role for the surface carbohydrates on cells? membrane?

How flexible are membranes? How ‘squishable are cells?’

(how small a diameter ‘hole’ could a cell fit through?)

In-class: How might cells pass

between layers of cells in a tissue?

What are the key structural properties

of cells that allow this to occur?


If flip-flop is slow (how slow?), how rapid is lateral diffusion?

Ask about lipids AND proteins



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