Reading Notes For Membrane Transport

Transport across membranes I: Passive Transport, no energy required

Words that you should be able to define:

simple diffusion

selectively permeable

facilitated diffusion

membrane potential (either chemical or electrical)
osmosis

transporters

permeases

aquaporins

Important concepts:

Why are the structures of many transporters not known?

What contributes to the thermodynamic barriers that impede membrane transport?

What driving forces allow passive diffusion to occur with transporters?

How are transporters analogous to enzymes?

How do tranporters differ from enzymes?

Give examples of locations of aquaporins.

How do aquaporins work? How is their proposed structure related to their function?

Where is the GluT1 glucose transporter found? How is the proposed structure of this transporter related to its function? How is it specific for glucose?

Michalis-Menton kinetics can be applied to transporters!

What is the value of Kt for D-glucose? For galactose? For L-glucose? What does Kt mean?

Comparison of Erythrocytes and Hepatocytes

The liver has GluT2 transporters and they have a Kt value of 66 mM. If blood glucose levels were 5 mM, which tissue would take up glucose? Make a graph of velocity of glucose uptake vs glucose concentration. Aren't you impatient to learn about the glucose transporters in muscle and adipocytes??

One last type of energy free (passive) transporters: Cotransport systems.

The chloride-bicarbonate exchanger works to shuttle CO2 out of the tissues, into the blood as bicarbonate and then out of the lungs.

Active transport (Energy is required)

Be able to describe and give examples of 1° and 2° active transport.

We will discuss the free energy equations that are used for the transport of uncharged species.

Why is delta G°' equal to zero for membrane transport?
We will also do some practice calculations for various ratios of stuff inside to stuff outside. The book uses C2 to indicate the side that stuff is going "to" (analogous to P for products), and C1 for the side that stuff came from (analogous to S for substrates).
What will the delta Gt be for transporting stuff into a cell that has more of the stuff inside than out?

Another addition to the equation is required for the transport of charged species.

delta psi = membrane potential
Delta psi= (from - to)
What is the Delta Gt for moving choride ions into a cell, if the concentration of chloride is the same outside and inside the cell?

If a reaction or process is spontaneous, what is the sign of the value for Delta G? What is the sign of Delta G for a non spontaneous process/reaction?

There are four types of ATP dependent transporters. Be familiar with these. Where are they found (cell type and membrane type), what inhibits them? What do they transport? Be sure to read about the multidrug transporter. Note: the Fo subunit of the F-type ATP coupled transporter is not F(zero) it is F(letter o). The "o" stands for oligomycin. Oligomycin inhibits this type of transporter by blocking the ion's access to the membrane channel. We'll learn more about these F-type transporters that both make and break ATP later this semester.

Box 11-3 describes the transporter that is defective in the genetic disease, cystic fibrosis. We'll discuss this briefly in class.

We will discuss how the Na+/K+ ATP pump works. About 25% of your resting energy consumption is due to this pump! We will practice some manipulations of the Delta Gt for maintaining the Na+ and K+ gradients.

Another important example of 1° active transport is the Ca+2 pump.

Why is Ca+2 pumped out of the cytosol?
Where is it pumped in myocytes?

What drives 2° active transport. Why is it "secondary"?

The Na+/glucose symporter allows intestinal cells to accumulate glucose at very high concentrations. How does this work? The book uses Delta E in place of delta psi, I think this is a mistake. Be able to calculate the excess glucose concentration on the inside for a given set of Na+ concentrations inside and outside and a given membrane potential. Note: The book states that this transporter works in conjunction with GluT2 in the intestinal epithelial cells. The GluT2 transporter is in the liver. The GluT5 glucose transporter is on the blood vessel side of intestinal epithelial cells.

deltaG = 8.314J/moleK*310K ln (145/12) + (1 *96480J/volt*mole*-50x10-3volts) = -13.2 kJ/mole

2 moles Na+ *-13.2kJ/mole = -26.4 kJ (the book has -25kJ)

To calculate the amount of glucose that can be accumulated, change the sign on the Delta G for the transport of Na+ and solve for the ratio of C2/C1.

-26.4kJ = 8.314J/moleK*310K ln (C2/C1)
-26.4/(8.314*310) = 10.22
10.22 = ln (C2/C1)
e10.22 = C2/C1
27,597 = C2/C1
If C1 = 1 then C2 = 27, 597 which is roughly 30,000 times more glucose on the inside than on the outside.

What is an ionophore?

Be able to describe ion selective channels.

What are the three major differences between ion selective channels and ion transporters? Where are these channels often found?

Be able to describe the K+ channel.

Why is it selective for K+ ?
Why is work on this channel a landmark in biochemical research?

What is a porin?

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