c340 Exercise on Membrane Function

Exercise on Membrane Function
	Return to Chemistry 340 Home Page

1. Fill in the table below which contrasts diffusion through the bilayer, diffusion through an ion channel, diffusion using a carrier, and active transport across a membrane.

Table 1: Movement Across Membranes
type of movement	Is energy required?	Is a protein required?	Is it specific?	Is it saturable?
diffusion through a bilayer
diffusion through an ion channel
diffusion using a carrier
active transport

2. Use the equation ΔG_T = RT ln (C₂/C₁ ) + Z F ΔV to determine ΔG_T for the
a. transporting glucose into the cell; b. transporting Na⁺ into the cell;
c. transporting Na⁺ out of the cell; d. transporting Ca²⁺ into the cell;
e. transporting Ca²⁺ out of the cell.
Assume ΔV = −70 mV (inside is more negative than outside). F = 96,480 J/V-mol

Table 2: Solute Concentrations for Question 2
solute	glucose	Na⁺	Ca²⁺
extracellular concentration	5 mM	145 mM	2 mM
intracellular concentration	1 mM	12 mM	1 x 10^-4 mM

3. Secondary active transport occurs when an electrochemical gradient that was created using active transport is used to move another solute against its electrochemical gradient. For example, transport of Na⁺ into a cell can be coupled to transport of Ca²⁺ in an unfavorable direction. This requires an overall negative ?G. Use your answers from question 2.
a. What is the combined ?G for transporting 1 Na⁺ into the cell and 1 Ca²⁺ out?
b. What is the combined ?G for transporting 2 Na⁺ into the cell and 1 Ca²⁺ out?
c. What is the combined ?G for transporting 3 Na⁺ into the cell and 1 Ca²⁺ out?
d. What is the combined ?G for transporting 4 Na⁺ into the cell and 1 Ca²⁺ out?
e. Which combination is the most efficient method of using the Na⁺ gradient to move
Ca²⁺ out of the cell? (Most efficient means the most economical; ?G is favorable, but
not by very much.)

4. GLUT1 and hexokinase indirectly work together to keep intracellular [glucose] low.
a. Draw the structures of glucose and glucose-6-phosphate. Include all of the phosphate.
b. Explain why GLUT1 transports glucose and galactose but not glucose-6-phosphate.
That is, consider why the GLUT1 binding site would not bind glucose-6-phosphate.

5. Intestinal epithelial cells and renal tubule cells both absorb glucose and transport it to capillaries. Because these cells do not modify most glucose, their intracellular glucose concentration can be significantly higher than extracellular concentration. Such cells use Na⁺–glucose symports to move glucose into the cell. Use your answer from question 2 for Na⁺ transport into the cell and calculate the combined ΔG for transporting 1 Na⁺ and 1 glucose into the cell when
a. C₂/C₁ for glucose = 1; b. C₂/C₁ for glucose = 10; c. C₂/C₁ for glucose = 100;
d. C₂/C₁ for glucose = 1,000.
e. At what C₂/C₁ ratio is co-transport of Na⁺ and glucose no longer favorable?

6. P-type ATPases all have a phosphoprotein intermediate and two conformations. For the Na⁺–K⁺ ATPase, one conformation has a high affinity for Na⁺, and the other has a low affinity for Na⁺, possibly as a result of changing the Na⁺ binding sites in size to resemble K⁺ binding sites. The proposed mechanism for the Na⁺–K⁺ ATPase has the following steps, listed alphabetically:
(A) ATP binds in the nucleotide-binding domain;
(B) first eversion (conformation change) so that Na⁺ sites change exposure and affinity;
(C) H₂O is added to remove phosphate from Asp side chain;
(D) K⁺ (2) from extracellular fluid bind sites for high affinity for K⁺;
(E) Na⁺ (3) from cytosol bind sites with high affinity for Na⁺;
(F) phosphate us transferred to an Asp side chain in the P domain;
(G) second eversion (conformation change) so that K⁺ sites change exposure and affinity.
a. Put the steps in order.
b. The diagrams below represent part of a membrane with the Na⁺–K⁺ ATPase going
through conformation and binding site changes.

Identify the cytosol and extracellular sides of the membrane.
Number the diagrams 1–6 to put them into the correct order.

top of page