In individuals with normal blood sugar levels, glucose is reabsorbed into the bloodstream in the kidney by members of the GLUT transporter family, which form an aqueous pore across the membrane through which glucose can move passively. As a result, no glucose is excreted in urine. However, in people with untreated diabetes mellitus, blood sugar levels are high and glucose is often present in the urine. What can explain this occurrence?

Question

A.  High blood glucose levels reverse the concentration gradient, allowing untransported glucose to be excreted in urine.
B.  High blood glucose levels interfere with the coupled transport of water and glucose, allowing untransported glucose to be excreted in urine.
C.  The GLUT transporters become saturated, allowing untransported glucose to be excreted in urine.
D.  The GLUT transporters cannot hydrolyze ATP quickly enough for ATP to transport the extra glucose, thereby allowing untransported glucose to be excreted in urine.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?

Gather Content
· What do you know about glucose movement through the GLUT transporter? What other information is related to the question?

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  The GLUT transporters become saturated, allowing untransported glucose to be excreted in urine.

Clarify Question
· What is the key concept addressed by the question?
        o The question asks about glucose movement through the GLUT transporter.
· What type of thinking is required?
        o You are being asked to apply your knowledge about glucose movement through the GLUT transporter.

Gather Content
· What do you know about glucose movement through the GLUT transporter? What other information is related to the question?
        o Passive transport of glucose indicates that glucose is moving down its concentration gradient through the GLUT transporter. This means that it does not require ATP or cotransport of water. The GLUT transporter is a protein that makes a pore through the membrane of kidney cells.

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Because the GLUT transporter forms a pore of a given size, only so many glucose molecules can move through it at a time. In diabetes blood glucose levels increase dramatically. So if glucose levels increase, not all of the glucose molecules may be reabsorbed in time and some will be secreted in the urine.
 Reflect on Process

· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to apply your knowledge about glucose movement through the GLUT transporter. If you got the correct answer, great job! If you got an incorrect answer, where did the process break down? Did you think that passive movement of glucose through the GLUT transporter required ATP? Did you think that elevating the glucose level would reverse the glucose concentration gradient?