Glucose transport in liposomes

Authors

  • József Szeberényi

    Corresponding author
    1. Department of Medical Biology, School of Medicine, University of Pécs, H-7624 Pécs, Szigeti 12, Hungary
    • Department of Medical Biology, School of Medicine, University of Pécs, H-7624 Pécs, Szigeti 12, Hungary
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Abstract

Terms to be familiar with before you start to solve the test: lipoprotein membranes, liposomes, Na+,K+-ATPase, Na+/glucose symporter, glucose transporter.

The mechanism of glucose transport across biological membranes was studied in this experiment using artificial liposomes. An aqueous solution containing sodium, potassium, and magnesium ions, glucose, ATP, Na+,K+-ATPase, Na+/glucose symporter, and glucose transporter was used to produce liposomes. Under the conditions used the proteins were incorporated into the phospholipid bilayer in their natural orientation. At the moment of liposome formation the intraliposomal and extraliposomal concentrations of Na+, K+, Mg2+, glucose, and ATP were identical.

FIVE-CHOICE COMPLETION

Select the one best answer.

  • 1.____ What is the order of events in the liposomes?

    • ATP hydrolysis → Na+ outflux and K+ influx → Na+ influx and glucose influx → glucose outflux

    • ATP hydrolysis → Na+ outflux and K+ influx → glucose outflux

    • ATP hydrolysis → Na+ influx → glucose influx → K+ influx

    • ATP hydrolysis → Na+ influx and glucose influx → glucose outflux

    • The processes described in D start at the same time

The composition of the solution used for liposome production can be varied; it is thus possible to study the effect of the concentration of constituents on the transport processes.

VARIATION RELATIONSHIP

This type of question consists of pairs of phrases that describe conditions or quantities that may or may not vary in relation to each other. Select

  • A if increase in the first is accompanied by increase in the second or if decrease in the first is accompanied by decrease in the second;

  • B if increase in the first is accompanied by decrease in the second or if decrease in the first is accompanied by increase in the second;

  • C if changes in the second are independent of changes in the first.

    • 2.____ I. Intraliposomal ATP concentration

                  II. The outflux of Na+ ions across the liposomal membrane

    • 3.____ I. Intraliposomal ATP concentration

                  II. The influx of Na+ ions across the liposomal membrane

    • 4.____ I. Intraliposomal ATP concentration

                  II. The influx of glucose molecules across the liposomal membrane

    • 5.____ I. Intraliposomal ATP concentration

                  II. The outflux of glucose molecules across the liposomal membrane

Since the liposomes studied in this experiment are unable to synthesize ATP, sooner or later ATP will be completely consumed. What will be the situation in this system at the moment of the hydrolysis of the last ATP molecule?

QUANTITATIVE COMPARISON

In this type of question paired statements describe two entities that are to be compared in a quantitative sense. Select

  • A if A is greater than B;

  • B if B is greater than A;

  • C if the two are equal or very nearly equal.

    • 6.____ A. Extraliposomal K+ concentration

                  B. Intraliposomal K+ concentration

    • 7.____ A. Extraliposomal glucose concentration

                  B. Intraliposomal glucose concentration

MULTIPLE COMPLETION

A question or incomplete statement is followed by four numbered completions, one or more of which are correct. Select

  • A if 1, 2 and 3 are correct;

  • B if 1 and 3 are correct;

  • C if 2 and 4 are correct;

  • D if only 4 is correct;

  • E if all four are correct.

    • 8.____ Ouabain (an inhibitor of Na+,K+-ATPase) is added intraliposomally. Which of the following processes are inhibited?

      • 1. Outflux of Na+

      • 2. Influx of Na+

      • 3. Influx of K+

      • 4. Influx of glucose

CORRECT ANSWERS

  • 1. A        5. A

  • 2. A        6. B

  • 3. A        7. B

  • 4. A        8. E

EXPLANATIONS

Under the experimental conditions described in this test, Na+,K+-ATPase pumps Na+ and K+ outwards and inwards, respectively, across the liposomal membrane (Fig. 1). These transport processes are driven by ATP hydrolysis (MCQ11 2: A; MCQ 6: B). The Na+ gradient created by the Na+,K+-ATPase will drive the liposomal uptake of glucose via Na+/glucose symporter (MCQ 3: A; MCQ 4: A). The resultant intraliposomal accumulation of glucose will lead to the passive outflux of glucose through glucose transporter (MCQ 1: A; MCQ 5: A). All the transport processes of these liposomes thus are directly (active transport of Na+ and K+) or indirectly (Na+/glucose symport, passive transport of glucose) driven by ATP hydrolysis by Na+,K+-ATPase (MCQ 7: B; MCQ 8: E).

Figure FIGURE 1..

Transport processes across the liposomal membrane of the experiment described in this test.

Footnotes

  1. 1

    The abbreviation used is: MCQ, multiple choice question.

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