Aim for FiveBiology

Analyze a model or visual representation

A model of a membrane protein

  • Units 1 and 2
  • 4 points
  • About 10 minutes

You can use a calculator on this question, just like on exam day.

A short question built around a model or diagram of a biological process. You describe what the model shows, explain how its parts relate, add to or use the model, and connect it to a bigger biological idea. On the exam: Question 5 of 6 in Section II; about 10 minutes suggested.

The question and its sources

Figure 1 is a model of a protein embedded in the plasma membrane of a cell.

Figure 1 (described)

The model shows a single polypeptide chain that crosses the phospholipid bilayer once. Its N-terminal section, about 120 amino acids long, is outside the cell. Next comes a section of 22 amino acids that passes through the membrane, coiled into an α-helix. The C-terminal section, about 60 amino acids long, is in the cytosol.

Each amino acid is shown as a circle, shaded to show its R group. In the section that passes through the membrane, 20 of the 22 amino acids have nonpolar R groups. In the sections outside the cell and in the cytosol, most amino acids have polar or charged R groups. The section outside the cell also contains one disulfide bond between two cysteine R groups.

Source: Simplified model

Suggested time: 10 minutes

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Part (a)

1 point

Describe the type of bond that links each amino acid to the next one in the polypeptide chain shown in Figure 1.

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Part (b)

1 point

Identify the level of protein structure represented by the α-helix in Figure 1, and describe the type of interaction that holds the α-helix in its shape.

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Part (c)

1 point

Using Figure 1, explain why most of the amino acids in the section that passes through the membrane have nonpolar R groups.

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Part (d)

1 point

A mutation changes many of the amino acids in the section that passes through the membrane to amino acids with charged R groups. Using Figure 1, predict where the mutant protein would be found in the cell, and explain your prediction.

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