Question 1

[emma]

Hi,
Can you explain on what part of the passage that say P1 and P2 are located outside of the membrane. I keep reading the passage but still can't figure it out. Thank you.

[admin]

Experiment 1 contains resealed cells. They are placed in an environment (= incubated) with an enzyme that can label protein. We know the enzyme cannot get into the cell because these cell's have been "resealed" so they are not permeable to macromolecules like enzymes. In other words, the enzyme is only able to label a protein that at least shows all or part of itself on the outside of the cell. Because Experiment 1 revealed that P-1 and 2 were labelled, that confirms that those 2 proteins must be either on the exterior of the cell's membrane or at least they must have a component of those proteins on the exterior of the cell's membrane.

Experiment 2 is designed so that labeling can only occur on the inside of the cell. It reveals that P-1 and 3 have been labeled which means that are either completely or partially inside the cell (on the inner part of the cell's membrane).

Putting the results together, P-1 has been shown to be in part on the exterior and the interior of the membrane. This can only be explained by the fact that it must span the membrane.

Analogy: the Fluid Mosaic model of the plasma membrane states that the phospholipid bilayer is fluid and the proteins form a mosaic in the membrane and their mobility is more limited. Consider the protein in the plasma membrane like raisins in raisin bread. If you look carefully, some raisins can only be seen on one side of the slice of bread, other raisins can only be seen on the other side of the bread while some raisins span the slice of bread so you can see part of the same raisin on both sides of the slice of bread.

[admin]

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