Question 40 Answer and Explanation _ACT December 2010 Form 62E Science Section

Passage VII

All cells have a thin outer layer called the plasma membrane. This membrane separates the cell's cytoplasm from the surrounding environment (see Figure 1).

Two models of the structure of the plasma membrane are proposed.

Model 1

Phospholipids are the most abundant type of molecule in the plasma membrane. These molecules consist of hydrophobic (water-repelling) tails composed of hydrocarbons and a hydrophilic (water-attracting)head containing a phosphate group (see Figure 2).

The phospholipids orient themselves to form a bilayer, with the tails on the inside of the bilayer and the heads forming the outer surfaces of the bilayer. A single layer of proteins coats both the inner and outer surfaces of the plasma membrane, rigidly holding the phospholipids in place. Small, protein-coated pores in the phospholipid bilayer allow small water-soluble molecules to pass in and out of the cell (see Figure 3).

Model 2

Model 2 agrees with Model 1 about the abundance and orientation of phospholipids in the bilayer; however, the way proteins are distributed differs. Some proteins are located in the inner half of the plasma membrane, some in the outer half. Transmembrane proteins span from the outside of the cell through the bilayer to the inside of the cell.

The proteins are distributed unevenly, giving the plasma membrane a rough appearance (see Figure 4). Since the phospholipids are not held in place by a layer of proteins, the plasma membrane is highly fluid, allowing phospholipids and proteins to move within the plasma membrane.

Figures 3 and 4 adapted from William T. Keeton and James L. Gould, Biological Science, 4th ed. 1986 by W. W. Norton & Company, Inc.

40. Which of the following observations of plasma membranes would weaken Model 1, but NOT Model 2?

F. Most proteins are located among the phospholipid tails.

G. Two different layers of phospholipid heads exist in the plasma membrane.

H. Proteins are determined to be a major component of the plasma membrane.

J. Plasma membranes are highly porous.

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

According to Model 1, the protein should be wrapped with phospholipids from the inside and outside, and should be outside the heads; but if you think about option F, many proteins are between the tails, then this is the same as that of Model 1 There was a contradiction between the statements. But Model 2 indicates that proteins are embedded in the bilayer structure, so it is normal to find proteins in the middle of tails.