Chapter 7, Problem 14A

Summary Introduction

Introduction : Most of the molecules in the plasma membrane are made of lipids. A phospholipid is a molecule that has a glycerol backbone, two fatty acid chains and a phosphate group. The basic structural molecule of a plasma membrane is a phospholipid bilayer.

Answer to Problem 14A

Correct answer :

The correct answer is option A.

Explanation of Solution

Explanation/justification for the correct answer:

Option A. The two layers of phospholipids are arranged tail- to- tail. Each phospholipid I is diagrammed as a head with two tails. The phosphate group makes the head polar. The polar head is attracted to water because water is also polar. The two fatty acids are nonpolar and are repelled by water. The phospholipids are arranged in such a way, that the polar heads are closest to water molecules. So the fatty acid tails form the interior of the plasma membrane and the heads face the watery environment found inside and outside the cell. This allows the plasma membrane to exist in a watery environment. Hence, this option represents the correct orientation of phospholipids.

Explanation for incorrect answer :

Option B. This is not correct orientation as the polar heads are seen in the inside and the fatty acid tails are present on the outside. The fatty acid tails should form the interior of the plasma membrane and the heads should face the watery environment found inside and outside the cell.

Option C. This is not correct orientation as the polar heads and the fatty acid tails are present in both directions. The fatty acid tails should form the interior of the plasma membrane and the heads should face the watery environment found inside and outside the cell.

Option D. This is not correct orientation as the polar heads are not together and the tail ends are alternating with polar heads. The fatty acid tails should form the interior of the plasma membrane and the heads should face the watery environment found inside and outside the cell.