Chapter 9, Problem 47A

Summary Introduction

To describe:

The basic structure of plasma membrane

Introduction: All cells have a special membrane called plasma membrane that acts as a thin flexible boundary between a cell and its watery environment. It controls what enters and leave the cell. The cells of our skin and that of a plant stem, all have a plasma membrane. It also helps in maintaining homeostasis in an organism. Plasma membranes are selectively permeable as they allow only certain substances to pass through while keeping others out.All prokaryotes and eukaryotes have plasma membrane.

Answer to Problem 47A

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.

Explanation of Solution

The following diagram shows the structure of a plasma membrane.

  

The two layers of phospholipids are arranged tail- to- tail. Each phospholipid isrepresented 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.

Along with phospholipids, there are other molecules like cholesterol, proteins and carbohydrates. Cholesterol molecules are nonpolar and are positioned among the phospholipids in such a way that they prevent the fatty acid tails of the phospholipid bilayer from sticking together. This causes fluidity in the plasma membrane.

Transport proteins help in transport of substances and waste materials through the plasma membrane.

The carbohydrates are attached to proteins and they define the cell’s characteristics and help cells identify chemical signals.