Chapter 8, Problem 13STP

Summary Introduction

To name:

Three components of a cell’s plasma membrane and explain the importance of each component

Introduction:

A cell is a structural and functional unit of an organism. Every cell is surrounded by a protective covering called plasma membrane. It is a thin, flexible boundary between a cell and its outer environment that allows nutrients into the cell and wastes to leave the cell. An important property of plasma membrane is selective permeability, by which a membrane allows some substances to pass through while keeping others out. This property is important to the survival of a cell.

Answer to Problem 13STP

Three components of a plasma membrane are:

1. Phospholipids- Helps plasma membrane to exist in watery environment.
2. Cholesterol- Imparts fluidity to the plasma membrane.
3. Proteins- These contribute to the selective permeability of the plasma membrane.

Explanation of Solution

1. Phospholipids- 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.The two layers of phospholipids are arranged tail- to- tail. Each phospholipid can be 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.
2. Cholesterol- 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.
3. Proteins- An important property of plasma membrane is selective permeability, by which a membrane allows some substances to pass through while keeping others out. This property is important to the survival of a cell. Proteins that are found on the outer surface of plasma membrane called receptors transmit signals to the inside of the cell. Some proteins span the entire membrane and act as tunnels through which certain substances enter and exit the cell. These transport proteins contribute to the selective permeability of the plasma membrane. They transport the needed substances and waste materials through the membrane.