Question

State the functions of the organic molecules of cell membranes: cholesterol, proteins, and phospholipids. (p. 50)

Short answer

Answer: Cholesterol regulates the fluidity and flexibility of the cell membrane, preventing fatty acid chains from sticking together and maintaining its fluidity at different temperatures. Proteins have various functions in the membrane, including transport, signal transduction, and cell adhesion, facilitating the movement of substances, signal relay, and communication between cells. Phospholipids, the main components of the cell membrane, provide its basic structure and form a bilayer, creating a selective barrier for substance passage and regulating the cell's internal environment.

Step-by-step solution

Introduction

Cell membranes serve as a barrier that protects the inside of the cell from the external environment and controls the movement of substances into and out of the cell. They are mainly composed of organic molecules, including cholesterol, proteins, and phospholipids. We will discuss each of their functions in the membrane.

Function of Cholesterol

Cholesterol is a steroid molecule that plays a crucial role in the cell membrane. It helps to regulate the fluidity and flexibility of the cell membrane by preventing fatty acid chains in phospholipids from sticking together. Cholesterol acts as a buffer: when the temperature is high, it reduces fluidity by restraining excess movement of the fatty acid chains, and when the temperature is low, it prevents crystallization and maintains the fluidity of the membrane by preventing the phospholipids from getting too close together.

Function of Proteins

Proteins in the cell membrane serve various functions including transport, signal transduction, and cell adhesion. Transport proteins help to move substances (such as ions, nutrients, and wastes) across the membrane through channels, pumps, or carriers, either by passive diffusion or active transport which requires energy. Receptor proteins detect chemical signals from outside the cell and relay these signals inside the cell to initiate a response. Cell adhesion proteins help cells stick together and communicate with each other, or bind cells to the extracellular matrix.

Function of Phospholipids

Phospholipids are the main components of the cell membrane, providing its basic structure. A phospholipid molecule has a hydrophilic head containing a negatively charged phosphate group and two hydrophobic fatty acid tails. Due to the hydrophobic effect and hydrogen bonding, phospholipids spontaneously form a bilayer with the hydrophilic heads facing the aqueous environment and the hydrophobic tails facing inward. This structure creates a selective barrier that permits the passage of some substances while blocking others, ensuring proper regulation of the cell's internal environment.