A Level Biology Practice Exam

What does the plasma membrane consist of?

1. A rigid structure that protects the cell

2. A single layer of phospholipids only

3. All membranes of cells, with a fluid-mosaic model structure

4. A complex of proteins and carbohydrates only

The correct answer is: All membranes of cells, with a fluid-mosaic model structure

The plasma membrane is best described as consisting of all cell membranes, characterized by a fluid-mosaic model structure. This model illustrates that the plasma membrane is not a static structure but rather dynamic, with various components such as phospholipids, proteins, and carbohydrates that can move laterally within the layer, resembling a mosaic. In this model, phospholipids form a bilayer, with hydrophilic heads facing outwards towards the water inside and outside the cell, while the hydrophobic tails are oriented inward. This arrangement creates a semi-permeable barrier that regulates the movement of substances in and out of the cell. Embedded within this bilayer are proteins that serve various functions, including transport, signaling, and structural support. Additionally, carbohydrates may be attached to proteins or lipids, playing crucial roles in cell recognition and communication. The fluid-mosaic model aptly describes the complex and versatile nature of the plasma membrane, highlighting its components and the interactions that occur within it, making it integral to the cell's functionality.