The cell membrane, also known as the plasma membrane, is a thin and flexible barrier that surrounds and protects the interior of a cell. It is made up of a lipid bilayer, which consists of two layers of phospholipid molecules, and is found in both plant and animal cells.

The cell membrane plays a critical role in the function of a cell, and its functions include:

Regulating the movement of substances in and out of the cell: The cell membrane acts as a selective barrier, allowing only certain substances, such as oxygen and nutrients, to enter the cell while keeping other substances, such as toxins, out.
Maintaining the shape of the cell: The cell membrane provides structure and support, helping to maintain the shape of the cell and preventing it from collapsing.
Facilitating cell-to-cell communication: The cell membrane contains proteins that are involved in cell-to-cell communication, allowing cells to communicate with each other and coordinate their activities.
Protecting the cell: The cell membrane provides a barrier that protects the interior of the cell from damage and helps to maintain a stable internal environment.

In A-level Biology, students learn about the properties of the cell membrane, including:

Fluidity: The cell membrane is fluid, allowing its components to move and flow, which is necessary for its function.
Selectivity: The cell membrane is selectively permeable, meaning it allows certain substances to pass through while blocking others.
Dynamic: The cell membrane is dynamic, with its components constantly moving and rearranging.
Integral proteins: The cell membrane contains proteins that are integrated into the lipid bilayer and play a role in the regulation of substance movement and cell-to-cell communication.

The cell membrane is a fundamental concept in A-level Biology and is essential for understanding the structure and function of cells. By studying the cell membrane, students gain an understanding of the role of membranes in maintaining the stability and function of cells, and learn how cells communicate and interact with each other. This knowledge is critical for understanding the functioning of organisms, the mechanism of diseases, and the design of treatments.