The cell membrane, also known as the plasma membrane, is a complex structure that serves as a selective barrier between the interior of the cell and its external environment. The fluid mosaic model is the widely accepted framework for understanding the cell membrane’s structure and function.
Fluid-Mosaic Model
The term “fluid” refers to the ability of the lipid and protein components of the membrane to move laterally within the layer. The phospholipid bilayer is not a rigid structure; instead, it behaves like a fluid, allowing the lipids and proteins to diffuse sideways. This fluidity is important for self healing, allowing the membrane to quickly repair itself if damaged, as the fluid nature allows lipids and proteins to flow back into place. It also enables the membrane to expand and contract as the cell grows or changes shape.
The “mosaic” aspect of the model refers to the diverse array of proteins embedded within the phospholipid bilayer. These proteins vary in shape, size, and function and are distributed unevenly, creating a mosaic-like pattern on the membrane surface.
Cell Membrane Structure
Phospholipid bilayer: This forms the fundamental structural component of the cell membrane. Each phospholipid molecule consists of a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails. In the bilayer, the hydrophobic tails face inward, shielded from water, while the hydrophilic heads face outward towards the aqueous environments inside and outside the cell. This arrangement creates a semi-permeable barrier that regulates the passage of substances into and out of the cell.
Protein channels and carriers: Embedded within the phospholipid bilayer are various proteins, which contribute to the “mosaic” aspect of the model. These proteins can be integral/intrinsic (spanning the membrane) or peripheral/extrinsic (attached to the surface). Integral proteins often function as channels or carriers, facilitating the transport of molecules that cannot pass directly through the phospholipid bilayer, such as ions, glucose and other polar substances. These protein channels are highly selective, allowing only specific molecules to cross the membrane, thus playing a crucial role in maintaining the cell’s internal environment.
Cholesterol: Cholesterol molecules are interspersed among the phospholipids in the bilayer, adding to the “fluid” nature of the membrane. Cholesterol serves two main functions: it provides stability to the membrane by preventing it from becoming too fluid at high temperatures and too rigid at low temperatures, and it helps maintain membrane integrity and flexibility. By modulating the fluidity of the membrane, cholesterol ensures that the membrane remains functional under various conditions.
Glycoproteins: Glycoproteins are proteins that have carbohydrate chains attached to them. These molecules extend from the outer surface of the cell membrane and play key roles in cell recognition and communication. Glycoproteins are involved in various cellular processes, such as the immune response, where they help the immune system distinguish between self and non-self cells. They also participate in cell signalling, aiding in the reception of chemical signals from other cells, which can trigger specific cellular responses.
