What is the cell membrane?

The cell membrane (aka the plasma membrane) separates the cell from the external environment and controls the entry and exit of all substances to and from the cell. The cell membrane separates the cell from its surroundings with a structure, described by the fluid mosaic model, which allows for the movement of materials into and out of the cell by osmosis, simple diffusion, facilitated diffusion, active transport, and vesicular transport (endocytosis/exocytosis).

The Fluid Mosaic Model

The Fluid Mosaic Model is used to describe the structure of the plasma membrane.

The phospholipid molecules that make up the plasma membrane are not bound together by strong physical bonds but are attracted to each other by weaker forces of attraction. Therefore, the proteins, phospholipids themselves and other constituents in the membrane can move throughout the molecules in the membrane. This is why the membrane is described as fluid.

The cell membrane consists of many constituents including phospholipids, different types of proteins (some attached to either side of the surface, some creating channels through the width of the membrane), cholesterol molecules. The variety of components in the membrane is why it is described as a mosaic.

Structural Components of the Cell Membrane

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Phospholipids: Phospholipid molecules form the main structure of the cell membrane. Each molecule consists of lipid tails and a phosphate group ‘head’. The lipid tails are hydrophobic, meaning they repel water, while the phosphate groups are hydrophilic, meaning they attract water. The phospholipids are arranged in two layers, with the heads facing outwards and the tails pointing inwards. This arrangement of two layers is called a bilayer.

Cholesterol: Cholesterol molecules are in between the phospholipid molecules and help to maintain the fluidity of the membrane

Proteins: Different types of proteins exist in the cell membrane, all with different purposes. The four different types of proteins in the membrane are receptor proteins, channel proteins, carrier proteins and cell identity markers.

Some protein molecules stretch the entire width of the cell membrane. These are called integral proteins and can be Channel Proteins or Carrier Proteins. These function to transport molecules (such as ions, water, etc.) across the membrane through a central pore in the membrane.

Other proteins are attached to the membrane surface. Some are attached to carbohydrates, which perform other functions. Cell identity markers are an example of cells attached to the cell membrane's outer side.

Functions of the Cell Membrane

A Physical Barrier

The cell membrane acts as a physical barrier because it separates the cell and its contents from the extracellular fluid. This is important because the cytoplasm of the cell and the extracellular fluid have different compositions.

Regulation of Passage of Materials

The membrane also regulates the passage of materials in and out of the cell due to its semi-permeability. Semi-permeability means it only allows certain molecules to pass easily through the membrane. It allows for the entry of ions and nutrients, removal of wastes and secretions.

Sensitivity

The membrane contains receptor proteins that are sensitive to molecules. This is important because it is the first part of the cell to be affected by a change in extracellular fluid.

Support

The internal layer of the membrane is attached to microfilaments of the cytoskeleton, allowing it to provide structural support to the cell. The cell membrane also has connections to adjacent cells which allows for communication between cells and the formation of entire tissues.