Cell Wall and Extracellular Matrix: Structure and Function

The Cell Wall

The cell wall is an outer covering found in bacteria, plants, algae, and fungi.

Chemical Composition

In fungi, the wall is composed of chitin (a polymer). In most algae and higher plants, it consists of cellulose. Cellulose is a linear polymer made of thousands of glucose monomers that form long, strong, associated microfibrils of great length. These microfibrils are embedded in two other polysaccharides: hemicellulose and pectin.

Structure

The cell wall consists of two layers: the middle lamella and the primary wall. When the cell matures, it produces a third layer called the secondary wall, located between the plasma membrane and the primary wall.

• Middle Lamella: Located between the primary walls of neighboring cells, except in places where plasmodesmata (intercellular bridges) exist. It is composed mainly of pectin.
• Primary Wall: Characteristic of growing cells. It is thin and mainly composed of cellulose, hemicellulose, and pectin.
• Secondary Wall: Formed when growth ceases and the cell differentiates. It is a third layer composed of cellulose. Many also contain lignin, which is responsible for the hardness of wood.

Functions

The cell wall acts as an exoskeleton that protects the cell, maintains its shape, and confers resistance without impeding growth. It is responsible for keeping the plant upright, preventing breakage, and actively maintaining intracellular osmotic pressure.

Extracellular Matrix

Fibrous Proteins

• Collagen: Proteins highly resistant to mechanical stress that provide great consistency, constituting the main component.
• Elastin: Usually accompanies collagen, providing elastic capacity to tissues such as blood vessels or lungs.

Proteoglycans

These are complex molecules composed of a central polypeptide chain bound to glycosaminoglycans, such as hyaluronic acid.

Structural Glycoproteins

• Fibronectin: Promotes cell adhesion.
• Laminin: A protein that promotes cell adhesion during embryonic development.

Functions of the Extracellular Matrix

The extracellular matrix keeps cells bound together to form tissues, providing elasticity and resistance to mechanical stress. It intervenes in the three-dimensional organization of tissues and organs during embryonic development and serves as a communication channel, allowing the diffusion of various substances.