Molecular Expressions Cell Biology: Plant Cell Structure - Cell Wall
Overview and Historical Discovery of the Plant Cell Wall
The presence of a cell wall is one of the primary features distinguishing plant cells from animal cells. The rigidity of the cell wall renders plants sedentary, whereas the absence of such a structure in animal cells grants them the flexibility necessary for locomotion.
The plant cell wall performs several crucial functions:
Protects intracellular contents.
Imparts structural rigidity to the plant body.
Provides a porous matrix for the distribution and circulation of water, minerals, and other nutrients.
Houses specialized molecules that regulate growth and defend against pathogens.
Cell walls are significantly thicker than plasma membranes. Robert Hooke first identified cell walls in cork tissue and coined the term cells in the 1660s.
Primary Cell Wall Structure and Composition
Many plant cells possess a primary cell wall, which develops first and accommodates cell expansion during growth. The primary wall is relatively thin and pliant compared to the secondary wall. It can remain unchanged or slightly modified after growth ceases, without requiring a secondary wall.
The principal chemical constituents of the primary plant cell wall include:
Cellulose Microfibrils: Macromolecules consisting of several thousand glucose units linked end to end.
Cross-Linking Glycans: Branched polysaccharides organized into a structural matrix with cellulose microfibrils to increase tensile strength.
Pectins: A coextensive network of branched polysaccharides that endows the wall with resistance to compressive forces.
Proteins: Found in small quantities within all primary cell walls. Some proteins provide mechanical reinforcement, while others are enzymes that synthesize, remodel, or break down structural wall networks. These enzymatic modifications are critical for processes such as fruit ripening and leaf abscission in autumn.
Secondary Cell Wall Structure and Modification
The secondary cell wall is deposited inside the primary cell wall as the cell reaches maturity. While its composition can mirror that of the primary wall, it typically incorporates additional specialized compounds:
Lignin: A complex group of hard polymers composed of aromatic alcohols. Lignin imparts substantial structural strength and rigidity. It provides the characteristic physical properties of wood in fiber cells and is abundant in xylem vessels, which supply structural support.
Waxy Substances: Compounds such as cutin, suberin, and other waxy materials combine with lignin to reduce the cell wall's vulnerability to fungal and bacterial infections.
Intercellular Communication and Cell Junctions
Neighboring plant cells are connected and organized via specialized structural regions:
Middle Lamella: A pectin-rich region positioned between adjacent cell walls that cements neighboring cells firmly together.
Plasmodesmata: Specialized microscopic channels that penetrate the middle lamella as well as the primary and secondary cell walls. These conduits establish direct pathways for transport of cytoplasmic molecules and intercellular communication between adjacent plant cells.