The Structure and Function of a Cell Wall

Cell Wall

A section of the cell wall in a plant cell
By LadyofHats (Own work) [Public domain], via Wikimedia Commons

The cell wall is the rigid, semi-permeable protective layer in some cell types. This outer covering is positioned next to the cell membrane (plasma membrane) in most plant cells, fungi, bacteria, algae, and some archaea. Animal cells however, do not have a cell wall. The cell wall conducts many important functions in a cell including protection, structure, and support. Cell wall composition varies depending on the organism. In plants, the cell wall is composed mainly of strong fibers of the carbohydrate polymer cellulose. Cellulose is the major component of cotton fiber and wood and is used in paper production.

Plant Cell Wall Structure

The plant cell wall is multi-layered and consists of up to three sections. From the outermost layer of the cell wall, these layers are identified as the middle lamella, primary cell wall, and secondary cell wall. While all plant cells have a middle lamella and primary cell wall, not all have a secondary cell wall.

  • Middle lamella - outer cell wall layer that contains polysaccharides called pectins. Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another.​
  • Primary cell wall - layer formed between the middle lamella and plasma membrane in growing plant cells. It is primarily composed of cellulose microfibrils contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. The primary cell wall provides the strength and flexibility needed to allow for cell growth.​
  • Secondary cell wall - layer formed between the primary cell wall and plasma membrane in some plant cells. Once the primary cell wall has stopped dividing and growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and supports the cell. In addition to cellulose and hemicellulose, some secondary cell walls contain lignin. Lignin strengthens the cell wall and aids in water conductivity in plant vascular tissue cells.

Plant Cell Wall Function

A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include:

  • Support - the cell wall provides mechanical strength and support. It also controls the direction of cell growth.​
  • Withstand turgor pressure - turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture.​
  • Regulate growth - sends signals for the cell to enter the cell cycle in order to divide and grow.
  • Regulate diffusion - the cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out.​
  • Communication - cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells).​
  • Protection - provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss.​
  • Storage - stores carbohydrates for use in plant growth, especially in seeds.

Plant Cell: Structures and Organelles

To learn more about organelles that can be found in typical plant cells, see:

  • Cell (Plasma) Membrane - surrounds the cytoplasm of a cell, enclosing its contents.​
  • Cell Wall - outer covering of the cell that protects the plant cell and gives it shape.
  • Centrioles - organize the assembly of microtubules during cell division.​
  • Chloroplasts - the sites of photosynthesis in a plant cell.​
  • Cytoplasm - gel-like substance within the cell membrane composed.​
  • Cytoskeleton - a network of fibers throughout the cytoplasm.​
  • Endoplasmic Reticulum - extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER).​
  • Golgi Complex - responsible for manufacturing, storing and shipping certain cellular products.​
  • Lysosomes - sacs of enzymes that digest cellular macromolecules.​
  • Microtubules - hollow rods that function primarily to help support and shape the cell.​
  • Mitochondria - generate energy for the cell through respiration.​
  • Nucleus - membrane bound structure that contains the cell's hereditary information.​
  • Nucleolus - structure within the nucleus that helps in the synthesis of ribosomes.​
  • Nucleopore - tiny hole within the nuclear membrane that allows nucleic acids and proteins to move into and out of the nucleus.​
  • Peroxisomes - tiny structures bound by a single membrane that contain enzymes that produce hydrogen peroxide as a by-product.​
  • Plasmodesmata - pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells.​
  • Ribosomes - consisting of RNA and proteins, ribosomes are responsible for protein assembly.​
  • Vacuole - typically large structure in a plant cell that provides support and participates in a variety of cellular functions including storage, detoxification, protection, and growth.

Sources

  • Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 22.5, The Dynamic Plant Cell Wall. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21709/

The Cell Wall of Bacteria

Bacterial Cell
This is a diagram of a typical prokaryotic bacterial cell. By Ali Zifan (Own work)/ Wikimedia Commons/CC BY-SA 4.0

Unlike in plant cells, the cell wall in prokaryotic bacteria is composed of peptidoglycan. This molecule is unique to bacterial cell wall composition. Peptidoglycan is a polymer composed of double-sugars and amino acids (protein subunits). This molecule gives the cell wall rigidity and helps to give bacteria shape. Peptidoglycan molecules form sheets which enclose and protect the bacterial plasma membrane.

The cell wall in gram-positive bacteria contains several layers of peptidoglycan. These stacked layers increase the thickness of the cell wall. In gram-negative bacteria, the cell wall is not as thick because it contains a much lower percentage of peptidoglycan. The gram-negative bacterial cell wall also contains an outer layer of lipopolysaccharides (LPS). The LPS layer surrounds the peptidoglycan layer and acts as an endotoxin (poison) in pathogenic bacteria (disease causing bacteria). The LPS layer also protects gram-negative bacteria against certain antibiotics, such as penicillins.