We know that cells are basic units of life for every known living organism on our planet called Earth. We have also learned that each and every cell in the body of a living organism has several small structures responsible for carrying out various functions that are vital to life. These structures are known as cell organelles. Cell wall is another such organelle that has a very specific set of functions to perform. In this article on Cell Wall facts, we will learn about its composition, its structure and its functions. We will also learn the types of cells that have Cell Wall. So, without further ado, let’s take a quick look at the Cell Wall facts that we want to share with you.
Cell Wall Facts: 1-5 | What is Cell Wall?
1. Cell Wall is a cell organelle. It is ABSENT is animal cells but is distinctly PRESENT in plant cells.
2. Apart from plant cells, Cell Wall can also be found in the following:
- Some archaea
3. In case you what to know what makes up a Cell Wall, it is not so simple. Cell Wall varies in composition depending upon which type of cell we are look at. The table below gives the composition of Cell Wall in different types of cells:
|Type of Cell||Composition of Cell Wall|
|Plant Cell (Eukaryotic)||A complex combination of cellulose1, pectic polysaccharides2, hemicellulose3, some lipids4, structural protein, lignin5 and water. Also, one can find silica crystals, transglycosylases6, peroxidase7, esterases8 and hydrolases9.|
|Fungal Cell (Eukaryotic)||Cell Wall is mostly composed of chitin10 and several other polysaccharides.|
|Algal Cell (Eukaryotic)||Either made of polysaccharides or an assorted range of glycoproteins11 or both!|
|Bacterial Cell (Prokaryotic)||Bacterial cells are of two different types – “gram positive” and “gram negative”. These types are derived from the reactions that cells show to a testing process known as ‘Gram Staining’. Gram positive bacterial cells are known for having a thick Cell Wall made of layers of peptidoglycan12 and teichoic acid. Gram negative cells are known for a thin Cell Wall composed only of peptidoglycans. Gram negative cells have a thin lipid membrane layer encircling the Cell Wall. This lipid membrane layer is made of lipoproteins13 and lipopolysaccharides14.|
|Archaeal Cell (Prokaryotic)||Unlike bacterial cells, archaeal cells do not have peptidoglycan in their Cell Wall except for one group of archaea known as Methanogens. Methanogens have peptidoglycan but in a very different and modified form compared to that of bacterial cells. Archaeal cells also contain surface-layer proteins, glycoproteins and polysaccharides.|
While discussion on different types of Cell Walls found in various types of living organisms sounds really fascinating, we will deliberately leave that out for now and focus mostly on Cell Wall of plant cells. Discussion on other types will either continue at the end of this article or in a separate article.
4. Except for mycoplasma bacteria, Cell Wall is present in most prokaryotes. It is present only in few Eukaryotes.
5. Cell Wall in plants is designed to completely coat the plasma membrane or the cell membrane. Yes, plasma membrane is present in both animal and plant cells but Cell Wall is absent in animal cells.
Cell Wall Facts: 6-10 | History of Cell Wall
6. Robert Hooke was the first person to observe Cell Wall. He observed it back in 1665 and referred to it as ‘Wall’.
7. Despite such early discovery, it was forgotten for then nearly next 3 centuries but the discovery remained alive only in relation to human health or animal health or in industrial processing.
8. Previously it was thought that all cells had shared walls but the notion was completely changed when in 1804, J. H. F. Link and Karl Rudolphi went on to prove that every cell has its own individual Cell Wall.
9. Then came the question of formation of Cell Wall somewhere in 19th century. Two independent ideas came up in this matter:
- Apposition15 advocated by Hugo von Mohl in years 1853 and 1858.
- Intussusception16 advocated by Carl Nägeli in years 1858, 1862 and 1863.
10. Both theories were eventually improved. The Apposition theory was improved by Eduard Strasburger while Julius Wiesner was the one who improved the theory of Intussusception.
Cell Wall Facts: 11-15 | Structure of Cell Wall
11. In plant cells, the Cell Wall has a multi-layered structure. There are at least two layers and up to three layers in Cell Wall.
12. The outermost layer of Cell Wall is known as “Middle Lamella”. Between the Middle Lamella layer and the Plasma Membrane or Cell Membrane is the layer called “Primary Cell Wall”.
13. The third layer (if it is present) is known as the “Secondary Cell Wall”. It lies between the Primary Cell Wall and the Plasma Membrane or Cell Membrane.
14. The Middle Lamella: This outermost layer of Cell Wall works as an adhesive. It helps in cell adhesion by allowing the Cell Walls of adjacent cells to stick together or bind with each other. The Middle Lamella is made of a type of polysaccharide known as the Pectin. It is the Pectin that helps in cell adhesion.
15. The Primary Cell Wall: This is the second layer of the Cell Wall. Lying between the Plasma Membrane and the Middle Lamella, this layer is generally thin, extensible and flexible. This layer is formed when the cell is growing.
Cell Wall Facts: 16-20 | Structure of Cell Wall
16. As far as the composition of the Primary Cell Wall is concerned, it is made of cellulose microfibrils17. These cellulose microfibrils in turn stay encased in gel-like matrix.
17. This gel-like matrix in turn is composed of pectin (a type of polysaccharide) and hemicellulose fibers. It is this unique composition of the Primary Cell Wall that allows the necessary flexibility which is required for cell growth.
18. The Secondary Cell Wall: This layer may not exist in some cells but can be found in some cells. This layer known as the Secondary Cell Wall is found between the Primary Cell Wall and the Plasma Membrane. When the Primary Cell Wall stops dividing and growing, it may actually thicken. Once the Primary Cell Wall Thickens, it forms the Secondary Cell Wall.
19. The Secondary Cell Wall is rigid for a very good reason. It is this rigidity that provides support to the cell and strengthens it.
20. As far as the composition of the Secondary Cell Wall is considered, it is usually made of hemicellulose and cellulose. However, there may be some Secondary Cell Walls that may actually contain lignin, which is responsible for strengthening the cell wall and also for conductivity of water in what is known as plant vascular tissue18.
Cell Wall Facts: 21-25 | Functions of Cell Wall
21. The Cell Wall have many functions. One of the most important functions is to work as a skeleton and provide a framework which can prevent the plant cells from over expansion.
22. This framework and hence, over-expansion prevention is achieved with the help of polysaccharides, structural proteins and cellulose fibers.
23. The Cell Wall is also known for controlling cell growth direction. It also provides support and mechanical strength.
24. One of THE MOST IMPORTANT functions that Cell Wall plays is that of countering and withstanding the turgor pressure. This is a pressure that is generated from inside the cell. The components of the cell exert an outward pressure on the plasma membrane of the cell. The plasma membrane in turn pushes against the Cell Wall.
25. It is this turgor pressure that actually keeps the plants erect. However, turgor pressure, in absence of Cell Wall could lead to bursting of the cells because the plasma membrane is not designed to withstand such pressure. It is the Cell Wall that withstands the pressure and prevents the cells from bursting.
Cell Wall Facts: 26-30 | Functions of Cell Wall
26. Regulation of cell growth is yet another important function of Cell Wall. It is responsible for sending cell division and cell growth signals to cells so that they can enter into the cell cycle.
27. Yet another function of the Cell Wall is to regulate the entry of various material into the cell. This happens because of porous nature of the Cell Wall. Only some selected substances are allowed to pass through including a few proteins. Rest of the substances are kept out of the cell.
28. Carbohydrate storage is yet another important function of the Cell Wall. This carbohydrate stored especially in seeds is important for cell growth.
29. It is very important that cells communicate with each other. Cell Wall plays and important function in this cellular communication. Cell Walls have something called Plasmodesmata. These are small channels or pores between the Cell Walls. These pores allow communication signals and molecules to pass between cells.
30. Yet another important function of the Cell Wall is to protect the cell from pathogens and plant viruses. The Cell Wall prevents these pathogens and viruses from entering the cells. In addition to that, the Cell Wall also prevents water loss.
Cell Wall Facts: 31-35 | Random Cell Wall Facts
31. In plants, the Cell Wall is 0.2 µm thick.
32. We know that the Cell Wall is rigid and prevents cell expansion. So, the question is, “how a cell grows?”. This happens because the Cell Wall matrix has several proteins that actually loosen up the Cell Wall structure when the cell growth takes place.
33. We know that the Cell Wall matrix is porous and allows diffusion of certain substances across Cell Wall. What we didn’t say earlier was that the matrix allows diffusion of only soluble factors or material.
34. Water and ions can diffuse easily across Cell Wall but when it comes to diffusion of particles, Cell Wall reduces the diffusion of particles that have a diameter exceeding ≈ 4nm including those proteins that have molecular weight below 20,000.
35. Aforementioned selective permeability of the Cell Wall is one of the primary reasons why hormones found in plants are not only small molecules but are also water-soluble in nature.
Cell Wall Facts: 36-40 | Random Cell Wall Facts
36. Cell Wall undergoes changes. However, it must be noted that those changes are maximum at meristem a shoot tip or a root. The reason is simple. The aforementioned sites are the places where cell division and cell expansion take place.
37. In the aforementioned sites, that is, meristem of a root or a shoot tip, the thin primary Cell Walls connect the young cells. The primary Cell Walls are thin in those areas and are capable of loosening and stretching so that subsequent cell elongation can happen.
38. Once cell elongation stops, the Cell Walls will thicken. This thickening can take place in two ways:
- Additional macromolecules will be secreted into the primary Cell Wall.
- Or, a secondary Cell Wall will be formed. This secondary Cell Wall will have several layers.
39. Cellulose fibrils are the most abundant molecules found in Cell Walls. They play the most central role in the structure of a Cell Wall.
40. 20 to 30 percent of a Cell Wall’s dry weight actually comes from cellulose fibrils.
This completes our discussion on the Cell Wall facts. In case you think we have missed out something very important or something that you need in this article, feel free to contact us either through our contact forms or through the comments section. We will try to rectify the errors or add missing information.
Glossary of terms:
Cellulose1: Cellulose is a molecule that is found in almost every plants’ cellular structure. This molecule is made Oxygen, Carbon and Hydrogen. Thus, it is actually an organic compound and is thought to be the most abundant organic compound in whole world.
Polysaccharides2: A chain of many but defined number of sugar molecules which are bonded together are called as Polysaccharides. The most common types of polysaccharides are glycogen, cellulose and starch.
Hemicellulose3: Hemicellulose is also a polysaccharide which have simpler structure than cellulose and are formed in a different way. They are also present in the cell walls of the plants. They are often referred as polyose.
Lipids4: Lipids are a type of organic compounds made up of hydrocarbon chains which are insoluble in water but soluble in alcohol, chloroform etc. They, along with carbohydrates and proteins, make up the major constituents of the cells. Fats are a type of lipids.
Lignin5: Lignin is a complex organic polymer. Like lipids, it is insoluble in water and soluble in alcohol, benzene etc. They help in forming tissues in vascular plants (plants which have tissues which conduct water, minerals and food from one part of the plant to another). Lignin takes the second place in being the most abundant organic polymer present on Earth.
Transglycosylases6: These are a class of GH Enzymes. GH Enzymes or Glycoside Hydrolase Enzymes play a very important role as a catalyst in hydrolysis (chemical breakdown of a compound through reaction with water) of glycosidic bonds (these are basically covalent bonds responsible for joining a carbohydrate molecule to some other group, which may or may not be a carbohydrate) found in complex sugars. The function of Transglycosylases is that of a catalyst in the transformation of a glycoside into another glycoside. Now, glycoside is a compound in which a sugar molecule binds with another compound and this binding take place with glycosidic bonds.
Peroxidase7: Peroxidases are enzymes which accelerates the reaction which usually involves hydrogen peroxides. They help in increasing the defenses of the plant against the pathogens.
Esterases8: Esterase is an enzyme which splits the esters (type of organic compound) into alcohol and acid when it chemically reacts with water (such chemical process is known as hydrolysis).
Hydrolases9: These are basically enzymes that work as catalysts in chemical reactions where chemical bonds are broken using water.
Chitin10: This is a fibrous substance that contains polysaccharides. They are major constituents in fungi Cell Wall and in arthropod exoskeleton (it is the external skeleton that protects and supports and organism’s body). In more technical terms, Chitin is a long-chain polymer (a polymer is a large molecule, often referred to as macromolecule and are made of many subunits that are repeated and these subunits are known as monomers). This long-chain polymer is made of a glucose derivate known as N-acetylglucosamine.
Glycoproteins11: These are those proteins that have carbohydrate groups attached to them. The carbohydrates are attached to the polypeptide chains. A polypeptide chain is single linear chain consisting of several amino acids that are held together using amide bonds (also known as peptide bond, it is a covalent chemical bond responsible for linking two consecutive amino acid monomers). A polypeptide chain can have 50+ amino acids and there many be multiple polypeptide chains in a single protein.
Peptidoglycan12: Also known as murein, peptidoglycan is a polymer. It is one of the primary constituents that make up the Cell Wall in most of the known bacteria. This polymer is actually made of amino acids and sugars. When many molecules of this polymer join together they form a crystal lattice in an orderly fashion. It is the structure of the peptidoglycan Cell Wall that determines whether a bacterium is to be classified as gram positive or gram negative.
Lipoproteins13: Lipoproteins are biochemical particles made of several thousand molecules. They are present for transporting fat molecules. Fat molecules, also known as hydrophobic lipid, are insoluble in water and hence, they cannot be transported in water-based fluids like extracellular fluid and blood. Lipoproteins are so designed that one end of them is hydrophilic (loves water) and the other end is lipophilic (loves lipids). The hydrophilic end is projected outwards while the lipophilic end is oriented inwards. The fat molecules are encased inside the lipophilic side of the lipoproteins. This allows easy transport of fat molecules through water-based fluids.
Lipopolysaccharides14: Lipopolysaccharides are large molecules that are formed by the binding of lipids and polysaccharides. Lipopolysaccharides, also known as LPS have three components – the core, lipid A and O-polysaccharide. O-polysaccharide is also known as O-antigen. Lipopolysaccharides are present in bacteria, specifically on the outer membrane and work as endotoxins. They are known for eliciting immune response in animals.
Apposition15: In Biology, apposition is a process of thickening of Cell Wall through deposition of successive material layers.
Intussusception16: It means, growth in a cell’s surface area through deposition of new material between Cell Wall’s existing components.
Cellulose microfibrils17: A microfibril is a very fine fiber-like strand or fibril. Cellulose microfibril means that the microfibrils are made of cellulose.
Plant vascular tissue18: Vascular tissue is a type of conducting tissue found in plants, specifically in vascular plants. The vascular tissue is made of more than one cell types. The primary components of vascular tissue are the phloem and the xylem. The plant vascular tissue is responsible for transporting nutrients and fluids internally.