Introduction
Before understanding the cell wall of fungi, you must know that the cytoplasm of growing yeasts and mycelia has more salts and sugars than the fluid around it. This difference in osmotic pressure causes a net flow of water through the plasma membrane, which causes the cell to grow. Fungi, unlike many protists, don’t stop growing by sending water away through contractile vacuoles. Instead, they stop growing by building a cell wall on top of the plasma membrane. When water gets into a cell, it pushes the plasma membrane against the inside of the cell wall. This is called hydrostatic pressure or turgor. The increase in internal pressure helps the cell get closer to a state of homeostasis in which the amount of water coming into the cell matches the amount of growth in cell volume. The wall is a very flexible structure that resists growth on most of its surface but grows in places like hyphal tips and yeast buds.
The cell wall of fungi
The cell wall of a fungus is made up of porous macromolecules that are put together on the surface of the plasma membrane. It has chitin microfibrils that can handle stress, glucose polymers called glucans, and a number of cell wall proteins (CWP).
Chitin
Chitin is made up of N-acetyl-d-glucosamine monomers that are linked together by -14 bonds. Chitin chains next to each other form hydrogen-bonded antiparallel arrays. This makes microfibrils that can be longer than 1 m. When chitin is broken, the cell loses its osmotic stability and may break.
Chitosan also called -1,4-glucosamine, is a sugar polymer made from deacetylated sugar. Like chitin, it is made by many fungi. Most of the time, -1,3-glucan is the most common polymer in cell walls. The -13-glycosidic linkage in glucans makes the polymer twist, and three glucan chains form a triple helix that is held together by hydrogen bonds. In the mature wall structure, -13-glucans are linked to -16-glucans to make a very flexible network of polymers with many branches.
Mannoprotein in fungal cell wall
The structural proteins in a cell wall are glycoproteins that have carbohydrates linked to their N- and O-termini. There are also glycoproteins that have both mannose and galactose residues, such as mannoproteins and other glycoproteins. Cell wall glycoproteins are linked to the plasma membrane by a glycophosphatidylinositol (GPI) anchor and are cross-linked to chitin microfibrils and glucans.
Even though chitin is a stress-bearing part of most fungi’s extracellular matrix, the saying “fungi have chitinous walls” can be wrong. Most of the time, -1,3-glucan, is the most important wall polymer, and up to half of the wall of some fungi is made of protein. Different types of fungi have different amounts of chitin, glucans, and glycoproteins.
Chitin in yeast
Chitin is a small part of the yeast Saccharomyces cerevisiae’s cell wall. It is made when the mother cell sends out a bud. Most of the yeast cell wall is made up of -1,3-glucans, which are like a framework, and mannoproteins, which make up the top layer. Cross-links connect the -13-glucans to the -16-glucans and to a number of CWPs.
Shizosaccharomyces pombe, which is fission yeast, has a very different cell wall. It has glucans with different glycosidic linkages (-13-, -13-, and -13-glucan), but no chitin at all. Mycelial fungi have the same polymers, but chitin is more important in these species. It makes up 10% or more of the wall’s dry weight. In Neurospora crassa, for example, -1,3-glucans and chitin make up the inner layer. A protein–polysaccharide complex covers the inner layer. This ascomycete’s cell wall does not have -1,6-glucan.
How chitin synthesis in fungal cell wall
Chitin is made by a protein called chitin synthase, which is part of the cell membrane.
Chitin synthase pushes chitin chains through the plasma membrane, and the new polymers form microfibrils by hydrogen bonding with each other. Saccharomyces cerevisiae has three chitin synthases (Chs1p, Chs2p, and Chs3p); the filamentous ascomycete Aspergillus fumigatus has seven chitin synthases, and four have been identified in Neurospora crassa. Chitin synthases and glucan synthases both do the same thing. They are part of the membrane and make long chains of glucans by adding glucose residues one after the other. Crosslinks between branches in glucan molecules and links between glucans, chitin, and glycoproteins are very important for keeping the strength of the cell wall. Antifungal agents might try to stop glucan synthesis. A family of drugs called echinocandins that bind to β-1→3-glucan synthase show promise in the treatment of aspergillosis and candidiasis.
On their way through the secretory pathway, most of the wall proteins get sugar chains attached to them. Exocytosis brings them to the plasma membrane, where GPI anchors keep them in place. Their sugar residues form covalent bonds with other wall polymers, which helps the cell keep its shape. Glycoproteins also have roles in signaling and transport, in fusing with other cells (for example, agglutinins help cells recognize each other during mating), and in sticking to surfaces, making biofilms, and causing diseases. They also help the cell take in compounds from its environment and keep harmful substances out of the cell. There are also a lot of enzymes in the wall. Some of these enzymes help make other parts of the cell wall and are essential for the cell wall to keep changing as the cell grows.
Function
a. Support and protection: The fungal cell wall helps hold the cell together and keeps it safe. It helps the cell keep its shape and integrity and keeps it safe from things like mechanical pressure and harmful chemicals in the environment. The cell wall acts as a semi-permeable barrier that controls the flow of ions, nutrients, and waste products into and out of the cell and keeps the environment inside the cell stable.
b. Controlling the amount of water in the cell: The fungal cell wall is also important for controlling the amount of water in the cell. Polysaccharides, like chitin, which are naturally water-loving, make up the wall. This helps control how much water the cell takes in and keeps, so it doesn’t become dehydrated or swell up so much that it might burst. Fungi need to control the water balance in their environment in order to stay alive and grow.
c. Interaction with the environment: The cell wall of the fungus is also involved in how it interacts with its surroundings. Different surface parts, like glycoproteins and lipids, cover the wall. These surface parts help fungal cells recognize and stick to each other and to other surfaces. This makes it possible for fungi to live in complex communities and work together with other organisms. The cell wall is also where hormones and growth factors that come from the environment are attached. This lets the fungus respond to changes in its environment.
References
Watkinson, S. C., Boddy, L., Money, N. P., & Carlile, M. J. (2016). The Fungi. Elsevier, Academic Press.
