Fungal Biology Notes

Structure of Fungi

Course Learning Outcomes

• Explain modern taxonomy and phylogeny of fungi.
• Compare and contrast major taxonomic groups based on phenotypic and/or genotypic characteristics.
• Identify and describe morphological features of yeasts and filamentous fungi.
• Recall and describe various metabolic processes, growth, and reproduction patterns in fungi.
• Describe symbiotic relationships between fungi, plants, and humans, with examples.
• Explain disease cycles, epidemiology, and control of fungal pathogens.
• Provide examples of the economic importance of fungi in agriculture, medicine, and biotechnology.

Major Sections

• Structure of a fungal hypha
• Fungal ultrastructure
• Hypha as part of a colony
• Structure of yeasts
• Fungal walls and wall components
• Septa
• Fungal nucleus
• Cytoplasmic organelles
• Fungal cytoskeleton and molecular motors

Morphology and Structure

• Yeasts are unicellular, nucleated, and rounded fungi.
• Molds are multicellular, filamentous fungi.
• The main body of most fungi is made of fine, branching, usually colorless threads called hyphae.
• Hyphae intertwine to form a tangled web called the mycelium.
• A hypha is a tube with a rigid wall containing protoplasm.
• Most fungi have cell walls containing chitin or glucan, or both, unlike plant cell walls, which contain cellulose.

Structure of Fungal Hyphae

• Hyphae grow only at their tips, called the extension zone, which can be up to $30 \mu m$ long in the fastest-growing hyphae, such as Neurospora crassa, which can extend at up to $40 \mu m/min$.
• Older cells are found behind the actively growing tips and are loaded with nutrients.
• Most fungi are multinucleate and multicellular organisms with cross walls called septa or are aseptate (coenocytic).
• Hyphae of most fungi have cross walls (septa) at fairly regular intervals.
• Septa are absent from hyphae of most Oomycota and Zygomycota, except where they occur as complete walls to isolate old or reproductive regions.

Fungal Ultrastructure - Summary

• Fungal hyphae are surrounded by a wall of complex organization, composed of chitin, glucan, or both.
• Underneath the cell wall and firmly attached is the plasma membrane.
• Similar to all eukaryotes, each fungal cell has a large accumulation of membrane-bound vesicles but no other major organelles.
• These vesicles are thought to be derived from Golgi bodies.
• The collection of vesicles at the hyphal tip is termed the apical vesicle cluster (AVC), consisting of a network of actin microfilaments and microtubules.
• The apex of septate fungi, at various phases of growth, has dark particles/spots when viewed by phase-contrast microscopy, termed the Spitzenkörper (“apical body”).

Cell Organelles

• Fungal cells have mitochondria, the powerhouse of the cell.
• Endoplasmic reticulum and ribosomes assist in protein synthesis.
• One or more nuclei; can be multinucleate (coenocytic).
• At times, diplonucleated since male and female nuclei are present and become diploid.
• The presence or absence of septa makes it difficult to define these cells with regards to a fixed number of nuclei.
• Few fungi have very special septa called dolipore septa - thick-walled septa.
• Golgi bodies help in the transport channel.
• Cells also have vesicles and vacuoles.

Dolipore Septa

• The dolipore septum has a narrow central channel (about 100-150 nm diameter) bounded by two flanges of amorphous glucan.
• On either side of this septum are bracket-shaped membraneous structures termed parenthosomes, which have pores to allow cytoplasmic continuity but which prevent the passage of major organelles.
• These septa isolate compartments, or they can allow the free passage of organelles through the septal pores.
• They can be degraded to allow the mass translocation of nutrients and cytoplasmic components to sites of future development.

Hypha as Part of a Colony

• Fungal colonies typically develop from a single germinating spore, which produces a germ tube (a young hypha) that grows and branches behind the tip.
• As the original hypha produces further branches behind them, eventually, the colony develops a characteristic circular outline.
• Several fungi grow as budding, uninucleate yeasts, rather than as hyphae.

Unicellular Yeast & Pseudohyphae

• Illustrates the microscopic morphology of yeasts, including cell wall, cell membrane, endoplasmic reticulum, nucleus, nucleolus, storage vacuole, golgi apparatus, ribosomes and mitochondrion. Shows budding and bud scars.

Fungal Cell Structure

• Vacuole: Cytoplasm less dense in older parts.
• Mitochondrion.
• Rough endoplasmic reticulum.
• Growing tip.
• Nuclei: This hypha is coenocytic (aseptate).
• Cell wall.
• Cell membrane.
• Golgi apparatus.
• Nucleus.

Cell Walls of Fungi

• Rigid and provide structural support and shape.
• Different in chemical composition from prokaryotic cell walls.
• Thick layer of polysaccharide fibers composed of chitin or cellulose.
• Thin outer layer of mixed glycans.
• Most fungi have cell walls that contain chitin, unlike the cell walls of plants, which contain cellulose.

The Plasma Membrane

• Typical bilayer of phospholipids in which protein molecules are embedded.
• Enriched with lipids and proteins.
• Relative rigidity gives stability to the membrane.
• Cytoplasmic membrane serves as a selectively permeable barrier.
• The fluid mosaic model is based on eukaryotic membranes.

Plasma Membrane

• Plasma membrane participates in a variety of cell processes such as cell movement and transduction.
• However, the plasma membrane is unique and typically contains ergosterol as the main membrane sterol, in contrast to animals, which have cholesterol, and plants which have phytosterols such as β-sitosterol.

Endoplasmic Reticulum

• Folded transport network.
• Rough ER (RER): Studded with ribosomes; sites of protein synthesis.
• Smooth ER: No ribosomes; synthesizes cell membranes, fats, and hormones.
• RER allows transport of materials from the nucleus to the cytoplasm and ultimately to the cell's exterior.
• Ribosomes are attached to its membrane surface.

Ribosomes

• Sites of protein synthesis.
• 80S: Consists of the large 60S subunit and the small 40S subunit.
• 70S.
• Membrane-bound: Attached to endoplasmic reticulum.
• Free: In cytoplasm.
• Ribosomes are also found within mitochondria.
• Ribosomes are staging areas for protein synthesis.

Golgi Complex

• Transport organelle; modifies proteins from the ER; transports modified proteins via secretory vesicles to the plasma membrane.

Mitochondria

• Double membrane: Contains inner folds (cristae) and fluid (matrix).
• Involved in cellular respiration (ATP/Energy production).
• Generate energy for the cell - site where ATP is generated by the electron transport chain and is known as the power houses of all eukaryotic cells.
• Appear as round or elongated particles scattered throughout the cytoplasm.
• Inner membrane has tubular inner folds called cristae - hold the enzymes and electron carriers of aerobic (presence of $O_2$) respiration.

The Nucleus

• Fungal nuclei are usually small ($1-2 \mu m$ to $20-25 \mu m$ diameter).
• They are surrounded by a double nuclear membrane with pores, as in all eukaryotes.
• The vast majority of fungi are haploid with chromosome numbers ranging from about 6 - 20.
• Note fungi are the only major group of eukaryotic organisms that are haploid.

The Nucleus

• Most prominent organelle of eukaryotic cells.
• Separated from the cytoplasm by the nuclear envelope.
• Nuclear envelope: A double membrane structure (composed of two membranes) separated by a narrow space, penetrated by nuclear pores.
• Pores allow materials to be transported into or out of the nucleus.
• Macromolecules migrate through the pores to the cytoplasm and vice versa.

The Nucleus (cont'd)

• Nucleolus:
  • Found in the nucleoplasm.
  • Site of RNA synthesis.
  • Collection area for ribosomal subunits.
• Chromatin:
  • Membrane-bound structure that houses genetic material of eukaryotic cell.
  • Made of DNA and proteins: dense material.

Cytoplasm and Cytoskeleton

• Cytoplasm: Substance inside the plasma and outside the nucleus.
• Cytosol: Fluid portion of cytoplasm.
• Cytoplasmic streaming: Movement of the cytoplasm throughout a cell.
• Many organelles of eukaryotic cells/fungi lie in the cytoplasmic matrix.
• Filaments that form the cytoskeleton: microfilaments, microtubules, and intermediate filaments.
• Plays a role in both cell shape and cell movement.

Cytoskeleton

• The cytoskeleton plays a major role in the internal organization of eukaryotic cells, providing a dynamic structural framework for transporting organelles, for cytoplasmic streaming, and for chromosome separation during cell division.
• The three main elements of the cytoskeleton are:
  • Microtubules: Consisting of polymers of tubulin proteins
  • Microfilaments: Consisting of the contractile protein actin
  • Intermediate filaments: Provide tensile strength

Note for clarity:

• The fungal cell wall is a complex and flexible structure composed basically of chitin, α- and β- linked glucans, glycoproteins, and pigments.
• As nouns, the difference between glycan and glucan is that glycan is (chemistry) any polysaccharide or oligosaccharide, especially one that is part of a glycoprotein or glycolipid while glucan is (carbohydrate) any polysaccharide that is a polymer of glucose.

References

• Fungal Biology-JWDeacon 4th ed.
• Microbiology Fundamentals- Cowan, Mc Graw Hill
• Microbiology, A clinical Approach -Danielle Moszyk-Strelkauskas-Garland Science 2010
• http://en.wikipedia.org/wiki/Scientific_method
• https://files.kennesaw.edu/faculty/jhendrix/bio 2261/home.html
• http://www.cdc.gov/cmv/
• https://www.researchgate.net/figure/The-variation-of-colours-and-morphology-of-the-fungal-colonies-of-Trichoderma-spp-arefig1273338662
• http://en.wikipedia.org/wiki/Fungus#Growthandphysiology
• http://bioweb.uwlax.edu/bio203/f2012/deflorin_patr/classification.htm
• https://qph.fs.quoracdn.net/main-qimg-017087ccca925a147d1848c252b726e9
• http://image.slidesharecdn.com/fungusi-120920111141-phpapp01/95/fungus-part-i-4-728.jpg?cb\u003d1348139624