Question 8 of 9: Fungi and Comparative Virus Biology

Question 8: Fungi and Comparative Virus Biology (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Characteristics distinguishing fungi. Fungi are eukaryotic (true nucleus, membrane-bound organelles), typically filamentous (a network of hyphae forming a mycelium) or unicellular (yeasts), and are chemoheterotrophic by absorptive nutrition — they secrete extracellular hydrolytic enzymes to break down complex organic polymers (cellulose, lignin, starch, proteins) outside the cell and absorb the resulting small molecules, rather than ingesting food. Their cell walls are built from chitin (not peptidoglycan like bacteria, nor cellulose like plants), and most reproduce by spores (asexual conidia/sporangiospores or sexual ascospores/basidiospores), which are highly resistant to desiccation and enable wide dispersal. Unlike bacteria, most fungi tolerate lower water activity and lower pH, and many are strict or facultative aerobes.

Beneficial roles in food. Yeasts (Saccharomyces) drive bread leavening and alcoholic fermentation (beer, wine); molds ripen and flavor cheeses (Penicillium roqueforti/camemberti) and ferment soy products (Aspergillus oryzae in soy sauce/miso, Rhizopus in tempeh); fungi are also industrial sources of enzymes (amylases, pectinases) and organic acids (citric acid from Aspergillus niger) used in food processing, and of single-cell protein.

Harmful roles in food. Molds cause visible spoilage (discoloration, off-odors, textural breakdown) of stored grains, fruits, and bread; more seriously, many molds produce mycotoxins (e.g. aflatoxins from Aspergillus flavus, ochratoxins, patulin) that are carcinogenic or otherwise toxic even after the visible mold is removed, since the toxin diffuses beyond the visible colony and survives normal cooking temperatures; fungi can also degrade packaging and cause economic loss even without producing a toxin of concern.

(b) Animal, plant, and bacterial viruses — similarities and differences. All three share the basic virion architecture (nucleic acid genome in a protein capsid, obligate intracellular parasitism, reliance entirely on host machinery for replication, and strict host specificity via receptor recognition). They differ chiefly in how they enter the host cell and in typical structure: bacteriophages (bacterial viruses) commonly have a complex head-and-tail structure and inject only their nucleic acid through the rigid bacterial cell wall, leaving the capsid outside; animal viruses enter whole (by receptor-mediated endocytosis or, for enveloped viruses, membrane fusion) since animal cells lack a rigid wall, and many carry a lipid envelope acquired by budding through the host membrane; plant viruses generally cannot penetrate the rigid, thick plant cell wall directly and instead require a wound or a vector (commonly a sap-sucking insect, e.g. aphids, or fungal/nematode vectors) to be introduced into the cell, and typically spread cell-to-cell through plasmodesmata rather than through the extracellular space.