04-BS-13 · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2019 — 04-BS-13, Biology. Three-hour, closed-book exam (one double-sided aid sheet permitted, approved Casio/Sharp calculator allowed). Format: Part I offers 20-mark questions with an instruction to "solve 3 questions only out of the following 5 questions" — but six questions (Q1–Q6) are actually printed under Part I, one more than the instruction text states (an inconsistency in the paper itself). Part II offers 3 questions (any 2 constitute a complete answer, 20 marks each). All nine questions are solved below for completeness using the exam's own numbering (Q1–Q6 = Part I, Q7–Q9 = Part II, no renumbering needed). Q2, Q3, Q4, Q5, Q6, and Q9 are calculation/stoichiometry questions; Q1, Q7, and Q8 are essay/qualitative questions.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts (2nd ed., Prentice Hall) — elemental/electron balances, yield coefficients, fermenter mass balances, respiratory quotient, batch growth kinetics; Madigan et al., Brock Biology of Microorganisms (15th ed., Pearson) — bacterial classification, fungal spores, plasmids, water-activity/temperature effects on growth; Toledo, Fundamentals of Food Process Engineering (3rd ed., Springer) — plant/animal tissue morphology and processing.
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) Five major characteristics used to classify bacteria. (1) Cell morphology — shape (coccus, bacillus, spirillum, vibrio) and arrangement (chains, clusters, tetrads). (2) Gram-stain reaction — Gram-positive (thick peptidoglycan, retains crystal violet) vs. Gram-negative (thin peptidoglycan + outer membrane, counterstains pink/red), reflecting a fundamental cell-envelope difference. (3) Metabolic/physiological traits — oxygen requirement (obligate aerobe, facultative/obligate anaerobe, microaerophile), carbon/energy source (autotroph vs. heterotroph, phototroph vs. chemotroph), and specific biochemical tests (catalase, oxidase, fermentation profiles). (4) Genetic/molecular sequence data — 16S rRNA gene sequence comparison is now the primary tool for establishing phylogenetic relationships and species boundaries, supplemented by whole-genome (DNA-DNA hybridization/ANI) comparisons. (5) Serological and antigenic properties — surface antigens (O, H, K antigens in Enterobacteriaceae) used especially for strain-level typing in clinical/food-safety microbiology.
(b) Fungal spores — definition and asexual types. A fungal spore is a small, often thick-walled, typically dormant/resistant reproductive or dispersal unit produced by fungi, capable of germinating into a new mycelium under favorable conditions; spores may be sexual (formed after nuclear/genetic recombination between compatible mating types) or asexual (formed by mitosis, genetically identical to the parent). Five types of asexual fungal spores: (1) Conidia — spores borne externally (not enclosed in a sac) at the tips or sides of specialized hyphae called conidiophores; the dominant asexual spore type in molds such as Aspergillus and Penicillium. (2) Sporangiospores — spores formed inside an enclosed sac (the sporangium) at the tip of a sporangiophore, characteristic of the bread molds (Rhizopus, Mucor). (3) Arthrospores (arthroconidia) — formed by simple fragmentation of an existing hypha into separate thick-walled segments, without a specialized spore-bearing structure. (4) Chlamydospores — thick-walled, resting spores formed by rounding-up and enlargement of a hyphal cell segment in place, serving mainly as a survival structure under adverse conditions rather than for dispersal. (5) Blastospores (blastoconidia) — spores produced by budding directly off a parent cell or hypha, typical of yeasts such as Candida and Saccharomyces.
(c) Why plasmid number varies among bacteria. Plasmids are extrachromosomal, typically circular, self-replicating DNA elements that are not essential for baseline survival but confer conditional advantages — antibiotic resistance, heavy-metal resistance, degradation of unusual carbon sources, virulence factors (toxins, adhesins), or conjugative transfer ability. A bacterium carries multiple plasmids when its ecological niche repeatedly favors those accessory traits (e.g. a hospital-associated pathogen under constant antibiotic selection pressure benefits from carrying several resistance plasmids simultaneously, and different plasmids can be independently gained via separate conjugation/transformation events provided they are compatible, i.e. do not share the same replication-control/incompatibility group). A bacterium may carry no plasmids when (i) its environment does not favor any of the traits plasmids typically encode, so maintaining them is a net fitness cost (replication burden, metabolic load) with no offsetting benefit and they are lost over generations, or (ii) all necessary genetic information for its lifestyle is already encoded chromosomally. Plasmid carriage is therefore a cost–benefit outcome of the organism's specific selective environment, not a fixed taxonomic property.
