23-Chem-B4 Biochemical Engineering · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-Chem-B4, Biochemical Engineering — Dec 2014. 3 hours, Closed-Book Exam (any non-communicating calculator permitted). Per the exam notes, FIVE (5) questions constitute a complete paper and all five must be answered; most require a short-essay-format answer.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts, 2nd ed.; Bailey & Ollis, Biochemical Engineering Fundamentals, 2nd ed.; Madigan et al., Brock Biology of Microorganisms, 13th ed.
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.
Photosynthesis is the light-driven conversion of light energy into chemical energy (ATP and reducing power), used by autotrophic organisms to fix CO2 into biomass. The two modes differ fundamentally in the electron donor used to replenish the reaction centre and, consequently, in whether O2 is produced.
Oxygenic photosynthesis (cyanobacteria, algae, plants) uses water as the electron donor. Two photosystems act in series (the "Z-scheme"): Photosystem II extracts electrons from water, splitting it into O2, protons, and electrons (the water-splitting/oxygen-evolving complex), and Photosystem I re-energizes the electrons to reduce NADP+ to NADPH. Because water is a poor electron donor (requiring a very strong oxidant to pull electrons from it), extracting electrons from water demands the extra photochemical "push" of two photosystems working in series, and O2 is released as an obligatory by-product.
Anoxygenic photosynthesis (purple and green sulfur/non-sulfur bacteria) uses weaker, more easily oxidized electron donors — hydrogen sulfide (H2S, oxidized to elemental sulfur or sulfate), other reduced sulfur compounds, H2, or organic acids — and possesses only a single photosystem. Because these donors give up electrons more readily than water, one photosystem suffices, and since water is never split, no oxygen is produced.
| Feature | Oxygenic | Anoxygenic |
|---|---|---|
| Electron donor | H2O | H2S, S, H2, organic acids |
| Photosystems | Two, in series (PSII + PSI) | One |
| O2 evolved? | Yes (obligate by-product) | No |
| Typical organisms | Cyanobacteria, algae, plants | Purple/green sulfur and non-sulfur bacteria |
| Habitat implication | Aerobic or O2-producing environments | Often anaerobic/microaerophilic (e.g. sulfidic sediments) |
Both modes converge on the same downstream chemistry — the light reactions generate a proton-motive force across a membrane that drives ATP synthase, and the resulting ATP and NAD(P)H power carbon fixation (usually via the Calvin cycle) — but the choice of electron donor is what fundamentally separates them, and is itself set by which donor is available in the organism's ecological niche.
Fermentation is an energy-yielding metabolic pathway in which an organic substrate is both the electron donor and the ultimate electron acceptor, and ATP is generated exclusively by substrate-level phosphorylation — no electron transport chain and no external terminal electron acceptor (O2 or otherwise) is involved. It is the metabolic strategy used when no suitable external electron acceptor is available, and is far less energy-efficient than respiration because most of the substrate's chemical energy remains locked in the reduced end products rather than being extracted via an ETC.
The pathway begins with glycolysis, which oxidizes glucose to two pyruvate molecules, netting 2 ATP (substrate-level phosphorylation at the phosphoglycerate kinase and pyruvate kinase steps) and reducing 2 NAD+ to 2 NADH. Because there is no ETC to re-oxidize this NADH, glycolysis would stall after one turn (NAD+ would be exhausted) unless the cell regenerates NAD+ by using pyruvate (or a pyruvate derivative) itself as the electron acceptor — this regeneration step is the defining chemistry of fermentation and gives each fermentation pathway its name:
| Fermentation type | NADH-reoxidizing step | End product(s) | Example organisms |
|---|---|---|---|
| Lactic acid (homolactic) | Pyruvate + NADH → lactate + NAD+ | Lactate | Lactobacillus, muscle cells |
| Alcoholic | Pyruvate → acetaldehyde + CO2; acetaldehyde + NADH → ethanol + NAD+ | Ethanol, CO2 | Saccharomyces cerevisiae |
| Mixed-acid | Multiple branch points reduce pyruvate-derived intermediates | Acetate, ethanol, lactate, formate/H2+CO2, succinate | Escherichia coli |
| Butyric acid | Reduction of acetyl-CoA-derived intermediates | Butyrate, butanol, acetone (solventogenic strains) | Clostridium spp. |
The net ATP yield of fermentation (2 ATP per glucose from glycolysis alone) is far lower than aerobic respiration's ∼30–32 ATP, because fermentation captures only the substrate-level-phosphorylation energy of glycolysis; the remaining chemical energy in glucose stays trapped in the reduced fermentation end-products (lactate, ethanol, etc.), which is exactly why industrial fermentation processes (brewing, lactic-acid production, ABE fermentation) are valued for their products rather than for biomass or energy yield.
