04-BS-13 · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 04-BS-13, Biology. Three-hour, closed-book exam (one double-sided aid sheet permitted, approved Casio/Sharp calculator allowed). Format: Part I offers 6 questions (any 3 constitute a complete answer, 20 marks each) and Part II offers 4 questions (any 2 constitute a complete answer, 20 marks each) — a full paper is 5 questions. All 10 are solved below for completeness. Most questions require an essay-format answer; Q1–Q4 are calculation questions.
Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts (2nd ed., Prentice Hall) — elemental/electron balances, yield coefficients, fermenter energy balances; Madigan et al., Brock Biology of Microorganisms (15th ed., Pearson) — bacterial/viral/fungal morphology and physiology; Toledo, Fundamentals of Food Process Engineering (3rd ed., Springer) — plant/animal tissue structure, rheology, water activity.
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.
Biological tissues behave as cellular solids — a network of fluid-filled, pressurized cells bonded together — and their bulk rheology (stress–strain response, viscoelasticity, fracture mode) is governed jointly by individual cell geometry and by the void fraction between cells.
Cell size. At a fixed tissue volume, smaller cells pack more cell walls (and more turgor-pressurized units) per unit volume, giving a stiffer, denser bulk material with a higher elastic modulus and better resistance to bruising, whereas tissue built from large cells (a hallmark of over-mature or senescent parenchyma) has fewer walls per volume and fractures more readily under load — the textural basis of "mealiness."
Cell shape. Elongated, fibrous cells (sclerenchyma fibres, xylem tracheids) impose strongly anisotropic mechanical behaviour — stiff and strong along the fibre axis, weak across it — while roughly isodiametric (spherical/polyhedral) parenchyma cells give an approximately isotropic bulk response and pack more efficiently, reducing void fraction.
Cell volume and turgor. Each living cell behaves like a pressurized, thin-walled vessel: internal turgor pressure (opposed by wall tension) is the principal source of tissue rigidity and the sensory property of crispness in fresh produce. Loss of turgor (wilting, or membrane rupture from freeze–thaw or over-cooking) collapses this hydraulic stiffening mechanism and shifts the tissue's response from elastic/brittle to soft and plastic.
Intercellular spaces. The air-filled void fraction between cells varies enormously between tissues (roughly 25% by volume in apple parenchyma versus about 1–2% in potato tuber tissue) and dominates bulk compressibility: at low applied stress the gas-filled voids collapse first (a soft, foam-like regime, described by classical cellular-solids theory), and only at higher stress does the load transfer to the cell walls themselves, producing the characteristic bilinear stress–strain curve of many fruits and vegetables. Higher porosity also increases the effective diffusivity for gas exchange (respiration, controlled-atmosphere storage) and for heat/moisture transport during drying or blanching. Finally, the strength of the pectin-rich middle lamella cementing adjacent cells together determines the fracture mode: strong cell-to-cell adhesion causes cells to rupture across their walls (a juicy fracture, releasing cell contents), while weak adhesion (common in over-ripe or over-cooked tissue) lets cells separate and round off intact — the classic "mealy" texture.
