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04-BS-13 · May 2016

Question 1 of 9: Plant vs. Animal Cell Structure and the Plant Cell Wall

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams — May 2016 — 04-BS-13, Biology. Three-hour, closed-book exam (one double-sided aid sheet permitted, approved calculator allowed). Format: Part I offers 5 questions (any 3 constitute a complete answer, 20 marks each; Q2 itself offers two alternative sub-problems) and Part II offers 3 questions (any 2 constitute a complete answer, 20 marks each) — a full paper is 5 questions. All 8 numbered questions (with both alternatives of Q2) are solved below for completeness. Q2–Q7 are calculation/derivation questions; Q1, Q6(a)(b)(d), and Q8 are essay questions.

Reference texts: Shuler & Kargi, Bioprocess Engineering: Basic Concepts (2nd ed., Prentice Hall) — elemental/electron balances, yield coefficients, fermenter mass and energy balances, growth kinetics; Madigan et al., Brock Biology of Microorganisms (15th ed., Pearson) — bacterial/viral structure, rapid methods, MPN; Toledo, Fundamentals of Food Process Engineering (3rd ed., Springer) — plant tissue structure and mechanical properties.

Question 1: Plant vs. Animal Cell Structure and the Plant Cell Wall (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) Typical plant and animal cell structures. Both are eukaryotic cells sharing a plasma membrane, cytoplasm, nucleus (with nucleolus and chromatin), endoplasmic reticulum (rough and smooth), Golgi apparatus, mitochondria, ribosomes, and a cytoskeleton. The key structural differences are summarized below and shown schematically in Figure 1.

Plant cell cell wall (cellulose) nucleus central vacuole chloroplast Animal cell nucleus mitochondria centriole lysosome plasma membrane only, irregular shape
Figure 1. Schematic plant cell (rigid cellulose wall, large central vacuole, chloroplasts) vs. animal cell (flexible membrane only, small scattered mitochondria, centrioles, no wall/vacuole/plastids).
FeaturePlant cellAnimal cell
Cell wallPresent (cellulose, hemicellulose, pectin, lignin)Absent — plasma membrane only
ShapeFixed, regular (rectangular/polygonal)Irregular, flexible
VacuoleLarge central vacuole (turgor, storage)Small, transient vesicles only
PlastidsChloroplasts (photosynthesis), other plastidsNone
CentriolesUsually absentPresent (mitosis)
Energy storageStarchGlycogen
Intercellular connectionPlasmodesmata through the wallTight/gap junctions, desmosomes

(b) Plant cell wall structure and its effect on tissue mechanical properties. The wall is a layered fibre-composite laid down outside the plasma membrane:

  1. Middle lamella. Pectin-rich cement layer shared between adjacent cells; binds cells together and, when degraded by pectinase (ripening, cooking), causes cell separation and tissue softening.
  2. Primary wall. Thin, laid down while the cell is still growing: cellulose microfibrils (the load-bearing, high-tensile-strength phase) embedded in a matrix of hemicellulose and pectin. Microfibril orientation is not fixed, allowing wall extension during growth.
  3. Secondary wall (mature/non-growing cells only). Thick, deposited inside the primary wall in several sub-layers with cellulose microfibrils wound at different, fixed angles; often impregnated with lignin (woody tissue), which cements the fibrils and greatly increases rigidity and compressive strength while reducing extensibility.
middle lamella primary wall secondary wall (S1/S2/S3) plasma membrane cell interior
Figure 2. Layered wall structure (not to scale): middle lamella → primary wall (loose microfibril net) → secondary wall (fixed-angle, often lignified layers) → plasma membrane.

These wall contents govern the mechanical behaviour of plant tissue: rheological properties and elasticity come from the balance between stiff cellulose microfibrils (elastic, high modulus) and the hydrated pectin/hemicellulose matrix (viscoelastic, allows creep and stress relaxation) — the composite behaves as a viscoelastic solid, not a purely elastic one. Swelling and shrinkage are controlled mainly by the pectin matrix and cell turgor: pectin is highly hydrophilic and swells/de-swells with water content, and loss of turgor pressure (wilting, dehydration) causes visible tissue shrinkage independent of any wall damage. Rigidity is provided jointly by turgor pressure pressing the protoplast against the wall and by wall thickness/lignification; loss of turgor (flaccidity) softens tissue even with an intact wall. Tensile strength is dominated by the cellulose microfibrils (very high tensile strength along the fibril axis, like reinforcing fibres in a composite), while the matrix mainly resists compressive and shear loads — the specific microfibril winding angle in the secondary wall (steep vs. shallow) trades off tensile strength along the cell axis against wall extensibility, exactly analogous to filament winding angle in an engineered fibre-composite pressure vessel.

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