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1.4.8Inquiry Process

Eukaryotic Cell Structure: Comparisons and Atypical Examples — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 160

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Comparing Kingdoms

Animal cells
  • Animal cells No wall, small temporary vacuoles, centrioles present, and carbohydrate energy store held as glycogen.
Plant cells
  • Plant cells Cellulose wall, large permanent vacuole, chloroplasts in green tissue, and energy stored as starch.
Fungal cells
  • Fungal cells Wall made of chitin, no chloroplasts, and glycogen as the usual store; yeasts are single cells.
Shared features
  • Shared features All have a nucleus, 80S ribosomes, mitochondria, endomembrane system and a cytoskeleton.
FeatureAnimalPlantFungus
WallNoneCelluloseChitin
ChloroplastsNoYesNo
StoreGlycogenStarchGlycogen

Part 2 of 5Atypical Cells

Atypical Cells
Aseptate fungal hyphae
  • Aseptate fungal hyphae Long tubes without cross walls, so many nuclei share one continuous cytoplasm rather than being divided into cells.
Skeletal muscle fibre
  • Skeletal muscle fibre A long fused cell with many nuclei, formed when several precursor cells join during development.
Red blood cell
  • Red blood cell Loses its nucleus as it matures, leaving room for haemoglobin, though this means it cannot repair itself.
Phloem sieve tube
  • Phloem sieve tube Keeps little cytoplasm and no nucleus, relying on a companion cell to supply its needs.
Acetabularia
  • Acetabularia A giant single-celled alga several centimetres long, with one nucleus at its base.

Part 3 of 5Why Atypical Cells Matter

1
Cell theory says living things are made of cells, yet some tissues seem to break the idea of one nucleus per cell.
2
Aseptate hyphae and muscle fibres show that a cell can contain many nuclei and still be a single cell.
3
Giant cells such as Acetabularia show that cell size is not always limited to microscopic dimensions.
4
Anucleate cells such as mature red blood cells show that a nucleus is not essential for short-term function.
5
These exceptions are called counter-examples and force scientists to refine the statements of cell theory.

Part 4 of 5Worked Example

Question

  • Skeletal muscle fibres can be several centimetres long and contain hundreds of nuclei.
  • Explain why they are still described as single cells, and what this shows about cell theory.

  1. The fibre is enclosed by one continuous plasma membrane, which is the boundary defining a cell.
  2. It forms when many myoblasts fuse during development, so the nuclei come from several earlier cells but now share one cytoplasm.
  3. Cell theory states that organisms are made of cells;
    • the muscle fibre does not contradict this but shows that cells are not always small with one nucleus.

Answer A muscle fibre is a single multinucleate cell, an atypical example that refines the idea of a typical cell.

Part 5 of 5Exam Checklist & Tips

1.4.8 – Key Points for the Exam

  • Chitin A nitrogen-containing polysaccharide forming fungal cell walls.
  • Aseptate Lacking dividing walls between sections.
  • Multinucleate Containing more than one nucleus.
  • Anucleate Lacking a nucleus.
  • Counter-example A case that challenges a general rule.

Common Mistakes

  • Saying fungi have cellulose walls; theirs are chitin.
  • Stating that all plant cells contain chloroplasts, ignoring root and other non-green cells.
  • Describing a muscle fibre as many cells; it is one cell with many nuclei.
  • Claiming that cells without nuclei are not living at all.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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