Chapter 3 Part 2: Eukaryotic Cells

Eukaryotic Cells – Core Concept

  • Prefix breakdown
    • Eu- = true
    • -karyote = nut (historical term for nucleus)
    • → “true nucleus.”
  • Presence of a membrane-bound nucleus is the diagnostic feature
    • Rare exceptions inside multicellular animals: mature red blood cells.
  • Greater complexity than prokaryotes because membranes create internal compartments → specialized micro-environments for distinct jobs.
  • Domain-level placement
    • Domain Eukarya = all organisms that are not Bacteria or Archaea.
    • Clear binary: an organism fits one side or the other—no in-between.
  • Cellular lifestyles
    • Unicellular eukaryotes → mostly Protista
    • Multicellular eukaryotes → Fungi, Plantae, Animalia
    • Fun exception: yeast (unicellular fungus)
    • Some protists deviate and become multicellular—biology loves exceptions.
  • Ethical / practical tie-in: Understanding compartmentalization underpins modern drug design (target organelle-specific chemistry without harming whole cell).

Two Major Model Cell Types to Learn

  • We focus on animal and plant cells because they dominate human contexts; fungal and protist cells resemble one of these two templates.

Typical Animal Cell

  • Nucleus (envelope + pores, chromatin, nucleolus)
  • Endoplasmic reticulum (ER)
    • Rough ER: ribosomes attached → protein synthesis
    • Smooth ER: lipid metabolism, detox, Ca²⁺ storage
  • Ribosomes (bound & free; identical to prokaryotic ribosomes in catalytic core)
  • Golgi apparatus / vesicles (modifies, sorts, ships biomolecules)
  • Mitochondria (ATP generator; about the size of a bacterium!)
  • Lysosomes (digest old organelles—"cellular stomach")
  • Peroxisomes (briefly noted, not detailed)
  • Cytoplasm (cytosol + organelles)
  • Centrosome (contains centrioles; mitotic spindle origin)
  • Microtubules / cytoskeleton elements
  • NO flagellum in ordinary animal cells; sole exception = sperm.
  • Instructor’s fun observation: textbook diagram looks like a flayed mouse, eye and limbs included!

Typical Plant Cell

  • Everything listed for animal cells plus three hallmark additions:
    • Cell wall surrounding plasma membrane (structural support, made of cellulose)
    • Central vacuole (large, water storage; maintains turgor)
    • Chloroplasts (photosynthesis)
  • Vacuoles in animals exist but are far smaller (adipose cells are an outlier with a big lipid vacuole).

Compare / Contrast Cheat-Sheet

  • Shared organelles
    • Nucleus
    • Mitochondria
    • Ribosomes
    • Endoplasmic reticulum (rough & smooth)
    • Golgi apparatus
    • Cytoskeleton
  • Animal-specific
    • Lysosomes (intracellular digestion of worn-out parts)
    • Centrioles (organize mitotic spindle)
  • Plant-specific
    • Cell wall
    • Central vacuole
    • Chloroplasts

Relative Size Landscape

  • Typical plant / animal cell: 10 μm→100 μm10\,\mu\text{m} \rightarrow 100\,\mu\text{m}
  • Pollen grain ≈ human egg • Frog egg ≈ 10×10\times human egg • Ostrich egg = largest known single cell.
  • Red blood cell < average animal cell.
  • Mitochondrion ≈ average bacterium (key clue for endosymbiotic theory).
  • Viruses: Smallpox, Flu, Polio range ≈20 nm→300 nm\approx 20\,\text{nm} \rightarrow 300\,\text{nm}.
  • Proteins: 1 nm→10 nm1\,\text{nm} \rightarrow 10\,\text{nm}
  • DNA double helix diameter ≈ 2 nm2\,\text{nm}
  • Water molecule ≈ 0.28 nm0.28\,\text{nm}
  • Atom ≈ 0.1 nm0.1\,\text{nm}
  • Microscopy windows
    • Light microscope: cell to large viruses
    • Electron microscope: down to proteins & lipids
    • X-ray crystallography / advanced methods: atomic resolution.

Cytoplasm Essentials

  • Cytoplasm = cytosol (liquid) + all suspended organelles.
  • Cytosol: aqueous solution with salts, sugars, proteins, free nucleotides, ions; site of many enzyme-catalyzed reactions (glycolysis, etc.).
  • Storage function: quick access pool for enzymes, ions, nucleotides.

Cytoskeleton & Centrosome

  • Purpose: internal scaffolding, motility, intracellular transport.
  • Three filament classes
    • Microfilaments (actin) – smallest; two protein chains; muscle contraction, cell crawling.
    • Intermediate filaments – mid-size; diverse proteins; tension bearing, temporary scaffolds.
    • Microtubules (tubulin) – largest; hollow cylinders (think paper-towel tube); tracks for vesicle movement, make mitotic spindle.
  • Centrosome (animal cells)
    • Interphase: appears as paired “barrels.”
    • Prophase: duplicates, splits into centrioles, migrates to poles.
    • Generates spindle fibers (polymerized microtubules) to segregate chromosomes in mitosis & meiosis.
  • Plants lack centrioles; they organize spindle via alternative microtubule-organizing centers.

Study & Exam Tips (Instructor Advice)

  • Practice drawing a blank animal or plant cell, label every remembered part, then check against master list.
  • Use size comparisons to ground microscopic concepts (e.g., mitochondrion ≈ bacterium).
  • Remember unique exclusions (animal cell lacks flagellum except sperm) to avoid common diagram traps.
  • Connect structure → function: each membrane compartment solves a chemical problem (e.g., lysosome isolates acid hydrolases from cytoplasm).