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Prokaryotes, Eukaryotes & Eukaryotic Cell Organisation

Prokaryotic Cells

  • Lack a membrane-bound nucleus; genetic material resides in a nucleoid rather than an enclosed compartment.

    • DNA arrangement: \text{single, circular chromosome}.

  • Absence of complex, membrane-bound organelles.

  • Typical cell size: 0.5\text{–}5\,\mu m.

  • Representative groups:

    • Bacteria

    • Archaea

  • Physiological/medical significance (contextual):

    • Unique ribosome structure is a major target for antibiotics, exploiting the prokaryote–eukaryote difference.

Eukaryotic Cells

  • Possess a membrane-bound nucleus that encloses genetic material.

    • DNA arrangement: multiple linear chromosomes.

  • Contain an extensive array of membrane-bound organelles, each specialised for particular cellular functions.

  • Typical cell size: 5\text{–}100\,\mu m.

  • Representative groups:

    • Protozoa

    • Fungi

    • Plants

    • Animals

  • Evolutionary/functional significance: compartmentalisation enables simultaneous, incompatible biochemical reactions and sophisticated regulation.

Prokaryotes vs. Eukaryotes (Comparative Overview)

  • Nucleus:

    • Prokaryote – not fully developed; no nuclear envelope.

    • Eukaryote – well-developed, double-membrane nuclear envelope with nuclear pores.

  • Chromosomes:

    • Prokaryote – 1 circular DNA molecule.

    • Eukaryote – >1 linear DNA molecules (chromosomes).

  • Organelles:

    • Prokaryote – none that are membrane bound.

    • Eukaryote – mitochondria, ER, Golgi, lysosome, etc.

  • Size range:

    • Prokaryote – 0.5\text{–}5\,\mu m.

    • Eukaryote – 5\text{–}100\,\mu m.

Organisation of a Typical Eukaryotic Cell

  • Cytoskeletal elements (structural & motility):

    • Actin (microfilaments)

    • Intermediate filaments

    • Microtubules; organised at the centrosome (pair of centrioles).

  • Extracellular matrix (ECM) provides external support/signalling interface.

  • Core organelles:

    • Nucleus: chromatin, nucleolus, nuclear envelope with pores.

    • Ribosomes (free & membrane-bound).

    • Endoplasmic reticulum (rough and smooth).

    • Golgi apparatus (cis → trans cisternae sequence).

    • Vesicles (transport, secretory, endocytic).

    • Lysosomes.

    • Peroxisomes.

    • Mitochondria.

  • Plasma membrane: phospholipid bilayer ~5\,\text{nm} thick, containing pumps and carriers that regulate selective exchange.

Functional Themes of Eukaryotic Organelles

  • Material production – \text{ribosomes} (protein synthesis).

  • Material sorting – \text{ER} & \text{Golgi}.

  • Material degradation – \text{lysosomes}.

  • Material transport – \text{cytoskeleton}, motor proteins (myosin, kinesin, dynein).

  • Energy – \text{mitochondria} (ATP generation).

  • Information storage/signalling – receptors, protein & second messengers, nucleus (DNA).

  • Multifunctionality: each unit often fulfils several roles (e.g., ER synthesises lipids and stores \text{Ca}^{2+}).

Nucleus (Information Storage & Transcription)

  • Encloses chromosomes, protecting DNA and organising gene expression.

  • Houses nucleolus – site of rRNA transcription & ribosome subunit assembly.

  • Governs DNA → mRNA transcription.

  • Nuclear pores regulate bidirectional traffic (proteins in, mRNA/ribosomal subunits out).

Ribosomes (Material Production)

  • Molecular machines for polypeptide synthesis.

  • Composition:

    • Large and small subunits, each an RNA-protein complex.

  • Localisation:

    • Free in cytosol (cytosolic proteins).

    • Bound to rough ER (secretory & membrane proteins).

  • Universally conserved across all cell types.

Endoplasmic Reticulum & Golgi Apparatus (Packaging, Sorting, Shipping)

  • Rough ER (RER):

    • Studded with ribosomes; synthesises membrane proteins & lumenal proteins.

    • Quality control (folding, post-translational modification).

    • Dispatches proteins to Golgi via vesicles.

  • Smooth ER (SER):

    • Lipid, phospholipid, and steroid hormone synthesis.

    • Carbohydrate metabolism.

    • Detoxification (e.g., alcohol, drugs).

    • \text{Ca}^{2+} ion storage.

  • Golgi apparatus:

    • Cis → medial → trans cisternae progression.

    • Modifies (glycosylation, phosphorylation), sorts, and labels proteins received from ER.

    • Packages cargo into specific vesicles destined for plasma membrane, lysosomes, or secretion.

Lysosomes (Material Degradation & Recycling)

  • Acidic, enzyme-rich organelles for macromolecule digestion.

    • Proteases – proteins.

    • Nucleases – nucleic acids.

    • Carbohydrases – polysaccharides.

    • Lipases – lipids.

  • Degrade material from:

    • Phagocytosis (external particles).

    • Endocytosis (receptor-mediated uptake).

    • Autophagy (self-digestion of organelles).

  • Recycling nutrients; maintaining cellular homeostasis.

Cytoskeleton & Motility Apparatus (Material Transport System)

  • Filament systems:

    • Microfilaments (actin) – cell shape, muscle contraction, cytokinesis.

    • Intermediate filaments – mechanical strength, nuclear lamina.

    • Microtubules – vesicle tracks, chromosome segregation; emanate from centrosome.

  • Molecular motors:

    • Myosin (actin-based).

    • Kinesin (plus-end microtubule motor).

    • Dynein (minus-end microtubule motor, cilia/flagella beating).

  • Enables organelle positioning, vesicular traffic, cell motility.

Plasma Membrane (Structure & Selective Barrier)

  • Phospholipid bilayer thickness ≈ 5\,\text{nm}.

  • Dynamic fluid mosaic: lateral diffusion of lipids/proteins, yet asymmetrical leaflets.

  • Selectivity mechanisms:

    • Passive diffusion (small, non-polar molecules).

    • Pumps (energy-driven), carriers, and channels for regulated exchange.

  • Interface for signal transduction via membrane receptors.

Mitochondria (Energy Generation & Beyond)

  • Principal site of ATP synthesis through oxidative phosphorylation (respiratory chain).

  • Additional roles:

    • Metabolic regulation (fatty acid oxidation, Krebs cycle).

    • Signalling hub for apoptosis (cytochrome c release triggers caspase cascade).

    • Ageing processes and reactive oxygen species (ROS) management.

Structural Hallmarks

  • Double membrane system:

    • Outer membrane (OM).

    • Intermembrane space (IMS).

    • Inner membrane (IM) with cristae – invaginations increasing surface area.

  • Crista junctions connect cristae to peripheral IM regions.

  • Matrix:

    • Contains enzymes of the Krebs cycle, mitochondrial DNA, and ribosomes.

Mitochondrial DNA (mtDNA)

  • Circular, double-stranded; exclusively maternally inherited.

  • Gene content: 37 genes →

    • 13 protein-coding (respiratory chain subunits).

    • 22 tRNAs.

    • 2 rRNAs.

  • Multiple mtDNA copies per organelle.

  • Protein origin:

    • Many mitochondrial proteins are nuclear-encoded, synthesised in cytosol, and imported post-translationally into mitochondria.

Integrative Perspective & Study Tips

  • Remember the “flow” of biomolecules: DNA (nucleus) → mRNA → ribosomes (RER) → Golgi → vesicles → final destination.

  • Distinguish organelle functions by core themes: production, sorting, degradation, transport, energy, information.

  • Size, structure, and genetic organisation differences between prokaryotes and eukaryotes underpin many biomedical tools (e.g., antibiotic selectivity, recombinant protein expression systems).

  • Visual mnemonics:

    • “Rough ≈ Ribosome” (protein processing).

    • “Smooth ≈ Steroids” (lipids, detox).

    • “Golgi = GPS” (labels & sorts).

    • “Lysosome = Lysol” (clean-up).

  • Ethical/practical considerations: manipulation of mitochondrial DNA in disease therapy (e.g., three-parent IVF) highlights organelle genetics relevance.