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.