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.
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.
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.
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.
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+}).
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.