Cell Structure: Prokaryotic & Eukaryotic Cells

Every living organism is built from one or more cells, a principle established by the Cell Theory of Schleiden and Schwann (1838-39), later extended by Virchow's axiom "Omnis cellula e cellula" — every cell arises from a pre-existing cell. Cells fall into two fundamental categories: prokaryotic and eukaryotic, distinguished primarily by the presence or absence of a membrane-bound nucleus.

Prokaryotic cells (bacteria, cyanobacteria) lack a true nucleus; their genetic material exists as a naked circular DNA molecule concentrated in a nucleoid region. The cell envelope consists of a plasma membrane surrounded by a rigid cell wall made of peptidoglycan (murein). Mesosomes — infoldings of the plasma membrane — assist in respiration and cell division. Ribosomes are 70S type (50S + 30S subunits). Plasmids, small extrachromosomal circular DNA molecules, confer traits like antibiotic resistance. Prokaryotic flagella are structurally distinct from eukaryotic flagella, composed of the protein flagellin rather than tubulin.

Eukaryotic cells possess a well-defined nucleus enclosed by a double-membraned nuclear envelope perforated by nuclear pores that regulate nucleocytoplasmic transport. Within the nucleus, chromatin fibres condense into chromosomes during division, and the nucleolus is the site of ribosomal RNA synthesis. The cytoplasm houses 80S ribosomes (60S + 40S subunits) and a sophisticated endomembrane system: the endoplasmic reticulum (rough ER studded with ribosomes for protein synthesis; smooth ER for lipid synthesis and detoxification) connects functionally to the Golgi apparatus, which receives, modifies, and packages proteins from its cis face to its trans face. Lysosomes, derived from the Golgi, contain hydrolytic enzymes for intracellular digestion — earning them the title "suicide bags" — and carry out heterophagy and autophagy. Vacuoles serve storage and osmoregulation functions, being particularly large in plant cells.

Mitochondria and chloroplasts are semi-autonomous organelles, each possessing their own circular DNA and 70S ribosomes — evidence supporting the endosymbiotic theory. Mitochondria have a double membrane with inner membrane folds called cristae that increase surface area for oxidative phosphorylation. Chloroplasts contain thylakoid membranes organized into grana connected by stroma lamellae, where light reactions of photosynthesis occur, while the Calvin cycle operates in the stroma.

The plasma membrane follows the Fluid Mosaic Model proposed by Singer and Nicolson (1972): a phospholipid bilayer with integral (transmembrane) and peripheral proteins, cholesterol (in animal cells), and glycoproteins, creating a selectively permeable, dynamic structure. Plant cells additionally possess a rigid cellulose cell wall (fungi have chitin walls), with the middle lamella composed of calcium pectate cementing adjacent cells. The cytoskeleton — microtubules, microfilaments, and intermediate filaments — provides structural support and facilitates intracellular transport. Centrioles, present in animal cells but absent in most plant cells, organize the mitotic spindle. Cilia and flagella in eukaryotes exhibit the characteristic 9+2 arrangement of microtubules.

The key testable concept is the structural and functional differences between prokaryotic and eukaryotic cells, including the semi-autonomous nature of mitochondria and chloroplasts with their 70S ribosomes.