Cell biology begins with the Cell Theory, the foundational framework for understanding life at its most basic level. Proposed by Matthias Schleiden (1838, plants) and Theodor Schwann (1839, animals), the Cell Theory establishes that all living organisms are composed of cells and that the cell is the basic structural and functional unit of life. Rudolf Virchow completed the theory in 1855 by adding the axiom "Omnis cellula e cellula" — every cell arises from a pre-existing cell — eliminating the concept of spontaneous generation.

All cells belong to one of two fundamental categories. Prokaryotic cells lack a membrane-bound nucleus; their genetic material consists of a single circular, naked DNA molecule concentrated in a nucleoid region. These cells, found in bacteria and cyanobacteria, range from 1–10 µm in size. The cell envelope includes a plasma membrane surrounded by a rigid cell wall composed of peptidoglycan (murein). Mesosomes — infoldings of the plasma membrane — compensate for the absence of mitochondria by providing increased membrane surface area for respiration and cell division. Ribosomes are 70S in type, consisting of 50S and 30S subunits. Plasmids — small extrachromosomal circular DNA molecules — carry accessory genes such as antibiotic resistance. Prokaryotic flagella are made of the protein flagellin and rotate via a proton gradient, with no 9+2 microtubule arrangement. Division occurs by binary fission without a spindle.

Eukaryotic cells are larger (10–100 µm) and distinguished by a membrane-bound nucleus enclosed in a double-layered nuclear envelope perforated by nuclear pores that regulate nucleocytoplasmic transport. Within the nucleus, chromatin fibres condense into chromosomes during cell division, and the nucleolus — a non-membrane-bound structure — synthesizes ribosomal RNA. The cytoplasm contains 80S ribosomes (60S + 40S subunits) and a sophisticated endomembrane system consisting of the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles. The rough ER, studded with ribosomes, is the site of protein synthesis and transport; the smooth ER handles lipid synthesis and detoxification. The Golgi apparatus receives proteins from the ER at its cis (forming) face, modifies them through glycosylation, phosphorylation, and proteolytic processing, and dispatches them from its trans (maturing) face as secretory vesicles or lysosomes. Lysosomes, derived from the Golgi, contain over 50 hydrolytic enzymes that perform intracellular digestion — earning them the label "suicide bags" because rupture releases these enzymes and causes autolysis. They carry out both heterophagy (digestion of ingested material) and autophagy (recycling of damaged organelles).

Mitochondria and chloroplasts are semi-autonomous organelles supported by robust evidence for an endosymbiotic origin. Mitochondria have a double membrane: the outer is smooth, while the inner is folded into cristae that vastly increase the surface area for oxidative phosphorylation (ATP synthesis). The matrix contains circular DNA, 70S ribosomes, and Krebs cycle enzymes. Chloroplasts, found in plant and algal cells, have a double membrane envelope, internal thylakoid membranes stacked into grana (site of light-dependent reactions), and a surrounding stroma where the Calvin cycle fixes carbon dioxide into sugars. Both organelles possess their own circular DNA and 70S ribosomes — features identical to prokaryotes — providing compelling evidence for the endosymbiotic theory proposed by Lynn Margulis in 1967.

The plasma membrane follows the Fluid Mosaic Model of Singer and Nicolson (1972): a dynamic phospholipid bilayer with integral (transmembrane) and peripheral proteins, glycoproteins forming the glycocalyx, and cholesterol (in animal cells) regulating membrane fluidity. The model replaced the earlier Davson-Danielli sandwich model. Plant cells are additionally surrounded by a rigid cellulose cell wall, with the middle lamella composed of calcium pectate cementing adjacent cells. Three wall layers exist: middle lamella, primary cell wall (cellulose), and secondary cell wall (lignin, in specialized cells). Fungi have chitin cell walls; animal cells have no cell wall.

The cytoskeleton — composed of microtubules (tubulin), microfilaments (actin), and intermediate filaments — maintains cell shape, enables movement, and facilitates intracellular transport. Eukaryotic cilia and flagella exhibit the characteristic 9+2 arrangement of microtubules powered by dynein motors, fundamentally distinct from the flagellin-based prokaryotic flagellum. Centrioles (absent in most plant cells) organize the mitotic spindle in animal cells.

Plant and animal cells differ in several key features. Plant cells possess a cellulose cell wall, three types of plastids (chloroplasts, chromoplasts, leucoplasts), a large central vacuole bounded by the tonoplast, and plasmodesmata for cell-to-cell communication. They form new cell walls by centrifugal cell plate formation during cytokinesis. Animal cells, conversely, contain centrioles, lysosomes, and cholesterol in their membranes; they lack plastids, large vacuoles, and cell walls. Animal cell cytokinesis proceeds by centripetal cleavage furrow formation. The key NEET-tested concept throughout this topic is the structural distinction between cell types, especially the 70S ribosome presence in both prokaryotes and semi-autonomous organelles — a classic examination trap that rewards students who understand the endosymbiotic basis of organelle biology.