Glossary — Cell Biology Key Terms — Notes

Term	Definition
Autophagy	Digestion of the cell's own worn-out or damaged organelles by lysosomes
Axoneme	The core microtubule structure (9+2 arrangement) of eukaryotic cilia and flagella
Binary fission	Prokaryotic cell division: chromosome replicates, cell elongates, septum divides cell into two
Cell plate	Structure formed during plant cell cytokinesis from Golgi vesicles; becomes the new cell wall
Cell Theory	Three-part theory: all organisms made of cells; cell = basic unit; all cells from pre-existing cells
Chromatin	Complex of DNA and histone proteins in eukaryotic nucleus; condenses into chromosomes during division
Cristae	Infoldings of the inner mitochondrial membrane; increase surface area for oxidative phosphorylation
Cytoskeleton	Network of protein filaments (microtubules, microfilaments, intermediate filaments) giving cell structure
Endomembrane system	Interconnected membrane system: ER → Golgi → lysosomes → vacuoles
Endosymbiotic theory	Proposes mitochondria and chloroplasts evolved from prokaryotic endosymbionts
Euchromatin	Loosely packed, transcriptionally active chromatin
Flagellin	Protein subunit of prokaryotic flagella; distinct from tubulin in eukaryotic flagella
Fluid Mosaic Model	Model of membrane structure (Singer & Nicolson, 1972): fluid phospholipid bilayer + mosaic of proteins
Glycocalyx	Carbohydrate coat on cell surface formed by glycoproteins and glycolipids
Grana	Stacks of thylakoid discs in chloroplasts; site of light-dependent reactions
Heterochromatin	Densely packed, transcriptionally inactive chromatin
Heterophagy	Digestion of material taken from outside the cell via endocytosis (e.g., bacteria) by lysosomes
Integral protein	Protein embedded within the lipid bilayer of a membrane; many span the full bilayer
Mesosome	Infolding of prokaryotic plasma membrane; assists in respiration, DNA replication, cell division
Middle lamella	Layer of calcium pectate between adjacent plant cell walls; cements cells together
Nucleoid	Region of prokaryotic cytoplasm containing the circular chromosome; not membrane-bound
Nucleolus	Dense body inside eukaryotic nucleus; site of rRNA synthesis and ribosome subunit assembly
Omnis cellula e cellula	Latin: "every cell from a pre-existing cell" — Virchow's contribution to Cell Theory
Peptidoglycan	Bacterial cell wall polymer (also called murein); target of penicillin
Peripheral protein	Protein on the surface of a membrane; not embedded in the lipid bilayer
Plasmid	Small, circular, extrachromosomal DNA in prokaryotes; replicates independently
Plasmodesmata	Cytoplasmic channels through plant cell walls connecting adjacent cells
Semi-autonomous	Organelles (mitochondria, chloroplasts) with own DNA + ribosomes but still dependent on nucleus
Stroma	Aqueous matrix of chloroplast; site of Calvin cycle
Thylakoid	Flattened membrane sac in chloroplasts; stacked into grana; site of light reactions
Tonoplast	Single membrane surrounding the central vacuole in plant cells