Cell biology is the study of the structure, function, and behavior of cells. All living organisms are composed of cells, which can be broadly divided into prokaryotes (bacteria and archaea) and eukaryotes (protists, fungi, plants, and animals).
Prokaryotic Cell Structure
The nucleoid is a region within the cytoplasm containing a single, circular chromosome (dsDNA) supercoiled by topoisomerases; unlike eukaryotes, there is no nuclear membrane. Ribosomes are 70S (50S + 30S subunits) composed of rRNA and proteins, responsible for protein synthesis, and their 70S size distinguishes them from eukaryotic 80S ribosomes. The cytoplasmic membrane is a phospholipid bilayer with embedded proteins that functions as a selective permeability barrier, a site of electron transport and ATP synthesis (oxidative phosphorylation), and a secretion apparatus. The cell wall is a peptidoglycan layer (Gram-positive or Gram-negative) that maintains cell shape and resists osmotic pressure. Flagella provide motility via a rotary motor, while pili (fimbriae) mediate adhesion to surfaces and other cells.
Eukaryotic Cell Structure
The plasma membrane is a phospholipid bilayer with cholesterol (absent in prokaryotes) containing embedded proteins for transport, signaling, and cell-cell recognition; the fluid mosaic model describes its dynamic nature. The nucleus is enclosed by a double nuclear membrane (envelope) with nuclear pores regulating molecular traffic and contains chromatin (DNA + histones), the nucleolus (rRNA synthesis), and nucleoplasm.
Endomembrane System
The endoplasmic reticulum exists in two forms: rough ER (studded with ribosomes) for protein synthesis and folding, and smooth ER for lipid synthesis, carbohydrate metabolism, and calcium storage. The Golgi apparatus is a stack of flattened cisternae that modifies, sorts, and packages proteins for secretion (constitutive or regulated) or delivery to other organelles. Lysosomes are membrane-bound vesicles containing hydrolytic enzymes (acid hydrolases, optimal pH ~5) for degradation of macromolecules, pathogens, and damaged organelles via autophagy.
Energy-Producing Organelles
Mitochondria are double-membrane organelles with an inner membrane folded into cristae; they are the site of oxidative phosphorylation, generating ATP via the electron transport chain and chemiosmotic coupling. They contain their own circular DNA (mtDNA) and 70S ribosomes, evidence of their endosymbiotic origin. Chloroplasts (in algae and plants) are also double-membrane organelles with thylakoid stacks (grana) for photosynthesis and contain their own DNA and ribosomes.
Cytoskeleton
Microfilaments (actin) are 7 nm in diameter, involved in cell movement, cytokinesis, and maintaining cell shape, polymerizing from ATP-actin monomers. Microtubules are 25 nm diameter hollow tubes composed of α/β-tubulin dimers that form the mitotic spindle, cilia, and flagella (9+2 arrangement), and serve as tracks for intracellular transport via kinesin and dynein motors. Intermediate filaments are 10 nm in diameter and comprise diverse proteins (keratin, vimentin, lamin) providing mechanical strength and nuclear structural support.
Cell Division
Mitosis is nuclear division producing two genetically identical daughter cells (prophase, metaphase, anaphase, telophase) and is used for growth and tissue repair. Meiosis involves two rounds of division (meiosis I and II) producing four non-identical haploid gametes with genetic variation from crossing over and independent assortment. Cytokinesis is the division of the cytoplasm, accomplished by an actin-myosin contractile ring in animal cells or a cell plate in plant cells.
