Cell: The Unit of Life — Class 11 Biology

This chapter establishes the cell as the fundamental structural and functional unit of life and compares prokaryotic and eukaryotic cells. The cell theory, formulated by Schleiden and Schwann and completed by Virchow (who showed cells arise from pre-existing cells), states that all organisms are made of cells and products of cells. Prokaryotic cells, found in bacteria, cyanobacteria, mycoplasma and PPLO, lack a membrane-bound nucleus and organelles; they have a cell envelope (glycocalyx, cell wall, plasma membrane), genomic DNA with plasmids, mesosomes, ribosomes (70S), inclusion bodies, and surface structures such as flagella, pili and fimbriae. Eukaryotic cells have a true nucleus and membrane-bound organelles. The chapter describes the fluid mosaic model of the plasma membrane, the cell wall, and the endomembrane system (endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles). It then covers the double-membraned mitochondria (power houses producing ATP) and plastids (chloroplasts, chromoplasts, leucoplasts), ribosomes (80S in eukaryotes), the cytoskeleton, cilia and flagella with their 9+2 axoneme, centrosome and centrioles, and the nucleus with its envelope, nucleolus, chromatin and chromosomes classified by centromere position.