Cell: The Unit of Life
Class 11 Biology · Biology · Chapter 8
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Summary
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.
Cell theory and overview of the cellProkaryotic cell: envelope, ribosomes, inclusionsPlasma membrane and cell wallEndomembrane system: ER, Golgi, lysosomes, vacuolesMitochondria, plastids and ribosomesCytoskeleton, cilia, centrosome and nucleus
Key terms
Cell theory
The principle that all living organisms are composed of cells and that all cells arise from pre-existing cells.
Prokaryotic cell
A cell lacking a membrane-bound nucleus and membrane-bound organelles, as in bacteria.
Fluid mosaic model
The model of the plasma membrane (Singer and Nicolson) in which proteins move laterally within a fluid lipid bilayer.
Endomembrane system
The coordinated organelles ER, Golgi apparatus, lysosomes and vacuoles.
Mesosome
An infolding of the plasma membrane in prokaryotes aiding cell wall formation, DNA replication and respiration.
Centromere
The primary constriction of a chromosome bearing the kinetochores, used to classify chromosome types.
Extra questions & answers
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The cell theory was formulated by: