Cell: The Unit of Life — Core Principles
Core Principles
The cell is the fundamental unit of life, capable of independent existence and performing all vital functions. Robert Hooke first observed cells, and the Cell Theory, proposed by Schleiden, Schwann, and Virchow, states that all living organisms are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.
Cells are broadly classified into prokaryotic (lacking a true nucleus and membrane-bound organelles, e.g., bacteria) and eukaryotic (possessing a true nucleus and membrane-bound organelles, e.g., plants, animals).
Key eukaryotic organelles include the plasma membrane (selective barrier), cell wall (plant support), cytoplasm (site of reactions), nucleus (genetic control), endoplasmic reticulum (protein/lipid synthesis), ribosomes (protein synthesis), Golgi apparatus (packaging/sorting), lysosomes (waste disposal), vacuoles (storage/turgor), mitochondria (energy production), and chloroplasts (photosynthesis in plants).
These components work in concert to sustain life.
Important Differences
vs Eukaryotic Cell
| Aspect | This Topic | Eukaryotic Cell |
|---|---|---|
| Size | Generally smaller (0.1-5 µm) | Generally larger (10-100 µm) |
| Nucleus | Absent (genetic material in nucleoid region) | Present (true nucleus with nuclear envelope) |
| Membrane-bound organelles | Absent (e.g., mitochondria, ER, Golgi) | Present (e.g., mitochondria, ER, Golgi, lysosomes, chloroplasts) |
| Genetic material | Single, circular chromosome, no histones (usually) | Multiple, linear chromosomes, associated with histones |
| Ribosomes | 70S type | 80S type (in cytoplasm), 70S type (in mitochondria/chloroplasts) |
| Cell wall | Present (peptidoglycan in bacteria) | Present in plants (cellulose) and fungi (chitin); absent in animals |
| Cell division | Binary fission | Mitosis and Meiosis |
| Respiration | Occurs in cytoplasm and mesosomes | Occurs in mitochondria |