What is Life? Cell Theory
What Makes Something Alive?
Seven Properties of Life
All living organisms share seven core properties: cellular organisation (built of cells), metabolism (chemical reactions to obtain/transform energy), homeostasis (maintaining stable internal conditions), growth (increase in size or cell number), reproduction (producing offspring), heredity (passing traits to descendants), and response to stimuli (reacting to environmental signals). Non-living systems like fire or crystals may show some of these but never all seven.
Cell Theory
The Cell Theory, established by Schleiden (1838), Schwann (1839), and Virchow (1855), has three principles:
- All living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise only from pre-existing cells (omnis cellula e cellula).
Robert Hooke first observed and named 'cells' in cork tissue in 1665. Antonie van Leeuwenhoek observed living single-celled organisms ('animalcules') shortly after.
Why Cell Theory Matters
Cell theory unified biology as a science. Everything from diagnosing cancer to developing antibiotics depends on understanding cellular structure, division, and function.
Seven Properties of Life
| Property | Definition | Non-living counter-example |
|---|---|---|
| Cellular organisation | Composed of cells | Crystals grow but are not cellular |
| Metabolism | Transforms energy chemically | Fire releases energy but has no controlled chemistry |
| Homeostasis | Regulates internal state | A thermostat regulates temperature but is not alive |
| Reproduction | Produces heritable offspring | Crystals can precipitate but offspring are not heritable |
| Response to stimuli | Reacts to environment | Bimetallic strips bend with heat but have no biology |
| Growth | Increases in size/complexity | Stalactites grow by mineral deposition only |
| Heredity | Passes traits via DNA | No non-living system uses DNA-based inheritance |
Which statement is NOT a principle of Cell Theory?
Prokaryotic cells lack a membrane-bound nucleus — this is not part of Cell Theory. The three principles concern composition, the cell as unit, and cell origin.
Correct answer: All cells contain a membrane-bound nucleus
Who coined the term 'cell' after observing cork under a microscope?
Robert Hooke (1665) observed honeycomb-like compartments in cork and called them cells, from the Latin cellula (small room).
Correct answer: Robert Hooke
Name the three scientists most associated with establishing Cell Theory and their key contributions.
Together they formed the three-part Cell Theory that is foundational to modern biology.
Correct answer: Schleiden (all plants made of cells), Schwann (all animals made of cells), Virchow (cells only from pre-existing cells)