Cell Theory
High-Yield Summary
- Cell theory: all living things are made of cells, the cell is the basic structural/functional unit of life, and all cells arise from preexisting cells.
- A 4th, modern tenet: cells carry genetic information as DNA, passed from parent to daughter cell during division.
- Hooke (1665) and Leeuwenhoek (1674) made the key early microscope observations; Schleiden (1838), Schwann (1839), and Virchow (1855) formalized the three classical tenets.
- Viruses are the classic exception — not considered cells, not fully living, because they lack independent metabolism and can't replicate without hijacking a host cell.
Key Terms
- Cell theory
- All living things are made of cells; the cell is life's basic structural/functional unit; all cells come from preexisting cells.
- Omnis cellula e cellula
- Virchow's phrase ("every cell from a cell") summarizing the third tenet — cells arise only from division of existing cells.
- Obligate intracellular parasite
- An entity (e.g. a virus) that can only reproduce inside a host cell, using the host's machinery.
History of Cell Theory
- 1Hooke (1665) — views cork under microscope, sees box-like compartments (dead cell walls), names them "cells."
- 2Leeuwenhoek (1674) — first to observe a living cell, showing cells are dynamic, not just static structures.
- 3Schleiden (1838) — proposes all plant tissue is composed of cells.
- 4Schwann (1839) — extends this to animal tissue; formally proposes the cell as life's basic structural/functional unit (tenets 1–2).
- 5Virchow (1855) — proposes all cells arise from division of preexisting cells (tenet 3).
- 620th-century molecular biology — confirms DNA as the hereditary molecule, adding the 4th (modern) tenet.
Do Viruses Violate Cell Theory?
- Viruses have genetic material (DNA or RNA) in a protein coat, but are NOT cells and most biologists don't count them as fully living.
- They fail tenet 2 (basic functional unit of life): no ribosomes, can't make own proteins, can't generate own ATP/metabolize, can't replicate independently.
- Hence "obligate intracellular parasite": obligate (no other way to reproduce), intracellular (must happen inside a host), parasite (depends on host to survive).
Common MCAT Trap
- 3 vs. 4 tenets: both are "correct." The 3 classical tenets are from the 1800s (Schleiden/Schwann/Virchow); the 4th (DNA/heredity) is a modern addition. Unqualified "tenets of cell theory" usually means the original 3.
- "Structural" vs. "functional" unit of life — cell theory means both; MCAT passages can hinge on this exact wording.
- Expect application over recall: apply the tenets to an unfamiliar scenario (e.g. a newly discovered particle), don't just list them.
Quick Recall
What are the three classical tenets of cell theory?
What is the 4th, modern tenet?
Why don't viruses count as living cells under cell theory?
Who proposed that all cells arise from preexisting cells, and in what year?