Questions: Cell Theory

Question 1

Which of the following is NOT one of the three core tenets of classical cell theory?

• A. All living organisms are composed of one or more cells
• B. The cell is the basic structural and functional unit of life
• C. All cells arise from pre-existing cells
• D. Viruses reproduce by dividing cells in half

Answer: (D)

The three tenets of classical cell theory are: (1) all organisms are made of cells, (2) the cell is the basic unit of life, and (3) all cells come from pre-existing cells. Viruses are not addressed by cell theory — they are not cells and do not reproduce by dividing. In fact, viruses are a major boundary case that cell theory does not cover, because they require a host cell to reproduce and lack independent cellular organization.

Question 2

Viruses should be considered cells under cell theory because they contain genetic material and are capable of reproduction.

• T. True
• F. False

Answer: False

Viruses are not cells. They lack cell membranes, cytoplasm, ribosomes, and the metabolic machinery to reproduce independently. They carry genetic material (DNA or RNA) but must hijack a host cell's machinery to replicate. Cell theory applies to living organisms composed of cells; most biologists classify viruses as acellular entities, placing them in a grey zone outside the definition of life that cell theory describes.

Question 3

Why was Rudolf Virchow's addition 'Omnis cellula e cellula' (all cells from cells) a scientifically important claim in the 1850s?

Model answer: Virchow's principle directly refuted the doctrine of spontaneous generation — the widespread belief that living organisms could arise from non-living matter. By asserting that every cell must come from a pre-existing cell, cell theory placed biogenesis on a firm observational footing, anticipating Pasteur's later experimental proof and establishing cellular continuity as a fundamental law of biology.

Before Virchow, spontaneous generation was a serious scientific position: maggots seemed to appear from meat, mold from bread, organisms from pond water. The cell-from-cell principle forced biologists to trace every new cell back to a parent cell, making biological origin a matter of continuity rather than magical emergence. This set the stage for germ theory, evolutionary thinking about common descent, and our modern understanding that all life is genealogically connected.