Cell theory — Science definition

Defined in 2 courses:

Where it is taught

In use

Those three claims together are cell theory: every living thing is made of one or more cells; the cell is the basic unit of both structure and function; and new cells come only from existing cells.

Cell Theory & Discovering Cells

' Stitched together, their work became cell theory, which makes three claims: all living things are composed of one or more cells; the cell is the basic unit of structure and function in life; and every cell arises only from a pre-existing cell that divides.

Cell Theory & the Discovery of Cells

Questions using cell theory

Problem 1

Does cell theory require every organism to have many cells?
Answer and explanation

No. A single cell can be an organism; many organisms contain multiple cells.

Problem 2

Illustrative record: a living cell divides into two cells; each later divides once. A student claims the resulting four cells must be four times the original mass. Use cell theory to explain the number change and evaluate the mass claim.
Answer and explanation

The sequence fits new cells arising from existing cells and gives four cells after the two division rounds. The mass claim is unsupported: cells can differ in size, and growth requires material uptake. Cell count alone does not establish four times the starting mass. Additional mass or size observations would be needed.

Count one to two to four cells.

Connect new cells with previous cells dividing.

Distinguish cell production from total mass.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 3

A newly observed unicellular organism has no tissue. Does that contradict cell theory?
Answer and explanation

No. Unicellular organisms consist of one cell that carries out life processes. Tissues are not required in every organism.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 4

Which fits cell theory?
  1. Every cell has chloroplasts
  2. All organisms contain many cells
  3. New cells arise from existing cells
  4. Every living cell has an enclosed nucleus
Answer and explanation

New cells arise from existing cells

Cell theory includes cells arising from existing cells; the other statements are not universal.

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