Use evidence and models to explain the three parts of cell theory and the basic split between prokaryotic and eukaryotic cells (Ohio's Learning Standards for Science, Biology, B.C).

What this topic is asking

Ohio's Biology standards open the Cells strand (B.C) by treating the cell as the basic unit of life. Ohio's Biology EOC turns this into items about the three parts of cell theory, the history that built it, and what that history shows about the nature of science. Because the standards ask you to reason from evidence and to use models, the exam rarely just asks you to recite the theory; it asks you to explain the evidence behind it, often using a labelled diagram or a short passage as the stimulus.

The three parts of cell theory

Each part does real work on the exam. Part one is why a virus, which is not made of cells, sits in a gray area between living and nonliving. Part two is why biologists study the cell to understand life: the processes that keep an organism alive (getting energy, responding to the environment, reproducing) all happen at the cellular level, which is why Ohio's standards tie B.C to functions that "sustain life." Part three is why spontaneous generation, the old idea that living things could appear from mud or rotting meat, was rejected once experiments showed that cells only ever come from other cells.

How cell theory was built

Cell theory is the EOC's standard case study for the nature of science, because it was assembled piece by piece as the microscope got better.

• Robert Hooke (1665) looked at thin cork under an early microscope and named the little boxes "cells" (he was actually seeing the empty walls of dead plant cells).
• Antonie van Leeuwenhoek (1670s) built stronger lenses and was the first to see living single-celled organisms, which he called "animalcules," in pond water and other samples.
• Matthias Schleiden (1838) concluded that all plants are made of cells, and Theodor Schwann (1839) extended the conclusion to all animals, giving the first two parts of the theory.
• Rudolf Virchow (1855) added the third part, summarized as "all cells come from cells," which finished off spontaneous generation.

What "theory" means in science

In everyday speech a "theory" can mean a hunch. In science a theory is a broad explanation that is supported by a large body of evidence and has survived repeated testing. Cell theory, the theory of evolution, and the germ theory of disease are all theories in this strong sense. A hypothesis, by contrast, is a single testable prediction. EOC items often probe this distinction, asking you to identify what a theory is or to recognize that adding new evidence (a sharper image, a new experiment) refines rather than destroys a well-supported theory.

One more split: two kinds of cell

Even at this opening stage, the standards introduce the idea that not all cells are alike. All cells share four features: a cell membrane, cytoplasm, ribosomes, and DNA. They then divide into two types:

• Prokaryotic cells (bacteria and archaea) have no membrane-bound nucleus and no membrane-bound organelles; their DNA floats free in the cytoplasm.
• Eukaryotic cells (plants, animals, fungi, protists) have a true, membrane-bound nucleus that encloses their DNA, plus membrane-bound organelles.

This split is developed further in cell structure and organelles, but it belongs to cell theory because it shows that the "basic unit of life" comes in more than one design.

Try this

Q1. State the three parts of cell theory. [3]

• Cue. All living things are made of one or more cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells.

Q2. Explain why the development of cell theory depended on improvements in the microscope. [2]

• Cue. Cells are too small to see with the naked eye, so each improvement in the microscope let scientists observe cells (and structures inside them) that they could not see before, and each new observation refined the theory.