The Cell Theory and Its Evidence

In 1665, Robert Hooke cut an exceedingly thin slice of cork, put it under his microscope and saw rows of tiny walled boxes. He called them cells. He was looking at the walls left after the cork’s cells had died. About 175 years passed before scientists argued that plants and animals alike are built of cells. Why did it take so long, and what did better tools have to do with it?

Explore: zoom in and see what appears

The simulation gives you a microscope with a magnification slider and a switch for the type of tool: a hand lens, a light microscope or an electron microscope. A set of objects sits on the slide: a frog egg, a human cheek cell, a bacterium, a virus and a ribosome. An object shows up clearly only when the tool can separate details smaller than the object.

Hand lens Light microscope Electron microscope

Magnification: 10 times

Smallest detail this tool separates

about 0.1 mm

Smallest detail the eye separates

about 0.1 mm

Reset

In this sketch: an object is seen only when (1) the tool separates details finer than the object and (2) the enlarged image is at least 0.1 mm, the eye’s limit (this rule is the sketch’s own; the lesson does not state it). Sizes use the small end of the lesson’s ranges (cheek cell 50 µm, bacterium 1 µm, ribosome 0.02 µm; frog egg is the African clawed frog, 1 mm). The lesson gives no limit for a hand lens, so it gets the eye’s 0.1 mm here. The lesson gives no virus size, so the virus is placed at an assumed 0.1 µm, between the two microscope limits. Electron limit: 0.002 µm (the coarse end of 1 to 2 nm). The limits are typical values; real ones vary with the instrument and the sample. The electron microscope needs a very thin, specially prepared sample. The 1 to 10,000 times range is part of the sketch; real hand lenses and light microscopes do not reach the top of it. The shapes are labelled sketches, not to scale.

Making the image bigger does not change what a tool can resolve: a tool shows a structure only if its resolution is finer than the structure.

Try this.

1. Predict which objects you can see with the hand lens. Write your guess down, then test it.
2. Switch to the light microscope and turn the magnification all the way up. Predict whether the ribosome will appear, then check.
3. Find the lowest magnification at which the cheek cell first looks like a cell. Does turning it higher show the virus?
4. Switch to the electron microscope. Which objects appear now that did not appear before?

Explain: the six ideas of the cell theory

A cell is the smallest unit that can carry out all the processes of life. The cell theory is the scientific explanation of how cells relate to living things. Virginia lists six ideas:

1. All living organisms are composed of cells.
2. Cells are the basic unit of life.
3. All cells arise from pre-existing cells.
4. Metabolism occurs within cells.
5. Hereditary information (DNA) is passed from cell to cell.
6. All cells have the same basic composition.

Metabolism is all the chemical reactions that happen in a living thing, such as breaking down sugar to release energy for the cell to use. Enzymes speed these reactions inside cells (that lesson). DNA is the molecule that carries hereditary information. When a cell divides, each new cell gets a copy (that lesson).

“Same basic composition” means that cells are built from the same main kinds of molecules: proteins, lipids, carbohydrates and nucleic acids (that lesson). Typical cells have a membrane around them, a watery inside and DNA. The details still differ. Archaea, for example, build their membranes from lipids linked together a bit differently from those of bacteria, plants and animals.

The same basic plan does not mean the same size or shape. Many bacteria are about 1 to 2 micrometers long. A human cheek cell is about 50 to 60 micrometers across. The yolk of an unfertilized chicken egg is one cell. A micrometer (µm) is one thousandth of a millimeter.

Viruses are not made of cells (that lesson).

Explain: seeing smaller things

Magnification is how many times larger an image looks than the real object. Resolution is the smallest distance between two points at which you can still see them as two separate points. Magnification makes an image bigger. Resolution decides how much detail it holds.

Think of stretching a blurry phone photo until it fills the screen: it gets bigger but stays blurry. The comparison breaks down in one way. The photo blurs because of the camera’s pixels, while a microscope blurs because light spreads out as it passes through the lens.

Each tool has its own limit:

Tool	Smallest detail it can separate (about)	Can it show…
Unaided eye	about 0.1 mm (100 µm)	an African clawed frog egg, about 1 mm across
Ordinary light microscope	0.2 µm	a cheek cell or a bacterium
Electron microscope, biological samples	0.001 to 0.002 µm (1 to 2 nm)	a virus or a ribosome

The light microscope’s limit and the electron microscope’s range are typical values for biological samples, and real values vary with the instrument and the sample. An electron microscope forms its image with a beam of electrons instead of light. Electrons have much shorter wavelengths than visible light, so they can separate much finer details. Ribosomes, the cell’s protein builders, are about 0.02 to 0.03 µm across. That is far below the light microscope’s limit, which is why they were seen clearly only with electron microscopes.

Resolution is not the only problem. Many cells are almost clear, so their inside parts are hard to see. A stain is a dye that sticks to some cell parts more than others. Stains added contrast, which let scientists see parts inside clear animal cells, such as the nucleus.

Explain: a theory that grew and changed

Scientists use three words that people often mix up:

Word	What it is	Example
Hypothesis	A testable statement about nature	”If cork is made of cells, thin slices of other plants will show cells too.”
Theory	A well-tested explanation of some part of nature, supported by many lines of evidence	The cell theory
Law	A description of how something behaves under stated conditions	The law of conservation of mass

A hypothesis is a testable statement. A theory is a well-tested explanation that ties many observations together. A law describes a pattern under stated conditions without explaining why. These are different jobs, so a theory never turns into a law, however much evidence it gets.

The cell theory shows how a theory changes. In 1838 Matthias Schleiden described plant tissues as made of cells. In 1839 Theodor Schwann compared plant and animal tissues, such as tadpole cartilage, and found the same cellular plan. Schwann’s model, though, allowed new cells to form inside a structureless fluid, a little like crystals growing in a solution.

That part was wrong. In 1852 Robert Remak published evidence that new cells form when existing cells divide. In 1855 Rudolf Virchow spread the idea widely in a short article: every cell comes from a cell. The theory kept its core and dropped the wrong piece. The ideas about metabolism and DNA were added later.

Elaborate: tools and discoveries over time

This table pairs steps in the cell theory with what the tools of the time could do.

Year	Who or what	Magnification (times)	Smallest detail separated (µm)	What it showed
1665	Hooke, cork slice	not reported	not reported	Small chambers he called cells
Before 1723	Leeuwenhoek microscope V7017	118	not reported	A single lens, ground and polished
Before 1723	Leeuwenhoek microscope UM-1	266	1.35	A single bead-shaped lens
1838	Schleiden, plant tissues	not reported	not reported	Plant tissues are made of cells
1839	Schwann, plant and animal tissues	not reported	not reported	Animals share the cellular plan
1852	Remak	not reported	not reported	New cells form by division
1855	Virchow	not reported	not reported	Every cell comes from a cell
1933	Ruska’s electron microscope	12,000	not reported	Electrons can form a magnified image
1937	Electron image of a diatom shell	3,500	0.13	The caption gives 130 nm, finer than the light limit

Sources: the two Leeuwenhoek microscopes survive in museums in Leiden and Utrecht and were measured in a 2021 study (Cocquyt and colleagues, Science Advances). The 1933 magnification and the 1937 diatom image come from Ernst Ruska’s 1986 Nobel Lecture. Dates for Schleiden, Schwann, Remak and Virchow come from the OpenStax Microbiology textbook (2016). It is a compilation, not one measured data set.

1. How many times greater was the magnification of Ruska’s 1933 microscope than that of UM-1? Round to the nearest whole number.
2. UM-1 separated details 1.35 µm apart. An ordinary light microscope today separates about 0.2 µm. About how many times finer is the modern limit?
3. The rows from 1838 to 1855 have no numbers for the tools. What kind of work moved the theory forward in those years?

Design problem

Hooke cut his cork slice exceedingly thin. Does the thickness of a slice change how many separate cells you can count in one view of a light microscope? Plan a fair test using an onion or a potato, a light microscope and slides.

One way to do it

• Hypothesis: If the slice is thinner, then more separate cells will be countable in one view, because fewer layers of cells overlap.
• Independent variable: slice thickness, for example very thin, medium and thick.
• Dependent variable: the number of cells with walls you can trace all the way around in one field of view.
• Constants: the same onion or potato, microscope, magnification, light setting and stain.
• Comparison: the thickest slice serves as the baseline.
• Repeated trials: make five slides at each thickness, count three views on each, and calculate the mean for each thickness.
• Safety: cutting thin slices needs a sharp blade. Use a guarded blade or a peeler, cut away from your hand, and work only with an adult’s permission.

Evaluate: check your thinking

Hooke saw the walls of dead cork cells, one material from one plant. Building the cell theory took many comparisons of plants and animals, a corrected idea about how new cells form, and tools such as stains, which added contrast, and electron microscopes, which could resolve smaller parts. A theory built that way is a well-tested explanation, not a guess.

Science practices you used: making a prediction before testing, reading a data table and comparing values, telling a hypothesis from a theory and a law, using a model of resolution and naming its limits, and planning a fair test with repeated trials and safety steps.