Cell Theory & Discovering Cells · Grade 7 Science

Cell Theory & Discovering Cells

Learning goals

• I can explain cell theory & discovering cells.
• I can use evidence, a model, and stated assumptions to support a biological explanation.

In 1665 Robert Hooke shaved a sliver of cork thin enough for light to pass through, slid it under a microscope he had built himself, and saw something nobody had described before: row after row of tiny empty boxes, packed like a honeycomb. They reminded him of the bare little rooms monks slept in, which were called cellulae, so he borrowed the word and called them cells. The name has stuck for three and a half centuries, even though the thing Hooke was looking at was dead — cork is bark, and what he saw were the stiff leftover cell walls, made of a tough material called cellulose, that plants build on the outside of their cells. Ten years later, in Delft, Antoni van Leeuwenhoek ground lenses so good that he could see things Hooke could not, and he pointed one at a drop of pond water. It was crowded. Tiny shapes darted, spun and reversed, and he called them animalcules — little animals. He had found living single cells. Neither man understood what he had hold of. It took another 170 years of grinding lenses, slicing tissue and arguing before anyone could say what a cell is: not a curiosity in bark or pond scum, but the unit that every living thing on Earth is built from.

The claim arrived in three pieces, from three people, over seventeen years. In 1838 the botanist Matthias Schleiden concluded that the plant tissues he had been sectioning — groups of similar cells working together as one material — were all built of cells. The next year Theodor Schwann argued the same for animal tissue, a bolder claim, because animal cells are softer and harder to see, and nobody had expected a nerve and a leaf to share a building block. Then in 1855 Rudolf Virchow supplied the piece that made the theory predictive rather than merely descriptive: every cell comes from a cell that already existed. He put it in Latin as omnis cellula e cellula. 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. An organism made of a single cell is unicellular, and one built from many is multicellular. What makes the theory powerful is how little it cares what an organism looks like. An oak, an amoeba in a ditch, a blue whale and the bacteria in your gut are built from the same kind of unit, and that single fact is why biology can be one subject instead of a thousand unrelated descriptions.

Hooke's empty boxes are also the source of the most common misunderstanding about cells, and it is worth naming it directly: a living cell is not an empty box. Every cell is wrapped in a thin cell membrane, the flexible boundary that decides what gets in and out, and inside it the cell is crowded with small working parts called organelles — so crowded that the contents behave more like a gel than like water. The largest organelle in most cells is the nucleus, the compartment that holds the cell's DNA, the long molecule carrying the instructions for building and running the cell. Instruments have limits you must know to read an image honestly. A good light microscope cannot separate two points closer than about 200 nanometres — a nanometre is a millionth of a millimetre — which is enough to show a nucleus and too coarse to show a ribosome, the tiny organelle that builds proteins, at roughly 20 nanometres. The inside of a cell came into view only with the electron microscopes of the 1930s, which use beams of electrons instead of light and resolve detail about a thousand times finer. Scale matters too. A human cheek cell is roughly 60 micrometres across — a micrometre is a thousandth of a millimetre — while a typical bacterium is 1 to 2 micrometres, so thirty to sixty bacteria would fit end to end across one of your own cells.

Build your understanding

Where this fits

Chapter 1 established how to measure carefully and how to tell an observation from an inference. Now we turn those tools on something you cannot see without them. The question driving this chapter is what living things are actually built from, and this unit answers the first part of it: what a cell is, how anyone found out, and why three separate claims are needed rather than one.

Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.

Part 1 · Why cell theory needs three claims

Each of the three claims does a different job, which is why dropping one breaks the theory. The first claim — that every living thing is made of one or more cells — is what makes the theory universal. It is the claim that lets a biologist study yeast in order to learn something about you. It is also the claim that is tested every time someone finds an unfamiliar organism: the question asked is whether it is made of cells, not whether it looks familiar.

The second claim, that the cell is the basic unit of structure and function, says where to look for an explanation. If you want to know how a muscle contracts, how a leaf captures light, or how a kidney concentrates urine, the answer is found in what cells are doing and how their activity is combined into tissues and organs. Structure and function are named together on purpose: a cell's shape and contents are the reason it can do its particular job.

The third claim, that new cells come only from existing cells, is the one that predicts. It rules out living cells assembling spontaneously from non-living material under ordinary conditions, and it means that every cell in your body has an unbroken line of division behind it. Note the scope carefully: this is a claim about living systems and reproduction as we observe them, not a complete account of how the first life on Earth arose, which is a separate and much harder question.

Pause and explain

Does cell theory require every organism to have many cells?

Check your reasoning after answering

No. The first claim says one or more cells, so a single cell can be a complete organism — a Paramecium feeds, responds, and reproduces on its own. Many organisms are instead built from large numbers of cells that divide the work between them, and both arrangements satisfy cell theory.

Part 2 · What Hooke actually saw, and what an instrument can show

Cork, a leaf cross-section and a drop of pond water give three very different observations, and reading each one honestly means knowing what the instrument could resolve. Hooke's cork showed boundaries with nothing inside, because the cells were dead: the cellulose cell wall survived and the living contents did not. Plant and cork cells have both a wall and a membrane; animal cells have only the membrane, which is part of why animal tissue was harder to see and why Schwann's claim came second.

Every microscope has a resolution limit — the smallest separation at which two points still look like two points rather than one blur. For a light microscope that limit is around 200 nanometres, set by the wavelength of visible light itself, so no amount of polishing gets past it. That is small enough to show a nucleus, a chloroplast (the green organelle where a plant cell captures light) or a large vacuole (a fluid-filled storage sac), and too large to show a ribosome at roughly 20 nanometres.

So a labelled drawing is a record of selected features that one instrument could resolve, interpreted by the person drawing it. It is evidence, and it is partial. When an image seems to show an absence — no nucleus, nothing inside, no structure — the first question is whether the instrument could have shown that thing had it been present.

Pause and explain

Why is cork a limited model of a living cell?

Check your reasoning after answering

Cork is dead bark, so only the rigid cell walls remain and the living contents are missing. It can show cell shape, packing and the existence of boundaries, but it cannot show the nucleus, the organelles or the crowded interior of a living cell, and a light microscope could not have resolved the smallest of those structures anyway.

Part 3 · Division, growth, and why four cells are not four times the mass

Cell division is the process that turns one cell into two, and it is how Virchow's third claim shows up in practice. A multicellular organism grows in two ways at once: cells divide, increasing the number, and cells take in material and enlarge, increasing the size of each. Repair works the same way, replacing damaged cells with new ones produced by division. Both processes need raw material and energy from outside the organism.

That second point is where a common arithmetic slip happens. Suppose one cell divides into two, and each of those divides once more, giving four cells. The count has gone one to two to four, which is a fourfold increase in number. It does not follow that there is four times as much material. Immediately after a division, each daughter cell is roughly half the volume of the parent, so four small cells can hold about the same total matter as the one cell you started with. The mass only increases as those cells absorb nutrients and build new components.

To reason about mass you therefore need more than a count: you need the size of the cells and what they are made of. This is a habit worth generalising. A number that is easy to measure — how many — is often not the number the question is about, and saying which quantity you actually have is half of a good scientific answer.

Pause and explain

What other information is needed to infer mass from cell count?

Check your reasoning after answering

You need the volume or size of the cells and their composition or density. Four cells produced by two rounds of division start out at about a quarter of the parent's volume each, so the count alone cannot tell you the total mass; mass changes only as the cells take in material and grow.

Part 4 · Organisms that do not look like the diagram

The real test of cell theory is an organism that breaks the familiar picture. A Paramecium is a single cell, perhaps 200 micrometres long, and it swims with rows of beating hairs called cilia, sweeps food into a groove, digests it, pumps out excess water through a contractile vacuole, responds to obstacles and reproduces. It has no organs, because organs are an arrangement some multicellular organisms use; one cell is carrying out every interacting life process by itself.

A bacterium pushes harder. It is typically 1 to 2 micrometres long, has no membrane-enclosed nucleus, and keeps its DNA loose in the body of the cell rather than in a separate compartment. Judged against a labelled plant-cell diagram it looks like it is missing the most important part. Judged against cell theory it fits without difficulty: it is a cell, bounded by a membrane, carrying out life processes, and producing new cells by dividing.

The difference between those two judgements is the difference between recognising a picture and applying a theory. A multicellular organism divides labour among specialised cells and tissues; a unicellular organism does not; neither arrangement is more alive than the other. When something unfamiliar turns up, ask what evidence there is of cells and of life processes, and leave the diagram out of it.

Pause and explain

Why can a bacterium be an organism without organs?

Check your reasoning after answering

Because a single cell can carry out all the interacting processes life requires — taking in material, releasing energy, responding, and reproducing by division. Organs are a way some multicellular organisms organise specialised cells; they are not a requirement for being alive, and a bacterium's lack of a nucleus does not place it outside cell theory.

Apply the whole unit · Explain a sequence of observations

Illustrative record, not a measured experiment: a single living cell divides into two, and a short time later each of those two divides once. A student looks at the four cells and claims that there must now be four times as much living material as there was at the start.

Your task

Use cell theory to explain the change in cell number, then evaluate the claim about mass and say what measurement would settle it.

What your response should show

• 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.

Response guidance — after your attempt

1. Count the divisions: one cell becomes two, and two become four, so the number has doubled twice and 1 × 2 × 2 = 4 cells.

2. Attribute the increase to Virchow's claim — each new cell came from an existing cell dividing, so no cell appeared from nothing.

3. Separate number from mass: immediately after dividing, each daughter cell is about half its parent's volume, so four cells can together hold roughly the material of the original one cell.

4. Name the measurement that would settle it: the volume or mass of the cells at the start and at the end, since mass rises only as cells absorb nutrients and build new components.

Cell theory explains the number: 1 cell → 2 cells → 4 cells, each new cell produced by the division of a cell that already existed. It does not support the mass claim. Each division splits the parent's contents between two daughters, so each of the four cells begins at roughly a quarter of the original volume and the total living material is close to what it was. Mass increases only when the cells take in material from outside and build new components, which takes time and a supply of nutrients. To decide the question you would measure total volume or mass at the start and at the end, not the number of cells — and you would state the assumption that the cells were growing under the same conditions throughout.

Scientific references for this unit

• Topic reference or educational context; core explanation is supplied above

Parent / teacher teaching notes

Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.

Worked examples

Avoid these mistakes

• Mistake: a living cell is an empty box with walls. Hooke's cork was dead, so only walls remained; a living cell is crowded enough to behave like a gel.
• Mistake: an organism needs organs to be alive. A Paramecium carries out every life process in one cell, and organs are an arrangement found in some multicellular organisms.
• Mistake: four cells must hold four times the material. Each division halves the parent's contents between two daughters, so mass rises only as cells take in material and grow.
• Mistake: if a microscope shows no structure, there is none. A light microscope stops at about 200 nanometres, so absence in an image is only evidence of absence when the instrument could have resolved the feature.

Words to know

cell

The basic structural and functional unit of life, bounded by a membrane; Hooke named it in 1665 after the small rooms called cellulae.

cell theory

Three connected claims: every living thing is made of one or more cells, the cell is the basic unit of structure and function, and new cells come only from existing cells.

unicellular

Made of a single cell that carries out every life process itself, as a Paramecium or a bacterium does.

multicellular

Made of many cells that divide the work between them, organised into tissues and in some organisms into organs.

cell division

The process by which one existing cell produces new cells; it increases cell number, while growth in mass requires material taken in from outside.

Extend your scientific explanation

Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.

In the real world

Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.

Exit check

Why can a bacterium be an organism without organs?