Cells & cell theory

What you will learn

the three principles of cell theory and the history behind them,
the main organelles of plant and animal cells and what each does,
how prokaryotic cells differ from eukaryotic cells,
how microscopes reveal structures too small to see with the naked eye,
how to calculate magnification from image and object sizes.

Worked example 0 Real-world example: why a sunflower needs chloroplasts

A sunflower plant converts sunlight into sugars it can use for growth. Explain which part of its cells does this and why an animal cell could not do the same.

Sunflower leaf cells contain chloroplasts — green organelles packed with chlorophyll that captures light energy.
Chlorophyll inside chloroplasts runs photosynthesis, combining carbon dioxide and water to make glucose (sugar).
Animal cells do not have chloroplasts, so they cannot make their own food from sunlight — they must eat other organisms instead.

Key idea: a cell’s job is set by the organelles it contains. No chloroplasts, no photosynthesis.

1. Cell theory

Cell theory is one of the big unifying ideas in biology. It was developed over about 200 years of microscope work by scientists including Robert Hooke (who named “cells” in 1665), Anton van Leeuwenhoek, Schleiden, Schwann, and Virchow.

Cell theory has three main statements:

All living things are made of one or more cells.
The cell is the basic unit of structure and function in life.
All cells come from pre-existing cells (they do not appear spontaneously).

2. Parts of a cell (organelles)

Organelles are tiny specialised structures inside a cell, each with a specific job. Think of a cell as a tiny factory — each organelle is a different department.

Generalised animal cell (left) and plant cell (right). The plant cell has a cell wall, chloroplasts and a large central vacuole; the animal cell does not.

Organelles found in both plant and animal cells:

Cell membrane — thin boundary that controls what enters and leaves.
Cytoplasm — jelly-like fluid where organelles sit and reactions happen.
Nucleus — contains DNA; directs everything the cell does.
Mitochondria — release energy from glucose by respiration (“powerhouse”).
Ribosomes — tiny dots that build proteins.

Only in plant cells:

Cell wall — rigid outer layer of cellulose that gives shape and support.
Chloroplasts — carry out photosynthesis.
Large central vacuole — filled with cell sap; keeps the cell firm (turgid).

Worked example 1 Matching structure to function

You are told a cell has a cell wall, chloroplasts and a large central vacuole. Is it a plant cell or animal cell? Explain.

Animal cells have no cell wall, no chloroplasts, and only small vacuoles.
Plant cells have all three of these features.
So the cell is a plant cell.

Key idea: three “extra” structures (wall, chloroplasts, big vacuole) are the quick test for plant vs animal.

3. Prokaryotic vs eukaryotic cells

All cells fall into one of two broad groups.

Feature	Prokaryotic	Eukaryotic
Examples	Bacteria, archaea	Plants, animals, fungi, protists
Nucleus	No (DNA floats free)	Yes (DNA inside a nucleus)
Membrane-bound organelles	No	Yes
Size	Small ( $\sim 1$ – $10$ $\mu$ m)	Larger ( $\sim 10$ – $100$ $\mu$ m)
Cell wall	Usually yes	Plants and fungi yes, animals no

Worked example 2 Classifying a mystery cell

An organism is made of a single cell roughly $2$ $\mu$ m across. It has DNA but no nucleus, no mitochondria, and a cell wall. Prokaryote or eukaryote?

No nucleus and no mitochondria rules out eukaryote.
Small size and free DNA match prokaryote.
This is a prokaryotic cell (most likely a bacterium).

Key idea: the presence of a nucleus is the clearest marker of a eukaryote.

4. Microscopes and magnification

Most cells are far too small to see with the naked eye ( $< 0.1$ mm). Microscopes magnify images so we can study them. Magnification is how many times bigger the image is than the real object.

Magnification

M = \dfrac{\text{image size}}{\text{object size}}.

Both measurements must use the same units.

Worked example 3 Calculating magnification

Under a microscope, a cell that is really $25$ $\mu$ m wide appears $5$ mm across on a photograph. Find the magnification.

Convert to the same units: $5$ mm $= 5000$ $\mu$ m.
$M = \dfrac{5000}{25} = 200$ .
The image is $200$ times bigger than the real cell (written as $\times 200$ ).

Worked example 4 Finding the real size

An image shows a cell $12$ mm long at $\times 400$ magnification. Find the actual cell length in micrometres.

$\text{object} = \dfrac{\text{image}}{M} = \dfrac{12 \text{ mm}}{400} = 0.03$ mm.
Convert: $0.03$ mm $= 30$ $\mu$ m.

5. Unicellular vs multicellular life

A unicellular organism is one single cell that carries out every life process itself (e.g. Amoeba, bacteria, yeast).
A multicellular organism is built of many cells that specialise — nerve cells send signals, muscle cells contract, red blood cells carry oxygen.

Specialisation lets large organisms like humans do things no single cell could manage alone.

Practice: Year 8

Fluency

Cell theory & parts

State the three main points of cell theory.
Name the organelle that (a) controls the cell, (b) releases energy, (c) carries out photosynthesis.
List three structures found in plant cells but not animal cells.
What is the function of the cell membrane?
What is the cytoplasm?
Give one example of a unicellular organism and one of a multicellular organism.

Fluency

Prokaryote vs eukaryote

Do prokaryotic cells have a nucleus?
Which group is generally larger — prokaryotes or eukaryotes?
Give two examples of prokaryotic organisms.
Name three kingdoms that are made of eukaryotic cells.
Why is the absence of a nucleus not the same as having no DNA?

Fluency

Magnification

An image is $8$ mm long; the real object is $40$ $\mu$ m. Find the magnification.
A cell is $60$ $\mu$ m long. At $\times 500$ , how long is the image (in mm)?
A photograph shows a cell $24$ mm wide at $\times 300$ . Find the real width (in $\mu$ m).
Convert: $0.2$ mm to $\mu$ m; $4500$ $\mu$ m to mm.

Reasoning

Explain and analyse

Explain why red blood cells, which have lost their nucleus, cannot divide.
Why is it helpful that multicellular organisms have specialised cells?
A leaf cell is placed in the dark for three days. Predict what happens to the chloroplasts and why.
A student says “bacteria don’t count as alive because they have no nucleus.” Use cell theory to explain why they are wrong.

Problem solving

Applied contexts

A hospital lab photographs a cell at $\times 1000$ . On screen it is $30$ mm wide. How big is it in reality ( $\mu$ m)?
Yeast cells are roughly $5$ $\mu$ m across. How many could you line up across a $1$ mm gap?
A cheek cell (animal) is placed next to an onion cell (plant) under a microscope. List two differences a student should notice.
Explain, using cell theory, why hospitals sterilise surgical instruments.

Challenge

Reasoning

Harder reasoning

Mitochondria and chloroplasts both have their own DNA. Biologists believe they were once free-living bacteria absorbed by a larger cell (the endosymbiotic theory). Give two pieces of evidence from what you know about prokaryotes that support this theory.
A biologist observes a cell with a cell wall but no chloroplasts. List two possible identities for this cell and explain how you would decide between them.
Estimate how many cells are in a $1$ cm $^3$ cube of muscle tissue, assuming each cell is roughly a cube $20$ $\mu$ m on each side.
Explain why unicellular organisms are generally smaller than individual cells inside a multicellular organism.