Eukaryotic and Prokaryotic Cells

This lesson covers the two fundamental types of cell — eukaryotic and prokaryotic — as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the differences in structure and size between these cell types, and be able to identify key subcellular structures and their functions.

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Two Types of Cell

All living organisms are made of cells. Cells can be divided into two broad categories based on their internal structure:

1. Eukaryotic cells — complex cells with a nucleus enclosed in a nuclear membrane. Examples include animal cells, plant cells and fungal cells.
2. Prokaryotic cells — simpler, smaller cells that do not have a true nucleus. Their DNA floats freely in the cytoplasm. Examples include bacteria.

Exam Tip: The prefix eu- means "true" and karyon means "nucleus". So eukaryotic = "true nucleus". Pro- means "before", so prokaryotic = "before nucleus" — i.e. these cells evolved before the nucleus existed.

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Eukaryotic Cells — Structure

Eukaryotic cells are relatively large and complex. They contain membrane-bound organelles (subcellular structures enclosed by membranes).

Common Subcellular Structures

Subcellular Structure	Function
Nucleus	Contains genetic material (DNA) organised into chromosomes. Controls the activities of the cell.
Cytoplasm	A jelly-like substance where most chemical reactions take place.
Cell membrane	Controls what enters and leaves the cell. It is selectively permeable.
Mitochondria	The site of aerobic respiration, where energy is transferred from glucose.
Ribosomes	The site of protein synthesis — where proteins are assembled from amino acids.

Additional Structures in Plant Cells

Subcellular Structure	Function
Cell wall	Made of cellulose. Provides structural support and prevents the cell from bursting.
Permanent vacuole	A large, fluid-filled sac containing cell sap (a weak solution of sugars and salts). Keeps the cell turgid.
Chloroplasts	Contain chlorophyll, the green pigment that absorbs light energy for photosynthesis.

graph TD
    subgraph "Eukaryotic Cell Structures"
        A["Animal Cell"] --- B["Nucleus"]
        A --- C["Cytoplasm"]
        A --- D["Cell membrane"]
        A --- E["Mitochondria"]
        A --- F["Ribosomes"]
        G["Plant Cell"] --- B
        G --- C
        G --- D
        G --- E
        G --- F
        G --- H["Cell wall"]
        G --- I["Permanent vacuole"]
        G --- J["Chloroplasts"]
    end

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Prokaryotic Cells — Structure

Prokaryotic cells are much smaller and simpler than eukaryotic cells. They do not have a nucleus or membrane-bound organelles.

Key Features of Prokaryotic Cells (Bacteria)

Subcellular Structure	Description
Cell membrane	Surrounds the cytoplasm and controls what enters and leaves.
Cell wall	Provides structure and protection (not made of cellulose — bacterial cell walls are made of peptidoglycan).
Cytoplasm	Where chemical reactions occur.
Ribosomes	Smaller than eukaryotic ribosomes; used for protein synthesis.
Chromosomal DNA	A single, circular loop of DNA that floats freely in the cytoplasm (not enclosed in a nucleus).
Plasmid DNA	Small, extra circles of DNA that can carry additional genes (e.g. antibiotic resistance genes).
Flagellum (plural: flagella)	A whip-like tail used for movement (not present in all bacteria).

graph LR
    subgraph "Prokaryotic Cell — Bacterium"
        A["Circular chromosomal DNA<br/>(no nucleus)"]
        B["Plasmid DNA"]
        C["Cell membrane"]
        D["Cell wall<br/>(peptidoglycan)"]
        E["Cytoplasm"]
        F["Ribosomes<br/>(smaller)"]
        G["Flagellum<br/>(some bacteria)"]
    end

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Comparing Eukaryotic and Prokaryotic Cells

Feature	Eukaryotic Cells	Prokaryotic Cells
Size	Typically 10–100 μm	Typically 0.1–5 μm
Nucleus	Yes — DNA enclosed in a nuclear membrane	No — DNA is free in the cytoplasm
DNA structure	Linear chromosomes	Single circular chromosome + plasmids
Membrane-bound organelles	Yes (mitochondria, chloroplasts, etc.)	No
Ribosomes	Larger (80S)	Smaller (70S)
Cell wall	Plants: cellulose. Animals: none	Peptidoglycan
Examples	Animal, plant and fungal cells	Bacteria

Exam Tip: Remember that both eukaryotic and prokaryotic cells have a cell membrane, cytoplasm, ribosomes and DNA. The key differences are: (1) whether the DNA is enclosed in a nucleus, and (2) whether there are membrane-bound organelles.

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Size of Cells

Most cells are too small to be seen with the naked eye. Sizes are measured in micrometres (μm).

$1 \text{ mm} = 1000 \text{ μm}$1 mm=1000 μm

$1 \text{ μm} = 1000 \text{ nm}$1 μm=1000 nm

Item	Approximate Size
Most animal cells	10–30 μm
Most plant cells	10–100 μm
Bacteria	0.2–5 μm
Virus	20–300 nm (0.02–0.3 μm)

Exam Tip: Viruses are not cells. They are much smaller than bacteria and are not classified as prokaryotic or eukaryotic. They are covered in Topic 3 (Infection and response) of the AQA specification.

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Worked Example

Question: A student views a bacterium under an electron microscope and measures its length as 2 μm. Express this length in mm and in nm.

Solution:

To convert μm to mm, divide by 1000:

$2 \text{ μm} = \frac{2}{1000} \text{ mm} = 0.002 \text{ mm}$2 μm=10002​ mm=0.002 mm

To convert μm to nm, multiply by 1000:

$2 \text{ μm} = 2 \times 1000 \text{ nm} = 2000 \text{ nm}$2 μm=2×1000 nm=2000 nm

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Common Mistakes

1. Saying prokaryotic cells have no DNA — they do have DNA; it is just not enclosed in a nucleus. It exists as a single circular chromosome in the cytoplasm, along with smaller plasmids.
2. Confusing cell wall materials — plant cell walls are made of cellulose; bacterial cell walls are made of peptidoglycan. These are different substances.
3. Forgetting plasmids — plasmids are unique to prokaryotic cells and are an important feature that distinguishes them from eukaryotic cells.
4. Mixing up μm and nm — always check the units you are working with and remember: 1 μm = 1000 nm.
5. Stating that only plant cells have a cell wall — bacteria also have a cell wall, but it is made of a different material.

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Summary

• Eukaryotic cells have a nucleus containing DNA, and membrane-bound organelles such as mitochondria. Animal and plant cells are eukaryotic.
• Prokaryotic cells (bacteria) are smaller and simpler. They have no nucleus — their DNA is a single circular loop in the cytoplasm, plus small plasmid rings.
• Both cell types have a cell membrane, cytoplasm, ribosomes and DNA.
• Cell sizes are measured in micrometres (μm). Prokaryotic cells are typically 10–100 times smaller than eukaryotic cells.
• This topic is part of AQA GCSE Combined Science Trilogy (8464), Biology Paper 1, Section 4.1 — Cell Biology.

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Extended Practice: Worked Examples and Comparisons

Worked Example 1 — Comparing sizes using standard form

A typical animal cell measures 20 μm across; a typical bacterial cell measures 2 μm across. How many times larger is the animal cell?

• Animal cell: 20 μm = 20 × 10⁻⁶ m = 2 × 10⁻⁵ m
• Bacterial cell: 2 μm = 2 × 10⁻⁶ m
• Ratio: (2 × 10⁻⁵) ÷ (2 × 10⁻⁶) = 10 times larger in linear dimension.

Because cells are roughly three-dimensional, a 10× linear difference becomes a 1,000× volume difference (10³). This explains why a single eukaryote can house far more organelles than a prokaryote.

Worked Example 2 — Counting bacteria

If 1 mL of pond water contains 5,000 bacteria and each bacterium is on average 2 μm long, what is the total combined length if laid end-to-end?

• 5,000 × 2 μm = 10,000 μm
• Convert to mm: 10,000 ÷ 1,000 = 10 mm = 1 cm

This kind of calculation tests whether you can move between micrometres (μm), millimetres (mm) and metres (m) confidently — a skill examiners reuse across the microscopy lesson too.

Comparison Table — At-a-glance

Feature	Eukaryotic Cell	Prokaryotic Cell
Typical size	10–100 μm	0.2–10 μm
True nucleus	Yes	No (free DNA loop)
Membrane-bound organelles	Yes	No
Cell wall	Plant/fungi only (cellulose/chitin)	Yes (peptidoglycan)
Ribosomes	Larger (80S)	Smaller (70S)
Plasmids	Rare	Common
Examples	Animals, plants, fungi, protists	Bacteria, archaea

graph TD
    A["All cells"] --> B["Eukaryotic<br/>(true nucleus)"]
    A --> C["Prokaryotic<br/>(no nucleus)"]
    B --> D["Animal cells"]
    B --> E["Plant cells"]
    B --> F["Fungal cells"]
    C --> G["Bacteria"]

Exam Tip: When asked to "compare", you must mention both cell types in the same sentence using a linking word such as whereas or but. For example: "Eukaryotic cells contain a nucleus, whereas prokaryotic cells have a single loop of DNA in the cytoplasm."

Common mistake: Writing that prokaryotes "have no DNA". They absolutely do — it just is not enclosed in a nuclear membrane. Always say "no true nucleus" rather than "no DNA".

Quick-fire facts to memorise

• 1 mm = 1,000 μm
• 1 μm = 1,000 nm
• Animal cell ≈ 20 μm; bacterial cell ≈ 2 μm; virus ≈ 0.1 μm (not a cell — for context only)
• Light microscope can resolve down to about 200 nm; electron microscope down to about 0.2 nm.

Worked Example 3 — Magnification comparison

A student views a bacterium using a light microscope at ×1,000 total magnification. It appears 2 mm long.

• Image size = 2 mm = 2,000 μm
• Real size = image ÷ magnification = 2,000 ÷ 1,000 = 2 μm — exactly the typical bacterial size expected. This confirms that a light microscope can just about resolve a bacterium, though internal structure is not visible. An electron microscope would be needed to see the ribosomes and plasmid DNA within.

Why compartmentalisation matters

A defining advantage of the eukaryote cell is the separation of incompatible reactions into different compartments. Respiration happens inside mitochondria; photosynthesis happens inside chloroplasts; protein synthesis happens on ribosomes; DNA is safely stored inside the nuclear envelope. Prokaryotes perform all these activities in the same shared cytoplasm, which constrains them to a simpler lifestyle and smaller size. This compartmentalisation is the single biggest reason eukaryotes can build multicellular organisms with specialised organelles serving specialised cells.

Mini exam-style question

Question: A student claims that "prokaryotes are a simpler form of eukaryotes". Using precise biology terms, explain why this statement is incorrect. (3 marks)

Model answer: Prokaryotes and eukaryotes are two distinct cell types — prokaryotes are not a simpler form of eukaryotes but an entirely different, older lineage. Prokaryotes lack a true nucleus and membrane-bound organelles, and their circular DNA sits free in the cytoplasm. Eukaryotes have evolved compartmentalisation: linear chromosomes inside a nuclear envelope, mitochondria for respiration and (in plants) chloroplasts for photosynthesis. Current scientific consensus is that eukaryotes arose from an ancestral prokaryote that engulfed another — the endosymbiotic origin of mitochondria — so if anything eukaryotes are a more complex derivative of prokaryotes, not the other way round.

Answering at different grade levels

Grades 3–4 (basic recall): "Eukaryotic cells have a nucleus and prokaryotic cells do not. Animal and plant cells are eukaryotic. Bacteria are prokaryotic."

Grades 5–6 (applied description): "A eukaryote has membrane-bound organelles including a true nucleus that contains chromosomal DNA. A prokaryote is much smaller, has no nucleus, and its single circular chromosome floats in the cytoplasm. Prokaryotes also contain small extra rings of DNA called plasmids."

Grades 7–9 (analytical comparison): "Eukaryotic cells (10–100 μm) contain a nuclear envelope enclosing linear chromosomes and possess membrane-bound organelles such as mitochondria and (in plants) chloroplasts, allowing compartmentalisation of biochemical reactions. Prokaryotic cells (0.2–10 μm) lack these compartments; their single circular chromosome lies free in the cytoplasm and protein synthesis occurs on smaller (70S) ribosomes. Plasmids in prokaryotes can carry accessory genes such as antibiotic resistance, which can be transferred horizontally between bacteria."

AQA alignment: This content is aligned with AQA GCSE Combined Science: Trilogy (8464) specification section 4.1 Cell biology — specifically 4.1.1.1 Eukaryotes and prokaryotes, 4.1.1.2 Animal and plant cells. Assessed on Biology Paper 1.