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.

Two Types of Cell

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

Eukaryotic cells — complex cells with a nucleus enclosed in a nuclear membrane. Examples include animal cells, plant cells and fungal cells.
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.

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

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

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.

Size of Cells

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

$$1 \text{ mm} = 1000 \text{ μm}$$

$$1 \text{ μm} = 1000 \text{ nm}$$

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.

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:

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

To convert μm to nm, multiply by 1000:

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

Common Mistakes

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.
Confusing cell wall materials — plant cell walls are made of cellulose; bacterial cell walls are made of peptidoglycan. These are different substances.
Forgetting plasmids — plasmids are unique to prokaryotic cells and are an important feature that distinguishes them from eukaryotic cells.
Mixing up μm and nm — always check the units you are working with and remember: 1 μm = 1000 nm.
Stating that only plant cells have a cell wall — bacteria also have a cell wall, but it is made of a different material.

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.