Eukaryotic and Prokaryotic Cells

This lesson covers the two fundamental types of cell — eukaryotic and prokaryotic — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to describe the structures found in both cell types, explain their functions and compare the key differences between them.

What Are Cells?

All living organisms are made of cells. A cell is the basic structural and functional unit of life. Some organisms are made of just one cell (unicellular, e.g. bacteria), while others are made of many cells working together (multicellular, e.g. animals and plants).

Cells fall into two broad categories:

Eukaryotic cells — complex cells with a true nucleus and membrane-bound organelles.
Prokaryotic cells — simpler, smaller cells with no true nucleus.

Exam Tip: The word "eukaryotic" comes from Greek and means "true nucleus". The word "prokaryotic" means "before nucleus". These root words can help you remember the key difference.

Eukaryotic Cell Structure

Eukaryotic cells are found in animals, plants, fungi and protists. They contain a nucleus enclosed by a nuclear membrane, along with other membrane-bound organelles such as mitochondria. Eukaryotic cells are typically 10–100 μm in diameter.

Sub-cellular Structures (Organelles)

The following table summarises the organelles you need to know for both animal and plant eukaryotic cells:

Organelle	Structure	Function
Nucleus	Large, spherical, surrounded by a nuclear membrane	Contains genetic material (DNA) arranged into chromosomes; controls the cell's activities
Cell membrane	Thin, flexible phospholipid bilayer	Controls what enters and leaves the cell; partially permeable
Cytoplasm	Jelly-like substance filling the cell interior	Where most chemical reactions take place; contains enzymes
Mitochondria	Small, oval-shaped with a folded inner membrane (cristae)	Site of aerobic respiration — energy is transferred from glucose
Ribosomes	Very small structures (~25 nm), found free in cytoplasm	Site of protein synthesis — amino acids are assembled into proteins

Exam Tip: Never write that mitochondria "produce" or "create" energy. Energy cannot be created or destroyed. Say mitochondria are the site of aerobic respiration, where energy is transferred (or released) from glucose.

Prokaryotic Cell Structure

Prokaryotic cells are found in bacteria. They are much smaller than eukaryotic cells, typically only 1–5 μm in diameter — roughly one-tenth the size of a typical animal cell. They do not have a true nucleus; their genetic material is free in the cytoplasm.

Structure of a Bacterial Cell

Structure	Description	Function
Cell membrane	Phospholipid bilayer	Controls what enters and leaves the cell
Cell wall	Made of peptidoglycan (not cellulose)	Provides structural support and protection
Cytoplasm	Jelly-like substance	Where chemical reactions occur
Ribosomes	Smaller than eukaryotic ribosomes (70S vs 80S)	Site of protein synthesis
Chromosomal DNA	A single, circular loop of DNA free in the cytoplasm	Carries the main genetic information
Plasmids	Small, extra circles of DNA	Carry additional genes, e.g. antibiotic resistance
Flagellum (plural: flagella)	Long, whip-like tail (not all bacteria have one)	Used for movement

graph TD
    A[Bacterial Cell] --> B[Cell wall - peptidoglycan]
    B --> C[Cell membrane]
    C --> D[Cytoplasm]
    D --> E[Circular DNA - no nucleus]
    D --> F[Plasmids]
    D --> G[70S Ribosomes]
    A --> H[Flagellum - movement]

Exam Tip: Do not write that prokaryotic cells have "no DNA". They do have DNA — it is simply not enclosed in a nuclear membrane. Say they have "no true nucleus".

Comparing Eukaryotic and Prokaryotic Cells

Feature	Eukaryotic Cell	Prokaryotic Cell
Size	Larger (10–100 μm)	Smaller (1–5 μm)
Nucleus	True nucleus with nuclear membrane	No true nucleus — DNA free in cytoplasm
DNA arrangement	Linear chromosomes inside nucleus	Single circular DNA loop + plasmids
Membrane-bound organelles	Present (mitochondria, etc.)	Absent
Ribosomes	Larger (80S)	Smaller (70S)
Cell wall	Present in plants (cellulose); absent in animals	Present (peptidoglycan)
Plasmids	Not normally present	Present
Examples	Animal, plant, fungal cells	Bacteria

Size and Scale

Understanding relative sizes is important for the exam:

Object	Approximate Size
Animal cell	10–30 μm
Plant cell	10–100 μm
Bacterial cell	1–5 μm
Virus (not a cell)	20–300 nm

Unit Conversions

You need to be able to convert between units of length:

1 mm = 1000 μm (millimetres to micrometres)
1 μm = 1000 nm (micrometres to nanometres)
1 mm = 1,000,000 nm (millimetres to nanometres)

Worked Example:

A bacterium is 3.5 μm long. Convert this to nanometres.

3.5 μm × 1000 = 3500 nm

Convert 3.5 μm to millimetres.

3.5 μm ÷ 1000 = 0.0035 mm

Exam Tip: Remember the conversion chain: mm → μm → nm, each step is × 1000. Going the other way (nm → μm → mm) is ÷ 1000. Show every step of your working for full marks.

Summary

Eukaryotic cells have a true nucleus, membrane-bound organelles (mitochondria, ribosomes), and are 10–100 μm in size.
Prokaryotic cells (bacteria) have no true nucleus, no membrane-bound organelles, smaller 70S ribosomes, a single circular DNA loop, plasmids, and a peptidoglycan cell wall. They are 1–5 μm in size.
Both cell types have a cell membrane, cytoplasm and ribosomes.
Know the unit conversion chain: mm → μm → nm (× 1000 each step).
Mitochondria are the site of aerobic respiration — energy is transferred from glucose, not "produced".