Differences between eukaryotic and prokaryotic cells

What you'll learn

This revision guide covers the fundamental differences between eukaryotic and prokaryotic cells, a core topic in AQA GCSE Biology. You'll learn to identify the structural features that distinguish these two cell types, understand the organisation of genetic material in each, and apply this knowledge to exam questions. This topic underpins your understanding of cell biology and is frequently examined.

Key terms and definitions

Eukaryotic cell — a cell containing a nucleus and other membrane-bound organelles; found in animals, plants, fungi and protists

Prokaryotic cell — a cell lacking a nucleus and membrane-bound organelles; bacterial cells are prokaryotic

Nucleus — a membrane-bound organelle containing chromosomal DNA that controls the cell's activities

Plasmid — a small circular loop of DNA found in bacterial cells, separate from the chromosomal DNA

Chromosomal DNA — the main genetic material of a cell; in prokaryotes it exists as a single loop in the cytoplasm

Ribosomes — structures found in all cells where protein synthesis occurs; prokaryotic ribosomes are smaller than eukaryotic ribosomes

Cell wall — a rigid outer layer that provides structural support; made of cellulose in plants and fungi, peptidoglycan in bacteria

Flagellum — a tail-like structure used for movement in some bacterial cells (plural: flagella)

Core concepts

Overview of cell types

All living organisms are composed of cells, but not all cells are structured the same way. Cells fall into two fundamental categories based on their internal organisation.

Eukaryotic cells are larger and more complex. They contain a distinct nucleus surrounded by a nuclear membrane, plus other specialised organelles such as mitochondria and chloroplasts (in plant cells). Examples of eukaryotic organisms include:

• Animals (humans, fish, insects)
• Plants (flowering plants, trees, mosses)
• Fungi (mushrooms, yeast)
• Protists (amoeba, algae)

Prokaryotic cells are smaller and simpler in structure. They lack a nucleus and membrane-bound organelles. The main examples of prokaryotic organisms are:

• Bacteria (E. coli, Salmonella, cyanobacteria)

The distinction between these cell types is fundamental to understanding biological classification and cellular function.

Structural differences

Size

Prokaryotic cells are significantly smaller than eukaryotic cells:

• Prokaryotic cells: typically 0.2–2.0 micrometres (μm) in diameter
• Eukaryotic cells: typically 10–100 μm in diameter

This size difference is important because it affects the surface area to volume ratio and how efficiently substances can move into and out of the cell by diffusion.

Genetic material organisation

The organisation of DNA differs fundamentally between the two cell types.

In eukaryotic cells:

• DNA is linear and organised into chromosomes
• Chromosomes are found inside the nucleus
• DNA is associated with histone proteins
• The nuclear membrane separates genetic material from the cytoplasm

In prokaryotic cells:

• DNA exists as a single circular chromosome
• This chromosomal DNA floats freely in the cytoplasm (not enclosed in a nucleus)
• The region where DNA is located is called the nucleoid
• DNA is not associated with histone proteins
• Additional DNA is present in plasmids

Plasmids

Plasmids are a distinctive feature of prokaryotic cells, though not all bacterial cells contain them. Key characteristics include:

• Small, circular loops of DNA
• Separate from the main chromosomal DNA
• Contain a small number of genes (often 5–100 genes)
• Can replicate independently of chromosomal DNA
• Often carry genes for antibiotic resistance or other advantageous traits
• Can be transferred between bacterial cells
• Used extensively in genetic engineering

Eukaryotic cells do not naturally contain plasmids, though small circular DNA molecules exist in mitochondria and chloroplasts.

Organelles and cellular components

Nucleus

The presence or absence of a nucleus is the defining difference between eukaryotic and prokaryotic cells.

Eukaryotic cells contain a nucleus which:

• Is surrounded by a double membrane called the nuclear envelope
• Contains pores that allow substances to move in and out
• Houses chromosomes made of DNA and proteins
• Contains the nucleolus where ribosomes are made

Prokaryotic cells have no nucleus. Instead, their genetic material is located in the cytoplasm in a region called the nucleoid.

Ribosomes

Both cell types contain ribosomes, but they differ in size:

• Eukaryotic ribosomes: larger (80S)
• Prokaryotic ribosomes: smaller (70S)

The 'S' refers to Svedberg units, which measure sedimentation rate. You don't need to remember the numbers for GCSE, but you must know that prokaryotic ribosomes are smaller. Both types of ribosomes perform protein synthesis.

Other organelles

Eukaryotic cells contain many membrane-bound organelles that are absent in prokaryotic cells:

• Mitochondria — site of aerobic respiration
• Chloroplasts — site of photosynthesis (plant cells and algae only)
• Endoplasmic reticulum — involved in protein and lipid synthesis
• Golgi apparatus — modifies and packages proteins
• Lysosomes — contain digestive enzymes

Prokaryotic cells lack all of these membrane-bound organelles. The processes that occur in mitochondria in eukaryotic cells (such as respiration) occur in the cytoplasm and cell membrane in prokaryotic cells.

Cell walls and membranes

Cell membrane

All cells (both prokaryotic and eukaryotic) are surrounded by a cell surface membrane that:

• Controls the movement of substances in and out of the cell
• Is made of lipids and proteins
• Is selectively permeable

Cell walls

Cell walls provide structural support and protection, but their composition varies:

Bacterial (prokaryotic) cell walls:

• Made of peptidoglycan (also called murein)
• Rigid structure that prevents the cell from bursting
• Target for certain antibiotics (like penicillin)

Plant and fungal (eukaryotic) cell walls:

• Plant cells: made of cellulose
• Fungal cells: made of chitin
• Provide support and prevent bursting

Animal cells (eukaryotic) do not have cell walls.

Additional prokaryotic features

Some bacterial cells possess additional structures:

Flagella (singular: flagellum):

• Long, whip-like structures that rotate
• Used for locomotion, enabling the bacterium to move towards nutrients or away from toxins
• Not all bacteria have flagella
• Different in structure from eukaryotic flagella (such as sperm tails)

Capsule (slime layer):

• A protective layer outside the cell wall
• Made of polysaccharides
• Protects against desiccation and the immune system
• Helps bacteria stick to surfaces
• Not present in all bacteria

Pili (singular: pilus):

• Hair-like structures on the cell surface
• Used for attachment to surfaces
• Involved in transfer of genetic material between bacteria

Worked examples

Example 1: Identifying cell types

Question: A student observes a cell under a microscope. The cell contains ribosomes and a cell membrane but no nucleus. The genetic material appears as a single circular strand in the cytoplasm. Identify the type of cell and give two reasons for your answer. [3 marks]

Mark scheme answer:

The cell is prokaryotic / a bacterial cell [1 mark]

Reason 1: It has no nucleus / genetic material is not enclosed in a nucleus [1 mark]

Reason 2: It has a single circular chromosome / circular DNA [1 mark]

Examiner tip: Make sure you clearly state the cell type first, then provide distinct reasons. Don't repeat the same point in different words.

Example 2: Comparing structures

Question: Table 1 shows some features of cells.

Complete the table by writing 'Yes' or 'No' in boxes A and B. [2 marks]

Mark scheme answer:

Box A: No [1 mark] Box B: No [1 mark]

Explanation: Prokaryotic cells do not contain mitochondria or any membrane-bound organelles. Eukaryotic cells do not naturally contain plasmids (though mitochondria and chloroplasts have their own circular DNA).

Example 3: Extended response

Question: Bacterial cells are prokaryotic. Describe three ways in which the structure of a bacterial cell differs from an animal cell. [3 marks]

Mark scheme answer:

Any three from:

• Bacterial cells have no nucleus / animal cells have a nucleus [1 mark]
• Bacterial cells have circular/chromosomal DNA / animal cells have linear DNA / chromosomes [1 mark]
• Bacterial cells have plasmids / animal cells do not have plasmids [1 mark]
• Bacterial cells have smaller ribosomes / animal cells have larger ribosomes [1 mark]
• Bacterial cells have a cell wall (made of peptidoglycan) / animal cells have no cell wall [1 mark]
• Bacterial cells have no mitochondria / animal cells have mitochondria [1 mark]

Examiner tip: When questions ask for multiple differences, make sure each point is distinct. Use comparative statements (bacterial cells have X whereas animal cells have Y) to ensure you're making a clear comparison.

Common mistakes and how to avoid them

• Stating that prokaryotic cells have no DNA — Incorrect. Prokaryotic cells do contain DNA, but it's not enclosed in a nucleus. The DNA exists as a circular chromosome in the cytoplasm, plus additional DNA in plasmids.

• Confusing cell walls in different organisms — Plant cells, fungal cells and bacterial cells all have cell walls, but they're made of different materials (cellulose, chitin and peptidoglycan respectively). Don't state that only plant cells have cell walls.

• Claiming prokaryotic cells have no ribosomes — Wrong. Both prokaryotic and eukaryotic cells have ribosomes for protein synthesis. The difference is that prokaryotic ribosomes are smaller.

• Using vague statements like "prokaryotic cells are simpler" — In exams, you need to be specific. State exactly which structures are absent (e.g., "prokaryotic cells lack a nucleus and membrane-bound organelles") rather than using general terms like "simpler" or "more basic."

• Forgetting that plasmids are separate from chromosomal DNA — Plasmids are distinct from the main bacterial chromosome. They're additional genetic elements, not part of the main DNA loop.

• Stating that all bacteria have flagella or capsules — Only some bacterial cells possess these structures. The question might specify whether they're present, or you should use qualifying language like "some bacteria have flagella."

Exam technique for "Differences between eukaryotic and prokaryotic cells"

• "Describe" questions require you to state features without explanation. For 3 marks, give three distinct differences. Use comparative language: "Prokaryotic cells have X whereas eukaryotic cells have Y."

• "Explain" questions need you to provide reasons or link cause and effect. For example, explaining why bacterial cells can reproduce quickly might link to their simpler structure and smaller size requiring less time to replicate components.

• Comparison table questions are common. Read carefully whether you need to write Yes/No or tick boxes. Check each row relates to the correct cell type. If asked to complete a table, make sure you fill in all required boxes.

• Mark allocation indicates detail required — A 1-mark question needs one clear point. A 4-mark question typically needs four distinct points or two points with development/explanation. Don't waste time writing more than needed, but ensure each point is scientifically accurate and clearly expressed.

Quick revision summary

Prokaryotic cells (bacteria) are smaller and simpler than eukaryotic cells (animals, plants, fungi, protists). Key differences: prokaryotic cells lack a nucleus, have circular chromosomal DNA in the cytoplasm, contain plasmids, possess smaller ribosomes, and have no membrane-bound organelles like mitochondria. Both cell types have ribosomes and cell membranes. Bacterial cell walls contain peptidoglycan; plant cell walls contain cellulose. Some bacteria have flagella for movement. Understanding these structural differences is essential for explaining cell function and classification.