# Comparison of eukaryotic and prokaryotic cells

> CIE A-Level Biology · Unit 1: Cell Structure
> Source: https://www.owlsprep.com/study/cie-9700-u1-comparison-of-eukaryotic-and-prokaryotic/

This sub-topic covers key structural, genetic and functional differences between prokaryotic and eukaryotic cells, a core foundational concept for all further cell biology topics in CIE A-Level Biology. It is regularly tested in both multiple choice and structured questions.

**Prerequisites:** [Basic eukaryotic cell organelle structure and function](https://www.owlsprep.com/study/cie-9700-u1-eukaryotic-cell-organelles/)

## Learning objectives

- Distinguish between eukaryotic and prokaryotic cells by structural features
- Compare genetic material, organelles and cell division between the two groups
- Apply knowledge to identify cell types in exam questions
- Recall common marking points for comparison questions

## Cell Size and Outer Structural Layers

**Prokaryotic vs Eukaryotic Cells** — Two fundamental classes of cells distinguished by the presence or absence of a membrane-bound nucleus and membrane-bound organelles

*Example:* Prokaryotes: bacteria; Eukaryotes: plant, animal, fungi cells

The first observable difference between the two cell types is size. Most prokaryotic cells range from 0.5-10 μm in diameter, while eukaryotic cells are typically 10-100 μm. The smaller size of prokaryotes supports a faster metabolic rate and shorter generation time.

All prokaryotes have a cell wall made of peptidoglycan (murein). Among eukaryotes, only plants, fungi and algae have cell walls, made of cellulose (plants) or chitin (fungi). Animal eukaryotic cells never have a cell wall.

**Worked example:** A student measures a cell with a diameter of 0.7 μm. State whether this is most likely prokaryotic or eukaryotic, with reason.

1. Recall the typical size ranges for both cell types:
2. - Prokaryotic cells: 0.5-10 μm
- Eukaryotic cells: 10-100 μm
3. 0.7 μm falls within the prokaryotic size range.
4. Conclusion: The cell is prokaryotic.

> **Exam tip:** Size questions are often linked to surface area to volume ratio, so be prepared to follow up with that concept if asked.

## Genetic Material Organization

The defining difference between prokaryotic and eukaryotic cells is the organization of genetic material:

- Eukaryotes have their DNA enclosed within a double-membrane nuclear envelope, forming a distinct nucleus
- Prokaryotes have no nucleus; their main DNA is located in an irregular, membrane-unbound region called the nucleoid
- Eukaryotic DNA is linear and wrapped around histone proteins to form chromosomes
- Prokaryotic main DNA is circular and not associated with histones (in most species)
- Prokaryotes often have additional small circular DNA molecules called plasmids, which are rare in eukaryotic somatic cells

> **note**
>
> Eukaryotes have small circular DNA in their mitochondria and chloroplasts, matching prokaryotic DNA structure, which supports endosymbiotic theory.

**Worked example:** Compare the arrangement of DNA in a bacterial (prokaryotic) cell and a human (eukaryotic) cell.

1. Location: Bacterial DNA is in the nucleoid, with no surrounding nuclear envelope. Human DNA is enclosed in a membrane-bound nucleus.
2. Structure of main DNA: Bacterial main DNA is circular and not wrapped around histones. Human nuclear DNA is linear and wrapped around histone proteins to form chromosomes.
3. Additional DNA: Bacteria usually have plasmids (small extra circular DNA molecules). Human cells do not have plasmids, though they have small circular DNA in mitochondria.

## Organelles and Cell Division

Both cell types have ribosomes for protein synthesis, but ribosome size differs. Prokaryotes have smaller 70S ribosomes, while eukaryotes have larger 80S ribosomes in their cytoplasm.

**Membrane-bound organelles** — Specialized organelles surrounded by a phospholipid membrane, only found in eukaryotic cells

*Example:* Mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus

Prokaryotes lack all membrane-bound organelles. While some prokaryotes have infoldings of their cell membrane for respiration or photosynthesis, these are not discrete organelles like eukaryotic mitochondria or chloroplasts.

Prokaryotes divide by binary fission, a simple process that does not involve spindle formation. Eukaryotes divide by mitosis or meiosis, which requires spindle formation and chromosome condensation.

**Worked example:** A cell is observed to have mitochondria, 80S ribosomes and linear DNA. State if it is prokaryotic or eukaryotic, and give two supporting reasons.

1. Conclusion: The cell is eukaryotic.
2. Reason 1: Mitochondria are membrane-bound organelles, which are only present in eukaryotic cells. Prokaryotes have no membrane-bound organelles.
3. Reason 2: Eukaryotes have 80S ribosomes in their cytoplasm, while prokaryotes only have 70S ribosomes.
4. Alternative reason: Eukaryotic nuclear DNA is linear, while prokaryotic main DNA is circular.

> **Exam tip:** CIE examiners consistently award a mark for correctly stating ribosome size, so never mix up 70S and 80S.

## Common pitfalls

- **Wrong:** Stating that prokaryotes do not have ribosomes
  - Why it fails: Prokaryotes require ribosomes to synthesize proteins, so all prokaryotes have ribosomes
  - Correct: State that prokaryotes have smaller 70S ribosomes, while eukaryotes have 80S cytoplasmic ribosomes
- **Wrong:** Claiming all eukaryotes have a cell wall, and prokaryotes do not
  - Why it fails: All prokaryotes have a cell wall, while only some eukaryotes (plants, fungi) have cell walls; animal eukaryotes do not
  - Correct: State all prokaryotes have a peptidoglycan cell wall; eukaryotes only have cell walls in plants (cellulose) and fungi (chitin), with no cell wall in animals
- **Wrong:** Mixing up 70S and 80S ribosome sizes
  - Why it fails: This is a common 1-mark question that is easy to get wrong if you misremember
  - Correct: Use the mnemonic: *Eighty is Eukaryotic* to remember 80S = eukaryotic, 70S = prokaryotic
- **Wrong:** Stating that prokaryotes do not have DNA
  - Why it fails: All living cells have genetic material; prokaryotes just do not store DNA in a membrane-bound nucleus
  - Correct: State that prokaryotes have circular DNA located in the nucleoid region, not enclosed by a nuclear envelope
- **Wrong:** Claiming prokaryotes have linear DNA
  - Why it fails: Prokaryotic main chromosomal DNA is always circular, while eukaryotic nuclear DNA is linear
  - Correct: Correctly assign circular DNA to prokaryotes and linear DNA to eukaryotic nuclear DNA

## Cheatsheet

| Feature | Prokaryotic Cell | Eukaryotic Cell |
| --- | --- | --- |
| Cell diameter | 0.5 - 10 μm | 10 - 100 μm |
| Nucleus | No nuclear envelope, nucleoid region | Nucleus enclosed in double membrane |
| DNA structure | Circular, no histones, has plasmids | Linear, wrapped in histones, no plasmids |
| Ribosome size | 70S | 80S (cytoplasmic) |
| Membrane-bound organelles | Absent | Present |
| Cell wall | Present (peptidoglycan) | Present (cellulose/chitin) / Absent (animals) |
| Cell division | Binary fission, no spindle | Mitosis/meiosis, spindle forms |

## What's next

This comparison of prokaryotic and eukaryotic cells is a foundational concept that underpins all further topics in CIE A-Level Biology, from cell membrane transport to cell division, and the endosymbiotic theory for the origin of eukaryotic organelles. Understanding the key differences is essential for interpreting electron micrographs and answering classification questions later in the course. Mastering the marking points for comparison questions will help you secure easy marks in both paper 1 multiple choice and paper 2 structured questions. Next, you will build on this knowledge to learn about specialized eukaryotic cell structures.

- [Biological Molecules](https://www.owlsprep.com/study/cie-9700-u2-overview/)
- [Water and inorganic ions](https://www.owlsprep.com/study/cie-9700-u2-water-and-inorganic-ions/)
- [Carbohydrates](https://www.owlsprep.com/study/cie-9700-u2-carbohydrates/)

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