# Prokaryotic cell structure

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u1-prokaryotic-cell-structure/

This module covers key structures and functions of prokaryotic cells, compares them to eukaryotic cells, and addresses common exam questions on this core foundational cell biology topic.

**Prerequisites:** [Basic cell theory](https://www.owlsprep.com/study/cie-9700-u1-cell-theory/); [Eukaryotic cell structure](https://www.owlsprep.com/study/cie-9700-u1-eukaryotic-cell-structure/)

## Learning objectives

- Identify and label key prokaryotic cell structures
- Distinguish prokaryotic cell structure from eukaryotic cells
- State the function of each prokaryotic cell component
- Relate prokaryotic cell structure to function

## Key Structures of Prokaryotic Cells

Prokaryotic cells are typically 0.5-5 μm, much smaller than most eukaryotic cells (10-100 μm). All prokaryotes lack membrane-bound organelles and a nucleus, but share core structures: a cell wall, cell membrane, cytoplasm, a circular main chromosome, and 70S ribosomes. Many also have optional additional structures like capsules, plasmids, flagella, and pili.

**Prokaryote** — A unicellular organism that lacks a membrane-bound nucleus and any membrane-bound organelles. The two major groups are bacteria and archaea.

*Example:* *Escherichia coli* (E. coli), a common gut bacterium, is a model prokaryote.

**Worked example:** Label four common structures of a bacterial prokaryotic cell and state one function for each.

1. 1. Outermost protective layer (outside cell wall):
2. Name = Capsule. Function = Protects the cell from desiccation and phagocytosis by host immune cells.
3. 2. Small circular DNA molecule separate from main DNA:
4. Name = Plasmid. Function = Carries extra non-essential genes, often for antibiotic resistance.
5. 3. Region containing main genetic material:
6. Name = Nucleoid. Function = Stores the main circular chromosome with all essential genes for cell function.
7. 4. Site of protein synthesis:
8. Name = 70S ribosome. Function = Synthesises all proteins required for cell growth and function.

> **Exam tip:** Diagram labelling of prokaryotic cells is a very common short answer question, so memorise the relative position of each structure.

## Structure and Function Relationships

Each prokaryotic structure is adapted to its specific role. Since prokaryotes lack membrane-bound organelles like mitochondria, the cell membrane carries out many of these functions, including respiration and photosynthesis in photosynthetic prokaryotes.

- **Cell wall**: Prevents osmotic lysis, gives shape to the cell. Made of peptidoglycan in bacteria.
- **Cell surface membrane**: Controls movement of substances in and out of the cell, acts as a site for respiration.
- **Flagellum**: Long, rigid whip-like structure that enables cell locomotion.
- **Pili**: Short hair-like projections that attach bacteria to surfaces and enable DNA exchange (conjugation).

**Worked example:** Explain why the cell wall is essential for prokaryotic survival in pure water.

1. 1. Pure water is hypotonic relative to the prokaryotic cytoplasm, so water moves into the cell by osmosis.
2. 2. As water enters, the cell volume increases, pushing the cell membrane against the rigid cell wall.
3. 3. The inelastic cell wall resists further expansion, stopping the cell from bursting.
4. Conclusion: Without the cell wall, osmotic entry of water would cause the cell membrane to rupture and the cell to lyse (die).

## Comparison of Prokaryotic and Eukaryotic Cells

Comparing prokaryotic and eukaryotic cell structure is a common 3-5 mark exam question, so you need to remember the key differences:

| Feature | Prokaryotic Cell | Eukaryotic Cell |
| --- | --- | --- |
| Nucleus | No membrane-bound nucleus, nucleoid region | Membrane-bound nucleus present |
| Cell wall | Almost always present, peptidoglycan (bacteria) | Only in plants/fungi, cellulose/chitin |
| Ribosome size | 70S (smaller) | 80S (larger) |
| Membrane organelles | None | Mitochondria, ER, Golgi present |
| DNA structure | Circular, naked, no histones | Linear, histone-bound, in nucleus |
| Average size | 0.5-5 μm | 10-100 μm |

**Worked example:** A student observes a cell under an electron microscope and identifies it as prokaryotic. What three features would confirm this?

1. 1. The cell has no membrane-bound nucleus, with DNA found free in the cytoplasm in the nucleoid region.
2. 2. There are no membrane-bound organelles such as mitochondria or chloroplasts present.
3. 3. The cell has only small 70S ribosomes, rather than larger 80S ribosomes.
4. Other valid features include a peptidoglycan cell wall or presence of plasmids, which are unique to prokaryotes.

> **Exam tip:** When asked to compare, always refer to both cell types to gain full marks in CIE marking schemes.

## Common pitfalls

- **Wrong:** Claiming prokaryotes do not have ribosomes.
  - Why it fails: Students confuse the absence of membrane-bound organelles with the absence of all organelles. Ribosomes are non-membrane bound.
  - Correct: State that prokaryotes have 70S ribosomes that carry out protein synthesis.
- **Wrong:** Calling the prokaryotic DNA region a 'nucleus'.
  - Why it fails: Prokaryotes do not have a membrane-bound nucleus. The term for the DNA region is nucleoid.
  - Correct: Refer to the main genetic region as the nucleoid containing a circular chromosome.
- **Wrong:** Stating prokaryotic cell walls are made of cellulose.
  - Why it fails: Cellulose is only found in plant cell walls. Bacterial cell walls are made of peptidoglycan.
  - Correct: State that bacterial prokaryotic cell walls are made of peptidoglycan.
- **Wrong:** Claiming all prokaryotes have a capsule.
  - Why it fails: Capsules are optional extra structures, not present in all prokaryotic species.
  - Correct: Write that *many* prokaryotes have a capsule, not all.

## Cheatsheet

| Structure | Key Function | Key Exam Fact |
| --- | --- | --- |
| Nucleoid | Stores main genetic material | No membrane, circular DNA |
| Plasmid | Carries extra genes (e.g. antibiotic resistance) | Small circular DNA separate from main chromosome |
| 70S ribosome | Site of protein synthesis | Smaller than eukaryotic 80S ribosomes |
| Cell wall | Prevents osmotic lysis | Bacteria = peptidoglycan |
| Capsule | Protection from immune cells | Optional outer layer |
| Flagellum | Cell locomotion | Whip-like structure |

## What's next

Prokaryotic cell structure is a foundational topic for CIE A-Level Biology, underpinning later units on infection and immunity, antibiotic resistance, genetic engineering, and cell division. You will build on this knowledge when studying how prokaryotes cause disease, how they reproduce, and how they are used in biotechnology. Next, you can explore related topics to complete your understanding of cell structure.

- [Virus Structure and Classification](https://www.owlsprep.com/study/cie-9700-u1-virus-structure/)
- [Comparison of eukaryotic and prokaryotic cells](https://www.owlsprep.com/study/cie-9700-u1-comparison-of-eukaryotic-and-prokaryotic/)
- [Biological Molecules](https://www.owlsprep.com/study/cie-9700-u2-overview/)

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