Prokaryotic Cells and Cell Size

This lesson covers prokaryotic cells and the concept of cell size and scale, as required by AQA GCSE Biology specification 4.1.1. You need to understand how prokaryotic cells differ from eukaryotic cells, recognise the key features of bacterial cells, and be able to use standard form and unit conversions when dealing with very small measurements.

---

Prokaryotic vs Eukaryotic Cells

Living organisms can be divided into two fundamental groups based on their cell structure:

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus — DNA is free in the cytoplasm	True nucleus with a nuclear membrane
DNA arrangement	Single circular chromosome + small rings of DNA called plasmids	Linear chromosomes contained in the nucleus
Size	Typically 0.1–5.0 micrometres	Typically 10–100 micrometres
Examples	Bacteria, archaea	Animal cells, plant cells, fungi, protists
Cell wall	Yes (made of peptidoglycan, not cellulose)	Plant cells: yes (cellulose). Animal cells: no
Membrane-bound organelles	No mitochondria or chloroplasts	Yes — mitochondria, chloroplasts (plants), etc.
Ribosomes	Smaller ribosomes (70S)	Larger ribosomes (80S)
Flagellum	Some have one or more flagella for movement	Rare (e.g. sperm cells)

Prokaryotic cells are much simpler and smaller than eukaryotic cells. The word "prokaryotic" means "before the nucleus" — these cells evolved billions of years before eukaryotic cells appeared.

Exam Tip: The key distinction is the nucleus. Prokaryotic cells do NOT have a true nucleus — their DNA floats freely in the cytoplasm as a single circular chromosome. This is the most commonly tested difference.

---

Structure of a Bacterial Cell

Bacteria are the most well-known prokaryotes. A typical bacterial cell has the following structures:

Structure	Description and Function
Cell membrane	Controls what enters and leaves the cell; a phospholipid bilayer.
Cell wall	Made of peptidoglycan (not cellulose). Provides shape, support, and protection. Prevents the cell from bursting.
Cytoplasm	Where the cell's chemical reactions occur. Contains enzymes and ribosomes.
Chromosomal DNA	A single, circular loop of DNA that carries the genetic information for the cell. Not enclosed in a nucleus.
Plasmid DNA	Small, extra circles of DNA that can carry additional genes, such as antibiotic resistance genes. Plasmids can be transferred between bacteria.
Ribosomes	Smaller than eukaryotic ribosomes (70S vs 80S). Site of protein synthesis.
Flagellum (plural: flagella)	A tail-like structure used for movement. Not all bacteria have flagella.
Slime capsule	Some bacteria have an extra protective layer outside the cell wall that helps them avoid being destroyed by the immune system.

graph TD
    A["Bacterial Cell (Prokaryote)"] --> B["Cell Wall (peptidoglycan)"]
    A --> C["Cell Membrane"]
    A --> D["Cytoplasm"]
    A --> E["Circular DNA (no nucleus)"]
    A --> F["Plasmid DNA"]
    A --> G["Ribosomes (70S, smaller)"]
    A --> H["Flagellum (some bacteria)"]
    A --> I["Slime Capsule (some bacteria)"]

    style A fill:#8e44ad,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#e74c3c,color:#fff

---

Plasmids and Antibiotic Resistance

Plasmids are particularly important in biology and medicine. They are small rings of DNA that replicate independently of the main chromosome. Plasmids often carry genes that provide an advantage to the bacterium, such as:

• Antibiotic resistance — genes that allow the bacterium to survive treatment with antibiotics.
• Toxin production — genes that code for toxins that cause disease.

Plasmids can be transferred from one bacterium to another through a process called conjugation, which is one reason antibiotic resistance can spread rapidly through bacterial populations.

Exam Tip: You may be asked why plasmids are important in genetic engineering. Plasmids are used as vectors to transfer genes from one organism to another — for example, inserting the human insulin gene into bacteria so they produce insulin for diabetic patients.

---

Units of Measurement for Cells

Cells are extremely small, so biologists use very small units to measure them. You must be confident converting between these units.

Unit	Symbol	Relationship
Metre	m	1 m
Millimetre	mm	1 mm = 0.001 m = 1 x 10^-3 m
Micrometre	um	1 um = 0.001 mm = 1 x 10^-6 m
Nanometre	nm	1 nm = 0.001 um = 1 x 10^-9 m

Conversion Rules

• To convert from mm to um, multiply by 1000.
• To convert from um to mm, divide by 1000.
• To convert from um to nm, multiply by 1000.
• To convert from nm to um, divide by 1000.

Worked Example

A bacterial cell is 2.5 um long. Convert this to:

• Millimetres: 2.5 / 1000 = 0.0025 mm
• Nanometres: 2.5 x 1000 = 2500 nm
• Standard form (metres): 2.5 x 10^-6 m

---

Standard Form

Standard form (also called scientific notation) is used to express very large or very small numbers. A number in standard form is written as:

A x 10^n

where A is a number between 1 and 10, and n is a positive or negative whole number (the power of 10).

Examples

Measurement	Standard Form
0.005 mm	5.0 x 10^-3 mm
0.000002 m	2.0 x 10^-6 m
150 nm	1.5 x 10^2 nm
0.0025 mm	2.5 x 10^-3 mm

Typical Cell Sizes

Cell or Structure	Approximate Size
Animal cell	10–30 um
Plant cell	10–100 um
Bacterium	0.2–5.0 um
Virus	20–300 nm
Ribosome	~20 nm
Mitochondrion	1–10 um
Red blood cell	~7 um