Eukaryotes and prokaryotes (AQA GCSE Biology Combined Science): Revision Notes

Eukaryotes and prokaryotes

What are eukaryotes and prokaryotes?

All living things are made of cells. There are two main types of cells that form the foundation of all life on Earth.

Eukaryotic cells - These include animal cells and plant cells. They have their genetic material (DNA) stored inside a nucleus.

Prokaryotic cells - These are bacterial cells. They do not have a nucleus, so their genetic material floats freely in the cell.

Features both cell types share

Despite their differences, both eukaryotic and prokaryotic cells share some essential features that are necessary for life.

Both eukaryotic and prokaryotic cells have:

• Cell membrane - Controls what goes in and out of the cell
• Cytoplasm - Jelly-like substance where chemical reactions happen

Special features of bacterial cells (prokaryotes)

Bacterial cells have evolved unique characteristics that distinguish them from eukaryotic cells and help them survive in diverse environments.

Bacterial cells are different because they:

• Are surrounded by a cell wall for protection and shape
• Do not have a nucleus - their DNA floats freely in the cytoplasm

Inside a bacterial cell

Understanding the internal structure of bacterial cells helps explain how these simple organisms can be so successful and diverse.

Chromosomal DNA - A single loop of DNA that carries most of the bacterial genes. It lies free in the cytoplasm instead of being inside a nucleus.

Ribosomes - Tiny structures that make proteins. They are smaller than ribosomes in plant and animal cells.

Plasmids - Extra circles of DNA that contain additional genes. These genes are not found in the main chromosome. Plasmids help bacteria survive in different conditions.

Cell wall - Protects the bacterium, but it is made of different substances than plant cell walls.

Flagellum - Some bacteria have this whip-like tail that helps them move around.

Working with very small measurements

Cells are incredibly small, often measured in micrometres or even nanometers. Scientists use standard form to write these tiny numbers more easily and avoid confusion with many zeros.

Standard form basics

Standard form has two parts: $7.3 \times 10^{-6}$

• The first part (7.3) is a number between 1 and 10
• The second part ($10^{-6}$) is a power of 10

For large numbers: The power is positive (e.g., $920,000 = 9.2 \times 10^5$)

For small numbers: The power is negative (e.g., $0.00703 = 7.03 \times 10^{-3}$)

Useful measurement units

• Millimetre (mm) = $10^{-3}$ metres
• Micrometre (μm) = $10^{-6}$ metres
• Nanometre (nm) = $10^{-9}$ metres

Size examples

Understanding the relative sizes of different cells helps put their structure and function into perspective.

• Animal cell: $0.01 \text{ mm} = 1 \times 10^{-5} \text{ m}$
• Bacterial cell: $1 \text{ μm} = 1 \times 10^{-6} \text{ m}$

This means an animal cell is 10 times larger than a bacterial cell - that's one order of magnitude bigger.

Estimation tips

When you need a quick answer or want to check your calculation, estimation is a valuable skill that can save time and help identify errors.

1. Round all numbers to 1 significant figure
2. Do the calculation with the rounded numbers
3. This gives you a rough answer to compare with your detailed calculation