Size and scaleSpec 4.2.1
In short
Each level of organisation is bigger than the one before: a tissue is many cells, and an organ is many tissues. Most cells are only tens of micrometres across, so you need a microscope to see them, while tissues and organs are measured in millimetres or centimetres. To compare sizes, convert mm to µm by multiplying by 1000.
Each level of organisation is bigger than the one before. A tissue is many cells, an organ is many tissues, and an organ system is several organs.
Most cells are only tens of micrometres (µm) across, so you need a microscope to see them. Tissues and organs are large enough to see with the naked eye, and are measured in millimetres (mm) or centimetres (cm).
To compare sizes you must use the same unit for both. To change mm to µm, multiply by 1000.
How many cells fit across a tissue?
A strip of tissue is 2 mm wide. Each cell is 40 µm wide. How many cells fit across the strip?
- Convert 2 mm to µm: 2 × 1000 = 2000 µm.
- Divide the width of the tissue by the width of one cell: 2000 ÷ 40.
Answer: 50 cells
Always check the units match before you divide. Mixing mm and µm is the most common mistake in size and scale questions.
Quick check
What is a tissue?
Show answer
A group of cells with a similar structure and function.
Why is the stomach an organ?
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It is made of several tissues that work together to perform a specific function.
Put these in order from smallest to largest: organ, cell, organ system, tissue.
Show answer
Cell, tissue, organ, organ system.
How many micrometres are in 1 mm?
Show answer
1000 µm.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026