Principles of organisation — AQA GCSE Biology
How cells, tissues, organs and organ systems build up an organism, and how their sizes compare.
Revision notes
2 short notes, in the order of the specification. Each one in short:
A tissue is a group of cells with a similar structure and function, an organ is an aggregation of tissues performing specific functions, and organs are organised into organ systems. The stomach is an organ: muscular tissue churns food, glandular tissue makes digestive juices and epithelial tissue covers its surfaces. It is part of the digestive system.
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.
Exam questions
4 exam-style questions (6 marks), each with its mark scheme.
Answer the questionsFlashcards
4 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of organisation on one page, so you can see where this subtopic fits.
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Quick check questions
What is a tissue?
Show answer
A group of cells with a similar structure and function.
Why is the stomach an organ?
Show answer
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.
Frequently asked questions
What is the difference between a tissue and an organ?
A tissue is a group of cells with a similar structure and function, such as muscular tissue. An organ is an aggregation of several tissues performing specific functions. The stomach is an organ because muscular, glandular and epithelial tissues work together in it: one churns the food, one makes digestive juices and one covers its surfaces.
What are the levels of organisation in biology?
The levels of organisation, from smallest to largest, are cells, tissues, organs, organ systems and the whole organism. Each level is made from the one below it. For example, muscle cells form muscular tissue, which is part of the stomach, an organ in the digestive system. In a plant, a palisade cell is part of a leaf.
What is an organ system?
An organ system is a group of organs that work together, and organ systems work together to form organisms. The digestive system is an example: organs such as the stomach, pancreas and small intestine work together to digest and absorb food. In plants, the roots, stem and leaves form an organ system for transport.
How do you convert millimetres to micrometres?
To convert millimetres to micrometres, multiply by 1000, because 1 mm = 1000 µm. Always change both sizes to the same unit before you compare or divide them. For example, a strip of tissue 2 mm wide is 2000 µm wide, so 50 cells that are each 40 µm wide fit across it.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026