Edexcel IGCSE Biology · Spec 2.1
Levels of Organisation
How organelles, cells, tissues, organs and organ systems build up to a whole organism, with animal and plant examples.
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Levels of Organisation
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Explained
Levels of organisation, built one stage at a time
Five levels are named in the specification: organelles, cells, tissues, organs and systems. Each is built from the one before it, and the definitions say so, which is why learning them as a chain is far more useful than learning them as a list.
The five levels
- Organelle: a specialised structure inside a cell that carries out a particular function.
- Cell: the basic structural unit of living organisms.
- Tissue: a group of similar cells working together to carry out a function.
- Organ: a structure made from different tissues working together.
- System: a group of organs working together to carry out a major function.
The whole organism is useful context, but it is not a sixth level in the specification point. It is the complete individual containing the systems.
Similar, different, together
Three words separate the three middle definitions, and getting them the right way round is most of the topic.
Similar cells make a tissue. Different tissues make an organ. Different organs working together make a system. Writing that an organ is made of similar tissues loses the mark, because that describes a tissue rather than an organ.
Worked examples in both kingdoms
In an animal: a mitochondrion is an organelle, a muscle cell is a cell, muscle tissue is a tissue, the heart is an organ, and the circulatory system is a system.
In a plant: a chloroplast is an organelle, a palisade cell is a cell, palisade tissue is a tissue, a leaf is an organ, and the shoot system is a system.
Questions frequently give a plant example expecting an animal answer, or the reverse. A leaf is an organ for exactly the same reason a heart is: it is made of several different tissues working together.
Placing an unfamiliar structure
Ask what it is made from. If it is inside a single cell, it is an organelle. If it is made of one kind of cell, it is a tissue. If it is made of several kinds of tissue, it is an organ. If it is several organs cooperating, it is a system.
That test works even for structures you have never met, which is exactly the situation these questions are designed to create.
Why the order is worth knowing in both directions
You may be asked to arrange structures from smallest to largest, or to say what a named level is made from. Both are the same chain read in opposite directions, so learn it once and read it whichever way the question needs.
Spec 2.1
What you need to know
- List the levels from organelle up to organism
- Give an example of each level
- Put the levels in order of size
Active recall
Quick check
Answer each question before opening the answer.
Put these in order of size: organ, cell, organism, tissue, organ system.
Cell → tissue → organ → organ system → organism.
Define a tissue and an organ.
A tissue is a group of similar cells working together; an organ is a group of different tissues working together to perform a function.
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