How are cells organised into tissues, organs and systems in a large organism?
Describe the levels of organisation from cell to tissue to organ to organ system to organism
A focused answer to the O-Level Biology outcome on biological organisation. The sequence from cell to tissue to organ to system to organism, with clear examples and why division of labour matters.
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What this dot point is asking
SEAB wants you to describe how, in a multicellular organism, cells are organised into larger and larger structures: cells form tissues, tissues form organs, organs form organ systems, and systems make up the whole organism. You should be able to define each level and give an example, and explain why this organisation allows division of labour.
The answer
The five levels, smallest to largest
- Cell
- The basic unit of life. Each cell is specialised for a job, such as a muscle cell or a nerve cell.
- Tissue
- A group of similar cells that work together to carry out a particular function. For example, muscle tissue is made of many muscle cells that contract together.
- Organ
- A structure made of several different tissues working together to perform a function. The heart, for example, contains muscle tissue, nerve tissue and blood vessels.
- Organ system
- A group of organs that work together to carry out a major life process. The digestive system, for example, includes the stomach, intestines, liver and pancreas.
- Organism
- A complete living thing made up of several organ systems working together, such as a human or a plant.
The sequence
Cells join to make tissues; different tissues join to make an organ; several organs together form an organ system; and the systems together make the whole organism. Each step adds complexity and allows more specialised work.
Why this organisation matters: division of labour
Because cells are specialised, different cells, tissues and organs each do their own particular job. This is called division of labour, and it lets a large organism carry out many processes efficiently at the same time. A single cell could not do everything a whole body needs.
Examples in plants
The same idea applies to plants. A palisade cell is a cell; the palisade layer is a tissue; the leaf is an organ; the shoot (leaves, stem and buds) is part of the plant system; and the whole plant is the organism.
Examples in context
Example 1. The digestive system. The stomach and small intestine are organs; together with the liver and pancreas they form the digestive system; and the system as a whole breaks down and absorbs food for the organism. Each organ does its part.
Example 2. A leaf as an organ. A leaf contains several tissues, such as the palisade layer for photosynthesis and the xylem and phloem for transport. Because it combines different tissues for one function (making food), the leaf is an organ, not a tissue.
Try this
Q1. Write the levels of organisation in order from smallest to largest. [2 marks]
- Cue. Cell, tissue, organ, organ system, organism.
Q2. Define an organ. [2 marks]
- Cue. An organ is a structure made of several different tissues that work together to carry out a particular function.
Q3. Explain why division of labour is an advantage for a large organism. [2 marks]
- Cue. Different specialised cells, tissues and organs each carry out their own job, so many processes can happen efficiently at once, which a single cell could not manage.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksPlace the following terms in order of increasing size and complexity, then define the term tissue: organ, cell, organism, tissue, organ system.Show worked answer →
The correct order from smallest to largest is: cell, tissue, organ, organ system, organism.
A tissue is a group of similar cells that work together to carry out a particular function. For example, muscle tissue is made of muscle cells that contract together.
Markers reward the fully correct order and a definition of tissue that mentions a group of similar cells with a shared function. An order with two terms swapped scores partial credit only.
Original3 marksThe stomach is described as an organ. Explain what this means, naming one tissue it contains and the function of that tissue.Show worked answer →
An organ is a structure made of several different tissues working together to carry out a particular function. The stomach is an organ because it contains different tissues that together digest food.
One tissue it contains is muscle tissue, which contracts to churn and mix the food; or glandular tissue, which produces digestive juices; or epithelial tissue, which lines and protects the stomach.
Markers reward a correct definition of an organ as several tissues working together, plus one correctly named tissue paired with its function.
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