Cells, tissues, organs and organ systemsSpec 4.2.1
Living things are built up in levels. Each level is made from the one below it.
- Cell
- The basic building block of all living organisms.
- Tissue
- A group of cells with a similar structure and function.
- Organ
- An aggregation of tissues performing specific functions.
- Organ system
- Organs are organised into organ systems, which work together to form organisms.
The stomach is a good example of an organ. It is made of several tissues that work together. Muscular tissue churns the food, glandular tissue makes digestive juices (enzymes and acid), and epithelial tissue covers the inside and outside surfaces.
The stomach is one of the organs in the digestive system. This is an organ system: several organs working together to digest and absorb food.
| Level | Animal example | Plant example |
|---|---|---|
| Cell | A muscle cell | A palisade mesophyll cell |
| Tissue | Muscular tissue | Palisade mesophyll |
| Organ | The stomach or the heart | A leaf |
| Organ system | The digestive system or the circulatory system | Roots, stem and leaves (transport system) |
| Organism | A human | A tree |
Tap to enlargeSize and scaleSpec 4.2.1
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
Quick check
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.