Levels of Organisation (Edexcel IGCSE Biology)
Exam board Pearson Edexcel · IGCSEUpdated 2026-07-02
Cells, Tissues, Organs & Organ Systems
Spec point —
2.1· describe the levels of organisation in organisms: organelles, cells, tissues, organs and systems
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Cells are the basic building blocks of all living organisms
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Unicellular organisms are made from one cell, whereas multicellular organisms are made up of collections of cells
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Cells are made up of structures called organelles, such as the nucleus, mitochondria and ribosomes
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In complex multicellular organisms:
- Cells are specialised to carry out particular functions
- These specialised cells form tissues
- The tissues form organs
- The organs form organ systems
Different Levels of Organisation Table
| Level | Description |
|---|---|
| Organelles | - Specialised structures within a cell that carry out specific functions - An example of an organelle is the mitochondrion, which is where aerobic respiration occurs |
| Cells | - Basic structural and functional units of all living organisms |
| Tissues | - A group of similar cells that work together to carry out a particular function |
| Organs | - A structure made up of different tissues working together to perform a specific function |
| Organ systems | - A group of organs that work together to carry out a particular function |
Multicellular organisms have many levels of organisation
Examples of Organ Systems in Animals & Plants Table
| Organ system | Organs | Tissue examples |
|---|---|---|
| Shoot system | - Leaf - Stem - Flower - Fruit | - Epidermis - Mesophyll - Xylem - Phloem |
| Root system | - Root - Tuber | - Xylem - Phloem |
| Digestive system | - Oesophagus - Stomach - Small intestine - Large intestine | - Muscle - Connective - Nerve - Epithelial |
| Circulatory system | - Heart | - Muscle - Connective - Nerve - Epithelial - Veins - Arteries |
| Respiratory system | - Trachea - Lungs | - Connective - Muscle - Epithelial |
| Excretory system | - Lungs - Kidneys - Skin | - Muscle - Connective - Nerve - Epithelial |
| Nervous system | - Brain - Spinal cord | - Nerves |
| Reproductive system | - Ovary - Uterus - Testes | - Muscle - Connective - Nerve |
Worked Example
The answer is B: 1 is the leaf organ, 2 is a palisade mesophyll cell and 3 is the upper epidermis.
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