IGCSEBiologyEdexcelBiology (Modular)Unit 1Revision Notes2. Structure & Functions in Living Organisms: Part 1Levels of OrganisationLevels of Organisation

Levels of Organisation (Edexcel IGCSE Biology (Modular): Unit 1): Revision Note

Exam code: 4XBI1

Written by: Lára Marie McIvor

Reviewed by: Lucy Kirkham

Updated on 2 July 2026

Cells, Tissues, Organs & Organ Systems

Cells are the basic building blocks of all living organisms
Unicellular organisms are made from one cell, whereas multicellular organisms are made up of collections of cells
Cells are made up of structures called organelles, such as the nucleus, mitochondria and ribosomes
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

Different levels of organisation in organisms 1, downloadable AS & A Level Biology revision notes

downloadable AS & A Level Biology revision notes

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

Diagram showing levels of organisation: 1 is a leaf, 2 is a section through leaf showing tissue, 3 is a cell. A table lists possible categorisations.

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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