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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
The levels of organisation are: cell → tissue → organ → organ system → organism. Each level is built from the previous one, with increasing complexity.
Understanding how cells organise into the structures of a multicellular organism is a fundamental concept. This hierarchy appears throughout the syllabus: from digestion to gas exchange to coordination. Examiners test your ability to define each level and give appropriate examples.
Definitions and Examples
Level
Definition
Examples
Cell
The basic unit of life
Red blood cell, nerve cell, palisade cell
Tissue
A group of similar cells working together to perform a specific function
Muscle tissue, epithelial tissue, xylem tissue
Organ
A structure made of different tissues working together to perform a particular function
Heart, leaf, kidney, stomach
Organ system
A group of organs working together to perform a major body function
Circulatory system, digestive system, respiratory system
Organism
A complete living thing made up of organ systems
A human, a flowering plant, a fish
Applying Levels to a Specific Example
The digestive system as an example:
Cells: Epithelial cells lining the gut wall; goblet cells secreting mucus; muscle cells in the gut wall.
Organ system: The shoot system (stem + leaves) or root system.
Organism: The whole plant.
Exam tip: A leaf is an organ because it contains multiple tissues (epidermis, palisade mesophyll, spongy mesophyll, xylem, phloem) working together for photosynthesis.
Try a focused question (2 marks)
The stomach is an organ. Explain what this means in terms of tissues. (2 marks)
Show the answer and marking guidance
An organ is made up of different types of tissues.
These tissues work together to perform a specific function: e.g. The stomach contains muscle tissue (for churning), glandular tissue (for enzyme secretion) and epithelial tissue (for lining/protection).
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Defining a tissue as "a group of cells" without specifying they are SIMILAR cells with the SAME function.
Confusing organs with tissues: an organ contains multiple tissue types, a tissue is one cell type.
Forgetting plant examples: a leaf is an organ, not a tissue.