Building up from the cell
A complex body is organised in a clear order of increasing size and function. The levels of organisation are:
Key idea
Cell → Tissue → Organ → System → Organism
- A cell is the basic unit of life.
- A tissue is a group of similar cells doing the same job (e.g. muscle tissue).
- An organ is made of several tissues working together (e.g. the heart).
- A system is a group of organs carrying out a major function (e.g. the circulatory system).
- All the systems together form the whole organism.
Examples in humans and plants
In humans, muscle tissue and other tissues form the heart (an organ), which is part of the circulatory system. In plants, similar levels exist: cells form tissues such as xylem, which build organs such as the leaf.
Example
The stomach is an organ built from muscle tissue, glandular tissue and other tissues. It works with the mouth and intestines as part of the digestive system.
Systems that work together
The systems of a body do not act alone. The respiratory system takes in oxygen, the circulatory system carries that oxygen to every cell, and the digestive system supplies the food that cells break down for energy. If one system fails, the others are affected, because all of them serve the same living body. This is why doctors study how the systems interact, not just each organ on its own.
Why organisation matters
Organising cells into tissues, organs and systems allows a division of labour, so a large multicellular organism can perform many complex functions efficiently. Each level adds a new ability that a single cell could never manage alone, and together they build a body that can grow, sense its surroundings, move and reproduce as one coordinated whole.