2.4 Levels of Organisation
A single cell is the basic unit of life — but a tree or a tiger is not just one cell. How do tiny cells build something as complex as a whole living body? They do it in layers, each level built from the one below.
The Ladder of Life
\textbf{Cell} \rightarrow \textbf{Tissue} \rightarrow \textbf{Organ} \rightarrow \textbf{Organ system} \rightarrow \textbf{Organism}
- A cell is the basic unit of life — like a single brick.
- A group of similar cells working together forms a tissue — like a patch of wall.
- Different tissues combine to form an organ (such as the stomach or a leaf).
- Several organs that share a major job form an organ system (such as the digestive system).
- All the organ systems together make a complete organism — a whole plant or animal.
This is why the brick-and-wall comparison is so useful: a brick (cell) builds a wall (tissue), walls build a room (organ), rooms build a house (organ system), and houses make a whole building (organism). Each level is simple on its own, yet together they produce something remarkably complex.
Drag the idea into order below: put the five levels in sequence from the smallest building block to the whole living being.
From one cell to a whole being
Here is a wonderful fact: the life of a complex organism begins as a single cell — the egg. That one cell has the astonishing ability to divide again and again, building a complete body made of countless cells. Living things built from many cells this way — animals, humans, plants — are called multicellular organisms.
Science Talk One cell becomes 2, then 4, then 8, doubling each time it divides. After just 10 rounds of doubling, how many cells would there be? After 20? Notice how quickly a single egg can build a body of millions of cells — that is the power of repeated division.