What is growth?
Growth is the permanent increase in size and dry mass of an organism. It is irreversible, which distinguishes it from a temporary change such as swelling after drinking water. Growth results mainly from cell division by mitosis, followed by the new cells enlarging and becoming specialised (differentiation). It is measured using parameters such as height, body mass or length. Because water content changes from day to day, scientists often use dry mass as the most reliable measure, even though the organism must be killed to obtain it.
The human growth curve
If height or mass is plotted against age, human growth follows an S-shaped (sigmoid) curve. Growth is rapid in infancy, slows during childhood, speeds up again at the adolescent growth spurt during puberty, and then levels off in adulthood when growth stops. This single overall pattern is called continuous growth, typical of humans and other mammals.
Growth in insects
Some animals, such as insects, have a hard external skeleton (exoskeleton) that cannot stretch. They must shed it in a process called moulting (ecdysis) to grow. Their growth curve is step-like (intermittent): size jumps up at each moult and stays constant in between.
| Feature | Humans/mammals | Insects |
|---|---|---|
| Curve shape | Smooth S-shape | Step-like |
| Growth type | Continuous | Intermittent |
Example
A grasshopper stays the same size within one stage, then suddenly appears larger after moulting, giving a staircase-shaped growth curve, while a child grows steadily along a smooth curve.
Key idea
Growth = permanent increase in size and dry mass through mitosis and cell enlargement. Human growth is a smooth sigmoid curve; insect growth is a step-like curve caused by moulting.