Growth and development
Growth is a permanent increase in size, usually from an increase in the number of cells (by mitosis) and the amount of material in them. Development is the change in form and complexity as an organism matures — including differentiation, where unspecialised cells become specialised for particular jobs. The two go together: an organism grows larger and, at the same time, more organised.
Development in animals
After fertilisation, the single-celled zygote divides repeatedly by mitosis to form an embryo. Its cells differentiate into the tissues and organs of the body, and in humans the embryo becomes a fetus that grows until birth. Growth continues through childhood to adulthood, then slows; later, cells repair and replace themselves less effectively — the process of ageing.
Growth in plants
Plants grow throughout their lives, but only at special regions called meristems — at the tips of shoots and roots — where cells divide, then enlarge and differentiate. Growth begins with germination: given water, oxygen and warmth, a seed's embryo uses its stored food to grow a root and shoot, until the seedling opens its leaves and begins to photosynthesise for itself.
Growth is more than getting bigger
Growth (more cells and material) and development (differentiation into specialised cells and organs) happen together — an organism becomes both larger and more organised as it matures.
A worked example
Describe what happens to a fertilised egg (zygote) as it develops into a fetus.
- The zygote divides repeatedly by mitosis.Mitosis increases the number of identical cells.
- A ball of cells — the embryo — forms.Repeated division builds the early embryo.
- The cells differentiate into specialised tissues and organs.Development means cells becoming specialised.
- The embryo grows and develops into a fetus, which continues to grow until birth.Growth and development continue through pregnancy.
The zygote divides by mitosis into an embryo, whose cells differentiate into tissues and organs, growing into a fetus that develops until birth.
Measuring growth
Growth can be measured in different ways — by height or length, by mass, or by counting cell number — and each tells a slightly different story (a plant may gain mass while drying out lowers it temporarily). Plotting a measurement against time gives a growth curve, which for many organisms is S-shaped: slow at first, then rapid, then levelling off as the adult size is reached. Comparing growth is often done as a percentage change so different-sized organisms can be compared fairly.
Metamorphosis and life cycles
Not all animals simply get bigger — many change form as they develop, a process called metamorphosis. In complete metamorphosis (butterflies, beetles) the animal passes through very different stages — egg, larva, pupa, adult — so the young and adults eat different foods and do not compete. In incomplete metamorphosis (locusts) the young (nymphs) resemble small adults and change gradually. Amphibians show a similar dramatic change, from water-living tadpole to land-living frog.