Growth, Development, and Organism Life Cycles

Growth, development and life cycles describe how an organism increases in size, changes in form and passes through the stages of its life from beginning to end. In brief: growth is an increase in size or cell number, development is the change in structure and function over time, and a life cycle is the whole sequence of stages from one generation to the next. A frog, for example, goes from egg to tadpole to adult frog, which then produces new eggs.

Every organism has a life cycle, from a bacterium that divides in minutes to a tortoise that lives more than a century. This guide explains growth and development, how reproduction closes the cycle, what birth, growth, reproduction and death mean in biology, and how to compare life cycles of different organisms.

What is growth?

Growth is a permanent increase in the size or mass of an organism. In most multicellular organisms it happens in two ways: cells divide to produce more cells, and cells enlarge after dividing. Both need nutrients and energy.

Not all organisms grow the same way. Humans grow mostly during childhood and adolescence, then stop increasing in height. Many trees and some fish keep growing throughout life. Insects with a hard outer skeleton grow in steps, because they must shed the skeleton, a process called molting.

Growth is measured in different ways, such as height, length, mass or the number of leaves. Because water content changes, dry mass is a more reliable measure in plants than fresh mass.

What is development?

Development is the series of changes in structure and function that an organism goes through as it matures. Growth concerns size, while development concerns form and ability. A baby grows when it gets taller, and it develops when it learns to walk and its bones harden.

During development, cells become specialized. A fertilized egg divides to produce a ball of similar cells, and these later become muscle, nerve, skin and other cell types. Plants show the same idea: a seed develops roots, a stem, leaves and finally flowers.

Why is reproduction part of the life cycle?

Reproduction produces new organisms, so it connects one generation to the next. Without it, a species would disappear when its individuals die.

• Asexual reproduction: One parent produces offspring that are genetically identical to it, as when a bacterium divides in two.
• Sexual reproduction: Two parents contribute sex cells that join in fertilization. The offspring differ genetically from both parents.

Individuals do not need to reproduce to be alive, but the species needs reproduction to continue. Many organisms reproduce once, such as Pacific salmon, while others reproduce many times over their lives.

What is a life cycle?

A life cycle is the complete series of stages an organism passes through from the start of one generation to the start of the next. It is called a cycle because the final stage, the reproducing adult, leads back to the first stage.

Life cycles can be simple or complex. A simple one, like that of a mammal, has stages that look similar: a young animal is a smaller version of the adult. A complex one involves large changes of body form, called metamorphosis, which is covered in detail in a separate article.

What are birth, growth, reproduction, and death?

Most life cycles can be described using four general stages. The names vary by organism, but the idea is shared.

Some organisms do not fit neatly. Bacteria that divide may not have a clear death, because the parent cell becomes the two daughter cells. Biologists therefore treat the four-stage model as a useful tool, not a rule.

Worked example: tracing a bean plant’s life cycle

A bean plant is a good example of a complete life cycle in a few months. Follow the stages in order.

1. Seed: A dry bean seed holds an embryo and stored food. It is dormant until it has water, oxygen and a suitable temperature.
2. Germination: The seed absorbs water, the root emerges first and then the shoot grows upward.
3. Seedling and growth: Leaves open and begin photosynthesis. The plant grows taller and produces more leaves.
4. Flowering: The mature plant produces flowers, which are pollinated and fertilized.
5. Fruit and seed formation: Pods develop with new seeds inside.
6. Death and new generation: The bean plant is an annual, so it dies after seeding. The seeds can start the cycle again.

The cycle ends where it began, with seeds. Growth happened in steps 2 and 3, development in steps 3 to 5 as new organs formed, and reproduction in steps 4 and 5.

How do life cycles of organisms compare?

Life cycles differ widely in length and in how much the body changes. The table compares several typical examples. Values for lifespan are approximate and depend on the species and the conditions.

Comparing these cycles shows common patterns. Small organisms with short lives tend to produce many offspring, while larger organisms with long lives often produce fewer offspring and give them more care. These are general trends with many exceptions.

How did scientists learn about life cycles?

Aristotle described the development of chick embryos and many animals in ancient Greece, but some of his ideas were later corrected. In the seventeenth century, the microscope allowed people to see eggs, sperm and the earliest stages of development.

Maria Sibylla Merian, a naturalist and artist born in 1647, carefully recorded the stages of insects and the plants they feed on. Her detailed illustrations of caterpillars, pupae and adult butterflies helped to show that these stages belong to a single animal. Later, cell theory and genetics explained how growth and inheritance work at the level of cells.

Why do life cycles matter in practice?

Farmers plan planting and harvesting around crop life cycles. Gardeners distinguish annual plants, which complete their cycle in one season, from perennials, which live for several years.

Understanding life cycles also helps control pests and diseases. Public-health workers target a mosquito’s aquatic larval stage, and doctors time treatments for parasites according to their development. In conservation, protecting breeding sites matters most when a species is vulnerable at one stage, such as when turtle eggs are laid on a beach.

Which mistakes are common?

Mistake 1: Treating growth and development as the same thing

Growth is an increase in size. Development is a change in form or function. A tadpole that gains mass is growing, and a tadpole that loses its tail and develops legs is developing.

Mistake 2: Thinking every life cycle ends with death of the parent

Many cycles do, but some organisms divide and no individual dies in the usual sense. Others, such as perennial plants, reproduce many times before they die.

Mistake 3: Believing all young animals look like small adults

Mammals and birds do, but insects, amphibians and many marine animals have larvae that differ greatly from adults in appearance and diet.

Mistake 4: Starting a cycle diagram at the wrong place

A cycle has no true starting point. A diagram can start at the egg or seed only as a convention, because every adult once began as one of these stages.