Life Cycles

New Unit: Life Science

What are we learning this week?

The student is expected to investigate and compare how animals and plants undergo a series of orderly changes in their diverse life cycles, such as tomato plants, frogs, and lady bugs.

Key Concepts

• Organisms undergo observable changes during their life cycles including birth, growth, development, reproduction, and death.
• We can compare the life cycles of various plants and animals.
• Some animals, such as frogs and lady bugs, pass through distinctly different life stages with very different appearances. Most plants, such as tomato plants, develop from seeds into small plants that resemble the adult form.

Fundamental Questions

• What are the general stages of the life cycle of living things?
• What investigations can we conduct to learn about the diverse life cycles of different plants and animals?
• What comparisons can we make among the diverse life cycles of different plants and animals?

Key Concept 1: Organisms undergo observable changes during their life cycles, including birth, growth and development, reproduction, and death.

An organism’s life cycle refers to each stage of its life; birth, life, and death. By reproducing, a species’ life cycle is repeated over and over with the main goal is to continue to survive in its environment.

Key Concept 2: We can compare the life cycles of various plants and animals.

Most organisms change their appearance from birth to the adult stage. However, those changes can be simple, like the life cycle of tomato plants, or complicated, like the life cycle of ladybugs and frogs. Emerging from a seed or an egg, life begins small and fragile. A seedling or young animal must be nurtured to continue growth into the adult stage of life. Students can find examples of the various stages in the life cycle of plants and animals in the library, on the Internet, outside at home or school, or in local fields.
Remind students to respect the environment and protect the plants and animals they are observing. Students can record observations on digital cameras, in sketchbooks, or make descriptions on a tape recorder. They must have permission of property owners if they collect specimens. Also, discuss safe and proper collection methods and what type of specimens are appropriate to collect.

Key Concept 3: Some animals, such as frogs and lady bugs, pass through distinctly different life stages with very different appearances, while most plants, such as tomato plants, develop from seeds into small plants that resemble the adult form.

Plants demonstrate a relatively simple process of life. Beginning with the seed of the plant placed in the ground through an act of nature or by human hand, eventually it will begin to grow and break through the crust of the ground to become a seedling, which is just a small plant. Young plants (seedlings), such as tomato plants usually resemble their parents, though smaller in size. Their leaves, color, and shape will copy their parents. As a seedling becomes an adult plant, the general appearance will not change; it will only be larger with more limbs, branches, and leaves. Once a plant reaches the adult stage, it prepares for reproduction by creating seeds. A plant sometimes protects its seed by either encasing it in a hard shell, such as a walnut, or a soft, fleshy covering, such as a tomato. When the tomato falls from the vine, this protection gives the seeds a chance to begin to grow. Some plants use flowers to attract insects to carry their seeds to other plants for fertilization. When the seed falls to the ground, and if is fortunate enough to settle into good soil and receives water, it will begin to grow into a seedling and the life cycle will be repeated.

Many animals, including fish, mammals, reptiles, and birds have relatively simple life cycles: they are born (either alive from their mothers or hatched from eggs), grow up, and become adults. Most young animals are similar to their parents, just smaller, and slowly grow into adulthood. This form of life process is called direct development.

Amphibians, such as frogs, have completely different life cycles. Frogs change their appearances from childhood to adulthood. This is called complete metamorphosis. (Third grade students are not responsible for this term). Frogs lay floating clumps of soft, jellylike eggs in water. The eggs hatch into a water breathing, fishlike tadpole with gills and no limbs. Slowly, legs develop, and the tadpole tail shortens. Adult frogs become air breathing, land animals, with fully-grown legs and lungs.

Insects also experience a complete change in their appearances during their life cycle. For example, butterflies and ladybugs have four stages of life:
(1) Egg, or unborn stage
(2) Larva, the young stage when most feeding is done and the insects look like worms
(3) Pupa, the no-feeding stage when the insect is camouflaged insides a sac and changes from a worm shape to a full grown insect
(4) Adult, the stage when the insect leaves the pupa, usually has wings, and is ready for reproduction.