In this lesson, you’ll learn the definition of ‘life cycle’ and find out about the three basic stages of a life cycle. You’ll also explore examples of different life cycles.
Definition of a Life Cycle
Can you imagine if humans were born as full grown adults? Thankfully, we are born as infants and slowly go through stages before reaching adulthood.
These stages are called a life cycle. A life cycle is defined as the developmental stages that occur during an organism’s lifetime. A life cycle ends when an organism dies.In general, plants and animals go through three basic stages in their life cycles, starting as a fertilized egg or seed, developing into an immature juvenile, and then finally transforming into an adult.
During the adult stage, an organism will reproduce, giving rise to the next generation.A life cycle can be comprised of more than the three basic stages depending on the species. For example, the human life cycle is comprised of 5 main stages. The names of each stage can also vary slightly depending on the species. For example, an immature juvenile dragonfly is called a nymph.The time an organism spends at each stage can differ.
For example, one species of cicada can spend 17 years as an immature nymph. That is about the age we are when we graduate high school! Then once the cicada reaches adulthood, it only lives about 24 hours before it dies. Other organisms spend more time as adults. For example, elephants reach maturity after 15 years and then spend over 30 years as an adult.
Life Spans Vary
Did you know there are ancient trees that are thousands of years old? Bristlecone pine trees have a life span reaching over 4,000 years. A life span is the time it takes for an organism to complete its life cycle. Some life spans are short for example, like a variety of bees only live for 4 to 5 weeks.Let’s look closer at some examples of life cycles:
Flowering Plant Life Cycle
Flowering plants have a simple life cycle comprised of three main stages, a seed, seedling, and a mature plant.A germinated seed sprouts into an immature seedling.
Over time, the seedling will develop into a mature, reproducing adult where it will produce flowers and seeds. If these seeds are fertilized, it will give rise to the next generation. The time it takes for flowering plants to complete their life cycle ranges from one year to decades.
Salmon Life Cycle
Salmon have a complex life cycle comprised of 6 main stages. Salmon begin their life in a stream as a fertilized egg. Once the eggs hatch, they are infant salmon called alevin. Upon leaving their nest, they become juvenile fish called fry.
As they grow larger, fry start to swim toward the ocean and become smolt. After a few weeks, smolt develop into the next stage called ocean phase adult salmon. After a few years, salmon mature and become spawning phase adult salmon, and they return to they’re nesting grounds and find a mate.
Shortly after they mate, salmon die. The time it takes for salmon to complete their life cycle ranges from 3 to 7 years.
Human Life Cycle
The human life cycle is complex and is comprised of five main stages. The life cycle starts as a fertilized egg. Then after 40 weeks in utero an infant is born.
Infancy is considered from birth until approximately one year of age. After one year a human enters the next stage of its life cycle, childhood . Childhood lasts until puberty. Puberty starts at approximately 12 years old and is when humans enter adolescence . When a human reaches 18 years of age, it enters the last stage of the life cycle, adulthood .
In adulthood, humans reproduce, giving rise to the next generation. The total time it takes for a human to complete it’s life cycle is approximately 75 years.
A life cycle is defined as the developmental stages that occur during an organism’s lifetime. In general, the life cycles of plants and animals have three basic stages including a fertilized egg or seed, immature juvenile, and adult. However, some organisms may have more than 3 life cycle stages, and the exact names of each stage can slightly differ depending on the species.
The time an organism spends at each stage also varies. The time it takes for an organism to complete its life cycle is called a life span. The life span of organisms ranges from weeks to thousands of years.