DK life cycles everything from start to finish生命周期从开始到结束循环

What is a life cycle

Life is always changing, but it follows patterns too. Living things—including ourselves—grow, produce young, and die. There are repeating processes for nonliving things too, from mountains, rocks, and rivers to planets, comets, and stars. We call all these patterns life cycles.

Life cycles are interlinked. Plants take nutrients and water from the soil and energy from sunlight. Animals eat plants or other animals in order to grow.
Many plants rely on animals such as insects to spread their pollen so they can make seeds and reproduce. When plants and animals die, their remains rot and become part of the soil that will nourish new plants.

The life cycles we see around us appear endless. Birth and growth are balanced by breakdown and decay. Yet these cycles can be fragile.
Change—whether natural or caused by humans—can disrupt them. Vulnerable species may dwindle to extinction when their cycles get broken, while—over millions of years— new species emerge by evolution

Earth and space
Our Earth is constantly changing. Rocks wear away and get recycled into new rocks, while water circulates between sea, sky, and land. In space, comets and stars form from gas and dust, burn out, then return to gas and dust.

Earth’s life cycles are tiny fractions of the age of the universe: 13.8 billion years

Sexual life cycles
Sexual reproduction involves two parents—male and female—and each supplies a sex cell. The female egg cell fuses (joins) with the male sperm cell, in a process called fertilization. The fertilized egg develops into a new animal or plant.

Asexual life cycles Some plants and animals reproduce asexually, from a single parent. The young develop from unfertilized eggs or form from a small part of the parent. The young are exact replicas (copies) of their parent.

Space is unimaginably huge and full of exciting things, such as planets, moons, galaxies, and black holes. It is also constantly changing— stars form from beautiful clouds of dust and gas, evolve through their life cycle to slowly die away, or go out with an enormous bang. Everything that has or will ever be formed was created in space—including the building blocks of you!

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