DK 全球视觉系列 one million things science 一百万件事科学系列
FULL OF LIFE
A coral reef is packed with marine life. Different species of fish, crustaceans, marine plants, and mammals, as well as billions of microscopic plankton, make the waters around a coral reef their home
LIFE ON EARTH Life exists in rich abundance on Earth. A species is a group of living organisms, such as humans, golden eagles, and blue whales, which share common characteristics and can breed with each other.
Scientists have identified almost two million different species, although there may be many more. These are divided up into five kingdoms—plants, animals, fungi, bacteria, and single-celled organisms known as protists. All share some basic characteristics of life that enable them to survive.
RESPONSIVENESS All living things have an awareness of their surroundings and are able to respond to their environment. An animal may respond to sound, heat, or light with specially adapted nerve cells sending signals from one part of the body to another.
Plants are sensitive to gravity with stems growing upward and roots growing downward. They also respond to light by bending toward light sources. This is known as phototropism.
There are more than 300,000 species of plants, ranging in size from the minute species of Wolffia (members of the duckweed family) to giant sequoia trees, the largest of which stand more than 330 ft (100 m) high.
Botanists divide plants into those that produce flowers and those that are nonflowering. Along with some bacteria, plants are the only living things that manufacture their own food through photosynthesis.
Photosynthesis is the process in which energy from sunlight is used to convert carbon dioxide and water into glucose and oxygen.
Chlorophyll in a plant’s leaves absorbs energy from sunlight. The energy-rich glucose is transported around the plant as food, but can also be used to form more complex chemical substances. These include cellulose, which is used to build the walls of plant cells.
REPTILES AND AMPHIBIANS
Reptiles are land-dwelling creatures with tough skins covered with scales. All reptiles are vertebrates (they have backbones) and most have four legs, although snakes and worm lizards do not.
For about 200 million years, giant reptiles called dinosaurs were the world’s largest creatures. Amphibians were the first vertebrates to move from water to land.
Ecosystems are complete communities of different living things, together with their environment including the climate. An ecosystem can be as small as a pond or a single rotting tree stump, or as large as a tropical rain forest.
Ecosystems are treated as separate units since this makes them easier to chart and study. In reality, they are rarely closed systems, and food and energy may flow from one ecosystem to another.
The human body’s building blocks are some 100 trillion cells. Many of these microscopic units are grouped together to form tissues. An organ, which performs one or more key functions,
consists of two or more types of tissues and often works along with other cells, tissues, and organs in a system. The human body relies on 12 major systems, including the lymphatic system and the circulatory system.
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