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Korea Biodiversity Information System > Plant Resource > Plant class > What is a plant?
The Earth's biosystem is divided into animals, fungi, and plants.
A plant is a living organism that has cell walls and chlorophyll, and is capable of photosynthesis through autotrophism.
In addition, it is characteristically incapable of locomotion. However, there are some exceptions: for example, unicellular algae such as euglena have no cell walls, and fungi such as mushrooms get nutrition by heterotrophism.
The Earth is about 4.5 billion years old.
At that time, there were no living organisms on the Earth. There were only seas covering the ground and only carbon dioxide and ammonia in the sky. Then, 3.5 billion years ago, the carbon dioxide and ammonia gases were turned into small protein clots by receiving sunlight. When it rained, they dropped into the seawater with the rain. From this, the first living creatures, bacteria, appeared. Bacteria evolved over a long time, and the first green plants that appeared on the Earth were green algae such as chlamydomonas and chlorella.
Algae refers to plants that live in water, such as seaweed.
The linear cell structure of algae changed to a wide planar structure attached to the surface of the land when green algae began to live on dry land.
These early land plants,
lived by being attached to moist soil, so water that was used in metabolism or evaporated into the air could be supplemented by diffusion from the soil. There was rich sunlight and carbon dioxide and no competitors, so these early land plants could prosper as long as sufficient water was supplied. Green algae produced nutrients and oxygen, and thus produced the first large amounts of oxygen on Earth.
Then, plants that absorbed oxygen gradually appeared,
and after the development of measures to prevent dehydration, they came to grow in sites away from coasts, lakes, and rivers. After an enormous amount of time passed, these algae evolved into the various plants that exist today.
Simple land plants such as Sphaerocarpus were derived from green algae, and then the early land plants gymnosperms and angiosperms appeared.
