According to Newton's working method, before studying the essence of a thing, we must first define our research object, so let's define what a plant is first.
Plants, in biological classification, refer to living organisms, and autotrophic organisms that can lead a fixed life are called plants.
Plants can produce their own food through photosynthesis during their growth. In other words, they don't need to hunt like animals to survive and reproduce, so mobility is no longer an evolutionary advantage for them, but consumes energy. In the process of evolution, the function of movement is gradually lost, and plants can only survive in one place for a long time.
Because plants have lost the function of movement, they are easily attacked by microorganisms, germs and animals in the process of evolution. Over time, these germs and microorganisms will lead to the mass extinction of autotrophic plants. However, a few plants have evolved the structure of cell wall due to genetic variation. Generally, the cell wall consists of polysaccharide cellulose, hemicellulose and pectin. These substances can be closely woven into a specific grid, just like soldiers' armor, and can rely on physical barriers to prevent the invasion of microorganisms, so they can survive and pass on to the next generation.
In order to resist the predation of animals, plants often evolve into poisonous, spiny, inedible and unpalatable tall plants.
For example, eucalyptus is poisonous and most animals can't eat eucalyptus leaves. Roses have thorns. When living things eat roses, they don't eat the whole plant at once. Pine trees give off a strong pungent smell to drive away animals.
The wisdom of these plants has been handed down from generation to generation, and it has become the earth we see today.
Furthermore, plants need photosynthesis to survive, but in the process of photosynthesis, high concentration of carbohydrate solute will lead to the rapid absorption of water into cells, which will easily lead to cell membrane rupture.
Cell wall in the form of polysaccharide can effectively reduce osmotic pressure, provide mechanical support for cells like skeleton, prevent cells from breaking, and make plants survive.
Animals don't have cell walls because animals don't need photosynthesis, and there is no high osmotic pressure environment in their bodies, so cells can exist intact.
In addition, plants also need to fight the loss of water in the body. We know that plants will grow as many branches and leaves as possible for better photosynthesis, and spread as many leaves as possible to absorb sunlight. But at the same time, too many leaves also make the water in plants evaporate. If plants don't have cell walls, the water in plant cells will be quickly evaporated by the sun, so that plants can't live on land and can only grow in water.
From these perspectives, the reason why plants have cell walls is to resist the harsh environment and adapt to the environment of the earth.
As we know, the difference between animals and plants is that animals belong to heterotrophs and cannot produce energy through photosynthesis. This causes animals to need strong fluidity, and the fluidity of cell wall is not as strong as that of cell membrane.
In addition, compared with plants, animals have relatively small body surface area and limited water evaporation. In addition, most animals have preserved the characteristics of hair in evolution, so the evaporation of water is small, so animals do not need the evaporation of body fluids from cell walls.
Most importantly, in the process of evolution, animals can drive away the attacks of parasites and other animals by moving, flapping and biting, without relying on cell walls to resist external attacks.
Plants have cell walls because they can resist microorganisms, prevent water from evaporating too fast, and prevent cells from breaking due to excessive osmotic pressure.
Animals have no cell walls because they belong to heterotrophs and their cell structures are different from those of plants.