Why are plants green? Why is blood red? Do blue-blooded creatures exist in the real world?

Humans have lived on Earth for generations. If we were to use a color to describe the planet that gave birth to us, I think blue would be the most appropriate. About 70% of the Earth is covered by blue oceans. In addition to blue, the second color that can represent the Earth should be green. Most of the plants on Earth are green. Why are plants green instead of other colors?

Because plants contain chlorophyll, chlorophyll dominates photosynthesis in plants. The current ecosystem on our earth and the oxygen that humans rely on for survival are all due to photosynthesis. Green plants absorb light energy, synthesize carbon dioxide and water into energy-rich organic matter, and release oxygen at the same time. Starting from primitive algae, oxygen has accumulated for hundreds of millions of years. Today, the oxygen content in the atmosphere is stable at about 21%, allowing humans and animals to breathe freely. Plants appear green because they contain chlorophyll, so why is chlorophyll green? Chlorophyll can produce photosynthesis, and first of all, it must absorb sunlight, which is a mixture of multiple light rays. Chlorophyll can absorb most of the red and blue light, while the green light is reflected back. Our human eyes naturally see plants appear green.

In addition to green plants, the vibrant earth also has many different kinds of animals. Animals do not have green chlorophyll in their bodies, but they almost all have red blood flowing in their bodies. Blood can transport various nutrients needed by the human body to the required tissue cells, and at the same time transport the waste produced by the metabolism of tissues and organs to the lungs and kidneys for excretion. Blood contains a large amount of iron, which combines with oxygen in the body and plays a vital role in the transportation and utilization of oxygen. It is the iron element that makes blood look red.

In 2002, a science fiction film starring Andy Lau, Shu Qi and other movie stars was released, telling the story of aliens coming to Earth and fighting against villains with the help of their friends on Earth. This popular film is "Blue Blood Man", in which the aliens have blue blood flowing in their veins. Coincidentally, in another international science fiction masterpiece "Avatar" released in 2010, the Na'vi people on Pandora also have blue blood.

People can't help but ask, in the real world, are there creatures on Earth with blue blood?

There really is such a creature - horseshoe crab (hòu), which is an arthropod that lives in the ocean and is shaped like a crab. Its body is brown and covered with a hard shell. It moves with its gills or the last pair of legs. Horseshoe crab is a very ancient creature, known as a "living fossil". It first appeared in the Devonian period, when there was no trace of dinosaurs on Earth.

Horseshoe crabs, a creature with an unremarkable appearance, are the only animals on earth with blue blood. The reason why horseshoe crab blood is blue is that the copper element in it replaces the iron element in red blood, which is responsible for combining oxygen and transporting it to the whole body. In hundreds of millions of years of evolution, blue blood has developed a unique antibacterial ability. Although there are no white blood cells that fight viruses in blue blood, there is a special kind of amoeba-like cell. When this cell finds the bacteria, it will wrap it up and cause a small amount of local blood to coagulate to prevent the invasion of bacteria.

After humans discovered that the blue blood of horseshoe crabs has this magical ability, they used it to develop horseshoe crab reagents around the 1960s. Even if the solution being tested contains a little bit of impurity bacteria, the horseshoe crab reagent will wrap it up and produce a gel, and we can know that the solution is contaminated. People who have gone to the hospital for infusion should thank the ancient creature horseshoe crab. Horseshoe crab blood can be said to be the most expensive blue liquid, with a liter worth up to $15,000.

Horseshoe crabs, which have survived five mass extinction events, have been hunted by humans for hundreds of years. In the past, they were even made into feed and fertilizer. After the successful development of horseshoe crab reagents, millions of horseshoe crabs were "forced to donate blood". The number of horseshoe crabs has been greatly reduced. The population of three-spined horseshoe crabs has dropped by 90% in the past three or four decades and is now endangered. However, in recent years, humans have greatly strengthened their protection of horseshoe crabs, and it has been listed as a national second-class protected animal. Nowadays, humans usually do not extract more than 30% of the blood of horseshoe crabs. After they recover, they will be released to a location far away from the fishing point to prevent repeated fishing next time.

Plants with green "blood", humans and animals with red blood, horseshoe crabs with blue blood... they should all live equally on this magical planet that gives birth to life. The colorful nature is the most beautiful. What do you think?

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