"Little Green Man" occupies a prominent place in movies about aliens, from the intelligent Jedi leader in Star Wars, Yuda, to the petite, one-eyed big-eyed boy in Monster Power, from the big, powerful superhero Hulk in the Marvel world to the disgusting monster Shrek in Shrek.
Characters covered in green skin are given a variety of abilities, but why aren't humans green? Why can't we photosynthesize like plants? After all, this saves us a lot of trouble.
<h1 class = "pgc-h-arrow-right" > intraneasional symbiosis theory – cells live in cells</h1>
The most familiar green creatures are, of course, plants. Plants are green because their cells are filled with internal organelles— chloroplasts, which are the center of photosynthesis.
Chloroplasts have a rather interesting evolutionary history, because they were once cyanobacteria that lived freely independently of plants, and were swallowed into the stomach by the ancestors of plants hundreds of millions of years ago, beginning a mutually beneficial and symbiotic evolutionary life.
Cyanobacteria are known for "inventing" photosynthesis, a process of absorbing energy from sunlight to make organic matter using water and carbon dioxide. Any inventor can tell you that if you have a great idea, then soon, a lot of people will come to visit you and seek to work with you.
In a surprising discovery, American biologist Lynn Margulis, who proposed the theory of endosymbiosis, realized that chloroplasts inside plants were cyanobacteria that were captured and domesticated early in the evolution of the plant lineage.
The single-celled ancestor of this terrestrial plant appears to have devoured a cyanobacteria, but instead of digesting it, realized that the cyanobacteria were good for them and decided to use it.
For all higher organisms, more basic is a type of organelle called mitochondria. Margulis realized that this was also once a free-living bacterium — in this case, the bacteria could harness the chemical energy locked in sugary substrates like glucose.
So plant cells are actually chimeras —a single organism consisting of an original host plus two captured bacteria. This theory is called the theory of endosymbiosis.
<h1 class = "pgc-h-arrow-right" > the joy of chloroplasts</h1>
Chloroplasts were domesticated to bring great and immediate benefits to plants. Animals have only mitochondria and no chloroplasts, which allow cells to oxidize glucose and harness the chemical energy produced to boost metabolism. But they must find the source of glucose. This means that they spend most of their day looking for, ingesting, and breaking down food.
Plants don't have to bother with this. They can simply use their chloroplasts to make glucose, and then, when needed, they can deliver the glucose to the mitochondria for biochemical reactions to release chemical energy.
<h1 class="pgc-h-arrow-right" > "can catch male lions, but it is difficult to catch crickets." </h1>
Since plants can go around looking for glucose to make themselves, animals certainly can too. In fact, many animals do exactly that.
Chloroplasts are a good invention, and many other organisms manage to beg, borrow, or steal chloroplasts, mainly from single-celled algae that already have chloroplasts living freely. This process is known as secondary endosymbiosis to distinguish it from primary endosymbiosis, in which the plant ancestors devoured a free-living cyanobacteria.
It is not entirely clear why the phenomenon of secondary endosymphysis occurs so many times, while the phenomenon of primary endosymbiosis occurs only once, although scientists have found a second example of primary endosymbiosis that is forming. In this example, the host is a strange amoeba called Polyinela, which appears to be domesticating a new type of cyanobacteria, thus reproducing the ancient events that led to the production of terrestrial plants.
Biological evolutionary tree
Through secondary endophylaxis, the transfer of chloroplasts around the bioevolutionary tree produces a large number of ecologically significant organisms, most of which are single-celled organisms. These organisms, such as diatoms, dinoflagellates, and naked algae, are unknown to most of us and are produced independently from chloroplasts from algae.
Diatom micrograph
Perhaps most interesting to our story is that these single-celled photosynthetic organisms themselves have been occupied by multicellular animals. These symbiotes have evolved independently many times, and the relationship between host and symbionts can take many forms.
<h1 class="pgc-h-arrow-right" > animals that perform photosynthesis</h1>
Hermaphrodites living on the seabed, sea slugs, feed on algae and eat whatever color they eat. Sea slugs are the first animals scientists have discovered that can produce chlorophyll and can "steal" chloroplasts from the algae they feed on.
Sea slugs cannot maintain the working state of the chloroplast for a long time, so they need continuous supply, and there is still debate about whether the chloroplast is really necessary for the survival of slugs.
Undersea chameleon - sea slugs
At one end of the food chain, many marine life, such as corals, giant clams and sea squirts, depend entirely on their symbiotes and would perish without them.
So, both slugs and sea squirts can benefit from photosynthesis, so why can't we?
The answer lies in considering the energy balance of large, active, multicellular advanced animals like humans. Every day, an adult consumes a molecule called adenosine triphosphate, the equivalent of his own body weight, to maintain basic life activities, which stores the chemical energy released by glucose oxidation and releases it when needed.
<h1 class= "pgc-h-arrow-right" > half a basketball court-sized skin plus full-day exposure</h1>
To produce about 70 kilograms (more or less) of adenosine triphosphate, an adult male needs to consume and absorb about 700 grams of glucose per day.
Given the maximum known rate of photosynthesis in higher plants, and assuming that an adult male with a height of 175 kilograms and a weight of 70 kilograms has a surface area of about 1.96 square meters, a man with green skin all over his body will produce only 1% of his daily glucose needs through photosynthesis.
Body surface area (m2) = 0.0061 * Height (cm) + 0.0128 * Weight (kg) - 0.1529
Therefore, in order to meet the energy demand, an adult male who relies on photosynthesis to maintain basic activities of life must have more skin, the skin surface area must reach at least 196 square meters, roughly equivalent to half a basketball court, and ensure sufficient light time and a certain light intensity every day.
From an evolutionary point of view, it makes sense that humans do not perform photosynthesis, because the photosynthesis efficiency is too low, and it is clear to look at the nutritional composition list of cabbage and pork, far less fast than obtaining from the outside world. If you want to rely on photosynthesis to provide energy, it is estimated that humans will have to rest for half a day after taking one step.
Therefore, we must reluctantly conclude that either aliens look much weirder than depicted in the movie, or that photosynthesis that evolved on planets colonized by other little green people is more effective than photosynthesis that evolved on Earth.