Why can ants move objects heavier than themselves?

The power of insect muscles often surprises us. For example, Asian weaver ants can "mouth" objects that are 100 times heavier than their own weight; fleas can jump to heights that are 100 times higher than their own height; dung beetles (commonly known as "dung beetles") can push dung balls that are 1,140 times their own weight!

Why are these insects so "powerful"? Do they have special muscles? In order to clarify this problem, scientists observed the detailed structure of insect muscles (such as the flight muscles used by insects to fly and the leg muscles used for crawling) and found that these muscles, like human skeletal muscles, are "striated muscles". Its basic structural unit (sarcomere) and the striations inside are also very similar to those of humans. If you only look at the electron microscope photos without explanation, it is difficult to tell whether this is human striated muscle or insect striated muscle.

Does it mean that insect muscles just look similar to human muscles but have different compositions? To answer this question, Austrian and German scientists collaborated to examine the genes of these insects and found that they contain the same "core components" as human muscles, including myosin II and actin. , myosin light chain (including "essential light chain" and "regulatory light chain"), tropomyosin and calmodulin, indicating that the core components of insect muscles are not the same as those of humans. different.

The structure and composition of insect striated muscles are the same as those of humans. So what is the result of measuring the strength of insect muscles (the tensile force generated per unit area) and comparing it with the striated muscles of vertebrates? Actual measurement results show that the striated muscles of vertebrates can produce about 25 Newtons per square centimeter of cross section, which is about 2.5 kilograms of pulling force. Insect muscles have similar, or slightly less, strength. For example, a cockroach mainly uses its hind legs to push when it turns over. A single hind leg can produce 0.14 Newtons, which is about 14 grams of force. The cross-section of the cockroach's hind leg muscles is about 0.6 square millimeters, which translates into 2.3 kilograms of force per square centimeter.

Since the muscles of insects are not fundamentally different from those of humans, why does the relative strength of humans appear so small? For example, the men's 62kg snatch world record was set by China's Shi Zhiyong in 2002, at 153kg, less than 2.5 times his own body weight. Why can't humans lift objects 100 times heavier than themselves, like insects? You can think about this issue yourself first, and we will discuss it at the end of the article. Now let’s talk about muscle-related topics first. It turns out that the contraction principles used by human and insect muscles existed long before animals appeared. This principle is used not only for muscle contraction, but also for a variety of processes in cells that require power.

Single-celled eukaryotes already have “muscles”

When it comes to muscles, it seems to only be related to animals. In fact, single-cell eukaryotes (such as yeast and amoeba) already have the two most critical components in vertebrate striated muscle, myosin and actin. Actin can polymerize to form long filaments, and has a "plus end" and a "minus end"; while myosin can use the energy released by ATP hydrolysis as power to "walk" along the actin filament toward the plus end. It's one of those "miniature power trains" inside cells that can do a lot of things.

Why do single-celled eukaryotes need such a "power train"? This is because eukaryotic cells (generally tens of microns) are thousands of times larger than prokaryotic (such as bacteria) cells (generally about 1 micron), and there are various organelles, such as mitochondria, lysosomes, Golgi apparatus, endoplasmic reticulum, secretory vesicles, etc. Small molecules (such as oxygen molecules and glucose molecules) can reach the desired location in the cell by diffusion, but organelles are too inefficient to move by diffusion, and "porters" are needed to move them. In addition, mechanical force is also required for cell movement (the front end extends and the rear end contracts) and cell division (the middle of the cell shrinks and then divides into two).

Myosin has such a "skill". Myosin is composed of three parts: "head", "neck" and "tail", and is shaped like a golf club. The "head" is enlarged and can bind to the filaments of actin. It has an ATP binding site. When a molecule of ATP binds to the "head," the head deforms and detaches from the actin. Energy is released when ATP is hydrolyzed, causing the head to "deflect" away from the "neck" and bind to a farther position on the actin filament. The deflected "head" is like a bent spring, returning to its original position, thus generating a pulling force on the actin filaments. If the position of the actin chain is fixed, the "head" of myosin can "walk" along the filament in the direction of the plus end. If myosin's position is fixed, it can pull the actin filaments toward the minus end. ATP is continuously combined and hydrolyzed, and this movement process can continue.

It is no longer possible to determine when this exquisite mechanism appeared, because actin and myosin are present in the cells of all eukaryotic organisms on the earth, so it must have developed some time after the emergence of eukaryotic cells. of. And even in the stage of single-celled eukaryotes, this mechanism for generating pulling force had developed to such perfection that it rarely changed in the subsequent billions of years. The myosin in rabbit muscles can even combine with the actin of amoeba; the actin "tracks" of plants and animals are also very similar, so that the "head" of animal myosin slides on the track of plants at a very fast speed Almost the same speed as gliding on animal tracks.

This mechanism for generating pulling force is so valuable that in the process of evolution, organisms continue to copy the genes of these two proteins and modify them to allow them to do various things that require pulling force without changing the mechanism and efficiency of pulling force. work. For example, yeast already has five myosin genes. The proteins they produce have similar "heads" but different "tails" that can do different things. Humans have more than 40 myosin genes.

The "tails" of I-type myosin and V-type myosin can combine with biological membranes, so they can "carry" organelles (such as mitochondria, endoplasmic reticulum, Golgi apparatus, secretory vesicles) wrapped by biological membranes along the The "orbital" movement of actin plays a role in transport. Type I myosin acts as a monomer and type V myosin acts as a dimer.

In animal muscles, type II myosin first forms a double body. The "tails" of the two myosins are tightly wound together, and the two "heads" are at the same end of the double body. Multiple such double bodies are then put together, with half of the double bodies facing in the opposite direction to the other half, forming a structure like a "double-headed mace". Actin filaments are neatly "inserted" on a disk with their plus ends, and the filaments are parallel to each other. Two such structures face each other, just like the hairs of two electric toothbrush heads facing each other, with a distance in between. The "double-headed mace" of myosin is inserted into the middle of these actin filaments, and the "head" binds to the filaments. After ATP binds to the "head" of myosin and is hydrolyzed, the "head" pulls the actin filament toward the minus end. Since the two ends of the myosin "double-headed mace" pull the actin filaments in opposite directions, the two "toothbrush heads" move toward the middle of the "mace" (that is, the two "toothbrush heads" move closer to each other). ), the muscle contracts.

When the amoeba moves forward, actin filaments are formed in the extended "pseudopods", parallel to the direction of advancement, with the plus end facing outward, forming a "track." The "tail" of type I myosin binds to the cell membrane, and the "head" slides along the actin "track" to pull the cell membrane forward. At the rear of the cell, a "contractile chain" composed of type II myosin and actin (similar to the contractile unit in striated muscle) "pulls" the cell membrane away from the solid surface, and the rear of the cell can retract. .

During yeast cell division, a "contractile ring" composed of type II myosin and actin forms in the center of the cell. This "contraction ring" continues to tighten, causing the cell to split into two. Cells lacking type II myosin are unable to divide and form giant cells containing many nuclei.

So even in single-celled eukaryotes, "muscle proteins" have already begun to play an important role. The muscles of multicellular organisms are just developed on this basis.

Plant cells also have “muscle protein”?

Plants generally do not move and do not seem to need muscles. But plant cells also contain actin and myosin, and more than one kind. For example, types VIII, XI and XIII myosins are unique to plants. They are related to the transportation of various "cargoes" within plant cells. For example, type XIII myosin can transport chloroplasts to the top of new tissues.

Another function of plant myosin is to cause "cytoplasmic flow" in plant cells. If you observe green algae (Nitella) under a microscope, you can see that the cytoplasm flows around the central vacuole, and the flow speed is faster near the cell membrane and slower near the vacuole. Research shows that green algae cells form parallel actin "tracks" beneath the cell membrane. The "tail" of type XI myosin is bound to plant organelles (such as chloroplasts), and the "head" slides along the "tracks" of actin, driving the cytoplasm to flow together. In green algae, cytoplasmic flow can reach speeds of 7 microns per second.

So at the cellular level, plants and animals have more similarities, because they both require pulling forces to carry out certain activities, especially the transportation of "cargo" within the cell.

The "power train" in cells is not limited to actin-myosin

There are many transportation tasks in cells. For example, when cells divide, two copies of chromosomes need to be distributed into two cells, and force is needed to "pull" them. The "axons" of nerve cells (the nerve fibers that send nerve signals) can be more than 1 meter long, but the proteins of nerve cells are mainly synthesized in the cell body (the enlarged part containing the nucleus). As "neurotransmitters" (molecules that transmit information between nerve cells), after synthesis, they are wrapped in a membrane into secretory vesicles and then transported to the nerve terminals. These transport tasks are no longer performed by actin and myosin, but by another type of "power train".

The "tracks" of this type of "power train" are not filaments polymerized by actin, but hollow microtubules polymerized by "tubulin", and like actin filaments, There are positive terminals and negative terminals. There are two proteins that move cargo along this track. They both use the energy released when ATP is hydrolyzed as power, but they move in different directions. "Dynein" moves toward the minus end of microtubules, transporting "cargo" from the far end of the cell to the center of the cell. Another protein called "kinesin" transports "cargo" to the plus end of microtubules, that is, from the center of the cell to the distal end.

In addition to "cargo transport", this type of protein is also related to the separation of chromosomes during cell division. The two sets of replicated chromosomes are connected to the "centrioles" located at the two poles of the cell through "microtubules", and appear to be pulled into the two daughter cells by "dynein".

Like actin and myosin, tubulin, dynein, and kinesin already exist in single-celled eukaryotes (such as yeast), so this type of "power train" is already well known. long evolutionary history. This suggests that when eukaryotes emerged, there were already various cellular activities that required pulling force, and the actin-myosin system later developed into muscles.

Eukaryotes may have appeared 2.1 billion years ago. The fossilized "Grypania" at that time appears to have been a multicellular organism several centimeters in size. We can now have a heartbeat and breathe, walk, cook, eat, exercise, drive, write, paint, embroider, dance, sing, play musical instruments, etc., all thanks to the inventor of the actin-myosin system. Single cell ancestor.

​antThe relative "power" of ant muscles is actually due to simple geometric factors

At this point, we can see that all eukaryotes, including ants and humans, are able to use the same myosin-actin interaction to generate the pulling force required for animal movement, and this The mechanism is quite complete and very efficient. When each molecule of ATP is hydrolyzed into ADP and phosphoric acid, it can release 38.5 kilojoules of energy, which is equivalent to releasing 6.4 x 10-13 ergs of energy when each ATP molecule is hydrolyzed, and can pull 16 with a force of 4 pN (pico Newtons). nm distance. The measured energy generated by the hydrolysis of a single ATP by myosin can pull the actin filament 11 to 15 nm with a force of 3 to 4 pN! Since ants and humans use the same myosin-actin system, ants cannot have any "magic muscles."

In this case, why can ants lift objects 100 times heavier than themselves but humans cannot? This is simply due to the small size of ants' bodies. If the ant is "enlarged" to human size and the muscle structure remains unchanged, the ant will not be able to lift an object 100 times heavier than itself, or even its head (the ratio of the head to the body of most ants is much higher than people). On the other hand, if a person is shrunk to the size of an ant and his body structure remains unchanged, he will become a "Hercules".

Maybe you are a little confused, why is this? This is because when the size of an object changes, the length changes linearly, the area changes squarely, and the volume changes cubically. For example, if the linear dimension of an object of the same shape is reduced by 10 times, the area will be reduced by 100 times, and the volume will be reduced by 1000 times. For small animals, objects of the same proportions weigh much less.

Assume that the length of a human is 1.6 meters and that of an ant is 6.4 millimeters. The length of an ant is one 250th of that of a human. Assuming that the body structure of ants is the same as that of humans, the area of the cross-section of the leg muscles of ants will be one-62,500th of that of humans (one-250ths squared), and their weight will be 15,625,000ths of humans. One (one 250th cubed). Assuming that the human weight is 60 kilograms, the ant's weight should be 3.84 milligrams.

Since the cross-section of an ant's leg muscles is 1/62,500 of a human's, and the strength of the muscles is approximately proportional to the cross-section, and humans can generally lift a weight equivalent to their own body weight, theoretically ants can lift 62,500 of a human's body weight. The weight of one is 960 milligrams, which is 250 times the weight of the ant! Therefore, ants can lift objects 100 times heavier than themselves using their relatively thin legs.

This could explain why many insects such as ants and mosquitoes can have relatively thin legs, while large animals such as elephants need very thick legs, because as the size of the animal increases, the weight increases much faster. If the elephant did not have such thick legs, it would not be able to carry such a heavy weight and would not be able to move. The giant ape in the movie "Tarzan" is as tall as a several-story building, but moves as quickly as a real ape. In fact, this is impossible. If the gorilla were scaled up to a height of 10 stories, not only would it be unable to jump, but it would even be difficult to walk.

​The far-reaching influence of simple geometric principles

In fact, not only living things, this geometric principle has a profound impact on many things.

For example, dust is a trouble in our lives. Not only do we need to "clean" frequently and wipe away the dust on the table, PM2.5 will also penetrate deep into the lungs and affect our health. These dust particles can float in the wind and appear to be "light". In fact, each dust particle is much heavier than the same volume of air. For example, the specific gravity of air at one atmospheric pressure is approximately 1.21-1.25 kilograms per cubic meter, which is 1.21-1.25 milligrams per cubic centimeter. The density of general dust is 2 to 3 grams per cubic centimeter, and the cotton fibers shed from clothes also have a density of 1.5 grams per cubic centimeter, both of which are more than 1,000 times heavier than the same volume of air. The reason why they can float in the air is because their size is small and the ratio of surface area to volume becomes large, so the friction generated when the air flows through is enough to lift them into the air.

Objects can "fly" in the air if they are small enough to a certain extent. What if they are large enough? It will gradually become spherical, just like the earth (average radius 6364 kilometers) and the moon (average radius 1737 kilometers). This spherical shape is not "made" by anyone, but is the result of simple geometric relationships. Because when an object reaches a certain size, the ratio between volume (proportional to weight) and surface area becomes extremely large, and the gravity per unit surface area also becomes very large, while the strength of the rock does not change, so any excessive height The bulges will collapse. For example, there can only be mountains several thousand meters high on the earth, but there can be no bulges tens of kilometers high. For smaller planets, bulges tens of kilometers high are possible. For example, the asteroid "Eros", although it weighs 7 trillion tons, is still irregular in shape (13 x 13 x 33 kilometers). "Ceres" is the largest known asteroid in the solar system, with an average radius of 471 kilometers, a weight of 9 trillion tons, and a shape that is very close to a sphere.

​Summary

The larger size of eukaryotic cells compared to prokaryotic cells and the formation of various organelles require a "power system" in the cell to complete transportation tasks and other tasks that require mechanical force. The actin-myosin system was developed in the eukaryotic single-cell organism stage, and its basic principles and components have been used until now, so the muscles of insects and mammals are highly similar. Since the length, area, and volume change at different speeds when the size of an object changes, the physical properties of an object that is enlarged or reduced in proportion are no longer the same as the original object. Without changing muscle strength, the reduction in body size can make ants become "strong men". Under the condition that the density and strength remain unchanged, the rock can either become dust that can "fly" in the air (when the size is very small), or it can "automatically" become a sphere (when the size is extremely large). Therefore, a simple geometric principle can have a very profound impact on the "behavior" of an object.

animal tags： ants