To better understand, let’s start a little bit from afar.
Life of unicellular cells
Once upon a time, at the dawn of evolution, there were only one-celled organisms. They were completely independent organisms, you could say humans in miniature – they could find their own food, get to this food themselves, split this food themselves and get energy and building blocks from it, from which they could build the worn-out parts of their small bodies and perform all the other functions necessary for survival themselves. They were all completely different, differing from each other far more than we differ from a fly. Humans and flies eat different foods, because our bodies are different, and these bodies are made up of different parts.
Unicellular organisms do not prefer the same food either. And what for one unicellular organism may be absolutely unnecessary, what it would gladly get rid of and flush down the toilet, for another unicellular organism may be the most necessary.
And over time, two such single-celled cells realized that they could be useful to each other. The first cell produced such substance as pyruvate as a by-product in the process of its life activity. This cell did not need pyruvate, but it did not interfere with it either. But the other substance, which was also formed as a by-product – oxygen – was very much in the way, as it was toxic for this one-cell organism.
But for the second cell, both pyruvate and oxygen proved to be very valuable. This cell could produce a huge amount of energy from them. Therefore, their cooperation was beneficial to both sides.
Cell fusion
The one-cell power plant was able to produce so much energy that it could give the excess to the first cell. The first one, in gratitude, began to give it not only unnecessary pyruvate and oxygen, but in general everything it needed, so that it could concentrate only on energy production. And even let the unicellular power plant inside itself to save it from having to defend itself from the outside environment. In the end, the single-cell power plant completely lost all functions except one – to produce energy, like a concubine in a sultan’s chamber who has been deprived of all her cares for only one function.
The one-cell Sultan, on the other hand, completely abandoned independent energy production, because he was supplied with it in excessive quantities by the one-cell concubine.
This is how mitochondria appeared in our cells. Every cell in our body has an organelle responsible for energy production, which was once a completely foreign organism, but has now become part of it. Other organelles in our cells, and even the nucleus, appeared under approximately the same circumstances.
What is the maximum cell size
So, absorbing other organisms, the single-celled organism grew and evolved. And it grew 100,000 times more than the original, simplest version. And it turned out that it couldn’t grow any further. The fact is that the volume of the cell grows much faster than the surface area of the cell:
Imagine a wooden box 1 m high, 1 m long and 1 m wide. That is 1x1x1 or 1 m3. Don’t be confused by the number 1. A cubic meter is a very large volume. A box like that can hold 1,000 liters of water. So if we want to upholster this box with cloth, we need a cloth 1 meter wide and 1 meter long to cover the top of the box, which is 1 m2. The same square of fabric on the 4 sides and bottom. The total surface area of this box is 6 m2. That is, the area is more than 6 times the volume.
But if we increase the width, length and height 10 times, the volume will be 10x10x10=1000 m3. And the surface area will then be (10×10)x6=600 m2. That is, the volume has increased 1000 times, but the area is only 600 times. If before the area was 6 times greater than the volume, now the volume is almost 2 times greater than the area. And the more we increase our box, the faster the volume will grow in relation to the area.
What this means in practice. A cell receives nutrients from outside, and releases unnecessary substances into the environment. When it is small, it can do this freely. But as the cell grows, each piece of the membrane separating the inside of the cell from the outside world is under more and more strain. Imagine a small balloon. And draw a line from its surface to its center. That’s the distance you have to travel to get something from the surface of the balloon to the center or back. Now zoom in on that balloon 10 times. And again draw a line to its center. The line is much bigger, and the distance to the center is much longer.
The life of multicellulars
So the cell cannot increase indefinitely. That’s why further the cells decided to interact not by merging into one cell, but by being located next to each other, and by exchanging the necessary substances. Thus, self-organizing, already multicellular organisms became more and more complex, and finally they became humans and other living beings that we know. So you could say that we are trillions of little organisms who decided that they would be better off if they worked as one.
Just like organelles in a single cell, for better interaction, cells give up some of their functions to perform one function, but perform it as well as possible. For example, an adipocyte is a cell almost entirely occupied by a drop of fat. Everything it needs to live is supplied by other cells. Or an erythrocyte, a blood cell that has even lost its nucleus in order to fill its entire volume with the protein hemoglobin, which can carry oxygen to other cells in the body that need it.
It also turned out that it is better for a certain function to be performed not by one cell, but by many at once. And it is more effective if these cells are next to each other. That’s how organs were formed. Well, these organs still constantly need substances to work, which they themselves do not produce.
This is what blood is for – it serves as both a conduit and a sewer. It connects one organ to another, allowing them to easily transfer substances to each other, as well as to remove unnecessary substances.
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