By 1838, science had accumulated a wealth of information about the cells of living organisms. First of all, it became clear that the living substance is the cell’s contents, not its walls, as Hooke believed. Cells were found in the tissues of plants and animals. Scientists learned that cells could multiply by dividing in half.

The German biologists Matthias Schleiden and Theodor Schwann summarized all this mass of information in 1838-1839. They formulated the basic principle of the cell theory: the cell is the unit of structure and life activity of all living organisms.

All living things are composed of cells. As a building is made of bricks, so the tissues and organs of living things are made of cells. There is no life outside the cell. There was, however, an error in Schwann and Schleiden’s theory: the scientists believed that the cells of an organism arise from a cell-free germ mass. In 1855 their compatriot Rudolf Virchow rejected this position. “Every cell is only from a cell,” was his aphoristic formulation of a new biological law. A new cell can only come from other cells.

CELL STRUCTURE

THE NUCLEUS
In 1833 the English botanist Robert Brown discovered dense round cells and described them. He called them nuclei. Biologists later found that all cells of plants, animals, and fungi have a nucleus (or many nuclei). (Although certain types of cells lose the nucleus during development.)

The nucleus on the scale of a cell is quite large. But is it possible to see the nucleus without using magnifying devices? It would seem that even if human cells can be seen with the naked eye only rarely, let alone the details of their structure, such as the nucleus. Meanwhile, the nucleus of the unicellular seaweed Acetabularia (20kb) is perfectly visible to the naked eye. This alga is considered one of the largest single-celled creatures: 2 to 4 cm in height. It consists of a cap, a “stem” and a stalk. Its core resembles a small ball 1 mm in diameter.

A series of famous experiments with acetabularia was carried out by the German biologist Joachim Hemmerling in the 1930’s. He simply cut the algae with scissors. The nucleus remained in the cap, stalk or stem. The scientist noticed that only that part of the plant, where the nucleus was preserved, could restore a full organism and reproduce. The lost nucleus was no longer regenerated.

CELL MEMBRANES.
Biologists have long guessed that every cell is surrounded by a thin “skin”, a shell that separates it from the external environment. But to see this shell was able to see only in the fifties of the XX century. using an electron microscope. What is this cellular “skin”?

To get a visual idea of it, let’s remember an ordinary soap bubble. Water is gradually flowing down, the wall of the bubble is thinning. Here on it began to run from the top of the iridescent streaks. This means that the soap film is only a few hundred soap molecules thick and has become commensurate with the length of light waves. As the film gets thinner and thinner, all the color runs through the bubble a few times. And then an amazing thing happens. A “hole” forms at the top of the bubble, which grows rapidly. The bubble bursts. If the bubble hangs in the air, at some point it may seem like only the bottom half of the bubble is left. But the upper part of the soap film does not disappear.

LYSOSOME
Lysosomes were discovered in 1955. They are small membrane bubbles filled with specific enzyme proteins. These proteins are so good at decomposing and digesting organic substances that if they are “released” from the lysosomes, the cell “digests itself”. Lysosomes are like intracellular “stomachs” . Lysosomes digest not only the food that has entered the cell, but also the parts of the cell itself that have failed. Lysosomes have other “duties” as well.

RETICULUM COMPLEX
It is also called Golgi apparatus after an Italian scientist who discovered it in 1898. Here substances produced by cell (proteins, fats, carbohydrates) are collected and “packed”, as a rule intended for export to various organs. Lysosomes are also produced here. The reticular complex itself consists of flat membrane vesicles stacked on top of each other like pancakes in a stack.

MITOCHONDRIES
Once upon a time, billions of years ago, bacteria-like creatures found an “unusual habitat. They settled inside the cells of other living organisms. Gradually, “hosts” and “occupants” adapted to each other, and finally they got on so well that they could not exist without each other. Such mutual assistance in nature is known as symbiosis.

CELL CENTER
Cell division is often called the “dance of the chromosomes. But chromosomes are not independent in this “dance”: their movements are directed by an experienced “dancer” with the help of long threads (microtubules). It is about an organoid discovered in 1875 and called the “cell center”. It is present in the cells of animals and some plants and fungi.