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The cell boundary
In today's world, it is common to think of a country as a portion of land spatially bounded by the presence of a frontier.
Let's think about the case of Brazil. We are surrounded by sea in half of our territory, and in the other half, we have a land border with nine other South American countries. At their borders, all countries set up a customs, which is a government agency controlling the movement of inputs and exits of persons and goods to or from abroad.
With cells is no different. Each has a "boundary area", represented by the plasma membrane, and in this area, the cells also have their "customs post", the proteins. As with border customs between countries, these proteins are responsible for recognizing substances from inside or outside the cell, such as hormones.
The work done by a cell is similar to what happens in a factory, such as televisions, for example. Through gates, various types of parts for the assembly lines are entered. To manufacture and assemble the devices, energy and skilled workers are required. It also takes a packaging sector to prepare the shipment of what is produced and a board to run the entire factory complex and maintain the relationship with the outside world. All within the limits represented by the factory wall.
The cell has sectors similar to those of a factory. A cell boundary, represented by the plasma membrane, separates the contents of the cell, the cytoplasm, from the external environment. The cytoplasm, consisting of organoids and hyaloplasma (or cytosol), a viscous material represents the productive sector. A nucleus containing the genetic material represents "the board" of the cell.
The limits of the living cell
A living cell is a microscopic compartment, isolated from the environment by at least one barrier: the plasma membrane.
This is an extremely thin and delicate film that exerts severe "scrutiny" on all substances and particles entering and leaving the cell.
Given the relative fragility of the plasma membrane, most cells have some kind of envelope that gives protection and physical support to the membrane. These include the glycocalyx, present in most animal cells, and the cellulosic wall, present in plant cells and some algae.