In the previous post “Nature is conservative (besides being a big recycler)” I noticed that the most important life molecules, such as nucleic acids and proteins, are unchanged since the origin of life itself and are the same in all the organisms at present time and in the past.
But in all living beings there are even common structures!!!
Cell is the basic unit of life. Many organisms (bacteria and microalgae) consist of a single prokaryotic cell (i.e. a cell without a nucleus), others (protists) of a single eukaryotic cell (i.e. a cell whose DNA is within a nucleus), while all the others (fungi, plants and animals) consist of many eukaryotic cells.
Every cell is surrounded by a cell membrane (plasma-membrane, historically called plasmalemma)).
It is a very thin layer (5-10 nm thick) delimiting, separating and protecting the interior of the cell from the external environment, like the boundary wall of an ancient town.
The structure and the amazing properties of cell membrane are the same in all the cells of all the organisms.
Plasmamembrane consists a bi-layer of phospholipids with their hydrophobic regions toward the internal side and the hydrophilic regions toward the external side of the membrane. This bi-layer, that at the electron microscope appears continuous, prevents the emission of cellular material and the introduction of aqueous material from the exterior.
Cell membrane at the electron microscope. In this pictture the membranes of two diffrent cells can be seen very close to each other
But cells are not water tight compartments!!!!
For their metabolism cells need external material and have to eject their products and waste. Many of this substances are water-soluble and are not able to cross a continuous lipid bi-layer!!!!!
Yet there aren’t gaps in the membrane!!!
There are instead proteic and glycoproteic molecules (besides cholesterol and different glycolipids) crossing the lipidic bi-layer.
These macromolecules act as canals or pumps able to transfer molecules inside or outside the cells. Noteworthy their ability to freely move along the membrane itself ( for this the membrane structure is called fluid mosaic) in such a way that exchanges can be performed in the suitable membrane region.
Many receptors, mainly proteins and glycoproteins are located on the surface of the cell membrane where they pick up and recognize external signals which, transferred trough the canals, bind to internal receptors. In this way specific cell responses are put into operation.
Schematic rapresentation of cell membrane structure according to themosaic fluid model.
Therefore cell membrane is a sort of fence-filter able to regulate what enters and exits the cell, at any moment.
But there is more:
By means of a rupture and fusion mechanism cell membrane can transport in or out the cell even large molecules or particles without any interruption of the membrane itself. There is esocytosis when internal material, enclosed in membrane-bound vesicles (all the internal membranes of the cell have he same structure of cell membrane). There is endocytosis when the material moves into the cell. Finally during phagocytosis, cells engulf large particles such as bacteria, cell debris, or even intact cells
By means of this same mechanisms cell membrane is also able to increase its length to follow the increasing of cell size or its division.
Cell membrane, in conclusion, is a real masterpiece of nature!!
Probably for this is exploited unmodified since the origin of the cell.