The typical structure of the cell membrane consists of a phospholipid bilayer between two protein layers located at the level of the separation surfaces between the internal and external phases of the cell. The lipid layer is bimolecular, with the polar groups facing the protein layer, while the apolar groups face each other with an isolation function.
Cell membranes, with their thickness of only 90 A, are not visible under the transmitted light microscope. Before the advent of electron microscopy, cytologists assumed that the cell was surrounded by an invisible film, because if this hypothetical film was broken, the cell contents could be seen leaking out. Today with the electron microscope, the membrane can be visualized as a thin double solid line According to current hypotheses, the membrane essentially consists of molecules of phospholipids and cholesterol arranged in such a way that their hydrophobic tails are turned inwards.
The polypeptide chains of the membrane protein molecules are perpendicular to the lipid molecules and are believed to maintain cohesion between the different parts of the plasma membrane.
The membranous structure performs the task of having to separate the cellular environment from the extracellular one, the nucleus from the cytoplasm, and also the material inside the various organelles from the cytoplasmic matrix.
In every cell, be it animal or vegetable, the peripheral layer of the protoplasm has the morphological and functional characteristics of a membrane placed to separate two different environments, which can be identified with solutions that have different chemical-physical characteristics and compositions. The function of this diaphragm is to allow the passage of water and other small solutes inside the cell, while it opposes solutes of high molecular weight. In general, the direction of the flow is determined by the concentration of the composition of the solution at the sides of the membrane, the flow always occurs in the direction from the most diluted solution to the most concentrated: that is, it tends to balance the two concentrations and ceases when equality is reached. The pressure needed to completely stop this movement is called osmotic pressure. It is all the greater the more concentrated the solution is.
The cell membrane is not an ideal semi-permeable membrane, as it is impermeable to some, but not all, of the solutes present. The permeability or otherwise of the membrane to solutes does not depend exclusively on its chemical-physical structural characteristics, but largely on phenomena intimately linked to cellular metabolism.
The cells, in relation to their behavior relative to osmotic pressure and environmental pressure, are divided into: poikilosmotic and homoosmotic. The former have an osmotic pressure equal to or nearly that of their environment, the latter are able to maintain an osmotic pressure within a vast latitude of values, very different from the environmental ones. Taking into account these characteristics of the behavior of animal and plant cells, J. Traube created a special device, consisting precisely of a semipermeable membrane, which had to artificially reproduce the behavior of living cells in the face of given solutions. Initially a copper ferrocyanide film was used as a membrane; subsequently semipermeable membranes were introduced with which it was possible to ascertain the entity of considerable osmotic pressures.
Finally, it can be stated that the passage of the various substances through the plasma membrane can occur by simple diffusion, facilitated, or by active transport.
Simple diffusion: passive transport through the lipid bilayer. Diffusion is the movement of molecules from one area to another as a result of their random thermal agitation.In simple diffusion, the permeability of the membrane is determined by the following factors: (a) the fat solubility of the diffusing substance, (b) the size and shape of the diffusing molecules, (c) the temperature and (d) the thickness of the membrane .
Facilitated diffusion: passive transport through membrane proteins. Facilitated diffusion is operated by two types of transport proteins: (a) transporters, which bind molecules on one side of the membrane and transport them to the other thanks to a conformational modification, and (b) channels, which form pores that extend from one side of the membrane to the other. In facilitated diffusion, the permeability of the membrane is determined by two factors: (a) the transport speed of the individual carriers or channels and (b) the number of carriers or channels present in the membrane.
Active transport. There are two main types of active transport: primary active transport, which uses ATP or other forms of chemical energy, and secondary active transport, which uses the electrochemical gradient of a substance as an energy source to induce the active transport of a high substance.
Click on the names of the various organelles to read the in-depth study
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