The study of the structure of cells of prokaryotic organisms, as well as plants of animals and humans is engaged in a section of biology called cytology. Scientists have found that the contents of the cell that is inside it are built quite complicated. It is surrounded by the so-called surface apparatus, which includes the outer cell membrane, supmembrane structures: glycocalyx and the cell wall, as well as microfilaments, pelicula and microtubules forming its submembrane complex.
In this article, we will study the structure and functions of the outer cell membrane that enters the surface apparatus of various types of cells.
What are the functions of the outer cell membrane?
As described previously, the outer membrane is part of the surface apparatus of each cell, which successfully separates its internal contents and protects the cellular organelles from adverse environmental conditions. Another function is to ensure the metabolism between the cellular contents and tissue fluid, therefore, the outer cell membrane carries out transport of molecules and ions entering the cytoplasm, and also helps to remove toxins and excess toxic substances from the cell.
Cell membrane structure
Membranes, or plasmalemma of various types of cells are very different from each other. Mainly, the chemical structure, as well as the relative content of lipids, glycoproteins, proteins and, accordingly, the nature of the receptors in them. The outer cell membrane, the structure and functions of which are determined primarily by the individual composition of glycoproteins, takes part in the recognition of environmental stimuli and in the reactions of the cell itself to their actions. Some types of viruses can interact with proteins and glycolipids of cell membranes, as a result of which they penetrate the cell. Herpes and influenza viruses can use the host cell plasmalemma to build their protective membrane.
And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and dissolve it at the point of contact with a special enzyme. Then a viral DNA molecule passes into the hole formed.
Structural features of the eukaryotic plasmalemma
Recall that the outer cell membrane performs the function of transport, that is, the transfer of substances into the cytoplasm of the cell and from it into the external environment. To implement this process, a special structure is needed. Indeed, the plasmalemma is a permanent, universal for all eukaryotic cells system of the surface apparatus. This is a thin (2-10 Nm), but quite dense multilayer film that covers the entire cell. Its structure was studied in 1972 by such scientists as D. Singer and G. Nicholson, they also created a liquid-mosaic model of the cell membrane.
The main chemical compounds that form it are ordered molecules of proteins and certain phospholipids, which are interspersed in a liquid lipid medium and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the non-polar hydrophobic “tails” of which are located inside the membrane, and the polar hydrophilic heads are facing the cell cytoplasm and the intercellular fluid.
The lipid layer is penetrated by large protein molecules that form hydrophilic pores. It is through them that aqueous solutions of glucose and mineral salts are transported. Some protein molecules are located on both the external and internal surfaces of the plasmalemma. Thus, on the outer cell membrane in the cells of all organisms with a nucleus, there are carbohydrate molecules linked by covalent bonds with glycolipids and glycoproteins. The carbohydrate content in cell membranes ranges from 2 to 10%.
The structure of the plasmalemma of prokaryotic organisms
The outer cell membrane in prokaryotes performs similar functions with plasmalemma of cells of nuclear organisms, namely: the perception and transmission of information coming from the external environment, the transport of ions and solutions into and out of the cell, and the protection of the cytoplasm from foreign reagents from the outside. It can form mesosomes - structures that arise when the plasmalemma is invaded into the cell. They may contain enzymes involved in the metabolic reactions of prokaryotes, for example, in DNA replication, protein synthesis.
Mesosomes also contain redox enzymes, and photosynthetics contain bacteriochlorophyll (in bacteria) and phycobilin (in cyanobacteria).
The role of external membranes in intercellular contacts
Continuing to answer the question of what functions the outer cell membrane performs, let us dwell on its role in intercellular contacts. In plant cells, pores are formed in the walls of the outer cell membrane, passing into the cellulose layer. Through them, the cytoplasm of the cell may exit, such thin channels are called plasmodesmata.
Thanks to them, the connection between neighboring plant cells is very strong. In human and animal cells, the contact points of adjacent cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and are also found in cardiomyocytes.
Auxiliary plasmalemma formations
To understand how plant cells differ from animals, it helps to study the structural features of their plasmalemms, which depend on what functions the outer cell membrane performs. Above it in animal cells is a layer of glycocalyx. It is formed by polysaccharide molecules bound to proteins and lipids of the outer cell membrane. Due to the glycocalyx, adhesion (clumping) occurs between the cells, leading to the formation of tissues, therefore, it takes part in the signal function of the plasmalemma - recognition of environmental stimuli.
How passive transport of certain substances through cell membranes is carried out
As mentioned earlier, the outer cell membrane is involved in the transport of substances between the cell and the external environment. There are two types of transport through the plasmalemma: passive (diffusion) and active transport. The first is diffusion, facilitated diffusion, and osmosis. The movement of substances along the concentration gradient depends, first of all, on the mass and size of the molecules passing through the cell membrane. For example, small nonpolar molecules easily dissolve in the middle lipid layer of the plasmalemma, move through it and end up in the cytoplasm.
Large molecules of organic substances penetrate the cytoplasm using special carrier proteins. They have species specificity and, when combined with a particle or ion, passively pass them through the membrane without concentration of energy along the concentration gradient (passive transport). This process underlies such a plasmalemma property as selective permeability. In the process of passive transport, the energy of ATP molecules is not used, and the cell saves it for other metabolic reactions.
Active transport of chemical compounds through the plasmalemma
Since the outer cell membrane provides the transfer of molecules and ions from the external environment into the cell and vice versa, it becomes possible to remove the products of dissimilation, which are toxins, to the outside, that is, into the intercellular fluid. Active transport occurs against a concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATPases, which are both enzymes.
An example of such a transport is the sodium-potassium pump (sodium ions pass from the cytoplasm to the external environment, and potassium ions are pumped into the cytoplasm). Epithelial cells of the intestines and kidneys are capable of it. Varieties of this method of transfer are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the outer cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, such as leukocytes, are capable of pinot and phagocytosis.
Bioelectric processes in cell membranes
It was established that there is a potential difference between the outer surface of the plasmalemma (it is positively charged) and the parietal layer of the cytoplasm charged negatively. It was called the resting potential, and it is inherent in all living cells. And the nervous tissue has not only a resting potential, but is also capable of conducting weak biocurrents, which is called the excitation process. The outer membranes of neuronal neuron cells, taking on irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. And the parietal layer of the cytoplasm due to the excess of cations receives a positive charge. This explains the reason for the recharging of the outer cell membrane of a neuron, which causes the conduction of nerve impulses that underlie the excitation process.