What are the functions of the outer cell membrane? The structure of the outer cell membrane

The study of the structure of cells of prokaryotic organisms, as well as plants of animals and humans, is concerned with the division of biology, called cytology. Scientists have established that the contents of the cell, which is inside it, is built quite difficult. It is surrounded by a so-called surface device, which includes an outer cell membrane, superimposed structures: a glycocalyx and a cell wall, as well as micro-filaments, a pelicle and microtubules forming its sub-membrane complex.

In this article, we will study the structure and functions of the outer cell membrane, which enters the surface apparatus of various types of cells.

What functions does 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 cellular organelles from unfavorable environmental conditions. Another function is to ensure the exchange of substances between the cellular contents and the tissue fluid, so the outer cell membrane carries the transport of molecules and ions entering the cytoplasm, and also helps to remove toxins and excess toxic substances from the cell.

The structure of the cell membrane

Membranes, or plasmalemmas of different types of cells are very different. Mainly, the chemical structure, as well as the relative content of lipids, glycoproteins, proteins, and, accordingly, the nature of the receptors in them. External 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 into the cell. Herpes and influenza viruses can use the host cell's plasmalemma to build their own protective shell.

And viruses and bacteria, the so-called bacteriophages, attach to the cell membrane and in the place of contact dissolve it with a special enzyme. Then a molecule of viral DNA passes into the formed hole.

Features of the structure of the eukaryotic plasmalemma

Recall that the outer cell membrane serves as a transport, that is, the transfer of substances into the cytoplasm of the cell and out of it into the external environment. To implement such a process, a special structure is needed. Indeed, plasmalemma is a constant, universal for all eukaryotic cells, a system of surface apparatus. This is a thin (2-10 Nm), but fairly dense multilayer film that covers the entire cell. Its structure was studied in 1972 by scientists such 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 with a liquid lipid medium and resemble a mosaic. Thus, the cell membrane consists of two layers of lipids, the nonpolar hydrophobic "tails" of which are located inside the membrane, and the polar hydrophilic heads face the cytoplasm of the cell and the intercellular fluid.

The lipid layer is penetrated by large protein molecules forming hydrophilic pores. It is through them that water solutions of glucose and mineral salts are transported. Some protein molecules are found both on the outer and inner surfaces of the plasmalemma. Thus, on the outer cell membrane in the cells of all organisms having nuclei, there are carbohydrate molecules bound by covalent bonds with glycolipids and glycoproteins. The content of carbohydrates in the cell membranes varies from 2 to 10%.

The structure of the plasmalemma of prokaryotic organisms

The outer cell membrane in prokaryotes performs similar functions with the plasma cells of the 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 protection of the cytoplasm from foreign reagents from outside. It can form mesosomes - structures that arise when the plasmalemma is invaginated inside the cell. They can contain enzymes involved in the metabolic reactions of prokaryotes, for example, in DNA replication, protein synthesis.

Mesosomes also contain oxidation-reduction enzymes, while photosynthetic bacteria 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 external cell membrane performs, let us dwell on its role in cell-cell contacts. Plant cells in the walls of the outer cell membrane form pores that pass into the cellulose layer. Through them, the cytoplasm of the cell can escape to the outside, 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 sites of neighboring cell membranes are called desmosomes. They are characteristic of endothelial and epithelial cells, and also occur in cardiomyocytes.

Auxiliary formations of the plasmalemma

To understand what distinguishes plant cells from animals, the study of the structural features of their plasmalemmas helps, which depends on what functions the external cell membrane performs. Above it in the animal cells is a layer of glycocalyx. It is formed by molecules of polysaccharides bound to proteins and lipids of the outer cell membrane. Thanks to the glycocalysis, adhesion (adhesion) occurs between the cells, leading to the formation of tissues, so it takes part in the signaling function of the plasmalemma - the recognition of environmental stimuli.

How is passive transport of certain substances through cell membranes carried out

As it was said before, the outer cell membrane participates in the process of transportation of substances between the cell and the external environment. There are two types of transfer through plasmalemma: passive (diffusion) and active transport. The first includes diffusion, light diffusion and osmosis. The movement of substances along the concentration gradient depends, first of all, on the mass and magnitude of molecules passing through the cell membrane. For example, small nonpolar molecules easily dissolve in the average lipid layer of the plasmalemma, move through it and find themselves in the cytoplasm.

Large molecules of organic substances penetrate the cytoplasm with the help of special carrier proteins. They have specific specificity and, connecting to a particle or an ion, passively pass them through the membrane along the concentration gradient (passive transport) without spending energy. This process underlies the property of a plasmalemma, such 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 plasma membrane

Since the outer cell membrane provides for the transport of molecules and ions from the environment into the cell and back, it becomes possible to remove the products of dissimilation, which are toxins, outwards, that is, into the intercellular fluid. Active transport occurs against the concentration gradient and requires the use of energy in the form of ATP molecules. It also involves carrier proteins called ATP-ases, which are both enzymes.

An example of such a transport is a sodium-potassium pump (sodium ions pass from the cytoplasm to the external environment, and the potassium ions are pumped into the cytoplasm). Epithelial cells of the intestine and kidneys are capable of it. Varieties of this mode of transport are the processes of pinocytosis and phagocytosis. Thus, having studied what functions the external cell membrane performs, it can be established that heterotrophic protists, as well as cells of higher animal organisms, for example, leukocytes, are capable of the processes of pinot and phagocytosis.

Bioelectric processes in cell membranes

It is established that there is a potential difference between the outer surface of the plasmalemma (it is positively charged) and the near-wall layer of the cytoplasm, which is negatively charged. It was called a resting potential, and it is inherent in all living cells. A neural tissue has not only a resting potential, but is also capable of carrying out weak biocurrents, which is called the excitation process. External membranes of nerve cells-neurons, accepting irritation from receptors, begin to change charges: sodium ions massively enter the cell and the surface of the plasmalemma becomes electronegative. A near-wall layer of the cytoplasm, due to an excess of cations, receives a positive charge. This explains why the external cell membrane of the neuron is being recharged, which causes the nerve impulses that underlie the excitation process.