The cell membrane is absent from whom? Structure and functions of the cell membrane

Above-membrane complexes of plant cells, fungi and some bacteria include a structure such as their wall. In this article, the structure of the cell membrane in different groups of organisms will be studied, and the functions that it performs are clarified. As you know, this component was first discovered by the English scientist Robert Hooke in the 17th century. Note also that the cell membrane is absent in unicellular animals and multicellular organisms, beginning with coelenterates and ending with vertebrates: fish, amphibians, reptiles, birds and mammals.

Why it is needed

Despite the fact that the structure and chemical composition of the wall of fungi, plants and crushed bunches are not the same, the functions of the cell membrane are very similar to them. First of all, it is the protection of the cytoplasm and its organelles from the damaging influence of environmental factors.

Further: the supramembrane complex serves as a reliable support and provides strength of contacts in the tissues of plants and fungi. Below we will consider in more detail how the structure of the shell is interrelated with the functions it performs.

Features of the cell membrane of plants

This structure in plant organisms consists mainly of a polymer belonging to the class of polysaccharides - cellulose. Its molecular formula is the same as that of vegetable starch (C ₆ H ₁₀ O ₅ ) _n . Macromolecules of this polysaccharide contain beta-glucose residues and have only a linear structure, so they can form fibers collected in bundles. They form a strong framework of the cell wall, deepened into a colloidal matrix, which also consists mainly of carbohydrates - pectin and hemicellulose. Also cellulose is often found in other parts of plants, for example, cotton fibers are 99% pure pulp, flax and hemp contain 75-80%, in wood - up to 55%. As it was already said before, the functions of the cell membrane are conditioned by the fact, in the tissues of which organisms it enters.

In addition to cellulose, the wall contains proteins, lipids and inorganic substances. For example, the composition of cell membranes of higher spore plants - horsetails - includes silicon dioxide, so the plant itself is very hard and durable and is inedible for animals. One of the layers that form the stem of perennial plants and is called a stopper, accumulates a fat-like substance in the shells - the suberin. As a result, the cytoplasm and its organoids are destroyed, and the cells themselves can perform only a supporting function (culling of the stem).

If lignin accumulates between the cellulose fibers, together with hemicellulose, it strengthens the mechanical strength of the stems and tree trunks of plants, and the pigments contained in the lignin determine the color of the wood. The wall also contains pores lined with a membrane that provide transport of substances.

Structure and function of the cell membrane of fungi

In the representatives of different groups of fungi, the chitin is the basis of the wall, a polysaccharide, which also occurs in the outer skeleton of arthropods and in some bacteria. Supramembrane fungal complexes also contain cellulose and animal starch, glycogen. For example, the chemical structure of the yeast cell membrane is represented mainly by carbohydrates - glucan and mannan. The wall itself is strong enough and poorly digested in the gastrointestinal tract of animals, so yeast nutrients are inaccessible and not absorbed by the epithelium of the small intestine.

Features of bacteria

If the cell membrane is absent in protista, then in prokaryotes it has a very complex structure, including murine, lipoproteins and lipopolysaccharides, as well as teichoic acids. Lipopolysaccharides of the wall promote the adhesion of bacteria to various substrates: for example, to the enamel of the teeth or to the membranes of eukaryotes. Therefore, the bacterial cell membrane also has antigenic properties.

Often the bacterial wall is covered from above with a mucous capsule (capsid), over which there may be another protective layer - peplos. Depending on its structure, in microbiology all bacteria are divided into gram-positive and gram-negative.

Differentiation of bacteria by biochemical composition

The method that made it possible to distinguish prokaryotes from the peculiarities of the chemical structure of their shell was proposed by the Danish scientist G. Gram at the end of the 19th century. He found that some species of bacteria are well colored with aniline dyes and form strong violet compounds that form part of the cell membrane.

Such prokaryotes were called Gram-positive: for example, staphylococci and streptococci. All of them are sensitive to the antibiotics of a number of penicillin and actinomycin. Other bacteria called Gram-negative bacteria are not stained with methyl violet. They are resistant to penicillin, since they have a strong capsule and a low permeable cell wall. These include salmonella, shigella, helikobakter. The bacterial cell membrane, which has a different chemical composition, serves as an important microbiological characteristic, which is taken into account in pharmacology and medicine.

Features of mycoplasmas

Let us dwell on a group of very small bacteria - mycoplasmas. Microscopic studies have shown that the cell membrane is absent in them, therefore mycoplasmas are sensitive to certain antibiotics, for example, to tetracycline. Mycoplasmas are widely distributed in nature, are the causative agents of many diseases, including the genitourinary system of man.

Most mycoplasmas in their metabolism necessarily use oxygen and are strict aerobes. Being parasites of humans and mammals, they multiply rapidly, as in the cell membranes there is cholesterol, which is a favorable substrate for the growth and reproduction of mycoplasmas.

Adaptations in protozoa

Earlier, we noted the fact that the cell membrane is absent in the ciliates and other unicellular animals, for example, in the Karnenozes. Zoologists have established that protists are a full-fledged animal organism, which has all the functions: growth, reproduction, nutrition, respiration, secretion. Moreover, living in an aqueous medium or moistened soil, protists through a thin membrane transport the water and mineral salts in the external environment, and produce through the plasma membrane pores products of its own metabolism. Therefore, unicellular animals do not have complex supramembrane complexes, which is an idioadaptation to the characteristics of environmental conditions.

To protect and preserve the integrity of the shell protozoa have a peliculus - the outer denser layer of ectoplasm. Thanks to the pelican, which has elasticity and strength, the constant shape of the animal's body remains.

In this article, the structure and chemical composition of the cell membrane, characteristic of cells of plant organisms, as well as bacteria and fungi, has been studied.