Structure and properties of the cytoplasmic membrane
The human and animal organism, consisting of billions of cells, developed in such a way that the function of each of its systems was the result of the function of the sum of cells from which the organs and tissues of the given system consist. Each cell of the body has a set of structures and mechanisms that allow it to exercise its own metabolism and perform its inherent function.
The cell consists of a cytoplasmic or surface membrane; Cytoplasm, which has a number of organelles, inclusions, cytoskeleton elements; Nucleus containing a nuclear genome. Cell organelles and nucleus are delimited in the cytoplasm by internal membranes. Each cell structure performs its function in it, and all of these together ensure the viability of the cell and the fulfillment of its specific functions.
A key role in the realization of cellular functions and their regulation belongs to the cytoplasmic membrane of the cell.
General principles of the structure of the cytoplasmic membrane
All cell membranes are characterized by a single building principle (Figure 1), which is based on the physico-chemical properties of complex lipids and proteins that make up their composition. Cell membranes are located in the aquatic environment and to understand the physicochemical phenomena affecting their structural organization, it is useful to describe the interaction of lipid and protein molecules with water molecules and with each other. A number of properties of cell membranes also follow from consideration of this interaction.
It is known that the plasma membrane of the cell is represented by a double layer of complex lipids covering the cell surface throughout its entire length. To create a lipid bilayer in its structure, only those lipid molecules that possess amphiphilic (amphipathic) properties could be selected by nature. These conditions correspond to molecules of phospholipids and cholesterol. Their properties are such that one part of the molecule (glycerol for phospholipids and cyclopentane for cholesterol) has polar (hydrophilic) properties, while the other (fatty acid radicals) has nonpolar (hydrophobic) properties.
If a certain number of molecules of phospholipids and cholesterol are placed in an aqueous medium, they will spontaneously begin to assemble into ordered structures and form closed vesicles (liposomes) in which a part of the aqueous medium is enclosed and the surface becomes covered with a continuous double layer (bilayer) of phospholipid molecules and cholesterol . When considering the nature of the spatial arrangement of molecules of phospholipids and cholesterol in this bilayer it is clear that the molecules of these substances are located in their hydrophilic parts towards the outer and inner water spaces, and hydrophobic - in opposite directions - inside the bilayer.
What causes the molecules of these lipids to spontaneously form in the aqueous medium bilayer structures, similar to the structure of the cell membrane bilayer? The spatial arrangement of amphiphilic lipid molecules in an aqueous medium is dictated by one of the requirements of thermodynamics. The most likely spatial structure that will be formed in the aqueous medium of the lipid molecule is a structure that has a minimum of free energy.
Such a minimum of free energy in the spatial structure of lipids in water will be achieved in the case when both the hydrophilic and hydrophobic properties of the molecules are realized as appropriate intermolecular bonds.
When considering the behavior of complex amphiphilic lipid molecules in water, some properties of cell membranes can also be explained. It is known that if the plasma membrane is mechanically damaged (for example, by piercing it with an electrode or through a puncture, removing the nucleus and placing another nucleus in the cell), then in a moment, due to the forces of intermolecular interaction of lipids and water, the membrane spontaneously restores its integrity. Under the action of the same forces, it is possible to observe the fusion of bilayers of two membranes upon their contact (for example, vesicles and presynaptic membranes in synapses). The ability of membranes to merge when they are in direct contact is part of the mechanisms for updating the structure of membranes, transporting membrane components from one subcellular space to another, and also part of the mechanisms of endo- and exocytosis.
The energy of intermolecular bonds in the lipid bilayer is very low, so conditions are created for the rapid movement of lipid and protein molecules in the membrane and for changing the structure of the membrane when mechanical forces, pressures, temperature, and other factors are affected. The presence of a double lipid layer in the membrane forms a closed space, isolates the cytoplasm from the surrounding aqueous medium and creates an obstacle to the free passage of water and soluble substances through the cell membrane. The thickness of the lipid bilayer is about 5 nm.
Cellular membranes also contain proteins. Their molecules by volume and mass are 40-50 times larger than molecules of membrane lipids. Due to proteins, the membrane thickness reaches 7-10 nm. Despite the fact that the total masses of proteins and lipids in most membranes are almost equal, the number of protein molecules in the membrane is ten times smaller than that of lipid molecules.
What happens if a protein molecule is placed in a phospholipid bilayer of liposomes, the outer and inner surfaces of which are polar, and the intralipidic one is unpolarized? Under the influence of the forces of intermolecular interactions of lipids, protein and water, a spatial structure will develop in which the nonpolar segments of the peptide chain will tend to settle in the depth of the lipid bilayer, while the polar ones will occupy a position on one of the surfaces of the bilayer and may also be submerged Into the external or internal aqueous environment of the liposome. A very similar nature of the arrangement of protein molecules occurs in the lipid bilayer of cell membranes (Figure 1).
Typically, protein molecules are localized in the membrane separately from each other. The very weak forces of hydrophobic interactions between the hydrocarbon radicals of lipid molecules and the nonpolar regions of the protein molecule (lipid-lipid, lipid-protein interactions) that arise in the nonpolar part of the lipid bilayer do not interfere with the course of thermal diffusion of these molecules in the bilayer structure.
When using the fine methods of research, the structure of the cell membranes was studied, it turned out that it is very similar to that which is spontaneously formed by phospholipids, cholesterol and proteins in the aquatic environment. In 1972, Singer and Nichols proposed a liquid-mosaic model of the structure of the cell membrane and formulated its basic principles.
According to this model, the structural basis of all cell membranes is a liquid-like continuous double layer of amphipathic molecules of phospholipids, cholestrol, glycolipids, spontaneously forming it in an aqueous medium. Protein molecules are asymmetrically located in the lipid bilayer, carrying out specific receptor, enzymatic and transport functions. Protein and lipid molecules have mobility and can make rotational movements, diffuse in the plane of the bilayer. Protein molecules are able to change their spatial structure (conformation), shift and change their position in the lipid bilayer membrane, plunging to different depths or floating on its surface. The structure of the lipid bilayer of the membrane is not uniform. It has sites (domains), called "rafts", which are enriched with sphingolipids and cholesterol. "Rafts" differ in the phase state from the state of the rest of the membrane in which they are located. The features of membrane structure depend on the function they perform and the functional state.
The study of the composition of cell membranes confirmed that their main components are lipids, which constitute about 50% of the mass of the plasma membrane. About 40-48% of the membrane mass is in proteins and 2-10% in carbohydrates. Remains of carbohydrates are either part of proteins, forming glycoproteins, or lipids, forming glycolipids. Phospholipids are the main structural lipids of plasma membranes and constitute 30-50% of their mass.
Carbohydrate residues of glycolipid molecules are usually located on the outer surface of the membrane and immersed in an aqueous medium. They play an important role in intercellular, cell-matrix interactions and the recognition of antigens by the cells of the immune system. Cholesterol molecules, embedded in the phospholipid bilayer, contribute to the preservation of the ordered arrangement of fatty acid chains of phospholipids and their liquid crystal state. In connection with the high conformational mobility of acyl radicals of fatty acids of phospholipids, they form a rather loose packing of the lipid bilayer and structural defects can form in it.
Protein molecules are able to permeate the entire membrane so that their end regions protrude beyond these transverse limits. Such proteins are called transmembrane, or integral. The membrane also contains proteins that are only partially immersed in the membrane or located on its surface.
Many specific functions of membranes are determined by protein molecules, for which the lipid matrix is a direct microenvironment, and the function of protein molecules depends on its properties. Among the most important functions of membrane proteins, it is possible to distinguish: receptor-binding with signaling molecules such as neurotransmitters, hormones, ingreleukins, growth factors, and signal transmission to post-receptor cell structures; Enzymatic - catalysis of intracellular reactions; Structural - participation in the formation of the structure of the membrane itself; Transport - the transfer of substances through membranes; Channeling - the formation of ion and water channels. Proteins together with carbohydrates participate in adhesion-adherence, gluing cells in immune reactions, uniting cells in layers and tissues, providing interaction of cells with extracellular matrix.
The functional activity of membrane proteins (receptors, enzymes, carriers) is determined by their ability to easily change their spatial structure (conformation) when interacting with signaling molecules, the action of physical factors or changing the properties of the environment of the microenvironment. The energy required to effect these conformational changes in the structure of proteins depends both on the intramolecular interaction forces of individual sections of the peptide chain and on the degree of fluidity (microviscosity) of the membrane lipids immediately surrounding the protein.
Carbohydrates in the form of glycolipids and glycoproteins are only 2-10% of the membrane weight; The number of them in different cells is variable. Due to them some types of intercellular interactions are carried out, they take part in recognition of the cell by foreign antigens and together with proteins create a kind of antigenic structure of the surface membrane of the cell. For such antigens, the cells recognize each other, combine into a tissue and for a short time stick together to transfer the signal molecules to each other.
Due to the low energy of the interaction of the substances entering the membrane and the relative ordering of their location, the cell membrane acquires a number of properties and functions that are not reducible to a simple sum of the properties of its constituent substances. Insignificant effects on the membrane, comparable to the energy of intermolecular bonds of proteins and lipids, can lead to a change in the conformation of protein molecules, the permeability of ion channels, changes in the properties of membrane receptors, and numerous other functions of the membrane and the cell itself. High sensitivity of the structural components of the plasma membrane is crucial in the perception of the cell by information signals and their transformation into cellular responses.
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