The surface apparatus of the cell is a universal subsystem. They define the boundary between the external environment and the cytoplasm. PAC provides regulation of their interaction. Let us further consider the features of the structural and functional organization of the surface apparatus of the cell.
Components
The following components of the surface apparatus of eukaryotic cells are distinguished: the plasma membrane, supramembrane and submembrane complexes. The first is presented in the form of a spherically closed element. The plasmalemma is considered the basis of the surface cellular apparatus. The epimembrane complex (also called the glycocalyx) is an external element located above the plasma membrane. It contains various components. In particular, these include:
- Carbohydrate parts of glycoproteins and glycolipids.
- Membrane peripheral proteins.
- Specific carbohydrates.
- Semi-integral and integral proteins.
The submembrane complex is located under the plasmalemma. It contains the musculoskeletal system and peripheral hyaloplasm.
Elements of the submembranecomplex
Considering the structure of the surface apparatus of the cell, we should dwell separately on the peripheral hyaloplasm. It is a specialized cytoplasmic part and is located above the plasma membrane. Peripheral hyaloplasm is presented as a highly differentiated liquid heterogeneous substance. It contains a variety of high and low molecular weight elements in solution. In fact, it is a microenvironment in which specific and general metabolic processes take place. The peripheral hyaloplasm performs many functions of the surface apparatus.
Musculoskeletal system
It is located in the peripheral hyaloplasm. In the musculoskeletal system, there are:
- Microfibrils.
- Skeletal fibrils (intermediate filament).
- Microtubules.
Microfibrils are filamentous structures. Skeletal fibrils are formed due to the polymerization of a number of protein molecules. Their number and length is regulated by special mechanisms. When they change, anomalies of cellular functions occur. Microtubules are the furthest away from the plasmalemma. Their walls are formed by tubulin proteins.
Structure and functions of the surface apparatus of the cell
The metabolism is carried out due to the presence of transport mechanisms. The structure of the surface apparatus of the cell provides the ability to carry out the movement of compounds in several ways. In particular, the following typestransport:
- Simple diffusion.
- Passive transport.
- Active movement.
- Cytosis (membrane-packed exchange).
In addition to transport, such functions of the surface apparatus of the cell as:
- Barrier (delimitation).
- Receptor.
- Identification.
- The function of cell movement through the formation of filo-, pseudo- and lamellopodia.
Free movement
Simple diffusion through the surface apparatus of the cell is carried out exclusively in the presence of an electrical gradient on both sides of the membrane. Its size determines the speed and direction of movement. The bilipid layer can pass any molecules of the hydrophobic type. However, most of the biologically active elements are hydrophilic. Accordingly, their free movement is difficult.
Passive transport
This type of compound movement is also called facilitated diffusion. It is also carried out through the surface apparatus of the cell in the presence of a gradient and without the consumption of ATP. Passive transport is faster than free transport. In the process of increasing the concentration difference in the gradient, there comes a moment at which the speed of movement becomes constant.
Carriers
Transport through the surface apparatus of the cell is provided by special molecules. With the help of these carriers, large molecules of the hydrophilic type (amino acids, in particular) pass along the concentration gradient. Surfaceeukaryotic cell apparatus includes passive carriers for various ions: K+, Na+, Ca+, Cl-, HCO3-. These special molecules are characterized by high selectivity for the transported elements. In addition, their important property is a high speed of movement. It can reach 104 or more molecules per second.
Active transport
It is characterized by moving elements against a gradient. Molecules are transported from an area of low concentration to areas of higher concentration. Such a movement involves a certain cost of ATP. For the implementation of active transport, specific carriers are included in the structure of the surface apparatus of the animal cell. They were called "pumps" or "pumps". Many of these carriers are distinguished by their ATPase activity. This means that they are able to break down adenosine triphosphate and extract energy for their activities. Active transport creates ion gradients.
Cytosis
This method is used to move particles of different substances or large molecules. In the process of cytosis, the transported element is surrounded by a membrane vesicle. If the movement is carried out into the cell, then it is called endocytosis. Accordingly, the reverse direction is called exocytosis. In some cells, elements pass through. This type of transport is called transcytosis or diacyosis.
Plasmolemma
The structure of the surface apparatus of the cell includes the plasmaa membrane formed predominantly of lipids and proteins in a ratio of approximately 1:1. The first "sandwich model" of this element was proposed in 1935. According to the theory, the basis of the plasmolemma is formed by lipid molecules stacked in two layers (bilipid layer). They turn their tails (hydrophobic areas) to each other, and outward and inward - hydrophilic heads. These surfaces of the bilipid layer are covered with protein molecules. This model was confirmed in the 1950s by ultrastructural studies carried out using an electron microscope. In particular, it was found that the surface apparatus of an animal cell contains a three-layer membrane. Its thickness is 7.5-11 nm. It has a middle light and two dark peripheral layers. The first corresponds to the hydrophobic region of lipid molecules. Dark areas, in turn, are continuous surface layers of protein and hydrophilic heads.
Other theories
Various electron microscopy studies carried out in the late 50s - early 60s. pointed to the universality of the three-layer organization of membranes. This is reflected in the theory of J. Robertson. Meanwhile, by the end of the 1960s quite a lot of facts have accumulated that have not been explained from the point of view of the existing "sandwich model". This gave impetus to the development of new schemes, including models based on the presence of hydrophobic-hydrophilic bonds between protein and lipid molecules. Amongone of them was the "lipoprotein rug" theory. In accordance with it, the membrane contains two types of proteins: integral and peripheral. The latter are associated by electrostatic interactions with polar heads on lipid molecules. However, they never form a continuous layer. Globular proteins play a key role in membrane formation. They are partially immersed in it and are called semi-integral. The movement of these proteins is carried out in the lipid liquid phase. This ensures the lability and dynamism of the entire membrane system. Currently, this model is considered the most common.
Lipids
The key physical and chemical characteristics of the membrane are provided by a layer represented by elements - phospholipids, consisting of a non-polar (hydrophobic) tail and a polar (hydrophilic) head. The most common of these are phosphoglycerides and sphingolipids. The latter are concentrated mainly in the outer monolayer. They are linked to oligosaccharide chains. Due to the fact that the links protrude beyond the outer part of the plasmalemma, it acquires an asymmetric shape. Glycolipids play an important role in the implementation of the receptor function of the surface apparatus. Most membranes also contain cholesterol (cholesterol) - a steroid lipid. Its amount is different, which largely determines the fluidity of the membrane. The more cholesterol, the higher it is. The liquid level also depends on the ratio of unsaturated and saturated residues fromfatty acids. The more of them, the higher it is. Fluid affects the activity of enzymes in the membrane.
Proteins
Lipids determine mainly the barrier properties. Proteins, in contrast, contribute to the performance of key functions of the cell. In particular, we are talking about regulated transport of compounds, regulation of metabolism, reception, and so on. Protein molecules are distributed in the lipid bilayer in a mosaic pattern. They can move in depth. This movement is apparently controlled by the cell itself. Microfilaments are involved in the movement mechanism. They are attached to individual integral proteins. Membrane elements differ depending on their location in relation to the bilipid layer. Proteins, therefore, can be peripheral and integral. The first are localized outside the layer. They have a weak bond with the membrane surface. Integral proteins are completely immersed in it. They have a strong bond with lipids and are not released from the membrane without damaging the bilipid layer. Proteins that penetrate it through and through are called transmembrane. The interaction between protein molecules and lipids of different nature ensures the stability of the plasmalemma.
Glycocalyx
Lipoproteins have side chains. Oligosaccharide molecules can bind to lipids and form glycolipids. Their carbohydrate parts, together with similar elements of glycoproteins, give the cell surface a negative charge and form the basis of the glycocalyx. Herepresented by a loose layer with a moderate electron density. The glycocalyx covers the outer part of the plasmalemma. Its carbohydrate sites contribute to the recognition of neighboring cells and the substance between them, and also provides adhesive bonds with them. The glycocalyx also contains hormone and hetocompatibility receptors, enzymes.
Extra
Membrane receptors are represented mainly by glycoproteins. They have the ability to establish highly specific bonds with ligands. The receptors present in the membrane, in addition, can regulate the movement of certain molecules into the cell, the permeability of the plasmalemma. They are able to convert signals from the external environment into internal ones, to bind the elements of the extracellular matrix and the cytoskeleton. Some researchers believe that semi-integral protein molecules are also included in the glycocalyx. Their functional areas are located in the supramembrane region of the surface cell apparatus.
