Cell Surface Device: Structure and Functions

The surface apparatus of the cell is a universal subsystem. He defines the boundary between the external environment and the cytoplasm. The PAC ensures the 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: plasma membrane, supramembrane and submembrane complexes. The first is represented as a spherically closed element. Plasmolemma is considered the basis of the surface cellular apparatus. The supra-membrane complex (also referred to as glycocalysis) is the outer element located above the plasma membrane. It consists of various components. In particular, they include:

1. Carbohydrate parts of glycoproteins and glycolipids.
2. Membrane peripheral proteins.
3. Specific carbohydrates.
4. Semi-integral and integral proteins.

The submembrane complex is located beneath the plasmolemma. In its composition, the support-contractile system and the peripheral hyaloplasm are distinguished.

Elements of the submembrane complex

Considering the structure of the surface apparatus of the cell, one should separately stop on the peripheral hyaloplasma. It is a specialized cytoplasmic part and is located above the plasmolemma. Peripheral hyaloplasm is represented as a liquid highly differentiated heterogeneous substance. It contains a variety of high- and low-molecular elements in solution. In fact, it is a microenvironment in which specific and common metabolic processes occur. Peripheral hyaloplasm provides the performance of many functions of the surface apparatus.

Support-contractile system

It is located in the peripheral hyaloplasm. In the support-contractile system, there are:

1. Microfibrils.
2. Skeletal fibrils (intermediate filaments).
3. Microtubules.

Microfibrils are threadlike 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, there are anomalies of cellular functions. The microtubules are the most remote from the plasmalemma. Their walls are formed by proteins tubulins.

Structure and functions of the surface cell apparatus

The exchange of substances is carried out due to the presence of transport mechanisms. The structure of the surface apparatus of the cell makes it possible to carry out the movement of the joints in several ways. In particular, the following types of transport are carried out:

1. Simple diffusion.
2. Passive transport.
3. Active movement.
4. Cytosis (exchange in a membrane package).

In addition to transport, such functions of the surface cell apparatus as:

1. Barrier (demarcating).
2. Receptor.
3. Identifying.
4. The function of the cell's motion with the formation of filo, pseudo, and lamellopodia.

Free movement

Simple diffusion through the surface apparatus of the cell is carried out exclusively when there is an electric gradient on both sides of the membrane. Its size determines the speed and direction of travel. The bilipid layer can pass any hydrophobic type molecules. However, most of the biologically active elements are hydrophilic. Accordingly, their free movement is difficult.

Passive transport

This kind of displacement of a compound is also called light diffusion. It is also carried out through the surface apparatus of the cell in the presence of a gradient and without the expense of ATP. Passive transport is faster than free. In the process of increasing the concentration difference in the gradient, there comes a time at which the speed of movement becomes constant.

Carriers

Transport through the surface of the cell provides special molecules. With the help of these carriers, large molecules of the hydrophilic type (amino acids, in particular) pass through the concentration gradient . The surface apparatus of the eukaryotic cell includes passive carriers for a variety of ions: K +, Na +, Ca +, Cl-, HCO3-. These special molecules are highly selective for transported elements. In addition, an important property is the high speed of movement. It can reach 104 or more molecules per second.

Active transport

It is characterized by the displacement of elements against the gradient. Molecules are transported from a region of low concentration to areas with a higher concentration. This movement involves certain costs of ATP. To carry out active transport, specific vectors are included in the structure of the surface apparatus of the animal cell. They are called "pump" or "pumps." Many of these carriers are characterized by ATPase activity. This means that they are capable of cleaving adenosine triphosphate and extracting energy for their activities. Active transport provides the creation of ion gradients.

Cytosis

This method is used to move particles of different substances or large molecules. During the process of cytosis the transported element is surrounded by a membrane vesicle. If the movement is carried out in a cage, then it is called endocytosis. Accordingly, the opposite direction is called exocytosis. In some cells, the elements go through. This type of transport is called transcytosis or diacious.

Plasmolemma

The structure of the surface apparatus of the cell includes a plasma membrane formed predominantly by 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 laid in two layers (bilipid layer). They are turned by tails (hydrophobic areas) to each other, and outward and inward - by hydrophilic heads. These surfaces of the bilipid layer cover the protein molecules. This model was confirmed in the fifties of the last century 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 contains a medium 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 the protein and hydrophilic heads.

Other theories

A variety of electron microscopic studies conducted in the late 50's - early 60's. Pointed to the universality of the three-layer membrane organization. This was reflected in the theory of J. Robertson. Meanwhile, by the end of the 60's. A lot of facts accumulated, which were not explained from the point of view of the existing "sandwich model". This gave impetus to the development of new schemes, among which were models based on the presence of hydrophobic-hydrophilic bonds of protein and lipid molecules. One of them was the theory of a "lipoprotein rug". In accordance with it, the membrane contains two types of proteins: integral and peripheral. The latter are bound by electrostatic interactions with polar heads on lipid molecules. However, they never form a continuous layer. A key role in the formation of the membrane belongs to globular proteins. They are immersed in it in part 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 physicochemical characteristics of the membrane are provided by a layer represented by elements - phospholipids, consisting of the nonpolar (hydrophobic) tail and the polar (hydrophilic) head. The most common of these are phosphoglycerides and sphingolipids. The latter are mainly concentrated in the outer monolayer. They have a connection with oligosaccharide chains. Due to the fact that the links extend beyond the outer part of the plasmolemma, it acquires an asymmetric shape. Glycolipids play an important role in the realization of the receptor function of the surface device. Most of the membranes also contain cholesterol (cholesterol) - a steroid lipid. Its quantity is different, which, to a large extent, determines the fluidity of the membrane. The more cholesterol there is, the higher it is. The liquidity level also depends on the ratio of unsaturated and saturated fatty acid residues. The more of them, the higher it is. The fluid affects the activity of enzymes in the membrane.

Proteins

Lipids mainly determine barrier properties. Proteins, in contrast to them, contribute to the fulfillment of the key functions of the cell. In particular, we are talking about the regulated transport of compounds, the regulation of metabolism, reception and so on. Protein molecules are distributed in the lipid bilayer mosaic. They can move in the thickness. This movement is controlled, most likely, by the cell itself. Microfilaments are involved in the movement mechanism. They attach to individual integral proteins. Membrane elements differ depending on their location with respect to the bilipid layer. Proteins, therefore, can be peripheral and integral. The first are localized outside the layer. They have a weak connection 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 permeate it through are called transmembrane. The interaction between protein molecules and lipids of different nature ensures the stability of the plasmalemma.

Glycocalix

Lipoproteins have side chains. Oligosaccharide molecules can bind to lipids and form glycolipids. Their carbohydrate parts together with similar elements of glycoproteins impart a negative charge to the cell surface and form the basis of the glycocalyx. It is represented by a loose layer with an electronically moderate density. Glycocalix covers the outer part of the plasmolemma. Its carbohydrate sites promote the recognition of neighboring cells and the substance between them, and also provides adhesive bonds with them. In the glycocalyx there are also receptors of hormones and gytocompatibility, enzymes.

Additionally

Membrane receptors are represented mainly by glycoproteins. They have the ability to establish highly specific bonds with ligands. Receptors present in the membrane, in addition, can regulate the movement of some molecules inside the cell, the permeability of the plasmalemma. They are able to convert the signals of the external environment into internal ones, to bind the elements of the intercellular matrix and the cytoskeleton. Some researchers believe that the composition of the glycocalyx also includes semi-integrated protein molecules. Their functional areas are located in the superembral region of the surface cellular apparatus.