The surface apparatus of the cell is a universal subsystem. He determines the boundary between the external environment and the cytoplasm. PAA provides regulation of their interaction. Let us further consider the features of the structural and functional organization of the cell surface apparatus.
Components
The following components of the surface apparatus of eukaryotic cells are distinguished: a plasma membrane, a supra-membrane and sub-membrane complexes. The first is represented as a spherically closed element. Plasmolemma is considered the basis of the surface cell apparatus. The supmembrane complex (it is also called glycocalyx) is an external element located above the plasma membrane. It consists of various components. In particular, they include:
- Carbohydrate parts of glycoproteins and glycolipids.
- Membrane peripheral proteins.
- Specific carbohydrates.
- Semi-integral and integral proteins.
The submembrane complex is located under the plasmolemma. In its composition, the musculoskeletal system and peripheral hyaloplasm are isolated.
Elements of the submembrane complex
Considering the structure of the surface apparatus of the cell, we should separately dwell on the peripheral hyaloplasm. It is a specialized cytoplasmic part and is located above the plasmolemma. Peripheral hyaloplasm is presented in the form of a liquid highly differentiated 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 occur. Peripheral hyaloplasm provides 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 filaments).
- Microtubules.
Microfibrils are filamentary 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 most distant from the plasmalemma. Their walls are formed by tubulin proteins.
The structure and functions of the surface apparatus of the cell
Metabolism is due to the presence of transport mechanisms. The surface structure of the cell provides the ability to carry out the movement of compounds in several ways. In particular, the following modes of transport are carried out:
- Simple diffusion.
- Passive transport.
- Active moving.
- Cytosis (exchange in membrane packaging).
In addition to the transport, such functions of the surface apparatus of the cell were revealed as:
- Barrier (demarcation).
- 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 if there is an electric gradient on both sides of the membrane. Its size determines the speed and direction of movement. 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 type of movement of the 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 consumption of ATP. Passive transport is faster than free. In the process of increasing the concentration difference in the gradient, a moment arrives at which the speed of movement becomes constant.
Carriers
Special molecules provide transport through the cell’s surface apparatus. Using these carriers, large molecules of the hydrophilic type (amino acids, in particular) pass along the concentration gradient . The surface apparatus of eukaryotic cells includes passive carriers for a variety of ions: K +, Na +, Ca +, Cl-, HCO3-. These special molecules are highly selective for transported elements. In addition, their important property is the high speed of movement. It can reach 104 or more molecules per second.
Active transport
It is characterized by the movement of elements against the gradient. Molecules are transported from a low concentration region to higher regions. Such movement involves certain costs of ATP. For the implementation of active transport, specific carriers are included in the structure of the surface apparatus of the animal cell. They are called "pumps" or "pumps." Many of these carriers are characterized by ATPase activity. This means that they are able to break down adenosine triphosphate and extract 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. In the process of cytosis, the transported element is surrounded by a membrane bubble. If the movement is carried out into the cell, then it is called endocytosis. Accordingly, the opposite direction is called exocytosis. In some cells, elements pass through. This type of transport is called transcytosis or diacyosis.
Plasmolemma
The surface structure 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. In accordance with the theory, the basis of plasmolemma is formed by lipid molecules laid in two layers (bilipid layer). They are turned by tails (hydrophobic sites) to each other, and outward and inward - by hydrophilic heads. These surfaces of the bilipid layer are coated with protein molecules. This model was confirmed in the 50s 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 the 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
A variety of electron microscopic studies conducted 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 60s. quite a lot of facts accumulated that 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. Among them was the theory of a "lipoprotein mat." In accordance with it, two types of proteins are present in the membrane: 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 membrane formation belongs to globular proteins. 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 physicochemical characteristics of the membrane are provided by the 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 focus mainly on the outer monolayer. They are linked to oligosaccharide chains. Due to the fact that the links protrude beyond the outer part of the plasmolemma, 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 liquidity of the membrane. The more cholesterol there is, the higher it is. The liquid level also depends on the ratio of unsaturated and saturated residues from fatty acids. The more of them, the higher it is. Liquid affects the activity of enzymes in the membrane.
Squirrels
Lipids mainly determine barrier properties. Proteins, in contrast to them, contribute to the implementation of key cell functions. In particular, we are talking about the regulated transport of compounds, the regulation of metabolism, reception and so on. Protein molecules are distributed in a lipid bilayer mosaic. They can move in thickness. This movement is apparently controlled 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 do not stand out from the membrane without damaging the bilipid layer. The proteins that penetrate it 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 glycocalyx. It is represented by a loose layer with an electronic moderate density. Glycocalyx covers the outer part of the plasma membrane. Its carbohydrate sites facilitate the recognition of neighboring cells and substances between them, and also provides adhesive bonds with them. In glycocalyx there are also hormone and guitar compatibility receptors, enzymes.
Additionally
Membrane receptors are predominantly 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 inside the cell, the permeability of the plasmalemma. They are able to turn signals of the external environment into internal, to bind the elements of the intercellular matrix and the cytoskeleton. Some researchers believe that the composition of glycocalyx also includes semi-integral protein molecules. Their functional areas are located in the supramembrane region of the surface cell apparatus.