The outer layer of the cell. Biology: the structure of plant cells, the scheme

Cells that form tissues of representatives of flora and fauna, have significant differences in size, shape, constituent elements. However, they all show similarities in the main features of growth, metabolism, vital activity, irritability, ability to change, development. Next, we will consider in more detail the structure of the plant cell (the table of the main components will be given at the end of the article).

A Brief Historical Reference

With the help of an osmotic blow in 1925, Grendel and Gorter received empty shells of red blood cells, their so-called "shadows." They were stacked, determining the area of their surface. With the use of acetone, lipids were isolated. Their number per unit area of erythrocytes was also determined. Despite the errors in the calculations, an accidentally correct result was output and a lipid bilayer was discovered.

general information

The study of the development and growth of tissue elements of representatives of the flora and fauna is engaged in biology. The structure of a plant cell is a complex of three inextricably related components:

• Core. It is separated from the cytoplasm by means of a porous membrane. It contains the nucleolus, nuclear juice and chromatin.
• Cytoplasm and a complex of specialized structures - organoids. The latter include, in particular, plastids, mitochondria, lysosomes and the Golgi complex, the cellular center. Organoids are constantly present. Besides them, there are also temporary formations, called inclusions.
• The structure forming the surface is the shell of the plant cell.

Features of the surface apparatus

In leukocytes and unicellular organisms, the cell membrane ensures the penetration of water, ions, small molecules of other compounds. The process in which penetration of solid particles occurs is called phagocytosis. If drops of liquid compounds enter, then they speak of pinocytosis.

Organoids

They are present in eukaryotic cells. Organoids are associated with biological transformations that occur in the cell. They are covered by a double membrane - plastids and mitochondria. They have their own DNA, as well as a device that synthesizes protein. Reproduction is by division. In mitochondria, in addition to ATP, a protein is synthesized in small amounts. Plastids are present in plant cells. Their multiplication is carried out by division.

Membrane

It is a mistake to assume that the outer layer of the cell is the cytoplasm. The membrane is a molecular elastic structure. The outer layer of a cell is called a surface device, through which the content is separated from the external environment. There are different functions of the cell shell. One of the main tasks is to ensure the integrity of the entire element. Inside, there are also structures that separate the cell into so-called compartments. These closed zones are called organelles or compartments. Within them, certain conditions are maintained. The function of the cell membrane includes regulating the exchange between the medium and the cell.

Membrane

What is the structure of the cell membrane? The cell membrane is a bilayer (double) of molecules of the lipid class. Most of them are lipids of complex type - phospholipids. In molecules, there are hydrophobic (tail) and hydrophilic (head) parts. When the cell membrane is formed, the tails are inward, and the heads are in the opposite direction. Membranes are invariable structures. The shell of the animal cell has many similarities with the element of the representative of the flora. The thickness of the membrane is about 7-8 nm. The biological outer layer of the cell includes various protein compounds: semi-integrated (one end immersed in an external or internal lipid layer), integral (piercing through), superficial (adjacent to the inner sides or located on the outside). A number of proteins are the points of abutment of the membrane and the cytoskeleton within the cell and the outer wall (if present). Some integral compounds fulfill the role of ion channels, various receptors and transporters.

Protective task

The structure of the cell membrane largely determines its activity. In particular, the membrane has selective permeability. This means that the degree of patency of molecules through the membrane depends on their size, chemical properties, electrical charge. The main function performed by the outer layer of the cell is called the barrier function. Due to it, a selective, regulated, active and passive exchange of connections with the environment is ensured. For example, the peroxisome coating provides protection of the cytoplasm from dangerous peroxides.

Transport

Through the outer layer of the cell there is a transition of substances. Due to transport, delivery of nutrient components, elimination of the final products of the metabolic process, secretion of various substances, formation of ionic ingredients are provided. In addition, the cell maintains the optimal pH and concentration of ions necessary for the operation of enzymes. If the necessary particles for some reason can not pass through the bilayer from phospholipids, for example, due to hydrophilic properties, since the membrane is hydrophobic inside or because of its large size, they can cross the membrane by means of special conveyors (carrier proteins) , By endocytosis or by protein channels. In the process of passive transport, the compounds pass the outer layer of the cell without energy costs by diffusion along the concentration gradient. One of the options for this process is lightweight implementation. In this case, a specific molecule helps to cross the outer layer of the cell. It can have a channel that is capable of transmitting substances of only 1 type. Active transport requires energy. This is due to the fact that the motion in this case occurs back to the concentration gradient. On the membrane in this case there are special protein-pumps, ATPase including, which actively pumps into the cell potassium ions and deflates the sodium.

Other tasks

The outer layer of the cell performs the matrix function. Due to this, a certain mutual arrangement and orientation of the membrane protein compounds is ensured, as well as their optimal interaction. Due to the mechanical function, the autonomy of the cell and internal structures is ensured, as well as the connection with other cells. Great importance in this case in the representatives of the flora have the walls of structures. In animals, providing a mechanical function depends on the intercellular substance. Membranes fulfill energy tasks. In the process of photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems are activated in their walls. In them, as in many other cases, proteins are involved. One of the most important is the receptor function. Some proteins that are found in the membrane are receptors. Thanks to these molecules, the cell can perceive these or other signals. For example, steroids circulating with blood flow affect only those target cells that have receptors that correspond to one or another hormone. There are also neurotransmitters. These chemical compounds provide impulsive transmission. They also have a connection with specific target proteins. Membrane components are often enzymes. Hence the enzymatic function of the cell membrane. Digestive compounds are present in the plasma membranes of the intestinal epithelial elements. In the outer layer of the cell, biopotentials are generated and carried out.

The concentration of ions

With the help of the membrane, the internal K + ion content is maintained at a higher level than outside. At the same time, the Na + concentration is significantly lower than from the outside. This is of particular importance, since the potential difference on the wall and the generation of the nerve impulse are thus provided.

Marking

On the membrane there are antigens, which act as some "labels". Marking allows you to identify the cell. Glycoproteins - proteins with doped oligosaccharide branched side chains attached to them - play the role of "antennas". Since there are innumerable configurations of side chains, it is possible to make a marker for each group of cells. With the help of them, certain elements are recognized by others, which, in turn, allows them to act in concert. This happens, for example, in the formation of tissues and organs. By the same mechanism, the immune system works to recognize foreign antigens.

Composition and structure

As mentioned above, cell membranes consist of phospholipids. However, in addition to them, cholesterol and glycolipids are present in the structure. The latter are lipids with carbohydrates docked to them. Glyco- and phospholipids, mainly forming cell membranes, consist of 2 long carbohydrate hydrophobic "tails". They are associated with a hydrophilic, charged "head." Due to the presence of cholesterol, the membrane has the necessary level of rigidity. The connection occupies a free space between the lipid hydrophobic tails, thus preventing their bending. In this regard, those membranes in which less cholesterol, are more flexible and soft, and where there is more, on the contrary, more stiffness and brittleness in the walls. In addition, the compound acts as a stopper, which prevents the movement of polar molecules from the cell to the cell . Of particular importance are proteins that permeate the membrane and are responsible for its various properties. This or that shell of the plant cell has proteins that are specific in composition and orientation.

Annular lipids

These compounds are next to the proteins. However, annular lipids are more ordered and less mobile. They contain fatty acids with a higher saturation. Lipids come out of the membranes together with the protein compound. Without annular elements, membrane proteins will not work. Often the shells are asymmetric. In other words, this means that the layers have a different composition of lipids. In the outer one contains mainly glycolipids, sphingomyelins, phosphatidylcholine, phosphatidylinositol. In the inner layer, however, phosphatidylinositol, phosphatidylethanolamine and phosphatidylserine are present. The transition from one level to another of a certain molecule is somewhat difficult. Nevertheless, it may well happen spontaneously. This happens about once every six months. The transition can also be carried out with the help of protein-flippaz and scramblazy. When phosatidylseril appears in the outer layer, the macrophages take a defensive position and direct their activity to destroy the cell.

Organelles

These sites can be single and closed or connected to each other, separated by membranes from the hyaloplasm. Single-membrane organelles are perixisomes, vacuoles, lysosomes, Golgi apparatus, endoplasmic reticulum. To the two-membrane include plastids, mitochondria, nucleus. As for the structure of the membranes, the walls of the different organelles differ in the composition of proteins and lipids.

Selective permeability

Through the cell membranes slowly diffuse fatty acids and amino acids, ions and glycerol, glucose. At the same time, the walls themselves quite actively regulate this process, letting alone and restraining other substances. There are four main mechanisms for joining a cell. These include endo- or exocytosis, active transport, osmosis and diffusion. The last two have a passive character and do not require energy costs. But the first two are active. They need energy. With passive transport, the selective permeability is determined by integral proteins-special channels. The membrane is permeated through them. These channels form in some way a passage. Its proteins are for the elements Cl, Na, K. As for the concentration gradient, the molecules of the elements move into the cell from it. Against the background of stimulation, the channels of sodium ions are opened. They, in turn, begin to drastically enter the cage. This is accompanied by imbalance of the membrane potential. However, after that it is restored. Potassium channels remain open at all times. The ions enter through them into the cage slowly.

Finally

Below are briefly the problems and structure of the plant cell. The table also contains information on the composition of the biological element.