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

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

Brief historical background

With the help of an osmotic strike in 1925, Grendel and Gorter received empty shells of red blood cells, their so-called "shadows." They were stacked to determine their surface area. Lipids were isolated using acetone. They also determined their number per unit area of ​​red blood cells. Despite the errors in the calculations, the correct result was accidentally deduced and a lipid bilayer was discovered.

general information

Biology is engaged in the study of the development and growth of tissue elements of representatives of flora and fauna. The structure of a plant cell is a complex of three components inextricably linked with each other:

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

Features of the surface apparatus

In leukocytes and unicellular organisms, the cell membrane provides the penetration of water, ions, small molecules of other compounds. The process during which the penetration of solid particles is called phagocytosis. If drops of liquid compounds get in, then they talk about 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 protein synthesizing apparatus. Reproduction is carried out by division. In mitochondria, in addition to ATP, a small amount of protein is synthesized. Plastids are present in plant cells. Their reproduction 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 the cell is called the surface apparatus, through which the separation of the contents from the external environment is carried out. There are different functions of the cell membrane. One of the main tasks is to ensure the integrity of the entire element. There are also structures inside that divide the cell into so-called compartments. These enclosed areas are called organelles or compartments. Inside them, certain conditions are supported. The functions of the cell membrane include the regulation of the exchange between the environment and the cell.

Membrane

What is the structure of the cell membrane? A cell membrane is a bilayer (double) of molecules of the lipid class. Most of them are complex type lipids - phospholipids. Hydrophobic (tail) and hydrophilic (head) parts are present in the molecules. When the cell membrane is formed, the tails are turned inward, and the heads are in the opposite direction. Membranes are invariant 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-integral (one end immersed in the outer or inner lipid layer), integral (penetrating through), surface (adjacent to the inner sides or located on the outside). A number of proteins are the points of contact between the membrane and the cytoskeleton inside the cell and the outer wall (if present). Some integral compounds play the role of ion channels, a variety of 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 to which molecules pass through the membrane depends on their size, chemical properties, and electric charge. The main function that the outer layer of the cell performs is called the barrier. Due to it, a selective, regulated, active and passive exchange of compounds with the environment is ensured. For example, the peroxisome membrane protects the cytoplasm from dangerous peroxides.

Transport

Through the outer layer of the cell there is a transition of substances. Due to transport, the delivery of nutrients, elimination of end products of the metabolic process, the secretion of various substances, the formation of ionic ingredients is ensured. In addition, the optimal pH and concentration of ions necessary for the functioning of enzymes are maintained in the cell. If the necessary particles for some reason cannot pass through a bilayer of 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 transporters (carrier proteins) , by endocytosis or through protein channels. In the process of passive transport, the compounds pass through the outer layer of the cell without energy consumption by diffusion along the concentration gradient. One option for this process is lightweight deployment. In this case, a specific molecule helps the substance cross the outer layer of the cell. She may have a channel that is capable of passing only type 1 substances. Active transport requires energy. This is due to the fact that the movement in this case occurs inversely to the concentration gradient. In this case, there are special pump proteins on the membrane, including ATPase, which quite actively pumps potassium ions into the cell and pumps out sodium.

Other tasks

The outer layer of the cell performs a matrix function. Due to this, a certain mutual arrangement and orientation of 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 connections with other cells. Of great importance in this case among representatives of the flora are the walls of the structures. In animals, the provision of mechanical function depends on the intercellular substance. Membranes perform 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 take part. One of the most important is receptor function. Some proteins that are in the membrane are receptors. Thanks to these molecules, the cell can perceive certain signals. For example, steroids circulating with blood flow affect only those target cells that have receptors corresponding to certain hormones. There are also neurotransmitters. These chemical compounds provide impulse transmission. They also have a relationship 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.

Ion concentration

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

Marking

Antigens are present on the membrane, which act as some kind of “label”. Marking allows you to identify the cell. Glycoproteins — proteins with oligosaccharide branched side chains attached to them — play the role of “antennas”. Since the configurations of the side chains are countless, you can make your own marker for each group of cells. With the help of them there is recognition of some elements by others, which, in turn, allows them to act in concert. This happens, for example, in the formation of tissues and organs. According to the same mechanism, the immune system works to recognize foreign antigens.

Composition and structure

As mentioned above, cell membranes are composed of phospholipids. However, besides 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 hydrophobic carbohydrate “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 the free space between the lipid hydrophobic tails, thus preventing their bending. In this regard, those membranes in which there is less cholesterol are more flexible and soft, and where it is more, on the contrary, there is more rigidity and brittleness in the walls. In addition, the compound acts as a stopper preventing the movement of polar molecules from cell to cell . Of particular importance are proteins that penetrate the membrane and are responsible for its various properties. This or that shell of a plant cell has proteins defined in composition and orientation.

Annular lipids

These compounds are next to proteins. However, annular lipids are more ordered and less mobile. They contain fatty acids with greater saturation. Lipids exit the membranes along with a protein compound. Without annular elements, membrane proteins will not work. Often the shells are asymmetrical. In other words, this means that the layers have a different lipid composition. The external contains mainly glycolipids, sphingomyelins, phosphatidylcholine, phosphatidylnositol. In the inner layer, phosphatidylnositol, phosphatidylethanolamine and phosphatidylserine are present. The transition from one level to another specific molecule is somewhat difficult. However, it may very well happen spontaneously. This happens approximately every six months. The transition can also be accomplished using flippase proteins and scramblase. When phosphatidylseryl appears in the outer layer, macrophages take a protective 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. Perixisomes , vacuoles, lysosomes, Golgi apparatus, endoplasmic reticulum are considered as single- membrane organelles . Plastids, mitochondria, and the core are referred to as double membranes. As for the structure of membranes, the walls of different organelles differ in the composition of proteins and lipids.

Selective Permeability

Fatty and amino acids, ions and glycerol, glucose slowly diffuse through cell membranes. At the same time, the walls themselves quite actively regulate this process, passing one and retaining other substances. There are four main mechanisms for a compound to enter a cell. These include endo- or exocytosis, active transport, osmosis and diffusion. The last two are passive in nature and do not require energy costs. But the first two are active. They need energy. In passive transport, selective permeability is determined by integral proteins - special channels. The membrane is pierced through them. These channels form a kind of passage. There are proteins 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 irritation, the channels of sodium ions open. They, in turn, begin to abruptly enter the cell. This is accompanied by an imbalance in membrane potential. However, after that it is restored. Potash channels always remain open. Ions flow through them into the cell slowly.

Finally

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