What is eps in biology. Cell structure

A bit of history

The cell is considered the smallest structural unit of any organism, but it also consists of something. One of its components is the endoplasmic reticulum. Moreover, EPS is a mandatory component of any cell in principle (except for some viruses and bacteria). It was discovered by the American scientist K. Porter back in 1945. It was he who noticed the systems of tubules and vacuoles, which, as it were, accumulated around the nucleus. Also, Porter noticed that the sizes of EPS in cells of different creatures and even organs and tissues of one organism are not similar to each other. He came to the conclusion that this is due to the functions of a particular cell, the degree of its development, as well as the stage of differentiation. For example, in humans, EPS is very well developed in the cells of the intestine, mucous membranes and adrenal glands.

Concept

EPS is a system of tubules, tubules, vesicles and membranes located in the cytoplasm of a cell.

Endoplasmic reticulum: structure and function

Summary of the other most important cell constituents

Cytoplasm: structure and function

Cell membrane: structure and function

The core: structure and function

Ribosomes

They are organelles that provide basic protein synthesis. They can be found both in the free space of the cytoplasm of the cell, and in a complex with other organelles (endoplasmic reticulum, for example). If the ribosomes are located on the membranes of the rough EPS (being on the outer walls of the membranes, the ribosomes create roughness) , the efficiency of protein synthesis increases several times. This has been proven by numerous scientific experiments.

Golgi complex

Organoid, consisting of several cavities, constantly secreting bubbles of various sizes. The accumulated substances are also used for the needs of the cell and the body. The Golgi complex and the endoplasmic reticulum are often located nearby.

Lysosomes

Organoids, surrounded by a special membrane and performing the cell's digestive function, are called lysosomes.

Mitochondria

Organoids surrounded by several membranes and performing an energetic function, that is, ensuring the synthesis of ATP molecules and distributing the received energy throughout the cell.

Plastids. Types of plastids

Chloroplasts (photosynthetic function);

Chromoplasts (storage and preservation of carotenoids);

Leukoplasts (accumulation and storage of starch).

Organoids designed to move

They also perform some kind of movement (flagella, cilia, long processes, etc.).

Cell center: structure and function

The endoplasmic reticulum (ER), also called the endoplasmic reticulum, is an important eukaryotic cell. It plays a leading role in the production, processing and transport of proteins and lipids. EPR produces transmembrane proteins and lipids for its membrane, as well as for many other cellular components, including secretory vesicles, and plant cells.

The endoplasmic reticulum is a network of tubules and flattened sacs that perform many functions in and out. There are two parts of the EPR, which differ in both structure and function. One part is called granular (rough) ER because it has ribosomes attached to the cytoplasmic side of the membrane. The other part is called agranular (smooth) ER, since it lacks attached ribosomes.

Typically, a smooth ESR is a piping network, while a rough ESR consists of a series of flattened pouches. The space inside the EPR is called the lumen. The endoplasmic reticulum extends extensively from the cell membrane through and forms a continuous connection with the nuclear envelope. Since the EPR is associated with the nuclear envelope, the lumen and space within the nuclear envelope are part of the same compartment.

Granular endoplasmic reticulum

The granular (rough) endoplasmic reticulum produces membranes and secretory proteins. Ribosomes attached to the granular ER synthesize proteins during translation. In some leukocytes (white blood cells), the rough EPR produces antibodies. In the cells of the pancreas, it produces insulin.

Granular and agranular EPR are usually interconnected, and proteins and membranes produced by rough EPR move into smooth EPR. Some proteins are sent to the Golgi apparatus by special transport vesicles. Once the proteins have been modified in the Golgi, they are transported to their proper destinations internally or exported from the cell by way.

Agranular endoplasmic reticulum

Agranular (smooth) endoplasmic reticulum has a wide range of functions, including the synthesis of carbohydrates and lipids. Lipids such as phospholipids and cholesterol are required to build cell membranes. The smooth ESR also serves as a transition area for vesicles that transport endoplasmic reticulum products to various destinations.

In liver cells, agranular EPR produces enzymes that help detoxify certain compounds. In the muscles, it helps in the contraction of muscle cells, and in the brain cells it synthesizes male and female hormones.

Endoplasmic reticulum (EPS), or endoplasmic reticulum (ER), was discovered only with the advent of the electron microscope. EPS is present only in eukaryotic cells and is a complex system of membranes that form flattened cavities and tubules. Together, it looks like a network. EPS refers to one-membrane cell organelles.

Micrograph of the endoplasmic reticulum

The EPS membranes extend from the outer membrane of the nucleus and are structurally similar to it.

The endoplasmic reticulum is divided into smooth (agranular) and rough (granular). The latter is dotted with ribosomes attached to it (because of this, "roughness" occurs). The main function of both types is associated with the synthesis and transport of substances. Only the rough one is responsible for protein, and the smooth one is responsible for carbohydrates and fats.

By its structure, EPS is a set of paired parallel membranes that permeate almost the entire cytoplasm. A pair of membranes forms a plate (the cavity inside has different widths and heights), however, the smooth endoplasmic reticulum has a more tubular structure. Such flattened membrane sacs are called ePS tanks.

Ribosomes located on the rough EPS synthesize proteins that enter the EPS channels, ripen (acquire a tertiary structure) there and are transported. These proteins first synthesize a signal sequence (consisting mainly of non-polar amino acids), the configuration of which corresponds to a specific EPS receptor. As a result, the ribosome and the endoplasmic reticulum are linked. In this case, the receptor forms a channel for the transition of the synthesized protein into the EPS cisterns.

After the protein is in the channel of the endoplasmic reticulum, the signal sequence is separated from it. After that, it folds into its tertiary structure. When transported along EPS, the protein acquires a number of other changes (phosphorylation, the formation of a bond with a carbohydrate, i.e., conversion into a glycoprotein).

Most of the proteins trapped in the rough EPS, then enter the Golgi apparatus (complex). From there, proteins are either secreted from the cell, or enter other organelles (usually lysosomes), or are deposited as granules of storage substances.

It should be borne in mind that not all cell proteins are synthesized on the rough EPS. Part (usually a smaller one) is synthesized by free ribosomes in the hyaloplasm, such proteins are used by the cell itself. Their signal sequence is not synthesized as unnecessary.

The main function of the smooth endoplasmic reticulum is lipid synthesis (fat). For example, EPS of intestinal epithelium synthesizes them from fatty acids and glycerol absorbed from the intestine. Then lipids enter the Golgi complex. In addition to intestinal cells, smooth EPS is well developed in cells that secrete steroid hormones (steroids are lipids). For example, in cells of the adrenal glands, interstitial cells of the testes.

The synthesis and transport of proteins, fats and carbohydrates are not the only functions of EPS. In baking, the endoplasmic reticulum is involved in detoxification processes. A special form of smooth EPS - sarcoplasmic reticulum - is present in muscle cells and provides contraction due to the pumping of calcium ions.

The structure, volume and functionality of the endoplasmic reticulum of a cell is not constant throughout the cell cycle, but is subject to one or another change.

The structure of the endoplasmic reticulum

Definition 1

Endoplasmic reticulum (EPS, endoplasmic reticulum) is a complex ultramicroscopic, very ramified, interconnected system of membranes, which more or less evenly permeates the mass of the cytoplasm of all eukaryotic cells.

EPS is a membrane organelle consisting of flat membrane sacs - cisterns, canals and tubules. Due to this structure, the endoplasmic reticulum significantly increases the area of \u200b\u200bthe inner surface of the cell and divides the cell into sections. Inside she is filled matrix (moderately dense loose material (synthesis product)). The content of various chemicals in the sections is not the same, therefore, in the cell, both simultaneously and in a certain sequence, various chemical reactions can occur in an insignificant volume of the cell. The endoplasmic reticulum opens in perinuclear space (the cavity between the two membranes of the karyolem).

The membrane of the endoplasmic reticulum consists of proteins and lipids (mainly phospholipids), as well as enzymes: adenosine triphosphatase and enzymes for the synthesis of membrane lipids.

There are two types of endoplasmic reticulum:

• Smooth (agranular, nuclear power plant), represented by tubes that anastamose with each other and do not have ribosomes on the surface;
• Grungy (granular, GRES), which also consists of interconnected tanks, but they are covered with ribosomes.

Remark 1

Sometimes they also allocate carryover, or transitory (TPP) endoplasmic reticulum, which is located in the area of \u200b\u200btransition of one type of ES to another.

Granular ES is characteristic of all cells (except for spermatozoa), but the degree of its development is different and depends on the specialization of the cell.

A highly developed hydroelectric power station of epithelial glandular cells (pancreas, producing digestive enzymes, liver - synthesizing serum albumin), fibroblasts (connective tissue cells producing collagen protein), plasma cells (production of immunoglobulins).

Agranular ES predominates in adrenal cells (synthesis of steroid hormones), in muscle cells (calcium metabolism), and in the cells of the fundic glands of the stomach (secretion of chlorine ions).

Another type of EPS membranes are branched membrane tubules containing a large number of specific enzymes inside, and vesicles are small vesicles surrounded by a membrane, mainly located next to tubules and cisterns. They provide the transfer of those substances that are synthesized.

EPS functions

The endoplasmic reticulum is an apparatus for the synthesis and, in part, the transport of cytoplasmic substances, thanks to which the cell performs complex functions.

Remark 2

The functions of both types of EPS are associated with the synthesis and transport of substances. The endoplasmic reticulum is a universal transport system.

Smooth and rough endoplasmic reticulum with their membranes and contents (matrix) perform common functions:

• separating (structuring), due to which the cytoplasm is distributed in an orderly manner and does not mix, and also prevents random substances from entering the organelle;
• transmembrane transport, due to which the necessary substances are transported through the membrane wall;
• synthesis of membrane lipids with the participation of enzymes contained in the membrane itself and ensuring the reproduction of the endoplasmic reticulum;
• due to the potential difference arising between the two surfaces of the ES membranes, it is possible to ensure conduction of excitation pulses.

In addition, each type of network has its own specific functions.

Functions of the smooth (agranular) endoplasmic reticulum

The agranular endoplasmic reticulum, in addition to the named functions common to both types of ES, also performs functions characteristic only for it:

• calcium depot... In many cells (skeletal muscles, heart, eggs, neurons) there are mechanisms that can change the concentration of calcium ions. The striated muscle tissue contains a specialized endoplasmic reticulum called the sarcoplasmic reticulum. This is a reservoir of calcium ions, and the membranes of this network contain powerful calcium pumps capable of ejecting large amounts of calcium into the cytoplasm or transporting it into the cavities of the network channels in hundredths of a second;
• lipid synthesis, substances such as cholesterol and steroid hormones. Steroid hormones are synthesized mainly in the endocrine cells of the gonads and adrenal glands, in the cells of the kidneys and liver. Intestinal cells synthesize lipids, which are excreted into the lymph, and then into the blood;

detoxification function - neutralization of exogenous and endogenous toxins;

Example 1

Renal cells (hepatocytes) contain oxidase enzymes that can destroy phenobarbital.

organelle enzymes take part in glycogen synthesis (in liver cells).

Functions of the rough (granular) endoplasmic reticulum

For the granular endoplasmic reticulum, in addition to the listed general functions, special ones are also characteristic:

• protein synthesis at the GRES has some peculiarities. It starts with free polysomes, which are later linked to the ES Mebrans.
• The granular endoplasmic reticulum synthesizes: all proteins of the cell membrane (except for some hydrophobic proteins, proteins of the inner membranes of mitochondria and chloroplasts), specific proteins of the inner phase of membrane organelles, as well as secretory proteins that are transported through the cell and enter the extracellular space.
• post-translational protein modification: hydroxylation, sulfation, phosphorylation. An important process is glycosylation, which occurs under the action of a membrane bound enzyme, glycosyltransferase. Glycosylation occurs before the secretion or transport of substances to some parts of the cell (Golgi complex, lysosomes or plasmolemma).
• transport of substances along the intramembrane part of the network. The synthesized proteins move along the ES intervals to the Golgi complex, which removes substances from the cell.
• due to the participation of the granular endoplasmic reticulum the Golgi complex is formed.

The functions of the granular endoplasmic reticulum are associated with the transport of proteins that are synthesized in ribosomes and located on its surface. The synthesized proteins enter the EPS, twist and acquire a tertiary structure.

The protein that is transported to the cisterns changes significantly along the way. It can, for example, be phosphorylated or converted to a glycoprotein. The usual path for a protein is through the granular EPS into the Golgi apparatus, from where it either leaves the cell, or enters other organelles of the same cell, for example, lysosomes), or is deposited as spare granules.

In liver cells, both granular and non-granular endoplasmic reticulum take part in the detoxification of toxic substances, which are then removed from the cell.

Like the outer plasma membrane, the endoplasmic reticulum has selective permeability, as a result of which the concentration of substances inside and outside the channels of the reticulum is not the same. This is important for cell function.

Example 2

The endoplasmic reticulum of muscle cells contains more calcium ions than its cytoplasm. Leaving the channels of the endoplasmic reticulum, calcium ions start the process of contraction of muscle fibers.

Endoplasmic reticulum formation

The lipid components of the membranes of the endoplasmic reticulum are synthesized by the enzymes of the reticulum itself, the protein component comes from the ribosomes located on its membranes. In a smooth (agranular) endoplasmic reticulum, there are no intrinsic factors of protein synthesis, therefore it is believed that this organelle is formed as a result of the loss of ribosomes by the granular endoplasmic reticulum.

The endoplasmic reticulum is one of the most important organelles in the eukaryotic cell. Its second name is the endoplasmic reticulum. EPS is of two types: smooth (agranular) and rough (granular). The more active the metabolism in the cell, the greater the amount of EPS.

Structure

It is a vast labyrinth of canals, cavities, vesicles, "cisterns" that are closely connected and communicate with each other. This organoid is covered with a membrane that communicates with both the cytoplasm and the outer cell membrane. The volume of the cavities is different, but they all contain a homogeneous liquid, which allows interaction between the cell nucleus and the external environment. Sometimes there are branches from the main network in the form of single bubbles. A rough EPS differs from a smooth one by the presence of a large number of ribosomes on the outer surface of the membrane.

Functions

• Functions of agranular EPS. It takes part in the formation of steroid hormones (for example, in the cells of the adrenal cortex). EPS, contained in liver cells, is involved in the destruction of certain hormones, drugs and harmful substances, and in the conversion of glucose, which is formed from glycogen. Also, the agranular network produces phospholipids necessary for the construction of membranes of all types of cells. And in the reticulum of muscle tissue cells, calcium ions are deposited, which are necessary for muscle contraction. This type of smooth endoplasmic reticulum is also called the sarcoplasmic reticulum.
• Functions of granular EPS. First of all, in the granular reticulum, the production of proteins occurs, which will subsequently be removed from the cell (for example, the synthesis of secretion products of glandular cells). Also, in a rough EPS, synthesis and assembly of phospholipids and multi-chain proteins takes place, which are then transported to the Golgi apparatus.
• Common functions for both smooth endoplasmic reticulum and rough endoplasmic reticulum are the demarcation function. Due to these organelles, the cell is divided into compartments (compartments). And in addition, these organelles are transporters of substances from one part of the cell to another.