Functions of the plasma membrane. Mechanisms of transport of substances through the plasmalemma. Receptor function of the plasmalemma. Structure of the plasma membrane

Active

(with energy consumption)

Simple diffusion

(without proteins)

Energy source - ATP

Facilitated diffusion

(involving proteins)

Other types of sources

Through a channel in the protein

By way of a coup

protein with substance

Rice. 4. Classification of types of transport of substances through the membrane.

By active transfer, inorganic ions, amino acids and sugars, and almost all medicinal substances with polar molecules move through the membrane - para-aminobenzoic acid, sulfonamides, iodine, cardiac glycosides, B vitamins, corticosteroid hormones, etc.

To clearly illustrate the process of transfer of substances through the membrane, we present (with minor changes) Figure 5 taken from the book “Molecular Biology of the Cell” (1983) by B. Alberts and other scientists considered leaders in the development of the theory

Transported molecule

Channel Protein

protein transporter

Lipid Electrochemical

bilayer gradient

Simple diffusion Facilitated diffusion

Passive transport Active transport

Figure 5. Many small, uncharged molecules pass freely through the lipid bilayer. Charged molecules, large uncharged molecules, and some small uncharged molecules pass through membranes through channels or pores or with the help of specific transporter proteins. Passive transport is always directed against the electrochemical gradient towards the establishment of equilibrium. Active transport occurs against an electrochemical gradient and requires energy expenditure.

transmembrane transport, reflects the main types of transfer of substances across the membrane. It should be noted that the proteins involved in transmembrane transport belong to integral proteins and are most often represented by one complex protein.

The transfer of high molecular weight protein molecules and other large molecules through the membrane into the cell is carried out by endocytosis (pinocytosis, phagocytosis and endocytosis), and from the cell by exocytosis. In all cases, these processes differ from the above in that the transported substance (particle, water, microorganisms, etc.) is first packaged into a membrane and in this form is transferred into the cell or released from the cell. The packaging process can occur both on the surface of the plasma membrane and inside the cell.

b. Transfer of information across the plasma membrane.

In addition to the proteins involved in the transfer of substances across the membrane, complex complexes of several proteins have been identified. Spatially separated, they are united by one finite function. Complex protein assemblies include a complex of proteins responsible for the production of a very powerful biologically active substance in the cell - cAMP (cyclic adenosine monophosphate). This ensemble of proteins contains both surface and integral proteins. For example, on the inner surface of the membrane there is a surface protein called G protein. This protein maintains the relationship between two adjacent integral proteins - a protein called the adrenaline receptor and an enzyme protein - adenylate cyclase. The adrenergic receptor is able to connect with adrenaline, which enters the intercellular space from the blood and become excited. This excitation is transmitted by the G-protein to adenylate cyclase, an enzyme capable of producing the active substance - cAMP. The latter enters the cytoplasm of the cell and activates a variety of enzymes in it. For example, an enzyme that breaks down glycogen into glucose is activated. The formation of glucose leads to an increase in mitochondrial activity and an increase in the synthesis of ATP, which enters all cellular compartments as an energy carrier, enhancing the work of the lysosome, sodium-potassium and calcium membrane pumps, ribosomes, etc. ultimately increasing the vital activity of almost all organs, especially muscles. This example, although very simplified, shows how the activity of the membrane is related to the work of other elements of the cell. At the everyday level, this complex scheme looks quite simple. Imagine that a dog suddenly attacked a person. The resulting feeling of fear leads to the release of adrenaline into the blood. The latter binds to adrenergic receptors on the plasma membrane, thereby changing the chemical structure of the receptor. This, in turn, leads to a change in the structure of the G-protein. The altered G-protein becomes capable of activating adenylate cyclase, which enhances the production of cAMP. The latter stimulates the formation of glucose from glycogen. As a result, the synthesis of the energy-intensive ATP molecule is enhanced. The increased formation of energy in a person’s muscles leads to a quick and strong reaction to a dog’s attack (flight, defense, fight, etc.).

PLASMA MEMBRANE - (cell membrane plasmalemma), a biological membrane surrounding the protoplasm of plant and animal cells. Participates in the regulation of metabolism between the cell and its environment.

The cell membrane (also cytolemma, plasmalemma, or plasma membrane) is an elastic molecular structure consisting of proteins and lipids. The cell wall, if the cell has one (usually plant cells do), covers the cell membrane. The cell membrane is a double layer (bilayer) of molecules of the lipid class, most of which are so-called complex lipids - phospholipids. Lipid molecules have a hydrophilic (“head”) and a hydrophobic (“tail”) part. When membranes are formed, the hydrophobic regions of the molecules turn inward, and the hydrophilic regions turn outward.

Cell membrane structure

Some exceptions are, perhaps, archaea, whose membranes are formed by glycerol and terpenoid alcohols. Some proteins are the points of contact between the cell membrane and the cytoskeleton inside the cell, and the cell wall (if there is one) outside.

See what “plasma membrane” is in other dictionaries:

Experiments with artificial bilipid films have shown that they have high surface tension, much higher than in cell membranes. J. Robertson formulated in 1960 the theory of a unitary biological membrane, which postulated a three-layer structure of all cell membranes.

According to this model, proteins in the membrane do not form a continuous layer on the surface, but are divided into integral, semi-integral and peripheral. For example, the peroxisome membrane protects the cytoplasm from peroxides that are dangerous to the cell. Selective permeability means that the permeability of the membrane to different atoms or molecules depends on their size, electric charge and chemical properties.

A variant of this mechanism is facilitated diffusion, in which a specific molecule helps a substance pass through the membrane. For example, hormones circulating in the blood act only on target cells that have receptors corresponding to these hormones. Neurotransmitters ( chemicals, ensuring the conduction of nerve impulses) also bind to special receptor proteins of target cells.

With the help of markers, cells can recognize other cells and act in concert with them, for example, in the formation of organs and tissues. Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol.

Cholesterol gives the membrane rigidity by occupying the free space between the hydrophobic tails of lipids and preventing them from bending. Therefore, membranes with a low cholesterol content are more flexible, and those with a high cholesterol content are more rigid and fragile. Cholesterol also serves as a “stopper” that prevents the movement of polar molecules from the cell and into the cell. An important part of the membrane consists of proteins that penetrate it and are responsible for the various properties of membranes.

Features of metabolism in the membrane

Next to the proteins are annular lipids - they are more ordered, less mobile, contain more saturated fatty acids and are released from the membrane along with the protein. Without annular lipids, membrane proteins do not function. The selective permeability of the membrane during passive transport is due to special channels - integral proteins. They penetrate the membrane right through, forming a kind of passage.

Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, sodium ion channels open, and sodium ions suddenly enter the cell. It serves not only as a mechanical barrier, but, most importantly, it limits the free two-way flow of low- and high-molecular substances into and out of the cell. Moreover, the plasmalemma acts as a structure that “recognizes” various chemical substances and regulates the selective transport of these substances into the cell.

The mechanical stability of the plasma membrane is determined not only by the properties of the membrane itself, but also by the properties of the adjacent glycocalyx and the cortical layer of the cytoplasm. External surface The plasma membrane is covered with a loose fibrous layer of substance 3-4 nm thick - the glycocalyx.

In this case, some membrane transport proteins form molecular complexes, channels through which ions pass through the membrane by simple diffusion. In other cases, special membrane transport proteins selectively bind to one or another ion and transport it across the membrane.

PLASMA MEMBRANE - the outer layer of the cell cytoplasm of a denser consistency. Anchor junctions, or contacts, not only connect the plasma membranes of neighboring cells, but also communicate with the fibrillar elements of the cytoskeleton. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.

1. Barrier- provides regulated, selective, passive and active metabolism with the environment.

Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, the membranes themselves actively regulate this process - some substances pass through, but others do not.

2. Transport- transport of substances into and out of the cell occurs through the membrane. Transport through membranes ensures: delivery of nutrients, removal of metabolic end products, secretion of various substances, creation of ion gradients, maintenance of the appropriate pH and ionic concentration in the cell, which are necessary for the functioning of cellular enzymes.

There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside:

a) Passive (diffusion, osmosis) (does not require energy)

Diffusion

The distribution of molecules or atoms of one substance between the molecules or atoms of another, leading to spontaneous equalization of their concentrations throughout the occupied volume. In some situations, one of the substances already has an equalized concentration and they talk about diffusion of one substance in another. In this case, the substance is transferred from an area of ​​high concentration to an area of ​​low concentration (along the concentration gradient vector (Fig. 2.4).

Rice. 2.4. Diagram of the diffusion process

Osmosis

The process of one-way diffusion of solvent molecules through a semipermeable membrane towards a higher solute concentration from a volume with a lower solute concentration (Fig. 2.5).

Rice. 2.5. Diagram of the osmosis process

b) Active transport (requires energy expenditure)

Sodium-potassium pump- a mechanism of active coupled transmembrane transport of sodium ions (out of the cell) and potassium ions (inside the cell), which provides a concentration gradient and transmembrane potential difference. The latter serves as the basis for many functions of cells and organs: secretion of gland cells, muscle contraction, conduction of nerve impulses, etc. (Fig. 2.6).

Rice. 2.6. Scheme of operation of the potassium-sodium pump

At the first stage, the Na + /K + -ATPase enzyme attaches three Na + ions to the inner side of the membrane. These ions change the conformation of the active center of ATPase. After this, the enzyme is able to hydrolyze one molecule of ATP. The energy released after hydrolysis is spent on changing the conformation of the carrier, due to which three Na + ions and a PO 4 3− ion (phosphate) appear on the outer side of the membrane. Here, Na + ions are split off, and PO 4 3− is replaced by two K + ions. After this, the enzyme returns to its original conformation, and K + ions appear on inside membranes. Here the K+ ions are split off, and the carrier is ready for work again.

As a result, a high concentration of Na + ions is created in the extracellular environment, and a high concentration of K + is created inside the cell. This concentration difference is used in cells when conducting a nerve impulse.

c) Endocytosis (phagocytosis, pinocytosis)

Phagocytosis(eating by a cell) is the process of absorption by a cell of solid objects, such as eukaryotic cells, bacteria, viruses, remains of dead cells, etc. A large intracellular vacuole (phagosome) is formed around the absorbed object. The size of phagosomes is from 250 nm and more. By fusion of the phagosome with the primary lysosome, a secondary lysosome is formed. In an acidic environment, hydrolytic enzymes break down macromolecules trapped in the secondary lysosome. The breakdown products (amino acids, monosaccharides and other useful substances) are then transported through the lysosomal membrane into the cell cytoplasm. Phagocytosis is very widespread. In highly organized animals and humans, the process of phagocytosis plays a protective role. The phagocytic activity of leukocytes and macrophages is of great importance in protecting the body from pathogenic microbes and other unwanted particles entering it. Phagocytosis was first described by the Russian scientist I. I. Mechnikov (Fig. 2.7)

Pinocytosis(drinking by a cell) is the process of absorption by a cell of a liquid phase from the environment containing soluble substances, including large molecules (proteins, polysaccharides, etc.). During pinocytosis, small vesicles called endosomes are released from the membrane into the cell. They are smaller than phagosomes (their size is up to 150 nm) and usually do not contain large particles. After the formation of the endosome, the primary lysosome approaches it, and these two membrane vesicles fuse. The resulting organelle is called a secondary lysosome. The process of pinocytosis is constantly carried out by all eukaryotic cells. (Fig. 7)

Receptor-mediated endocytosis - active specific process, in which the cell membrane bulges into the cell, forming bordered pits. The intracellular side of the bordered pit contains a set of adaptive proteins. Macromolecules that bind to specific receptors on the cell surface pass inward at a much higher rate than substances entering cells through pinocytosis.

Rice. 2.7. Endocytosis

d) Exocytosis (negative phagocytosis and pinocytosis)

A cellular process in which intracellular vesicles (membrane vesicles) fuse with the outer cell membrane. During exocytosis, the contents of secretory vesicles (exocytosis vesicles) are released out, and their membrane merges with the cell membrane. Almost all macromolecular compounds (proteins, peptide hormones, etc.) are released from the cell in this way (Fig. 2.8)

Rice. 2.8. Exocytosis scheme

3. Generation and conduction of biopotentials- with the help of the membrane, a constant concentration of ions is maintained in the cell: the concentration of the K+ ion inside the cell is much higher than outside, and the concentration of Na+ is much lower, which is very important, since this ensures the maintenance of the potential difference on the membrane and the generation of a nerve impulse.

4. Mechanical- ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues).

5. Energy- during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate;

6. Receptor- some proteins located in the membrane are receptors (molecules with the help of which the cell perceives certain signals).

7. Enzymatic- membrane proteins are often enzymes. For example, the plasma membranes of intestinal epithelial cells contain digestive enzymes.

8. Matrix- ensures a certain relative position and orientation of membrane proteins, their optimal interaction;

9. Cell marking- there are antigens on the membrane that act as markers - “labels” that allow the cell to be identified. These are glycoproteins (that is, proteins with branched oligosaccharide side chains attached to them) that play the role of “antennas”. With the help of markers, cells can recognize other cells and act in concert with them, for example, in the formation of organs and tissues. This also allows the immune system to recognize foreign antigens.

Cellular inclusions

Cellular inclusions include carbohydrates, fats and proteins. All these substances accumulate in the cytoplasm of the cell in the form of drops and grains of various sizes and shapes. They are periodically synthesized in the cell and used in the metabolic process.

Cytoplasm

It is part of a living cell (protoplast) without a plasma membrane or nucleus. The composition of the cytoplasm includes: the cytoplasmic matrix, cytoskeleton, organelles and inclusions (sometimes inclusions and the contents of vacuoles are not considered to be the living substance of the cytoplasm). Demarcated from external environment plasma membrane, the cytoplasm is the internal semi-liquid environment of cells. The cytoplasm of eukaryotic cells contains the nucleus and various organelles. It also contains various inclusions - products of cellular activity, vacuoles, as well as tiny tubes and filaments that form the skeleton of the cell. Proteins predominate in the composition of the main substance of the cytoplasm.

Functions of the cytoplasm

1) the main metabolic processes take place in it.

2) unites the nucleus and all organelles into one whole, ensures their interaction.

3) mobility, irritability, metabolism and reproduction.

Mobility manifests itself in various forms:

Intracellular movement of the cell cytoplasm.

Amoeboid movement. This form of movement is expressed in the formation of pseudopodia by the cytoplasm towards or away from a particular stimulus. This form of movement is inherent in amoeba, blood leukocytes, and also some tissue cells.

Flickering movement. It appears in the form of beatings of tiny protoplasmic outgrowths - cilia and flagella (ciliates, epithelial cells of multicellular animals, sperm, etc.).

Contractive movement. It is ensured due to the presence in the cytoplasm of a special organelle of myofibrils, the shortening or lengthening of which contributes to the contraction and relaxation of the cell. The ability to contract is most developed in muscle cells.

Irritability is expressed in the ability of cells to respond to irritation by changing metabolism and energy.

Cytoskeleton

One of the distinctive features of a eukaryotic cell is the presence in its cytoplasm of skeletal formations in the form of microtubules and bundles of protein fibers. Cytoskeletal elements, closely associated with the outer cytoplasmic membrane and the nuclear envelope, form complex weaves in the cytoplasm.

The cytoskeleton is formed by microtubules, microfilaments and the microtrabecular system. The cytoskeleton determines the shape of the cell, participates in cell movements, division and movement of the cell itself, and in the intracellular transport of organelles.

Microtubules are found in all eukaryotic cells and are hollow, unbranched cylinders, the diameter of which does not exceed 30 nm, and the wall thickness is 5 nm. They can reach several micrometers in length. Easily disintegrate and reassemble. The microtubule wall is mainly composed of helical subunits of the protein tubulin (Fig. 2.09)

Functions of microtubules:

1) perform a support function;

2) form a spindle; ensure the divergence of chromosomes to the poles of the cell; responsible for the movement of cellular organelles;

3) take part in intracellular transport, secretion, cell wall formation;

4) are structural component cilia, flagella, basal bodies and centrioles.

Microfilaments are represented by filaments with a diameter of 6 nm, consisting of actin protein, close to muscle actin. Actin makes up 10-15% of the total cell protein. In most animal cells, a dense network of actin filaments and associated proteins forms just beneath the plasma membrane.

In addition to actin, myosin filaments are also found in the cell. However, their number is much smaller. The interaction between actin and myosin causes muscle contraction. Microfilaments are associated with the movement of the entire cell or its individual structures within it. In some cases, movement is provided only by actin filaments, in others by actin together with myosin.

Functions of microfilaments

1) mechanical strength

2) allows the cell to change its shape and move.

Rice. 2.09. Cytoskeleton

Organelles (or organelles)

Divided into non-membrane, single-membrane and double-membrane.

TO non-membrane organelles Eukaryotic cells include organelles that do not have their own closed membrane, namely: ribosomes and organelles built on the basis of tubulin microtubules - cell center (centrioles) And organelles of movement (flagella and cilia). In the cells of most unicellular organisms and the vast majority of higher (land) plants, centrioles are absent.

TO single-membrane organelles include: endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, spherosomes, vacuoles and some others. All single-membrane organelles are interconnected into a single cell system. Plant cells have special lysosomes, animal cells have special vacuoles: digestive, excretory, contractile, phagocytic, autophagocytic, etc.

TO double membrane organelles include mitochondria and plastids.

Non-membrane organelles

A) Ribosomes- organelles found in the cells of all organisms. These are small organelles, represented by globular particles with a diameter of about 20 nm. Ribosomes consist of two subunits of unequal size - large and small. Ribosomes contain proteins and ribosomal RNA (rRNA). There are two main types of ribosomes: eukaryotic (80S) and prokaryotic (70S).

Depending on the location in the cell, there are free ribosomes located in the cytoplasm that synthesize proteins and attached ribosomes - ribosomes connected by large subunits to the outer surface of the ER membranes, synthesizing proteins that enter the Golgi complex and are then secreted by the cell. During protein biosynthesis, ribosomes can combine into complexes - polyribosomes (polysomes).

Eukaryotic ribosomes are formed in the nucleolus. First, rRNAs are synthesized on nucleolar DNA, which are then covered with ribosomal proteins coming from the cytoplasm, cleaved to the required size and form ribosomal subunits. There are no fully formed ribosomes in the nucleus. The combination of subunits into a whole ribosome occurs in the cytoplasm, usually during protein biosynthesis.

Ribosomes are found in the cells of all organisms. Each consists of two particles, small and large. Ribosomes contain proteins and RNA.

Functions

protein synthesis.

Synthesized proteins first accumulate in the channels and cavities of the endoplasmic reticulum and are then transported to organelles and cell sites. EPS and ribosomes located on its membranes represent a single apparatus for the biosynthesis and transport of proteins (Fig. 2.10-2.11).

Rice. 2.10. Ribosome structure

Rice. 2.11. Structure of ribosomes

B) Cell center (centrioles)

The centriole is a cylinder (0.3 µm long and 0.1 µm in diameter), the wall of which is formed by nine groups of three fused microtubules (9 triplets), interconnected at certain intervals by cross-links. Often the centrioles are combined into pairs where they are located at right angles to each other. If the centriole lies at the base of the cilium or flagellum, then it is called the basal body.

Almost all animal cells have a pair of centrioles, which are the middle element of the cell center.

Before division, the centrioles diverge to opposite poles and a daughter centriole appears near each of them. From centrioles located at different poles of the cell, microtubules are formed that grow towards each other.

Functions

1) form a mitotic spindle, facilitating the uniform distribution of genetic material between daughter cells,

2) are the center of organization of the cytoskeleton. Some of the spindle threads are attached to the chromosomes.

Centrioles are self-replicating organelles of the cytoplasm. They arise as a result of duplication of existing ones. This occurs when the centrioles separate. The immature centriole contains 9 single microtubules; Apparently, each microtubule is a template for the assembly of triplets characteristic of a mature centriole (Fig. 2.12).

Cetrioles are found in the cells of lower plants (algae).

Rice. 2.12. Centrioles of the cell center

Single-membrane organelles

D) Endoplasmic reticulum (ER)

The entire internal zone of the cytoplasm is filled with numerous small channels and cavities, the walls of which are membranes similar in structure to the plasma membrane. These channels branch, connect with each other and form a network called the endoplasmic reticulum. The endoplasmic reticulum is heterogeneous in its structure. There are two known types of it - granular And smooth.

On the membranes of the channels and cavities of the granular network there are many small round bodies - ribosomes, which give the membranes a rough appearance. The membranes of the smooth endoplasmic reticulum do not carry ribosomes on their surface. EPS performs many different functions.

Functions

The main function of the granular endoplasmic reticulum is participation in protein synthesis, which occurs in ribosomes. The synthesis of lipids and carbohydrates occurs on the membranes of the smooth endoplasmic reticulum. All these synthesis products accumulate in channels and cavities, and are then transported to various organelles of the cell, where they are consumed or accumulate in the cytoplasm as cellular inclusions. EPS connects the main organelles of the cell with each other (Fig. 2.13).

Rice. 2.13. The structure of the endoplasmic reticulum (ER) or reticulum

D) Golgi apparatus

The structure of this organelle is similar in the cells of plant and animal organisms, despite the diversity of its shape. Performs many important functions.

Single membrane organelle. They are stacks of flattened “cisterns” with expanded edges, with which a system of small single-membrane vesicles (Golgi vesicles) is associated. Golgi vesicles are mainly concentrated on the side adjacent to the ER and along the periphery of the stacks. It is believed that they transfer proteins and lipids to the Golgi apparatus, the molecules of which, moving from tank to tank, undergo chemical modification.

All these substances first accumulate, become chemically complex, and then enter the cytoplasm in the form of large and small bubbles and are either used in the cell itself during its life, or removed from it and used in the body (Fig. 2.14-2.15).

Rice. 2.14. Structure of the Golgi apparatus

Functions:

Modification and accumulation of proteins, lipids, carbohydrates;

Packaging of incoming organic substances into membrane bubbles (vesicles);

Place of formation of lysosomes;

Secretory function, therefore the Golgi apparatus is well developed in secretory cells.

Rice. 2.15. Golgi complex

E) Lysosomes

They are small round bodies. Inside the lysosome there are enzymes that break down proteins, fats, carbohydrates, and nucleic acids. Lysosomes approach a food particle that has entered the cytoplasm, merge with it, and one digestive vacuole is formed, inside which there is a food particle surrounded by lysosome enzymes.

Lysosome enzymes are synthesized on the rough ER and move to the Golgi apparatus, where they are modified and packaged into membrane vesicles of lysosomes. A lysosome can contain from 20 to 60 various types hydrolytic enzymes. The breakdown of substances using enzymes is called lysis.

There are primary and secondary lysosomes. Primary lysosomes are those that bud from the Golgi apparatus.

Secondary are called lysosomes formed as a result of the fusion of primary lysosomes with endocytic vacuoles. In this case, they digest substances that enter the cell by phagocytosis or pinocytosis, so they can be called digestive vacuoles.

Functions of lysosomes:

1) digestion of substances or particles captured by the cell during endocytosis (bacteria, other cells),

2) autophagy - destruction of structures unnecessary for the cell, for example, during the replacement of old organelles with new ones, or the digestion of proteins and other substances produced inside the cell itself,

3) autolysis - self-digestion of a cell, leading to its death (sometimes this process is not pathological, but accompanies the development of the organism or the differentiation of some specialized cells) (Fig. 2.16-2.17).

Example: When a tadpole transforms into a frog, lysosomes located in the cells of the tail digest it: the tail disappears, and the substances formed during this process are absorbed and used by other cells of the body.

Rice. 2.16. Lysosome formation

Rice. 2.17. Functioning of lysosomes

G) Peroxisomes

Organelles similar in structure to lysosomes, vesicles with a diameter of up to 1.5 microns with a homogeneous matrix containing about 50 enzymes.

Catalase causes the breakdown of hydrogen peroxide 2H 2 O 2 → 2H 2 O + O 2 and prevents lipid peroxidation

Peroxisomes are formed by budding from previously existing ones, i.e. belong to self-replicating organelles, despite the fact that they do not contain DNA. They grow due to the intake of enzymes; peroxisomal enzymes are formed on the rough ER and in the hyaloplasm (Fig. 2.18).

Rice. 2.18. Peroxisome (crystalline nucleoid in the center)

H) Vacuoles

Single-membrane organelles. Vacuoles are “containers” filled with aqueous solutions of organic and inorganic substances. The ER and Golgi apparatus take part in the formation of vacuoles.

Young plant cells contain many small vacuoles, which then, as the cells grow and differentiate, fuse with each other and form one large central vacuole.

The central vacuole can occupy up to 95% of the volume of a mature cell; the nucleus and organelles are pushed towards the cell membrane. The membrane that bounds the plant vacuole is called tonoplast.

The liquid that fills the plant vacuole is called cell sap. The cell sap contains water-soluble organic and inorganic salts, monosaccharides, disaccharides, amino acids, final or toxic metabolic products (glycosides, alkaloids), some pigments (anthocyanins).

Sugars and proteins are most often stored from organic substances. Sugars are often in the form of solutions, proteins enter in the form of ER vesicles and the Golgi apparatus, after which the vacuoles are dehydrated, turning into aleurone grains.

Animal cells contain small digestive and autophagy vacuoles, which belong to the group of secondary lysosomes and contain hydrolytic enzymes. Unicellular animals also have contractile vacuoles that perform the function of osmoregulation and excretion.

Functions

In plants

1) accumulation of fluid and maintenance of turgor,

2) accumulation of reserve nutrients and mineral salts,

3) coloring flowers and fruits and thereby attracting pollinators and distributors of fruits and seeds.

In animals:

4) digestive vacuoles - destroy organic macromolecules;

5) contractile vacuoles regulate the osmotic pressure of the cell and remove unnecessary substances from the cell

6) phagocytic vacuoles are formed during phagocytosis of antigens by immune cells

7) autophagocytic vacuoles are formed during phagocytosis of their own tissues by immune cells

Double-membrane organelles (mitochondria and plastids)

These organelles are semi-autonomous because they have their own DNA and their own protein-synthesizing apparatus. Mitochondria are found in almost all eukaryotic cells. Plastids are found only in plant cells.

I) Mitochondria

These are organelles that supply energy to metabolic processes in the cell. In the hyaloplasm, mitochondria are usually distributed diffusely, but in specialized cells they are concentrated in those areas where there is the greatest need for energy. For example, in muscle cells, large numbers of mitochondria are concentrated along the contractile fibrils, along the sperm flagellum, in the epithelium of the renal tubules, in the area of ​​synapses, etc. This arrangement of mitochondria ensures less loss of ATP during its diffusion.

The outer membrane separates the mitochondrion from the cytoplasm, is closed on itself and does not form invaginations. The inner membrane limits the internal contents of mitochondria - the matrix. Feature– the formation of numerous invaginations – cristae, due to which the area of ​​the internal membranes increases. The number and degree of development of cristae depends on functional activity fabrics. Mitochondria have their own genetic material (Fig. 2.19).

Mitochondrial DNA is a closed circular double-stranded molecule; in human cells it has a size of 16,569 nucleotide pairs, which is approximately 105 times smaller than DNA localized in the nucleus. Mitochondria have their own protein synthesizing system, but the number of proteins translated from mitochondrial mRNA is limited. Mitochondrial DNA cannot code for all mitochondrial proteins. Most mitochondrial proteins are under genetic control of the nucleus.

Rice. 2.19. The structure of mitochondria

Functions of mitochondria

1) ATP formation

2) protein synthesis

3) participation in specific syntheses, for example, the synthesis of steroid hormones (adrenal glands)

4) spent mitochondria can also accumulate excretion products and harmful substances, i.e. capable of taking on the functions of other cell organelles

K) Plastids

Plastids-organelles characteristic only of plants.

There are three types of plastids:

1) chloroplasts(green plastids);

2) chromoplasts(plastids yellow, orange or red)

3) leucoplasts(colorless plastids).

Typically, only one type of plastid is found in a cell.

Chloroplasts

These organelles are found in the cells of leaves and other green organs of plants, as well as in a variety of algae. In higher plants, one cell usually contains several dozen chloroplasts. The green color of chloroplasts depends on the content of the chlorophyll pigment in them.

Chloroplast is the main organelle of plant cells in which photosynthesis occurs, i.e. the formation of organic substances (carbohydrates) from inorganic substances (CO 2 and H 2 O) using energy sunlight. Chloroplasts are similar in structure to mitochondria.

Chloroplasts have a complex structure. They are separated from the hyaloplasm by two membranes - external and internal. The internal contents are called stroma. The inner membrane forms inside the chloroplast a complex, strictly ordered system of membranes in the form of flat bubbles called thylakoids.

Thylakoids are collected in stacks - grains, resembling columns of coins . The grana are interconnected by stromal thylakoids that pass through them throughout the plastid. (Fig. 2.20-2.22). Chlorophyll and chloroplasts are formed only in light.

Rice. 2.20. Chloroplasts under a light microscope

Rice. 2.21. Structure of a chloroplast under an electron microscope

Rice. 2.22. Schematic structure of chloroplasts

Functions

1) photosynthesis(formation of organic substances from inorganic substances due to light energy). Chlorophyll plays a central role in this process. It absorbs light energy and directs it to carry out photosynthesis reactions. In chloroplasts, as in mitochondria, ATP synthesis occurs.

2) participate in the synthesis of amino acids and fatty acids,

3) serve as a storage facility for temporary starch reserves.

Leukoplasts- small colorless plastids that are found in the cells of organs hidden from sunlight (roots, rhizomes, tubers, seeds). Their structure is similar to the structure of chloroplasts (Fig. 2.23).

However, unlike chloroplasts, leucoplasts have a poorly developed internal membrane system, because they are involved in the synthesis and accumulation of reserve nutrients - starch, proteins and lipids. In the light, leucoplasts can turn into chloroplasts.

Rice. 2.23. Leukoplast structure

Chromoplasts- plastids are orange, red and yellow in color, which is caused by pigments belonging to the group of carotenoids. Chromoplasts are found in the cells of the petals of many plants, mature fruits, and rarely in root vegetables, as well as in autumn leaves. The internal membrane system in chromoplasts is usually absent (Fig. 24).

Rice. 2.24. Chromoplast structure

The significance of chromoplasts has not yet been fully elucidated. Most of them are aging plastids. They, as a rule, develop from chloroplasts, while chlorophyll and the internal membrane structure are destroyed in plastids, and carotenoids accumulate. This occurs when the fruits ripen and the leaves turn yellow in the fall. The biological significance of chromoplasts is that they determine bright color flowers and fruits, attracting insects for cross-pollination and other animals for distribution of fruits. Leucoplasts can also transform into chromoplasts.

Functions of plastids

Synthesis of organic substances in chlorophyll from simple inorganic compounds: carbon dioxide and water in the presence of quanta of sunlight - photosynthesis, ATP synthesis during the light phase of photosynthesis

Protein synthesis on ribosomes (between the inner membranes of the chloroplast there are DNA, RNA and ribosomes, therefore, in chloroplasts, as well as in mitochondria, the synthesis of the protein necessary for the activity of these organelles occurs).

The presence of chromoplasts explains the yellow, orange and red color of the corollas of flowers, fruits, and autumn leaves.

Leukoplasts contain storage substances (in stems, roots, tubers).

Chloroplasts, chromoplasts and leucoplasts are capable of cell interchange. So, when fruits ripen or leaves change color in autumn, chloroplasts turn into chromoplasts, and leucoplasts can turn into chloroplasts, for example, when potato tubers turn green.

In an evolutionary sense, the primary, original type of plastids are chloroplasts, from which the other two types of plastids originated. Plastids have many common features with mitochondria, distinguishing them from other components of the cytoplasm. This is, first of all, a shell of two membranes and relative genetic autonomy due to the presence of its own ribosomes and DNA. This uniqueness of organelles formed the basis for the idea that the predecessors of plastids and mitochondria were bacteria, which in the process of evolution were built into a eukaryotic cell and gradually turned into chloroplasts and mitochondria (Fig. 2.25).

Rice. 2.25. Formation of mitochondria and chloroplasts according to the theory of symbiogenesis

The cell has long been defined as the structural unit of all living things. And this is true. After all, billions of these structures, like bricks, form plants and animals, bacteria and microorganisms, and humans. Every organ, tissue, body system - everything is built from cells.

Therefore, it is very important to know all the subtleties of its internal structure, chemical composition and ongoing biochemical reactions. In this article we will look at what the plasma membrane is, the functions it performs, and its structure.

Cell organelles

Organelles are the smallest structural parts found inside a cell and providing its structure and vital functions. These include many different representatives:

1. Plasma membrane.
2. Nucleus and nucleoli with chromosomal material.
3. Cytoplasm with inclusions.
4. Lysosomes.
5. Mitochondria.
6. Ribosomes.
7. Vacuoles and chloroplasts, if the cell is plant.

Each of the listed structures has its own complex structure, formed by high molecular weight substances (HMCs), performs strictly defined functions and takes part in a complex of biochemical reactions that ensure the vital activity of the entire organism as a whole.

General structure of the membrane

The structure of the plasma membrane has been studied since the 18th century. It was then that its ability to selectively pass or retain substances was first discovered. With the development of microscopy, the study of the fine structure and structure of the membrane has become more possible, and therefore today almost everything is known about it.

A synonym for its main name is plasmalemma. The composition of the plasma membrane is represented by three main types of IUDs:

• proteins;
• lipids;
• carbohydrates.

The ratio of these compounds and location may vary in the cells of different organisms (plant, animal or bacterial).

Fluid mosaic model of the structure

Many scientists have tried to make assumptions about how lipids and proteins are located in the membrane. However, it was only in 1972 that scientists Singer and Nicholson proposed a model that is still relevant today, reflecting the structure of the plasma membrane. It is called liquid mosaic, and its essence is as follows: various types of lipids are arranged in two layers, oriented with the hydrophobic ends of the molecules inward and the hydrophilic ends outward. Moreover, the entire structure, like a mosaic, is permeated with different types of protein molecules, as well as a small amount of hexoses (carbohydrates).

The entire proposed system is in constant dynamics. Proteins are capable of not only penetrating the bilipid layer through and through, but also orienting themselves at one of its sides, being embedded inside. Or even freely “walk” along the membrane, changing location.

Microscopic analysis data provides evidence to support and justify this theory. In black and white photographs, the layers of the membrane are clearly visible, the top and bottom are equally dark, and the middle one is lighter. A number of experiments were also carried out proving that the layers are based precisely on lipids and proteins.

Plasma membrane proteins

If we consider the percentage of lipids and proteins in the membrane of a plant cell, it will be approximately the same - 40/40%. In the animal plasmalemma, up to 60% is made up of proteins, in the bacterial plasmalemma - up to 50%.

The plasma membrane consists of different types of proteins, and the functions of each of them are also specific.

1. Peripheral molecules. These are proteins that are oriented on the surface of the inner or outer parts of the lipid bilayer. The main types of interactions between the molecular structure and the layer are as follows:

• hydrogen bonds;
• ionic interactions or salt bridges;
• electrostatic attraction.

The peripheral proteins themselves are water-soluble compounds, so it is not difficult to separate them from the plasmalemma without damage. What substances belong to these structures? The most common and numerous is the fibrillar protein spectrin. It can be up to 75% in the mass of all membrane proteins in individual cellular plasmalemmas.

Why are they needed and how does the plasma membrane depend on them? The functions are as follows:

• formation of the cell cytoskeleton;
• maintaining constant shape;
• restriction of excessive mobility of integral proteins;
• coordination and implementation of ion transport through the plasmalemma;
• can bind to oligosaccharide chains and participate in receptor signaling from and to the membrane.

2. Semi-integral proteins. Such molecules are those that are fully or half immersed in the lipid bilayer, to varying depths. Examples include bacteriorhodopsin, cytochrome oxidase, and others. They are also called “anchored” proteins, that is, as if attached inside the layer. What can they come into contact with and how do they take root and hold on? Most often thanks to special molecules, which can be myristic or palmitic acids, isoprenes or sterols. For example, in the plasmalemma of animals there are semi-integral proteins associated with cholesterol. These have not yet been found in plants or bacteria.

3. Integral proteins. Some of the most important in the plasmalemma. They are structures that form something like channels that penetrate both lipid layers through and through. It is through these pathways that many molecules enter the cell, those that lipids do not allow through. Therefore, the main role of integral structures is the formation of ion channels for transport.

There are two types of lipid layer penetration:

• monotopic - once;
• polytopic - in several places.

Varieties of integral proteins include glycophorin, proteolipids, proteoglycans and others. All of them are insoluble in water and are closely embedded in the lipid layer, so it is impossible to remove them without damaging the structure of the plasmalemma. These proteins are globular in structure, their hydrophobic end is located inside the lipid layer, and the hydrophilic end is located above it, and can rise above the entire structure. Due to what interactions are integral proteins held inside? In this they are helped by hydrophobic attractions to fatty acid radicals.

Thus, there are a number of different protein molecules that the plasma membrane includes. The structure and functions of these molecules can be combined into several general points.

1. Structural peripheral proteins.
2. Catalytic enzyme proteins (semi-integral and integral).
3. Receptor (peripheral, integral).
4. Transport (integral).

Plasmalemma lipids

The liquid lipid bilayer that makes up the plasma membrane can be very mobile. The fact is that different molecules can move from the upper layer to the lower one and vice versa, that is, the structure is dynamic. Such transitions have their own name in science - “flip-flop”. It was formed from the name of an enzyme that catalyzes the processes of rearrangement of molecules within one monolayer or from upper to lower and vice versa, flipase.

The amount of lipids that a cell's plasma membrane contains is approximately the same as the number of proteins. Species diversity is wide. The following main groups can be distinguished:

• phospholipids;
• sphingophospholipids;
• glycolipids;
• cholesterol

The first group of phospholipids includes molecules such as glycerophospholipids and sphingomyelins. These molecules form the basis of the membrane bilayer. The hydrophobic ends of the compounds are directed into the layer, the hydrophilic ends are directed outward. Connection examples:

• phosphatidylcholine;
• phosphatidylserine;
• cardiolipin;
• phosphatidylinositol;
• sphingomyelin;
• phosphatidylglycerol;
• Phosphatidylethanolamine.

To study these molecules, a method is used to destroy the membrane layer in some parts by phospholipase, a special enzyme that catalyzes the process of phospholipid breakdown.

The functions of the listed connections are as follows:

1. Provide general structure and the structure of the plasma membrane bilayer.
2. They come into contact with proteins on the surface and inside the layer.
3. Define physical state, which the plasma membrane of a cell will have under different temperature conditions.
4. They participate in the limited permeability of the plasmalemma for various molecules.
5. Form different types interactions of cell membranes with each other (desmosome, slit-like space, tight junction).

Sphingophospholipids and membrane glycolipids

Sphingomyelins or sphingophospholipids in their chemical nature- derivatives of the amino alcohol sphingosine. Along with phospholipids, they take part in the formation of the bilipid layer of the membrane.

Glycolipids include the glycocalyx, a substance that largely determines the properties of the plasma membrane. It is a jelly-like compound consisting mainly of oligosaccharides. The glycocalyx occupies 10% of the total mass of the plasmalemma. The plasma membrane, the structure and functions it performs, are directly related to this substance. For example, the glycocalyx carries out:

• membrane marker function;
• receptor;
• processes of parietal digestion of particles inside the cell.

It should be noted that the presence of glycocalyx lipid is characteristic only of animal cells, but not of plant, bacterial and fungi.

Cholesterol (membrane sterol)

It is an important component of the cell bilayer in mammals. It is not found in plants, nor in bacteria or fungi. From a chemical point of view, it is an alcohol, cyclic, monohydric.

Like other lipids, it has amphiphilic properties (the presence of a hydrophilic and hydrophobic end of the molecule). In the membrane, it plays an important role as a limiter and controller of bilayer fluidity. It also participates in the production of vitamin D and is an accomplice in the formation of sex hormones.

Plant cells contain phytosterols, which do not participate in the formation of animal membranes. According to some data, it is known that these substances provide plant resistance to certain types of diseases.

The plasma membrane is formed by cholesterol and other lipids in a common interaction, a complex.

Carbohydrates membrane

This group of substances makes up approximately 10% of the total composition of plasma membrane compounds. In their simple form, mono-, di-, and polysaccharides are not found, but only in the form of glycoproteins and glycolipids.

Their functions are to exercise control over intra- and intercellular interactions, maintain a certain structure and position of protein molecules in the membrane, as well as perform reception.

Main functions of the plasmalemma

The plasma membrane plays a very important role in the cell. Its functions are multifaceted and important. Let's take a closer look at them.

1. Separates the contents of the cell from the environment and protects it from external influences. Thanks to the presence of a membrane, it is maintained at a constant level chemical composition cytoplasm and its contents.
2. The plasmalemma contains a number of proteins, carbohydrates and lipids that give and maintain the specific shape of the cell.
3. Every cellular organelle has a membrane, which is called a membrane vesicle (vesicle).
4. The component composition of the plasmalemma allows it to play the role of a “guard” of the cell, carrying out selective transport into it.
5. Receptors, enzymes, biologically active substances function in the cell and penetrate it, cooperate with its surface shell only thanks to the proteins and lipids of the membrane.
6. Through the plasma membrane, not only compounds of various natures are transported, but also ions important for life (sodium, potassium, calcium and others).
7. The membrane maintains osmotic balance outside and inside the cell.
8. With the help of the plasma membrane, ions and compounds of various natures, electrons, and hormones are transferred from the cytoplasm to organelles.
9. Through it, sunlight is absorbed in the form of quanta and signals are awakened inside the cell.
10. It is this structure that generates impulses of action and rest.
11. Mechanical protection of the cell and its structures from minor deformations and physical impacts.
12. Cell adhesion, that is, adhesion, and keeping them close to each other is also carried out thanks to the membrane.

The cellular plasmalemma and cytoplasm are very closely interconnected. The plasma membrane is in close contact with all substances and molecules, ions that penetrate into the cell and are freely located in the viscous internal environment. These compounds try to penetrate into all cellular structures, but the barrier is the membrane, which is capable of carrying out different types of transport through itself. Or don't skip some types of connections at all.

Types of transport across the cell barrier

Transport across the plasma membrane occurs in several ways, all of which have one thing in common: physical feature- the law of diffusion of substances.

1. Passive transport or diffusion and osmosis. It involves the free movement of ions and solvent through the membrane along a gradient from an area of ​​high concentration to an area of ​​low. Does not require energy consumption, as it proceeds on its own. This is how the sodium-potassium pump operates, the change of oxygen and carbon dioxide during breathing, the release of glucose into the blood, and so on. A very common phenomenon is facilitated diffusion. This process implies the presence of some kind of assistant substance that grabs the desired compound and drags it along a protein channel or through the lipid layer into the cell.
2. Active transport involves the expenditure of energy on the processes of absorption and excretion through the membrane. There are two main ways: exocytosis - the removal of molecules and ions outside. Endocytosis is the capture and passage of solid and liquid particles into the cell. In turn, the second method of active transport includes two types of process. Phagocytosis, which involves the ingestion of solid molecules, substances, compounds and ions by a membrane vesicle and carrying them into the cell. During this process, large vesicles are formed. Pinocytosis, on the other hand, involves taking up droplets of liquids, solvents and other substances and carrying them into the cell. It involves the formation of small bubbles.

Both processes - pinocytosis and phagocytosis - play a large role not only in the transport of compounds and liquids, but also in protecting the cell from debris of dead cells, microorganisms and harmful compounds. We can say that these methods of active transport are also options for the immunological protection of the cell and its structures from various dangers.

The vast majority of organisms living on Earth consists of cells that are largely similar in their chemical composition, structure and vital functions. Metabolism and energy conversion occur in every cell. Cell division underlies the processes of growth and reproduction of organisms. Thus, the cell is a unit of structure, development and reproduction of organisms.

A cell can only exist as an integral system, indivisible into parts. Cell integrity is ensured by biological membranes. A cell is an element of a system of a higher rank - an organism. Cell parts and organelles, consisting of complex molecules, represent integral systems of a lower rank.

The cell is an open system connected with the environment by the exchange of substances and energy. It is a functional system in which each molecule performs specific functions. The cell has stability, the ability to self-regulate and self-reproduce.

The cell is a self-governing system. The control genetic system of a cell is represented by complex macromolecules - nucleic acids (DNA and RNA).

In 1838-1839 German biologists M. Schleiden and T. Schwann summarized knowledge about the cell and formulated the main position of the cell theory, the essence of which is that all organisms, both plant and animal, consist of cells.

In 1859, R. Virchow described the process of cell division and formulated one of the most important provisions of cell theory: “Every cell comes from another cell.” New cells are formed as a result of division of the mother cell, and not from non-cellular substance, as was previously thought.

The discovery of mammalian eggs by the Russian scientist K. Baer in 1826 led to the conclusion that the cell underlies the development of multicellular organisms.

Modern cell theory includes the following provisions:

1) cell - the unit of structure and development of all organisms;

2) cells of organisms from different kingdoms of living nature are similar in structure, chemical composition, metabolism, and basic manifestations of life activity;

3) new cells are formed as a result of division of the mother cell;

4) in a multicellular organism, cells form tissues;

5) organs are made up of tissues.

With the introduction of modern biological, physical and chemical research methods into biology, it has become possible to study the structure and functioning of various components of the cell. One of the methods for studying cells is microscopy. A modern light microscope magnifies objects 3000 times and allows you to see the largest cell organelles, observe the movement of the cytoplasm, and cell division.

Invented in the 40s. XX century An electron microscope provides magnification of tens and hundreds of thousands of times. An electron microscope uses a stream of electrons instead of light, and electromagnetic fields instead of lenses. Therefore, an electron microscope produces clear images at much higher magnifications. Using such a microscope, it was possible to study the structure of cell organelles.

The structure and composition of cell organelles is studied using the method centrifugation. Chopped tissues with destroyed cell membranes are placed in test tubes and rotated in a centrifuge at high speed. The method is based on the fact that different cellular organoids have different mass and density. Denser organelles precipitate in vitro when low speeds centrifugation, less dense - at high. These layers are studied separately.

Widely used cell and tissue culture method, which consists in the fact that from one or several cells on a special nutrient medium one can obtain a group of the same type of animal or plant cells and even grow a whole plant. Using this method, you can get an answer to the question of how various tissues and organs of the body are formed from one cell.

The basic principles of cell theory were first formulated by M. Schleiden and T. Schwann. A cell is a unit of structure, vital activity, reproduction and development of all living organisms. To study cells, methods of microscopy, centrifugation, cell and tissue culture, etc. are used.

The cells of fungi, plants and animals have much in common not only in chemical composition, but also in structure. When examining a cell under a microscope, various structures are visible in it - organoids. Each organelle performs specific functions. There are three main parts in a cell: the plasma membrane, the nucleus and the cytoplasm (Figure 1).

Plasma membrane separates the cell and its contents from the environment. In Figure 2 you see: the membrane is formed by two layers of lipids, and protein molecules penetrate the thickness of the membrane.

Main function of the plasma membrane transport. It ensures the flow of nutrients into the cell and the removal of metabolic products from it.

An important property of the membrane is selective permeability, or semi-permeability, allows the cell to interact with the environment: only certain substances enter and are removed from it. Small molecules of water and some other substances penetrate the cell by diffusion, partly through pores in the membrane.

Sugars, organic acids, and salts are dissolved in the cytoplasm, the cell sap of the vacuoles of a plant cell. Moreover, their concentration in the cell is significantly higher than in environment. The higher the concentration of these substances in the cell, the more water it absorbs. It is known that water is constantly consumed by the cell, due to which the concentration of cell sap increases and water again enters the cell.

The entry of larger molecules (glucose, amino acids) into the cell is ensured by membrane transport proteins, which, combining with the molecules of transported substances, transport them across the membrane. This process involves enzymes that break down ATP.

Figure 1. Generalized diagram of the structure of a eukaryotic cell.
(to enlarge the image, click on the picture)

Figure 2. Structure of the plasma membrane.
1 - piercing proteins, 2 - submerged proteins, 3 - external proteins

Figure 3. Diagram of pinocytosis and phagocytosis.

Even larger molecules of proteins and polysaccharides enter the cell by phagocytosis (from the Greek. phagos- devouring and kitos- vessel, cell), and drops of liquid - by pinocytosis (from the Greek. pinot- I drink and kitos) (Figure 3).

Animal cells, unlike plant cells, are surrounded by a soft and flexible “coat” formed mainly by polysaccharide molecules, which, joining some membrane proteins and lipids, surround the cell from the outside. The composition of polysaccharides is specific to different tissues, due to which cells “recognize” each other and connect with each other.

Plant cells do not have such a “coat”. They have a pore-ridden plasma membrane above them. cell membrane, consisting predominantly of cellulose. Through the pores, threads of cytoplasm stretch from cell to cell, connecting the cells to each other. This is how communication between cells is achieved and the integrity of the body is achieved.

The cell membrane in plants plays the role of a strong skeleton and protects the cell from damage.

Most bacteria and all fungi have a cell membrane, only its chemical composition is different. In fungi it consists of a chitin-like substance.

The cells of fungi, plants and animals have a similar structure. A cell has three main parts: the nucleus, the cytoplasm, and the plasma membrane. The plasma membrane is composed of lipids and proteins. It ensures the entry of substances into the cell and their release from the cell. In the cells of plants, fungi and most bacteria there is a cell membrane above the plasma membrane. It performs a protective function and plays the role of a skeleton. In plants, the cell wall consists of cellulose, and in fungi, it is made of a chitin-like substance. Animal cells are covered with polysaccharides that provide contacts between cells of the same tissue.

Do you know that the main part of the cell is cytoplasm. It consists of water, amino acids, proteins, carbohydrates, ATP, and ions of inorganic substances. The cytoplasm contains the nucleus and organelles of the cell. In it, substances move from one part of the cell to another. Cytoplasm ensures the interaction of all organelles. Chemical reactions take place here.

The entire cytoplasm is permeated with thin protein microtubules that form cell cytoskeleton, thanks to which it maintains a constant shape. The cell cytoskeleton is flexible, since microtubules are able to change their position, move from one end and shorten from the other. They enter the cell different substances. What happens to them in the cage?

In lysosomes - small round membrane vesicles (see Fig. 1) molecules of complex organic substances are broken down into simpler molecules with the help of hydrolytic enzymes. For example, proteins are broken down into amino acids, polysaccharides into monosaccharides, fats into glycyrin and fatty acids. For this function, lysosomes are often called the “digestive stations” of the cell.

If the membrane of lysosomes is destroyed, the enzymes contained in them can digest the cell itself. Therefore, lysosomes are sometimes called “cell killing weapons.”

The enzymatic oxidation of small molecules of amino acids, monosaccharides, fatty acids and alcohols formed in lysosomes to carbon dioxide and water begins in the cytoplasm and ends in other organelles - mitochondria. Mitochondria are rod-shaped, thread-like or spherical organelles, delimited from the cytoplasm by two membranes (Fig. 4). The outer membrane is smooth, and the inner one forms folds - cristas, which increase its surface. The inner membrane contains enzymes that participate in the oxidation of organic substances to carbon dioxide and water. This releases energy that is stored by the cell in ATP molecules. Therefore, mitochondria are called the “power stations” of the cell.

In the cell, organic substances are not only oxidized, but also synthesized. The synthesis of lipids and carbohydrates is carried out on the endoplasmic reticulum - EPS (Fig. 5), and proteins - on ribosomes. What is EPS? This is a system of tubules and cisterns, the walls of which are formed by a membrane. They permeate the entire cytoplasm. Substances move through the ER channels to different parts of the cell.

There is smooth and rough EPS. On the surface of the smooth ER, carbohydrates and lipids are synthesized with the participation of enzymes. The roughness of the ER is given by the small round bodies located on it - ribosomes(see Fig. 1), which are involved in protein synthesis.

The synthesis of organic substances also occurs in plastids, which are found only in plant cells.

Rice. 4. Scheme of the structure of mitochondria.
1.- outer membrane; 2.- inner membrane; 3.- folds of the inner membrane - cristae.

Rice. 5. Scheme of the structure of rough EPS.

Rice. 6. Diagram of the structure of a chloroplast.
1.- outer membrane; 2.- inner membrane; 3.- internal contents of the chloroplast; 4.- folds of the inner membrane, collected in “stacks” and forming grana.

In colorless plastids - leucoplasts(from Greek leukos- white and plastos- created) starch accumulates. Potato tubers are very rich in leucoplasts. Yellow, orange, and red colors are given to fruits and flowers. chromoplasts(from Greek chromium- color and plastos). They synthesize pigments involved in photosynthesis - carotenoids. In plant life, it is especially important chloroplasts(from Greek chloros- greenish and plastos) - green plastids. In Figure 6 you see that chloroplasts are covered with two membranes: an outer and an inner. The inner membrane forms folds; between the folds there are bubbles arranged in stacks - grains. Granas contain chlorophyll molecules, which are involved in photosynthesis. Each chloroplast has about 50 grains arranged in a checkerboard pattern. This arrangement ensures maximum illumination of each face.

In the cytoplasm, proteins, lipids, and carbohydrates can accumulate in the form of grains, crystals, and droplets. These inclusion- spare nutrients, which are consumed by the cell as needed.

In plant cells, some of the reserve nutrients, as well as breakdown products, accumulate in the cell sap of vacuoles (see Fig. 1). They can account for up to 90% of the volume of a plant cell. Animal cells have temporary vacuoles that occupy no more than 5% of their volume.

Rice. 7. Scheme of the structure of the Golgi complex.

In Figure 7 you see a system of cavities surrounded by a membrane. This Golgi complex, which performs various functions in the cell: participates in the accumulation and transportation of substances, their removal from the cell, the formation of lysosomes and the cell membrane. For example, cellulose molecules enter the cavity of the Golgi complex, which, using vesicles, move to the cell surface and are included in the cell membrane.

Most cells reproduce by division. Participating in this process cell center. It consists of two centrioles surrounded by dense cytoplasm (see Fig. 1). At the beginning of division, the centrioles move toward the poles of the cell. Protein threads emanate from them, which connect to the chromosomes and ensure their uniform distribution between the two daughter cells.

All cell organelles are closely interconnected. For example, protein molecules are synthesized in ribosomes, they are transported through ER channels to different parts of the cell, and proteins are destroyed in lysosomes. Newly synthesized molecules are used to build cell structures or accumulate in the cytoplasm and vacuoles as reserve nutrients.

The cell is filled with cytoplasm. The cytoplasm contains the nucleus and various organelles: lysosomes, mitochondria, plastids, vacuoles, ER, cell center, Golgi complex. They differ in their structure and functions. All organelles of the cytoplasm interact with each other, ensuring the normal functioning of the cell.

Table 1. CELL STRUCTURE

The most important role in the life activity and division of cells of fungi, plants and animals belongs to the nucleus and the chromosomes located in it. Most cells of these organisms have a single nucleus, but there are also multinucleated cells, such as muscle cells. The nucleus is located in the cytoplasm and has a round or oval shape. It is covered with a shell consisting of two membranes. The nuclear envelope has pores through which the exchange of substances occurs between the nucleus and the cytoplasm. The nucleus is filled with nuclear juice, in which nucleoli and chromosomes are located.

Nucleoli- these are “workshops for the production” of ribosomes, which are formed from ribosomal RNAs formed in the nucleus and proteins synthesized in the cytoplasm.

The main function of the nucleus - storage and transmission of hereditary information - is associated with chromosomes. Each type of organism has its own set of chromosomes: a certain number, shape and size.

All cells of the body, except the sex cells, are called somatic(from Greek soma- body). Cells of an organism of the same species contain the same set of chromosomes. For example, in humans, each cell of the body contains 46 chromosomes, in the fruit fly Drosophila - 8 chromosomes.

Somatic cells, as a rule, have a double set of chromosomes. It's called diploid and is denoted by 2 n. So, a person has 23 pairs of chromosomes, that is, 2 n= 46. Sex cells contain half as many chromosomes. Is it single, or haploid, kit. Person has 1 n = 23.

All chromosomes in somatic cells, unlike chromosomes in germ cells, are paired. The chromosomes that make up one pair are identical to each other. Paired chromosomes are called homologous. Chromosomes that belong to different couples and vary in shape and size, called non-homologous(Fig. 8).

In some species, the number of chromosomes may be the same. For example, red clover and peas have 2 n= 14. However, their chromosomes differ in shape, size, and nucleotide composition of DNA molecules.

Rice. 8. Set of chromosomes in Drosophila cells.

Rice. 9. Chromosome structure.

To understand the role of chromosomes in the transmission of hereditary information, it is necessary to become familiar with their structure and chemical composition.

The chromosomes of a non-dividing cell look like long thin threads. Before cell division, each chromosome consists of two identical strands - chromatid, which are connected between the waists of the waist - (Fig. 9).

Chromosomes are made up of DNA and proteins. Because the nucleotide composition of DNA varies among species, the composition of chromosomes is unique to each species.

Every cell, except bacterial cells, has a nucleus in which nucleoli and chromosomes are located. Each species is characterized by a certain set of chromosomes: number, shape and size. In the somatic cells of most organisms the set of chromosomes is diploid, in the sex cells it is haploid. Paired chromosomes are called homologous. Chromosomes are made up of DNA and proteins. DNA molecules ensure the storage and transmission of hereditary information from cell to cell and from organism to organism.

Having worked through these topics, you should be able to:

1. Explain in what cases a light microscope (structure) or a transmission electron microscope should be used.
2. Describe the structure of the cell membrane and explain the relationship between the structure of the membrane and its ability to exchange substances between the cell and its environment.
3. Define the processes: diffusion, facilitated diffusion, active transport, endocytosis, exocytosis and osmosis. Indicate the differences between these processes.
4. Name the functions of the structures and indicate in which cells (plant, animal or prokaryotic) they are located: nucleus, nuclear membrane, nucleoplasm, chromosomes, plasma membrane, ribosome, mitochondrion, cell wall, chloroplast, vacuole, lysosome, smooth endoplasmic reticulum (agranular) and rough (granular), cell center, Golgi apparatus, cilium, flagellum, mesosoma, pili or fimbriae.
5. Name at least three signs by which a plant cell can be distinguished from an animal cell.
6. List the most important differences between prokaryotic and eukaryotic cells.

Ivanova T.V., Kalinova G.S., Myagkova A.N. "General Biology". Moscow, "Enlightenment", 2000

• Topic 1. "Plasma membrane." §1, §8 pp. 5;20
• Topic 2. "Cage." §8-10 pp. 20-30
• Topic 3. "Prokaryotic cell. Viruses." §11 pp. 31-34