Cell membrane
Image of a cell membrane. The small blue and white balls correspond to the hydrophobic “heads” of the phospholipids, and the lines attached to them correspond to the hydrophilic “tails”. The figure shows only integral membrane proteins (red globules and yellow helices). Yellow oval dots inside the membrane - cholesterol molecules Yellow-green chains of beads on the outside of the membrane - chains of oligosaccharides forming the glycocalyx
A biological membrane also includes various proteins: integral (penetrating the membrane through), semi-integral (immersed at one end in the outer or inner lipid layer), surface (located on the outer or adjacent to the inner sides of the membrane). 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. Some of the integral proteins function as ion channels, various transporters and receptors.
Membranes are composed of three classes of lipids: phospholipids, glycolipids and cholesterol. Phospholipids and glycolipids (lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon tails that are connected to a charged hydrophilic head. 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. Their composition and orientation differ in different membranes.
Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the transition of an individual molecule from one layer to another (the so-called flip flop) is difficult.
These are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, peroxisomes; to double membranes - nucleus, mitochondria, plastids. The structure of the membranes of various organelles differs in the composition of lipids and membrane proteins.
Cell membranes have selective permeability: glucose, amino acids, fatty acids, glycerol and ions slowly diffuse through them, and the membranes themselves, to a certain extent, actively regulate this process - some substances pass through, but others do not. There are four main mechanisms for the entry of substances into the cell or their removal from the cell to the outside: diffusion, osmosis, active transport and exo- or endocytosis. The first two processes are passive in nature, that is, they do not require energy; the last two are active processes associated with energy consumption.
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. The elements K, Na and Cl have their own channels. Relative to the concentration gradient, the molecules of these elements move in and out of the cell. When irritated, the sodium ion channels open and a sudden influx of sodium ions into the cell occurs. In this case, an imbalance of membrane potential occurs. After which the membrane potential is restored. Potassium channels are always open, allowing potassium ions to slowly enter the cell.
Plasma membrane , or plasmalemma,- the most permanent, basic, universal membrane for all cells. It is a thin (about 10 nm) film covering the entire cell. The plasmalemma consists of protein molecules and phospholipids (Fig. 1.6).
Phospholipid molecules are arranged in two rows - with hydrophobic ends inward, hydrophilic heads towards the internal and external aqueous environment. In some places, the bilayer (double layer) of phospholipids is penetrated through and through by protein molecules (integral proteins). Inside such protein molecules there are channels - pores through which water-soluble substances pass. Other protein molecules penetrate the lipid bilayer halfway on one side or the other (semi-integral proteins). There are peripheral proteins on the surface of the membranes of eukaryotic cells. Lipid and protein molecules are held together due to hydrophilic-hydrophobic interactions.
Properties and functions of membranes. All cell membranes are mobile fluid structures, since lipid and protein molecules are not interconnected by covalent bonds and are able to move quite quickly in the plane of the membrane. Thanks to this, membranes can change their configuration, i.e., they have fluidity.
Membranes are very dynamic structures. They quickly recover from damage and also stretch and contract with cellular movements.
Membranes of different types of cells differ significantly both in chemical composition and in the relative content of proteins, glycoproteins, lipids in them, and, consequently, in the nature of the receptors they contain. Each cell type is therefore characterized by an individuality, which is determined mainly glycoproteins. Branched chain glycoproteins protruding from the cell membrane are involved in factor recognition external environment, as well as in mutual recognition of related cells. For example, an egg and a sperm recognize each other by cell surface glycoproteins, which fit together as separate elements of a whole structure. Such mutual recognition is a necessary stage preceding fertilization.
A similar phenomenon is observed in the process of tissue differentiation. In this case, cells similar in structure, with the help of recognition areas of the plasmalemma, are correctly oriented relative to each other, thereby ensuring their adhesion and tissue formation. Associated with recognition transport regulation molecules and ions through the membrane, as well as an immunological response in which glycoproteins play the role of antigens. Sugars can thus function as information molecules (like proteins and nucleic acids). The membranes also contain specific receptors, electron carriers, energy converters, and enzyme proteins. Proteins are involved in ensuring the transport of certain molecules into or out of the cell, provide a structural connection between the cytoskeleton and cell membranes, or serve as receptors for receiving and converting chemical signals from the environment.
The most important property of the membrane is also selective permeability. This means that molecules and ions pass through it at different speeds, and the larger the size of the molecules, the slower the speed at which they pass through the membrane. This property defines the plasma membrane as osmotic barrier. Water and gases dissolved in it have the maximum penetrating ability; Ions pass through the membrane much more slowly. The diffusion of water through a membrane is called by osmosis.
There are several mechanisms for transporting substances across the membrane.
Diffusion- penetration of substances through a membrane along a concentration gradient (from an area where their concentration is higher to an area where their concentration is lower). Diffuse transport of substances (water, ions) is carried out with the participation of membrane proteins, which have molecular pores, or with the participation of the lipid phase (for fat-soluble substances).
With facilitated diffusion special membrane transport proteins selectively bind to one or another ion or molecule and transport them across the membrane along a concentration gradient.
Active transport involves energy costs and serves to transport substances against their concentration gradient. He carried out by special carrier proteins that form the so-called ion pumps. The most studied is the Na - / K - pump in animal cells, which actively pumps Na + ions out while absorbing K - ions. Due to this, a higher concentration of K - and a lower concentration of Na + is maintained in the cell compared to the environment. This process requires ATP energy.
As a result of active transport using a membrane pump in the cell, the concentration of Mg 2- and Ca 2+ is also regulated.
During the process of active transport of ions into the cell, various sugars, nucleotides, and amino acids penetrate through the cytoplasmic membrane.
Macromolecules of proteins, nucleic acids, polysaccharides, lipoprotein complexes, etc. do not pass through cell membranes, unlike ions and monomers. Transport of macromolecules, their complexes and particles into the cell occurs in a completely different way - through endocytosis. At endocytosis (endo...- inward) a certain area of the plasmalemma captures and, as it were, envelops extracellular material, enclosing it in a membrane vacuole that arises as a result of invagination of the membrane. Subsequently, such a vacuole connects with a lysosome, the enzymes of which break down macromolecules into monomers.
The reverse process of endocytosis is exocytosis (exo...- out). Thanks to it, the cell removes intracellular products or undigested residues enclosed in vacuoles or pu-
zyryki. The vesicle approaches the cytoplasmic membrane, merges with it, and its contents are released into the environment. This is how digestive enzymes, hormones, hemicellulose, etc. are removed.
Thus, biological membranes, as the main structural elements of a cell, serve not just as physical boundaries, but are dynamic functional surfaces. Numerous biochemical processes take place on the membranes of organelles, such as active absorption of substances, energy conversion, ATP synthesis, etc.
Functions of biological membranes the following:
They delimit the contents of the cell from the external environment and the contents of organelles from the cytoplasm.
They ensure the transport of substances into and out of the cell, from the cytoplasm to organelles and vice versa.
Act as receptors (receiving and converting chemicals from the environment, recognizing cell substances, etc.).
They are catalysts (providing for near-membrane chemical processes).
Participate in energy conversion.
Cell- this is not only liquid, enzymes and other substances, but also highly organized structures called intracellular organelles. Organelles for a cell are no less important than its chemical components. Thus, in the absence of organelles such as mitochondria, the supply of energy extracted from nutrients will immediately decrease by 95%.
Most organelles in a cell are covered membranes consisting mainly of lipids and proteins. There are membranes of cells, endoplasmic reticulum, mitochondria, lysosomes, and Golgi apparatus.
Lipids are insoluble in water, so they create a barrier in the cell that prevents the movement of water and water-soluble substances from one compartment to another. Protein molecules, however, make the membrane permeable to various substances through specialized structures called pores. Many other membrane proteins are enzymes that catalyze numerous chemical reactions, which will be discussed in subsequent chapters.
Cell (or plasma) membrane is a thin, flexible and elastic structure with a thickness of only 7.5-10 nm. It consists mainly of proteins and lipids. The approximate ratio of its components is as follows: proteins - 55%, phospholipids - 25%, cholesterol - 13%, other lipids - 4%, carbohydrates - 3%.
Lipid layer of the cell membrane prevents water penetration. The basis of the membrane is a lipid bilayer - a thin lipid film consisting of two monolayers and completely covering the cell. Proteins are located throughout the membrane in the form of large globules.
Schematic representation of a cell membrane, reflecting its main elementsLipid bilayer consists mainly of phospholipid molecules. One end of such a molecule is hydrophilic, i.e. soluble in water (a phosphate group is located on it), the other is hydrophobic, i.e. soluble only in fats (it contains a fatty acid).
Due to the fact that the hydrophobic part of the molecule phospholipid repels water, but is attracted to similar parts of the same molecules, phospholipids have a natural property of attaching to each other in the thickness of the membrane, as shown in Fig. 2-3. The hydrophilic part with the phosphate group forms two membrane surfaces: the outer one, which is in contact with the extracellular fluid, and the inner one, which is in contact with the intracellular fluid.
Middle of the lipid layer impermeable to ions and aqueous solutions of glucose and urea. Fat-soluble substances, including oxygen, carbon dioxide, and alcohol, on the contrary, easily penetrate this area of the membrane.
Molecules cholesterol, which is part of the membrane, also belongs to lipids by nature, since their steroid group is highly soluble in fats. These molecules seem to be dissolved in the lipid bilayer. Their main purpose is to regulate the permeability (or impermeability) of membranes for water-soluble components of body fluids. In addition, cholesterol is the main regulator of membrane viscosity.
Cell membrane proteins. In the figure, globular particles are visible in the lipid bilayer - these are membrane proteins, most of which are glycoproteins. There are two types of membrane proteins: (1) integral, which penetrate the membrane through; (2) peripheral, which protrude only above one of its surfaces, without reaching the other.
Many integral proteins form channels (or pores) through which water and water-soluble substances, especially ions, can diffuse into the intra- and extracellular fluid. Due to the selectivity of the channels, some substances diffuse better than others.
Other integral proteins function as carrier proteins, transporting substances for which the lipid bilayer is impermeable. Sometimes carrier proteins act in the direction opposite to diffusion; such transport is called active transport. Some integral proteins are enzymes.
Integral membrane proteins can also serve as receptors for water-soluble substances, including peptide hormones, since the membrane is impermeable to them. The interaction of a receptor protein with a specific ligand leads to conformational changes in the protein molecule, which, in turn, stimulates the enzymatic activity of the intracellular segment of the protein molecule or the transmission of a signal from the receptor into the cell using a second messenger. Thus, integral proteins embedded in the cell membrane involve it in the process of transmitting information about the external environment into the cell.
Molecules of peripheral membrane proteins often associated with integral proteins. Most peripheral proteins are enzymes or play the role of dispatcher of the transport of substances through membrane pores.
Delimitative ( barrier) - separate cellular contents from the external environment;
Regulate the exchange between the cell and the environment;
They divide cells into compartments, or compartments, intended for certain specialized metabolic pathways ( dividing);
It is the site of some chemical reactions (light reactions of photosynthesis in chloroplasts, oxidative phosphorylation during respiration in mitochondria);
Provide communication between cells in the tissues of multicellular organisms;
Transport- carries out transmembrane transport.
Receptor- are the location of receptor sites that recognize external stimuli.
Transport of substances through the membrane - one of the leading functions of the membrane, ensuring the exchange of substances between the cell and the external environment. Depending on the energy consumption for the transfer of substances, they are distinguished:
passive transport, or facilitated diffusion;
active (selective) transport with the participation of ATP and enzymes.
transport in membrane packaging. There are endocytosis (into the cell) and exocytosis (out of the cell) - mechanisms that transport large particles and macromolecules through the membrane. During endocytosis, the plasma membrane forms an invagination, its edges merge, and a vesicle is released into the cytoplasm. The vesicle is delimited from the cytoplasm by a single membrane, which is part of the outer cytoplasmic membrane. There are phagocytosis and pinocytosis. Phagocytosis is the absorption of large particles that are quite hard. For example, phagocytosis of lymphocytes, protozoa, etc. Pinocytosis is the process of capturing and absorbing droplets of liquid with substances dissolved in it.
Exocytosis is the process of removing various substances from the cell. During exocytosis, the membrane of the vesicle, or vacuole, fuses with the outer cytoplasmic membrane. The contents of the vesicle are removed beyond the cell surface, and the membrane is included in the outer cytoplasmic membrane.
At the core passive transport of uncharged molecules lies in the difference between the concentrations of hydrogen and charges, i.e. electrochemical gradient. Substances will move from an area with a higher gradient to an area with a lower one. The speed of transport depends on the difference in gradients.
Simple diffusion is the transport of substances directly through the lipid bilayer. Characteristic of gases, non-polar or small uncharged polar molecules, soluble in fats. Water quickly penetrates the bilayer because its molecule is small and electrically neutral. The diffusion of water through membranes is called osmosis.
Diffusion through membrane channels is the transport of charged molecules and ions (Na, K, Ca, Cl) penetrating through the membrane due to the presence of special channel-forming proteins that form water pores.
Facilitated diffusion is the transport of substances using special transport proteins. Each protein is responsible for a strictly defined molecule or group of related molecules, interacts with it and moves through the membrane. For example, sugars, amino acids, nucleotides and other polar molecules.
Active transport carried out by carrier proteins (ATPase) against an electrochemical gradient, with energy consumption. Its source is ATP molecules. For example, sodium is a potassium pump.
The concentration of potassium inside the cell is much higher than outside it, and sodium - vice versa. Therefore, potassium and sodium cations passively diffuse through the water pores of the membrane along a concentration gradient. This is explained by the fact that the permeability of the membrane for potassium ions is higher than for sodium ions. Accordingly, potassium diffuses out of the cell faster than sodium into the cell. However, for normal cell functioning a certain ratio of 3 potassium and 2 sodium ions is necessary. Therefore, there is a sodium-potassium pump in the membrane that actively pumps sodium out of the cell and potassium into the cell. This pump is a transmembrane membrane protein capable of conformational rearrangements. Therefore, it can attach to itself both potassium and sodium ions (antiport). The process is energy intensive:
From the inside of the membrane, sodium ions and an ATP molecule enter the pump protein, and potassium ions come from the outside.
Sodium ions combine with a protein molecule, and the protein acquires ATPase activity, i.e. the ability to cause ATP hydrolysis, which is accompanied by the release of energy that drives the pump.
The phosphate released during ATP hydrolysis attaches to the protein, i.e. phosphorylates the protein.
Phosphorylation causes conformational changes in the protein; it becomes unable to retain sodium ions. They are released and move outside the cell.
The new conformation of the protein promotes the attachment of potassium ions to it.
The addition of potassium ions causes dephosphorylation of the protein. It changes its conformation again.
A change in protein conformation leads to the release of potassium ions inside the cell.
The protein is again ready to attach sodium ions to itself.
In one cycle of operation, the pump pumps out 3 sodium ions from the cell and pumps in 2 potassium ions.
Cytoplasm– an obligatory component of the cell, located between the surface apparatus of the cell and the nucleus. This is a complex heterogeneous structural complex consisting of:
hyaloplasma
organelles (permanent components of the cytoplasm)
inclusions are temporary components of the cytoplasm.
Cytoplasmic matrix(hyaloplasm) is the internal contents of the cell - a colorless, thick and transparent colloidal solution. The components of the cytoplasmic matrix carry out biosynthesis processes in the cell and contain enzymes necessary for energy production, mainly due to anaerobic glycolysis.
Basic properties of the cytoplasmic matrix.
Determines the colloidal properties of the cell. Together with the intracellular membranes of the vacuolar system, it can be considered a highly heterogeneous or multiphase colloidal system.
Provides a change in the viscosity of the cytoplasm, a transition from a gel (thicker) to a sol (more liquid), which occurs under the influence of external and internal factors.
Provides cyclosis, amoeboid movement, cell division and movement of pigment in chromatophores.
Determines the polarity of the location of intracellular components.
Provides mechanical properties of cells - elasticity, ability to merge, rigidity.
Organelles– permanent cellular structures that ensure the cell performs specific functions. Depending on the structural features, they are distinguished:
membrane organelles - have a membrane structure. They can be single-membrane (ER, Golgi apparatus, lysosomes, vacuoles of plant cells). Double-membrane (mitochondria, plastids, nucleus).
Non-membrane organelles - do not have a membrane structure (chromosomes, ribosomes, cell center, cytoskeleton).
General-purpose organelles are characteristic of all cells: nucleus, mitochondria, cell center, Golgi apparatus, ribosomes, EPS, lysosomes. When organelles are characteristic of certain cell types, they are called specialty organelles (for example, myofibrils that contract a muscle fiber).
Endoplasmic reticulum- a single continuous structure, the membrane of which forms many invaginations and folds that look like tubules, microvacuoles and large cisterns. The ER membranes are, on the one hand, connected to the cell cytoplasmic membrane, and on the other, to the outer shell of the nuclear membrane.
There are two types of EPS - rough and smooth.
In rough or granular ER, cisterns and tubules are associated with ribosomes. is the outer side of the membrane. Smooth or agranular ER has no connection with ribosomes. This is the inner side of the membrane.
Cytoplasm- an obligatory part of the cell, enclosed between the plasma membrane and the nucleus; is divided into hyaloplasm (the main substance of the cytoplasm), organelles (permanent components of the cytoplasm) and inclusions (temporary components of the cytoplasm). Chemical composition of the cytoplasm: the basis is water (60-90% of the total mass of the cytoplasm), various organic and inorganic compounds. The cytoplasm has an alkaline reaction. A characteristic feature of the cytoplasm of a eukaryotic cell is constant movement ( cyclosis). It is detected primarily by the movement of cell organelles, such as chloroplasts. If the movement of the cytoplasm stops, the cell dies, since only by being in constant motion can it perform its functions.
Hyaloplasma ( cytosol) is a colorless, slimy, thick and transparent colloidal solution. It is in it that all metabolic processes take place, it ensures the interconnection of the nucleus and all organelles. Depending on the predominance of the liquid part or large molecules in the hyaloplasm, two forms of hyaloplasm are distinguished: sol- more liquid hyaloplasm and gel- thicker hyaloplasm. Mutual transitions are possible between them: the gel turns into a sol and vice versa.
Functions of the cytoplasm:
Cell membranes limit eukaryotic cells. In each cell membrane, at least two layers can be distinguished. The inner layer is adjacent to the cytoplasm and is represented by plasma membrane(synonyms - plasmalemma, cell membrane, cytoplasmic membrane), over which the outer layer is formed. In an animal cell it is thin and is called glycocalyx(formed by glycoproteins, glycolipids, lipoproteins), in a plant cell - thick, called cell wall(formed by cellulose).
All biological membranes have common structural features and properties. It is currently generally accepted fluid mosaic model of membrane structure. The basis of the membrane is a lipid bilayer formed mainly by phospholipids. Phospholipids are triglycerides in which one fatty acid residue is replaced by a phosphoric acid residue; the section of the molecule containing the phosphoric acid residue is called the hydrophilic head, the sections containing the fatty acid residues are called the hydrophobic tails. In the membrane, phospholipids are arranged in a strictly ordered manner: the hydrophobic tails of the molecules face each other, and the hydrophilic heads face outward, towards the water.
In addition to lipids, the membrane contains proteins (on average ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, receiving and converting signals from the environment, etc.). There are: 1) peripheral proteins(located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins(immersed in the lipid bilayer to varying depths), 3) integral, or transmembrane, proteins(pierce the membrane through, contacting both the external and internal environment of the cell). Integral proteins are in some cases called channel-forming or channel proteins, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them through).
A - hydrophilic phospholipid head; B - hydrophobic phospholipid tails; 1 - hydrophobic regions of proteins E and F; 2 — hydrophilic regions of protein F; 3 - branched oligosaccharide chain attached to a lipid in a glycolipid molecule (glycolipids are less common than glycoproteins); 4 - branched oligosaccharide chain attached to a protein in a glycoprotein molecule; 5 - hydrophilic channel (functions as a pore through which ions and some polar molecules can pass).
The membrane may contain carbohydrates (up to 10%). The carbohydrate component of membranes is represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form a supra-membrane complex, the glycocalyx, which is several tens of nanometers thick. It contains many cell receptors, and with its help cell adhesion occurs.
Molecules of proteins, carbohydrates and lipids are mobile, capable of moving in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.
Membranes perform the following functions:
The most important membrane property— selective permeability, i.e. membranes are highly permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, ensuring the exchange of substances between the cell and the external environment. The process of substances passing through the cell membrane is called transport of substances. There are: 1) passive transport- the process of passing substances without energy consumption; 2) active transport- the process of passage of substances that occurs with the expenditure of energy.
At passive transport substances move from an area of higher concentration to an area of lower, i.e. along the concentration gradient. In any solution there are solvent and solute molecules. The process of moving solute molecules is called diffusion, and the movement of solvent molecules is called osmosis. If the molecule is charged, then its transport is also affected by the electrical gradient. Therefore, people often talk about an electrochemical gradient, combining both gradients together. The speed of transport depends on the magnitude of the gradient.
The following types of passive transport can be distinguished: 1) simple diffusion— transport of substances directly through the lipid bilayer (oxygen, carbon dioxide); 2) diffusion through membrane channels— transport through channel-forming proteins (Na +, K +, Ca 2+, Cl -); 3) facilitated diffusion- transport of substances using special transport proteins, each of which is responsible for the movement of certain molecules or groups of related molecules (glucose, amino acids, nucleotides); 4) osmosis— transport of water molecules (in all biological systems the solvent is water).
Necessity active transport occurs when it is necessary to ensure the transport of molecules across a membrane against an electrochemical gradient. This transport is carried out by special carrier proteins, the activity of which requires energy expenditure. The energy source is ATP molecules. Active transport includes: 1) Na + /K + pump (sodium-potassium pump), 2) endocytosis, 3) exocytosis.
Operation of Na + /K + pump. For normal functioning, the cell must maintain a certain ratio of K + and Na + ions in the cytoplasm and in the external environment. The concentration of K + inside the cell should be significantly higher than outside it, and Na + - vice versa. It should be noted that Na + and K + can diffuse freely through the membrane pores. The Na + /K + pump counteracts the equalization of the concentrations of these ions and actively pumps Na + out of the cell and K + into the cell. The Na + /K + pump is a transmembrane protein capable of conformational changes, as a result of which it can attach both K + and Na +. The Na + /K + pump cycle can be divided into the following phases: 1) addition of Na + from the inside of the membrane, 2) phosphorylation of the pump protein, 3) release of Na + in the extracellular space, 4) addition of K + from the outside of the membrane , 5) dephosphorylation of the pump protein, 6) release of K + in the intracellular space. Almost a third of all energy required for cell functioning is spent on the operation of the sodium-potassium pump. In one cycle of operation, the pump pumps out 3Na + from the cell and pumps in 2K +.
Endocytosis- the process of absorption of large particles and macromolecules by the cell. There are two types of endocytosis: 1) phagocytosis- capture and absorption of large particles (cells, parts of cells, macromolecules) and 2) pinocytosis— capture and absorption of liquid material (solution, colloidal solution, suspension). The phenomenon of phagocytosis was discovered by I.I. Mechnikov in 1882. During endocytosis, the plasma membrane forms an invagination, its edges merge, and structures delimited from the cytoplasm by a single membrane are laced into the cytoplasm. Many protozoa and some leukocytes are capable of phagocytosis. Pinocytosis is observed in intestinal epithelial cells and in the endothelium of blood capillaries.
Exocytosis- a process reverse to endocytosis: the removal of various substances from the cell. During exocytosis, the vesicle membrane merges with the outer cytoplasmic membrane, the contents of the vesicle are removed outside the cell, and its membrane is included in the outer cytoplasmic membrane. In this way, hormones are removed from the cells of the endocrine glands; in protozoa, undigested food remains are removed.
Go to lectures No. 5"Cell theory. Types of cellular organization"
Go to lectures No. 7“Eukaryotic cell: structure and functions of organelles”
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