9.5.1. One of the main functions of membranes is participation in the transport of substances. This process is provided by three main mechanisms: simple diffusion, facilitated diffusion and active transport (Figure 9.10). Remember the most important features of these mechanisms and examples of the transported substances in each case.
Figure 9.10. Mechanisms of transport of molecules across the membrane
simple diffusion- transport of substances through the membrane without participation special arrangements. Transport occurs along a concentration gradient without energy consumption. Small biomolecules - H2O, CO2, O2, urea, hydrophobic low molecular weight substances are transported by simple diffusion. The rate of simple diffusion is proportional to the concentration gradient.
Facilitated diffusion- the transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along the concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transferred substance, all carrier molecules take part in the transfer and the transport speed reaches the limit value.
active transport- also requires the participation of special carrier proteins, but the transfer occurs against a concentration gradient and therefore requires energy. With the help of this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons through the mitochondrial membrane. The active transport of substances is characterized by saturation kinetics.
9.5.2. An example of a transport system that performs active ion transport is Na+,K+ -adenosine triphosphatase (Na+,K+ -ATPase or Na+,K+ -pump). This protein is located in the thickness of the plasma membrane and is able to catalyze the reaction of ATP hydrolysis. The energy released during the hydrolysis of 1 ATP molecule is used to transfer 3 Na + ions from the cell to the extracellular space and 2 K + ions in the opposite direction (Figure 9.11). As a result of the action of Na + , K + -ATPase, a concentration difference is created between the cytosol of the cell and the extracellular fluid. Since the transport of ions is non-equivalent, a difference in electrical potentials arises. Thus, an electrochemical potential arises, which is the sum of the energy of the difference in electric potentials Δφ and the energy of the difference in the concentrations of substances ΔС on both sides of the membrane.
Figure 9.11. Scheme of Na+, K+ -pump.
9.5.3. Transfer through membranes of particles and macromolecular compounds
Along with transport organic matter and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb and remove macromolecular compounds from the cell by changing the shape of the biomembrane. Such a mechanism is called vesicular transport.
Figure 9.12. Types of vesicular transport: 1 - endocytosis; 2 - exocytosis.
During the transfer of macromolecules, sequential formation and fusion of vesicles (vesicles) surrounded by a membrane occur. According to the direction of transport and the nature of the substances carried, they are distinguished the following types vesicular transport:
Endocytosis(Figure 9.12, 1) - the transfer of substances into the cell. Depending on the size of the resulting vesicles, there are:
a) pinocytosis - absorption of liquid and dissolved macromolecules (proteins, polysaccharides, nucleic acids) using small bubbles (150 nm in diameter);
b) phagocytosis — absorption of large particles, such as microorganisms or cell debris. In this case, large vesicles are formed, called phagosomes with a diameter of more than 250 nm.
Pinocytosis is characteristic of most eukaryotic cells, while large particles are absorbed by specialized cells - leukocytes and macrophages. At the first stage of endocytosis, substances or particles are adsorbed on the membrane surface; this process occurs without energy consumption. At the next stage, the membrane with the adsorbed substance deepens into the cytoplasm; the resulting local invaginations of the plasma membrane are laced from the cell surface, forming vesicles, which then migrate into the cell. This process is connected by a system of microfilaments and is energy dependent. The vesicles and phagosomes that enter the cell can merge with lysosomes. Enzymes contained in lysosomes break down substances contained in vesicles and phagosomes to low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported to the cytosol, where they can be used by the cell.
Exocytosis(Figure 9.12, 2) - the transfer of particles and large compounds from the cell. This process, like endocytosis, proceeds with the absorption of energy. The main types of exocytosis are:
a) secretion - removal from the cell of water-soluble compounds that are used or affect other cells of the body. It can be carried out both by non-specialized cells and cells of the endocrine glands, the mucosa of the gastrointestinal tract, adapted for the secretion of the substances they produce (hormones, neurotransmitters, proenzymes), depending on the specific needs of the body.
Secreted proteins are synthesized on ribosomes associated with the membranes of the rough endoplasmic reticulum. These proteins are then transported to the Golgi apparatus, where they are modified, concentrated, sorted, and then packaged into vesicles, which are cleaved into the cytosol and subsequently fuse with the plasma membrane so that the contents of the vesicles are outside the cell.
Unlike macromolecules, small secreted particles, such as protons, are transported out of the cell using facilitated diffusion and active transport mechanisms.
b) excretion - removal from the cell of substances that cannot be used (for example, the removal of a reticular substance from reticulocytes during erythropoiesis, which is an aggregated remnant of organelles). The mechanism of excretion, apparently, consists in the fact that at first the excreted particles are in the cytoplasmic vesicle, which then merges with the plasma membrane.
Membranes are extremely viscous and at the same time plastic structures that surround all living cells. Functions cell membranes:
1. The plasma membrane is a barrier that maintains a different composition of the extra- and intracellular environment.
2. Membranes form specialized compartments inside the cell, i.e. numerous organelles - mitochondria, lysosomes, Golgi complex, endoplasmic reticulum, nuclear membranes.
3. Enzymes involved in energy conversion in processes such as oxidative phosphorylation and photosynthesis are localized in membranes.
Structure and composition of membranes
The basis of the membrane is a lipid bilayer, in the formation of which phospholipids and glycolipids participate. The lipid bilayer is formed by two rows of lipids, the hydrophobic radicals of which are hidden inside, and the hydrophilic groups are turned outward and are in contact with the aqueous medium. Protein molecules seem to be “dissolved” in the lipid bilayer.
Structure of membrane lipids
Membrane lipids are amphiphilic molecules, because the molecule has both a hydrophilic region (polar heads) and a hydrophobic region, represented by hydrocarbon radicals of fatty acids, spontaneously forming a bilayer. There are three main types of lipids in membranes: phospholipids, glycolipids, and cholesterol.
The lipid composition is different. The content of one or another lipid, apparently, is determined by the variety of functions performed by these lipids in membranes.
Phospholipids. All phospholipids can be divided into two groups - glycerophospholipids and sphingophospholipids. Glycerophospholipids are classified as derivatives of phosphatidic acid. The most common glycerophospholipids are phosphatidylcholines and phosphatidylethanolamines. Sphingophospholipids are based on the amino alcohol sphingosine.
Glycolipids. In glycolipids, the hydrophobic part is represented by alcohol ceramide, and the hydrophilic part is represented by a carbohydrate residue. Depending on the length and structure of the carbohydrate part, cerebrosides and gangliosides are distinguished. Polar "heads" of glycolipids are located on the outer surface of plasma membranes.
Cholesterol (CS). CS is present in all membranes of animal cells. Its molecule consists of a rigid hydrophobic core and a flexible hydrocarbon chain. The only hydroxyl group at the 3-position is the "polar head". For an animal cell, the average molar ratio of cholesterol / phospholipids is 0.3-0.4, but in the plasma membrane this ratio is much higher (0.8-0.9). The presence of cholesterol in membranes reduces the mobility of fatty acids, reduces the lateral diffusion of lipids, and therefore can affect the functions of membrane proteins.
Membrane Properties:
1. Selective permeability. The closed bilayer provides one of the main properties of the membrane: it is impermeable to most water-soluble molecules, since they do not dissolve in its hydrophobic core. Gases such as oxygen, CO 2 and nitrogen have the ability to easily penetrate into the cell due to the small size of the molecules and weak interaction with solvents. Also, molecules of a lipid nature, for example, steroid hormones, easily penetrate through the bilayer.
2. Liquidity. The membranes are characterized by fluidity (fluidity), the ability of lipids and proteins to move. Two types of movements of phospholipids are possible - this is a somersault (in scientific literature called “flip flop”) and lateral diffusion. In the first case, phospholipid molecules opposing each other in the bimolecular layer turn over (or somersault) towards each other and change places in the membrane, i.e. the outside becomes the inside and vice versa. Such jumps are associated with the expenditure of energy. More often, rotations around the axis (rotation) and lateral diffusion are observed - movement within the layer parallel to the membrane surface. The speed of movement of molecules depends on the microviscosity of membranes, which, in turn, is determined by the relative content of saturated and unsaturated fatty acids in lipids. Microviscosity is lower if unsaturated fatty acids predominate in the composition of lipids, and higher if the content of saturated fatty acids is high.
3. Asymmetry of membranes. The surfaces of the same membrane differ in the composition of lipids, proteins and carbohydrates (transverse asymmetry). For example, phosphatidylcholines predominate in the outer layer, while phosphatidylethanolamines and phosphatidylserines predominate in the inner layer. The carbohydrate components of glycoproteins and glycolipids come to the outer surface, forming a continuous pouch called the glycocalyx. There are no carbohydrates on the inner surface. Proteins - hormone receptors are located on the outer surface of the plasma membrane, and the enzymes regulated by them - adenylate cyclase, phospholipase C - on the inside, etc.
Membrane proteins
Membrane phospholipids act as a solvent for membrane proteins, creating a microenvironment in which the latter can function. Proteins account for 30 to 70% of the mass of membranes. The number of different proteins in the membrane varies from 6-8 in the sarcoplasmic reticulum to more than 100 in the plasma membrane. These are enzymes, transport proteins, structural proteins, antigens, including antigens of the main histocompatibility system, receptors for various molecules.
By localization in the membrane, proteins are divided into integral (partially or completely immersed in the membrane) and peripheral (located on its surface). Some integral proteins cross the membrane once (glycophorin), while others cross the membrane many times. For example, the retinal photoreceptor and β 2 -adrenergic receptor crosses the bilayer 7 times.
Peripheral proteins and domains of integral proteins located on the outer surface of all membranes are almost always glycosylated. Oligosaccharide residues protect the protein from proteolysis and are also involved in ligand recognition or adhesion.
The cell membrane has a rather complex structure which can be seen with an electron microscope. Roughly speaking, it consists of a double layer of lipids (fats), in which different peptides (proteins) are included in different places. The total thickness of the membrane is about 5-10 nm.
General plan of the building cell membrane universal for all living things. However, animal membranes contain inclusions of cholesterol, which determines its rigidity. The difference between the membranes of different kingdoms of organisms mainly concerns the supra-membrane formations (layers). So in plants and fungi above the membrane (on the outside) there is a cell wall. In plants, it consists mainly of cellulose, and in fungi - of the substance of chitin. In animals, the epimembrane layer is called the glycocalyx.
Another name for the cell membrane is cytoplasmic membrane or plasma membrane.
A deeper study of the structure of the cell membrane reveals many of its features associated with the functions performed.
The lipid bilayer is mainly composed of phospholipids. These are fats, one end of which contains a residue phosphoric acid, which has hydrophilic properties (i.e., attracts water molecules). The second end of the phospholipid is a chain of fatty acids that have hydrophobic properties (do not form hydrogen bonds with water).
Phospholipid molecules in the cell membrane line up in two rows so that their hydrophobic "ends" are on the inside and the hydrophilic "heads" are on the outside. It turns out a fairly strong structure that protects the contents of the cell from the external environment.
Protein inclusions in the cell membrane are unevenly distributed, in addition, they are mobile (since phospholipids in the bilayer have lateral mobility). Since the 70s of the XX century, people began to talk about fluid-mosaic structure of the cell membrane.
Depending on how the protein is part of the membrane, there are three types of proteins: integral, semi-integral and peripheral. Integral proteins pass through the entire thickness of the membrane, and their ends stick out on both sides of it. They mainly perform a transport function. In semi-integral proteins, one end is located in the thickness of the membrane, and the second goes out (from the outside or inside) side. They perform enzymatic and receptor functions. Peripheral proteins are located on the outer or inner surface membranes.
The structural features of the cell membrane indicate that it is the main component of the surface complex of the cell, but not the only one. Its other components are the supra-membrane layer and the sub-membrane layer.
The glycocalyx (supramembrane layer of animals) is formed by oligosaccharides and polysaccharides, as well as peripheral proteins and protruding parts of integral proteins. The components of the glycocalyx perform a receptor function.
In addition to the glycocalyx, animal cells also have other supra-membrane formations: mucus, chitin, perilemma (similar to a membrane).
The supra-membrane formation in plants and fungi is the cell wall.
The submembrane layer of the cell is the surface cytoplasm (hyaloplasm) with the supporting-contractile system of the cell included in it, the fibrils of which interact with the proteins that make up the cell membrane. Various signals are transmitted through such compounds of molecules.
Outside, the cell is covered with a plasma membrane (or outer cell membrane) about 6-10 nm thick.
The cell membrane is a dense film of proteins and lipids (mainly phospholipids). Lipid molecules are arranged in an orderly manner - perpendicular to the surface, in two layers, so that their parts that interact intensively with water (hydrophilic) are directed outward, and the parts that are inert to water (hydrophobic) are directed inward.
Protein molecules are located in a non-continuous layer on the surface of the lipid framework on both sides. Some of them are immersed in the lipid layer, and some pass through it, forming areas permeable to water. These proteins perform various functions - some of them are enzymes, others are transport proteins involved in the transfer of certain substances from the environment to the cytoplasm and vice versa.
Basic Functions of the Cell Membrane
One of the main properties biological membranes is the selective permeability (semipermeability)- some substances pass through them with difficulty, others easily and even towards a higher concentration. So, for most cells, the concentration of Na ions inside is much lower than in environment. For K ions, the reverse ratio is characteristic: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to enter the cell, and K ions - to go outside. The equalization of the concentrations of these ions is prevented by the presence in the membrane of a special system that plays the role of a pump that pumps Na ions out of the cell and simultaneously pumps K ions inside.
The desire of Na ions to move from outside to inside is used to transport sugars and amino acids into the cell. With the active removal of Na ions from the cell, conditions are created for the entry of glucose and amino acids into it.
In many cells, absorption of substances also occurs by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression where the captured particle enters. This recess increases, and, surrounded by a portion of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoeba and some other protozoa, as well as leukocytes (phagocytes). Similarly, the cells absorb liquids containing the substances necessary for the cell. This phenomenon has been called pinocytosis.
The outer membranes of various cells differ significantly both in the chemical composition of their proteins and lipids, and in their relative content. It is these features that determine the diversity in the physiological activity of the membranes of various cells and their role in the life of cells and tissues.
The endoplasmic reticulum of the cell is connected to the outer membrane. With the help of outer membranes, different types intercellular contacts, i.e. communication between individual cells.
Many types of cells are characterized by the presence on their surface a large number protrusions, folds, microvilli. They contribute both to a significant increase in the surface area of cells and improve metabolism, as well as to stronger bonds of individual cells with each other.
At plant cells outside the cell membrane there are thick shells, clearly visible in an optical microscope, consisting of fiber (cellulose). They create a strong support for plant tissues (wood).
Some cells of animal origin also have a number of external structures that are located on top of the cell membrane and have a protective character. An example is the chitin of the integumentary cells of insects.
Functions of the cell membrane (briefly)
| Function | Description |
|---|---|
| protective barrier | Separates the internal organelles of the cell from the external environment |
| Regulatory | It regulates the exchange of substances between the internal contents of the cell and the external environment. |
| Delimiting (compartmentalization) | Separation of the internal space of the cell into independent blocks (compartments) |
| Energy | - Accumulation and transformation of energy; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion. |
| Receptor (information) | Participates in the formation of excitation and its conduct. |
| Motor | Carries out the movement of the cell or its individual parts. |
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