Membrane structures. Cell membrane: structure and functions
Cell membrane (also cytolemma, plasmalemma, or plasma membrane) is an elastic molecular structure consisting of proteins and lipids. Separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments - compartments or organelles that maintain certain conditions environment.
If the cell has one (usually plant cells do), it 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. The biological membrane also includes various proteins:
- integral (piercing the membrane through),
- semi-integral (immersed at one end in the outer or inner lipid layer),
- superficial (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 outside.
Membrane functions:
- Barrier - provides regulated, selective, passive and active metabolism with the environment.
- Transport - transport of substances into and out of the cell occurs through the membrane. Transport through membranes provides: delivery of nutrients, removal final products metabolism, secretion of various substances, creation of ion gradients, maintenance of optimal pH and ion concentrations in the cell, which are necessary for the functioning of cellular enzymes.
- Matrix - ensures a certain relative position and orientation of membrane proteins, their optimal interaction.
- Mechanical - ensures the autonomy of the cell, its intracellular structures, as well as connection with other cells (in tissues). Cell walls play a major role in ensuring mechanical function.
- Energy - during photosynthesis in chloroplasts and cellular respiration in mitochondria, energy transfer systems operate in their membranes, in which proteins also participate.
Membranes consist of three classes of lipids:
- phospholipids,
- glycolipids,
- cholesterol
Phospholipids and glycolipids(lipids with carbohydrates attached) consist of two long hydrophobic hydrocarbon “tails” that are connected to a charged hydrophilic “head”.
Cholesterol imparts rigidity to the membrane 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 and into the cell.
An important part of the membrane is proteins, penetrating it and responsible for the various properties of membranes. Their composition and orientation differ in different membranes. 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.
Cell membranes are often asymmetrical, that is, the layers differ in lipid composition, the outer one contains mainly phosphatidylinositol, phosphatidylcholine, sphingomyelins and glycolipids, the inner one contains phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol. The transition of an individual molecule from one layer to another (the so-called flip-flop) is difficult, but can occur spontaneously, approximately once every 6 months, or with the help of flippases proteins and scramblase of the plasma membrane. If phosphatidylserine appears in the outer layer, this is a signal for macrophages to destroy the cell.
Membrane organelles- these are closed single or interconnected sections of the cytoplasm, separated from the hyaloplasm by membranes. Single-membrane organelles include 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.
9.5.1. One of the main functions of membranes is participation in the transfer of substances. This process is achieved through 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 substances transported in each case.
Figure 9.10. Mechanisms of transport of molecules across the membrane
Simple diffusion- transfer of substances through the membrane without the participation of special mechanisms. Transport occurs along a concentration gradient without energy consumption. By simple diffusion, small biomolecules are transported - H2O, CO2, O2, urea, hydrophobic low-molecular substances. The rate of simple diffusion is proportional to the concentration gradient.
Facilitated diffusion- transfer of substances across the membrane using protein channels or special carrier proteins. It is carried out along a concentration gradient without energy consumption. Monosaccharides, amino acids, nucleotides, glycerol, and some ions are transported. Saturation kinetics is characteristic - at a certain (saturating) concentration of the transported substance, all molecules of the carrier take part in the transfer and the transport speed reaches a maximum value.
Active transport- also requires the participation of special transport proteins, but transport occurs against a concentration gradient and therefore requires energy expenditure. Using this mechanism, Na+, K+, Ca2+, Mg2+ ions are transported through the cell membrane, and protons are transported through the mitochondrial membrane. Active transport of substances is characterized by saturation kinetics.
9.5.2. An example of a transport system that carries out active transport of ions is Na+,K+-adenosine triphosphatase (Na+,K+-ATPase or Na+,K+-pump). This protein is located deep in the plasma membrane and is capable of catalyzing 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 cell cytosol and the extracellular fluid. Since the transfer of ions is not equivalent, an electrical potential difference occurs. Thus, an electrochemical potential arises, which consists of the energy of the difference in electrical potentials Δφ and the energy of the difference in the concentrations of substances ΔC on both sides of the membrane.
Figure 9.11. Na+, K+ pump diagram.
9.5.3. Transport of particles and high molecular weight compounds across membranes
Along with transport organic matter and ions carried out by carriers, there is a very special mechanism in the cell designed to absorb high-molecular compounds into the cell and remove high-molecular compounds from it by changing the shape of the biomembrane. This 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 membrane-surrounded vesicles (vesicles) occurs. Depending on the direction of transport and the nature of the substances transported, they distinguish following types vesicular transport:
Endocytosis(Figure 9.12, 1) - transfer of substances into the cell. Depending on the size of the vesicles formed, they are distinguished:
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 called phagosomes with a diameter of more than 250 nm are formed.
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 surface of the membrane; 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 detached 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 into low molecular weight products (amino acids, monosaccharides, nucleotides), which are transported into the cytosol, where they can be used by the cell.
Exocytosis(Figure 9.12, 2) - transfer of particles and large compounds from the cell. This process, like endocytosis, occurs 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 unspecialized cells and by cells of the endocrine glands, the mucous membrane 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 released 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 mechanisms of facilitated diffusion and active transport.
b) excretion - removal from the cell of substances that cannot be used (for example, during erythropoiesis, removal from reticulocytes of the mesh substance, which is aggregated remains of organelles). The mechanism of excretion appears to be that the excreted particles are initially trapped in a cytoplasmic vesicle, which then fuses with the plasma membrane.
The outside of 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 their parts 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 environment into the cytoplasm and in the opposite direction.
Basic functions of the cell membrane
One of the main properties of biological membranes is selective permeability (semi-permeability)- some substances pass through them with difficulty, others easily and even towards higher concentrations. Thus, for most cells, the concentration of Na ions inside is significantly lower than in the environment. The opposite relationship is typical for K ions: their concentration inside the cell is higher than outside. Therefore, Na ions always tend to penetrate the cell, and K ions always tend to exit. 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, which pumps Na ions out of the cell and simultaneously pumps K ions inside.
The tendency 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, substances are also absorbed by phagocytosis and pinocytosis. At phagocytosis the flexible outer membrane forms a small depression into which the captured particle falls. This recess increases, and, surrounded by a section of the outer membrane, the particle is immersed in the cytoplasm of the cell. The phenomenon of phagocytosis is characteristic of amoebas and some other protozoa, as well as leukocytes (phagocytes). Cells absorb liquids containing substances necessary for the cell in a similar way. This phenomenon was called pinocytosis.
The outer membranes of different 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 they are carried out various types intercellular contacts, i.e. communication between individual cells.
Many types of cells are characterized by the presence on their surface large quantity protrusions, folds, microvilli. They contribute to both a significant increase in cell surface area and improved metabolism, as well as stronger connections between individual cells and each other.
Plant cells have thick membranes on the outside of the cell membrane, clearly visible under an optical microscope, consisting of fiber (cellulose). They create a strong support for plant tissues (wood).
Some animal cells also have a number of external structures located on top of the cell membrane and have a protective nature. An example is the chitin of insect integumentary cells.
Functions of the cell membrane (briefly)
| Function | Description |
|---|---|
| Protective Barrier | Separates internal cell organelles from the external environment |
| Regulatory | Regulates the metabolism between the internal contents of the cell and the external environment |
| Dividing (compartmentalization) | Division of the internal space of the cell into independent blocks(compartments) |
| Energy | - Energy accumulation and transformation; - light reactions of photosynthesis in chloroplasts; - Absorption and secretion. |
| Receptor (informational) | Participates in the formation of arousal and its conduct. |
| Motor | Carries out the movement of the cell or its individual parts. |
Among The main functions of the cell membrane can be distinguished: barrier, transport, enzymatic and receptor. The cellular (biological) membrane (also known as plasmalemma, plasma or cytoplasmic membrane) protects the contents of the cell or its organelles from the environment, provides selective permeability for substances, enzymes are located on it, as well as molecules that can “catch” various chemical and physical signals.
This functionality is ensured by the special structure of the cell membrane.
In the evolution of life on Earth, a cell could generally form only after the appearance of a membrane, which separated and stabilized the internal contents and prevented them from disintegrating.
In terms of maintaining homeostasis (self-regulation of the relative constancy of the internal environment) the barrier function of the cell membrane is closely related to transport.
Small molecules are able to pass through the plasmalemma without any “helpers”, along a concentration gradient, i.e., from an area with a high concentration of a given substance to an area with a low concentration. This is the case, for example, for gases involved in respiration. Oxygen and carbon dioxide diffuse through the cell membrane in the direction where their concentration is in at the moment less.
Since the membrane is mostly hydrophobic (due to the lipid double layer), polar (hydrophilic) molecules, even small ones, often cannot penetrate through it. Therefore, a number of membrane proteins act as carriers of such molecules, binding to them and transporting them through the plasmalemma.
Integral (membrane-permeating) proteins often operate on the principle of opening and closing channels. When any molecule approaches such a protein, it binds to it and the channel opens. This substance or another passes through the protein channel, after which its conformation changes, and the channel closes to this substance, but can open to allow the passage of another. The sodium-potassium pump works on this principle, pumping potassium ions into the cell and pumping sodium ions out of it.
Enzymatic function of the cell membrane to a greater extent realized on the membranes of cell organelles. Most proteins synthesized in the cell perform an enzymatic function. “Sitting” on the membrane in a certain order, they organize a conveyor when the reaction product catalyzed by one enzyme protein moves on to the next. This “conveyor” is stabilized by surface proteins of the plasmalemma.
Despite the universality of the structure of all biological membranes (they are built according to a single principle, almost identical in all organisms and in different membrane cell structures), they chemical composition may still differ. There are more liquid and more solid ones, some have more of certain proteins, others have less. In addition, different sides (inner and outer) of the same membrane also differ.
The membrane that surrounds the cell (cytoplasmic) has on its outer side many carbohydrate chains attached to lipids or proteins (resulting in the formation of glycolipids and glycoproteins). Many of these carbohydrates serve receptor function, being susceptible to certain hormones, detecting changes in physical and chemical indicators in the environment.
If, for example, a hormone connects with its cellular receptor, then the carbohydrate part of the receptor molecule changes its structure, followed by a change in the structure of the associated protein part that penetrates the membrane. At the next stage, various biochemical reactions are started or suspended in the cell, i.e. its metabolism changes, and the cellular response to the “stimulus” begins.
In addition to the listed four functions of the cell membrane, others are also distinguished: matrix, energy, marking, formation of intercellular contacts, etc. However, they can be considered as “subfunctions” of those already considered.
