MINISTRY OF EDUCATION AND SCIENCE
MBOU "ACADEMIC LYCEUM"
ABSTRACT
Membrane cell organelles
Subject: biology
PERFORMED:
10th grade student
Kuzmina Anastasia
SUPERVISOR:
Tomsk 2014
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Types of organelles by structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Kinds membrane organelles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Endoplasmic reticulum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
Golgi apparatus (complex). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Lysosomes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5
Vacuoles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Cell vacuoles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6
Plastids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
Mitochondria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Literature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Introduction
Organelles (from the Greek organon - tool, organ and idos - type, likeness) organelles are supramolecular structures of the cytoplasm that perform specific functions, without which normal cell activity is impossible.
Membrane organelles are hollow structures whose walls are formed by a single or double membrane.
Single membrane: endoplasmic reticulum, Golgi complex, lysosomes, vacuoles . These organelles form an intracellular system for the synthesis and transport of substances.
Double-membrane: mitochondria and plastids
Endoplasmic reticulum
EPS is a single-membrane organelle consisting of cavities and tubules connected to each other. The endoplasmic reticulum is structurally connected to the nucleus: a membrane extending from the outer membrane of the nucleus forms the walls endoplasmic reticulum. EPS is more characteristic of eukorotic cells (i.e., those that have a nucleus).
There are 2 types of EPS, available in both plant and animal cell:
· rough (granular)
smooth (agranular)
On membranes rough XPS There are numerous small granules - ribosomes, special organelles with the help of which proteins are synthesized, which then penetrate inside and can move through the cavities to any place in the cell.
Structure:
Vacuoles
Ribosomes
Records
Internal cavities
On membranes smooth EPS there are no ribosomes, but there are enzymes that synthesize carbohydrates and lipids. After synthesis, carbohydrates and lipids can move along the ER membranes to any location in the cell.
The degree of development of the EPS type depends on the specialization of the cell.
granular ER is better developed in cells that synthesize protein hormones
agranular EPS in cells that synthesize fat-like substances.
EPS functions:
· Synthesis of substances.
· Transport function. Through the cavities of the ER, synthesized substances move to any place in the cell.
Golgi complex
The Golgi complex (dictyosome) is a stack of flat membrane sacs called cisternae. The tanks are completely isolated from each other and are not connected to each other. Along the edges of the tanks numerous tubes and bubbles branch off. From time to time, vacuoles (vesicles) with synthesized substances are detached from the EPS, which move to the Golgi complex and connect with it. Substances synthesized in the ER become more complex and accumulate in the Golgi complex.
· In the tanks of the Golgi complex, further chemical transformation and complication of substances received from the EPS occurs. For example, substances necessary to renew the cell membrane (glycoproteins, glycolipids) and polysaccharides are formed.
· In the Golgi complex, substances accumulate and are temporarily “stored”
· The formed substances are “packed” into vesicles (vacuoles) and in this form move throughout the cell.
· Lysosomes (spherical organelles with digestive enzymes) are formed in the Golgi complex.
· Removal of secretions (hormones, enzymes) from cells
Lysosomes
(“lysis” - disintegration, dissolution)
Lysosomes are small spherical organelles, the walls of which are formed by a single membrane; contain lytic (breaking down) enzymes. First, lysosomes detached from the Golgi complex contain inactive enzymes. Under certain conditions, their enzymes are activated. When a lysosome merges with a phagocytotic or pinocytotic vacuole, a digestive vacuole is formed, in which intracellular digestion of various substances occurs.
Functions of lysosomes:
1. They break down substances absorbed as a result of phagocytosis and pinocytosis. Biopolymers are broken down into monomers, which enter the cell and are used for its needs. For example, they can be used to synthesize new organic matter or can be further broken down to produce energy.
2. Destroy old, damaged, redundant organelles. The breakdown of organelles can also occur during cell starvation.
3. Carry out autolysis (splitting) of the cell (resorption of the tail in tadpoles, liquefaction of tissues in the area of inflammation, destruction of cartilage cells in the process of formation bone tissue and etc.).
Vacuoles
Vacuoles are spherical single-membrane organelles that are reservoirs of water and substances dissolved in it.
(vesicles detached from the ER and Golgi complex).
Vacuoles: phagocytotic,
pinocytotic,
digestive vacuoles
Cell vacuoles
Animal cell vacuoles are small and numerous, but their volume does not exceed 5% of the total volume of the cell.
Functions of vacuoles in an animal cell:
transport of substances throughout the cell,
· implementation of the relationship between organelles.
In a plant cell, vacuoles account for up to 90% of the volume. In mature plant cell one vacuole, occupies central position. The membrane of the plant cell vacuole is the tonoplast, its contents are cell sap.
Functions of vacuoles in a plant cell:
maintaining the cell membrane in tension,
accumulation of various substances, including waste cell activity,
· supply of water for photosynthesis processes.
Cell sap may contain:
Reserve substances that can be used by the cell itself ( organic acids, amino acids, sugars, proteins).
Substances that are removed from cell metabolism and accumulate in vacuoles (phenols, tannins, alkaloids, etc.)
Phytohormones, phytoncides,
Pigments (dyes) that give cell sap its purple, red, blue, violet, and sometimes yellow or cream color. It is the pigments of cell sap that color flower petals, fruits, and roots.
Plastids
Plant cells have special double-membrane organelles - plastids. There are 3 types of plastids: chloroplasts, chromoplasts, leucoplasts.
Chloroplasts have a shell of 2 membranes. Outer shell smooth, and the inner one forms numerous vesicles (thylakoids). A stack of thylakoids is a grana. Granules are staggered for better penetration sunlight. The thylakoid membranes contain molecules of the green pigment chlorophyll, so chloroplasts have green color. Photosynthesis occurs with the help of chlorophyll. Thus, main function chloroplasts - carrying out the process of photosynthesis.
The space between the grains is filled with matrix. The matrix contains DNA, RNA, ribosomes (small, like those of prokaryotes), lipid droplets, and starch grains.
Chloroplasts, like mitochondria, are semi-autonomous organelles of a plant cell, since they can independently synthesize their own proteins and are able to divide regardless of cell division.
Chromoplasts are plastids that are red, orange or yellow in color. Chromoplasts are colored by carotenoid pigments located in the matrix. Thylakoids are poorly developed or absent altogether. The exact function of chromoplasts is unknown. Perhaps they attract animals to the ripe fruits.
Leukoplasts are colorless plastids located in the cells of colorless tissues. Thylakoids are undeveloped. Leukoplasts accumulate starch, lipids and proteins.
Plastids can mutually transform into each other: leucoplasts - chloroplasts - chromoplasts.
Mitochondria
The mitochondrion is a two-membrane semi-autonomous organelle that synthesizes ATP.
The shape of mitochondria is varied; they can be rod-shaped, filamentous or spherical. The walls of mitochondria are formed by two membranes: outer and inner. The outer membrane is smooth, and the inner one forms numerous folds - cristas. The inner membrane contains numerous enzyme complexes that carry out the synthesis of ATP.
The folding of the inner membrane has great importance. More enzyme complexes can be located on a folded surface than on a smooth surface. The number of folds in the mitochondria can change depending on the energy needs of the cells. If the cell needs energy, then the number of cristae increases. Accordingly, the number of enzyme complexes located on the cristae increases. As a result, more ATP will be formed. In addition, the cell may increase total mitochondria. If the cell does not need a large amount of energy, then the number of mitochondria in the cell decreases and the number of cristae within the mitochondria decreases.
The internal space of mitochondria is filled with a structureless homogeneous substance (matrix). The matrix contains circular molecules of DNA, RNA and small ribosomes (like in prokaryotes). Mitochondrial DNA contains information about the structure of mitochondrial proteins. RNA and ribosomes carry out their synthesis. The ribosomes of mitochondria are small, their structure is very similar to the ribosomes of bacteria.
Mitochondria are called semi-autonomous organelles. This means that they are dependent on the cell, but at the same time retain some independence. For example, mitochondria themselves synthesize their own proteins, including the enzymes of their enzyme complexes. In addition, mitochondria can multiply by fission independently of cell division.
Conclusion
Literature
1. http://ppt4web. ru/
2. http://biofile. ru/bio/5032.html
3. http://becmology. blogspot. ru/2011/04/blog-post_6850.html
4. http://ru. wikipedia. org
5. http://biofile. ru/bio/5091.html
6. http://www. vedu. ru/bigencdic/
Biological membranes located at the border of the cell and the extracellular space, as well as at the border of the membrane organelles of the cell (mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, peroxisomes, nucleus, membrane vesicles) and the cytosol, are important for the functioning of not only the cell as a whole, but also its organelles. Cell membranes have a fundamentally similar molecular organization. In this chapter, biological membranes are examined primarily using the example of the plasma membrane (plasmolemma), which separates the cell from the extracellular environment.
Plasma membrane
Any biological membrane (Fig. 2-1) consists of phospholipids (~50%) and proteins (up to 40%). In smaller quantities, the membrane contains other lipids, cholesterol and carbohydrates.
Phospholipids. A phospholipid molecule consists of a polar (hydrophilic) part (head) and an apolar (hydrophobic) double hydrocarbon tail. In the aqueous phase, phospholipid molecules automatically aggregate tail to tail, forming the framework of the biological membrane (Fig. 2-1 and 2-2) in the form of a double layer (bilayer). Thus, in the membrane, the tails of phospholipids (fatty acids) are directed into the bilayer, and the heads containing phosphate groups are directed outward.
Squirrels biological membranes are divided into integral (including transmembrane) and peripheral (see Fig. 2-1, 2-2).
Integral membrane proteins (globular) embedded in the lipid bilayer. Their hydrophilic amino acids are mutually
Rice. 2-1. Biological membrane consists of a double layer of phospholipids, the hydrophilic parts of which (heads) are directed towards the surface of the membrane, and the hydrophobic parts (tails that stabilize the membrane in the form of a bilayer) are directed into the membrane. And - integral proteins are immersed in the membrane. T - transmembrane proteins penetrate the entire thickness of the membrane. Π - peripheral proteins are located either on the outer or inner surface of the membrane.
interact with phosphate groups of phospholipids, and hydrophobic amino acids - with chains fatty acids. Integral membrane proteins include adhesion proteins, some receptor proteins(membrane receptors). Transmembrane protein- a protein molecule that passes through the entire thickness of the membrane and protrudes from it on both the outer and inner surfaces. Transmembrane proteins include pores, ion channels, transporters, pumps, some receptor proteins.
Hydrophilic area
Rice. 2-2. Plasma membrane. Explanations in the text.
♦ Pores And channels- transmembrane pathways along which water, ions and metabolite molecules move between the cytosol and the intercellular space (and in the opposite direction).
♦ Vectors carry out transmembrane movement of specific molecules (including in combination with the transfer of ions or molecules of another type).
♦ Pumps move ions against their concentration and energy gradients (electrochemical gradient) using the energy released by ATP hydrolysis.
Peripheral membrane proteins (fibrillar and globular) are located on one of the surfaces of the cell membrane (external or internal) and are non-covalently associated with integral membrane proteins.
♦ Examples of peripheral membrane proteins associated with the outer surface of the membrane are - receptor proteins And adhesion proteins.
♦ Examples of peripheral membrane proteins associated with the inner surface of the membrane are - cytoskeleton proteins, second messenger system proteins, enzymes and other proteins.
Carbohydrates(mainly oligosaccharides) are part of the glycoproteins and glycolipids of the membrane, accounting for 2-10% of its mass (see Fig. 2-2). Interact with cell surface carbohydrates lectins. Oligosaccharide chains protrude onto outer surface cell membranes and form the surface membrane - glycocalyx.
Membrane permeability
The membrane bilayer separates the two aqueous phases. Thus, the plasma membrane separates the intercellular (interstitial) fluid from the cytosol, and the membranes of lysosomes, peroxisomes, mitochondria and other membranous intracellular organelles separate their contents from the cytosol. Biological membrane- semi-permeable barrier.
Semi-permeable membrane. A biological membrane is defined as semi-permeable, i.e. a barrier impenetrable to water, but permeable to substances dissolved in it (ions and molecules).
Semi-permeable tissue structures. Semi-permeable tissue structures also include the wall of blood capillaries and various barriers (for example, the filtration barrier of the renal corpuscles, the aerohematic barrier of the respiratory part of the lung, the blood-brain barrier and many others, although such barriers, in addition to biological membranes (plasmolemma), also include non-membrane components. The permeability of such tissue structures is discussed in the section “Transcellular permeability” in Chapter 4.
The physicochemical parameters of the intercellular fluid and cytosol are significantly different (see Table 2-1), as are the parameters of each membrane intracellular organelle and cytosol. The outer and inner surfaces of a biological membrane are polar and hydrophilic, but the nonpolar core of the membrane is hydrophobic. Therefore, nonpolar substances can penetrate the lipid bilayer. At the same time, it is the hydrophobic nature of the core of a biological membrane that determines the fundamental impossibility of direct penetration of polar substances through the membrane.
Non-polar substances(for example, water-insoluble cholesterol and its derivatives) penetrate freely through biological membranes. In particular, it is for this reason that steroid hormone receptors are located inside the cell.
Polar substances(for example, Na +, K +, Cl -, Ca 2 + ions; various small but polar metabolites, as well as sugars, nucleotides, protein and nucleic acid macromolecules) themselves do not penetrate through biological membranes. That is why receptors of polar molecules (for example, peptide hormones) built into plasma membrane, and the transmission of the hormonal signal to other cellular compartments is carried out by second messengers.
Selective permeability - the permeability of the biological membrane in relation to specific chemicals is important for maintaining cellular homeostasis, the optimal content of ions, water, metabolites and macromolecules in the cell. The movement of specific substances across a biological membrane is called transmembrane transport (transmembrane transport).
Transmembrane transport
Selective permeability is carried out using passive transport, facilitated diffusion and active transport.
Passive transport
Passive transport (passive diffusion) - the movement of small non-polar and polar molecules in both directions along a concentration gradient (difference in chemical potential) or along an electrochemical gradient (transport of charged substances - electrolytes) occurs without energy expenditure and is characterized by low specificity. Simple diffusion is described by Fick's law. An example of passive transport is passive (simple) diffusion of gases during respiration.
Concentration gradient. The determining factor in the diffusion of gases is their partial pressure (for example, the partial pressure of oxygen - Po 2 and the partial pressure of carbon dioxide - PCO 2). In other words, with simple diffusion, the flow of an uncharged substance (for example, gases, steroid hormones, anesthetics) through the lipid bilayer is directly proportional to the difference in the concentration of this substance on both sides of the membrane (Fig. 2-3).
Electrochemical gradient(Δμ x). Passive transport of a charged solute X depends on the difference in the concentration of the substance in the cell ([X] B) and outside (outside) the cell ([X] C) and on the difference electric potential outside (Ψ C) and inside the cell (Ψ B). In other words, Δμ χ takes into account the contribution of both the concentration gradient of the substance (chemical potential difference) and the electrical potential on both sides of the membrane (electric potential difference).
Φ Thus, driving force passive transport of electrolytes is an electrochemical gradient - the difference in electrochemical potential (Δμ x) on both sides of the biological membrane.
Facilitated diffusion
For facilitated diffusion of substances (see Fig. 2-3), protein components built into the membrane (pores, carriers, channels) are required. All these components are integral
Rice. 2-3. Passive transport by diffusion across the plasma membrane. A - the direction of transport of the substance in both simple and facilitated diffusion occurs along the concentration gradient of the substance on both sides of the plasmalemma. B - transport kinetics. Along the ordinate - the amount of diffused substance, along the ordinate - time. Simple diffusion does not require direct energy expenditure, is an unsaturated process, and its speed linearly depends on the concentration gradient of the substance.
(transmembrane) proteins. Facilitated diffusion occurs along a concentration gradient for non-polar substances or along an electrochemical gradient for polar substances.
Pores. By definition, filled with water the pore channel is always open(Fig. 2-4). Pores are formed by different proteins (porins, perforins, aquaporins, connexins, etc.). In some cases, giant complexes (such as nuclear pores) are formed, consisting of many different proteins.
Vectors(transporters) transport through biological membranes many different ions (Na +, Cl -, H +, HCO 3 -, etc.) and organic substances (glucose, amino acids, creatine, norepinephrine, folate, lactate, pyruvate, etc.). Conveyors specific: each specific re-
Rice. 2-4. It's time in the plasmalemma .
The pore channel is always open, so Chemical substance X passes through the membrane along its concentration gradient or (if the substance X is charged) along an electrochemical gradient. IN in this case substance X moves from the extracellular space into the cytosol.
the carrier carries, as a rule and predominantly, one substance through the lipid bilayer. There are unidirectional (uniport), combined (symport) and multidirectional (antiport) transport (Fig. 2-5).
Carriers that carry out both combined (symport) and multidirectional (antiport) transmembrane transport, from the point of view of energy costs, function in such a way that the energy accumulated during the transfer of one substance (usually Na+) is spent on the transport of another substance. This type of transmembrane transport is called secondary active transport (see below). Ion channels consist of interconnected protein SEs that form a hydrophilic pore in the membrane (Fig. 2-6). Ions diffuse through an open pore along an electrochemical gradient. The properties of ion channels (including specificity and conductance) are determined by both the amino acid sequence of a particular polypeptide and the conformational changes that occur with in different parts polypeptides in the integral protein of the channel. Specificity. Ion channels are specific (selective) in relation to specific cations and anions [for example, for Na+ (sodium channel), K+ (potassium
Rice. 2-5. Model of variants of transmembrane transport of different molecules .
Rice. 2-6. Potassium channel model. The integral protein (protein fragments are marked with numbers in the figure) penetrates the entire thickness of the lipid bilayer, forming a channel pore filled with water (in the figure, three potassium ions are visible in the channel, the lower of them is located in the pore cavity).
channel), Ca 2+ (calcium channel), Cl - (chloride channel) and
etc.].
Φ Conductivity is determined by the number of ions that can pass through the channel per unit time. The conductance of a channel changes depending on whether the channel is open or closed.
Φ Gates. The channel can be either open or closed (Figure 2-7). Therefore, the channel model provides for the presence of a device that opens and closes the channel - a gate mechanism, or channel gate (by analogy with open and closed gates).
Φ Functional components. In addition to the gate, the ion channel model provides for the existence of such functional components as a sensor, a selective filter, and an open channel pore.
Rice. 2-7. Model of the ion channel gating mechanism . A. The gate of the channel is closed, the X ion cannot pass through the membrane. B. The channel gate is open, X ions pass through the membrane through the channel pore.
♦ Sensor. Each channel has one (sometimes more) sensors for different types of signals: changes in membrane potential (MP), second messengers (from the cytoplasmic side of the membrane), different ligands (from the extracellular side of the membrane). These signals regulate the transition between the open and closed states of the channel.
■ Channel classification according to sensitivity to different signals. Based on this feature, channels are divided into voltage-dependent, mechanosensitive, receptor-dependent, G-protein-dependent, Ca 2 +-dependent.
♦ Selective filter determines which types of ions (anions or cations) or specific ions (for example, Na +, K +, Ca 2 +, Cl -) have access to the channel pore.
♦ It's time for an open channel. After the integral channel protein acquires a conformation corresponding to the open state of the channel, a transmembrane pore is formed, within which ions move.
Φ Channel states. Due to the presence of a gate, sensor, selective filter and pore, ion channels can be in a state of rest, activation and inactivation.
♦ State of rest- the channel is closed, but is ready to open in response to chemical, mechanical or electrical stimuli.
♦ Activation state- the channel is open and allows ions to pass through.
♦ Inactivation state- the channel is closed and is not capable of activation. Inactivation occurs immediately after the channel opens in response to a stimulus and lasts from several to several hundred milliseconds (depending on the type of channel).
Φ Examples. The most common channels are for Na+, K+, Ca 2 +, Cl -, HCO - 3.
♦ Sodium channels are present in almost any cell. Since the transmembrane electrochemical potential difference for Na+ (Δμ?a) negative, when the Na + channel is open, sodium ions rush from the intercellular space into the cytosol (on the left in Fig. 2-8).
Rice. 2-8. Na+-, K+ -pump . Model of Na+-, K+-ATPase built into the plasma membrane. The Na+-, K+-pump is an integral membrane protein consisting of four SEs (two catalytic subunits α and two glycoprotein β forming the channel). The Na+-, K+-pump transports cations against the electrochemical gradient (μ x) - transports Na+ from the cell in exchange for K+ (during the hydrolysis of one ATP molecule, three Na+ ions are pumped out of the cell and two K+ ions are pumped into it). To the left and right of the pump, arrows show the directions of the transmembrane flow of ions and water into the cell (Na+) and out of the cell (K+, Cl - and water) due to their differences Δμ x. ADP - adenosine diphosphate, Fn - inorganic phosphate.
■ In electrically excitable structures (for example, skeletal MVs, cardiomyocytes, SMCs, neurons), sodium channels generate AP, more precisely the initial stage of membrane depolarization. Potentially excitable sodium channels are heterodimers; they contain a large α-subunit (Mr about 260 kDa) and several β-subunits (Mr 32-38 kDa). Transmembrane α-CE determines the properties of the channel.
■ In the nephron tubules and intestine, Na+ channels are concentrated at the apex of epithelial cells, so Na+ enters these cells from the lumen and then enters the blood, allowing sodium reabsorption in the kidney and sodium absorption in the gastrointestinal tract.
♦ Potassium channels(see Fig. 2-6) - integral membrane proteins, these channels are found in the plasmalemma of all cells. The transmembrane electrochemical potential difference for K+ (Δμ κ) is close to zero (or slightly positive) therefore, when the K+ channel is open, potassium ions move from the cytosol to the extracellular space (“leakage” of potassium from the cell, on the right in Fig. 2-8). Functions K+ channels - maintenance of resting MP (negative on the inner surface of the membrane), regulation of cell volume, participation in the completion of AP, modulation of electrical excitability of nerve and muscle structures, insulin secretion from β-cells of the islets of Langerhans.
♦ Calcium channels- protein complexes, consisting of several SEs (α ρ α 2, β, γ, δ). Since the transmembrane electrochemical potential difference for Ca 2 + (Δμ ca) is significantly negative, then when the Ca^ channel is open, calcium ions rush out from inside cell membranes nal “calcium depots” and intercellular space into the cytosol. When channels are activated, membrane depolarization occurs, as well as interaction of ligands with their receptors. Ca 2+ channels are divided into voltage-gated and receptor-gated (for example, adrenergic) channels.
♦ Anion channels. Many cells contain different types anion-selective channels through which passive transport of Cl - and, to a lesser extent, HCO - 3 occurs. Since the transmembrane electrochemical potential difference for Cl - (Δμ α) is moderate negative, when the anion channel is open, chlorine ions diffuse from the cytosol into the intercellular space (right in Fig. 2-8).
Active transport
Active transport - energy-dependent transmembrane transport against an electrochemical gradient. There are primary and secondary active transport. Primary active transport is carried out pumps(various ATPases), secondary - symporters(combined unidirectional transport) and antiporters(oncoming multidirectional traffic).
Primary active transport provide the following pumps: sodium-, potassium ATPases, proton and potassium ATPases, Ca 2+ -transporting ATPases, mitochondrial ATPases, lysosomal proton pumps, etc.
Φ Sodium-, potassium ATPase(see Fig. 2-8) regulates the transmembrane flows of the main cations (Na +, K +) and indirectly - water (which maintains a constant cell volume), provides?+-related transmembrane transport (symport and antiport) of many organic and inorganic molecules , participates in the creation of resting MF and generation of PD of nerve and muscle elements.
Φ Proton And potassium ATPase(H+-, K+-pump). With the help of this enzyme, the parietal cells of the glands of the gastric mucosa participate in the formation of hydrochloric acid (electronically neutral exchange of two extracellular K + ions for two intracellular H + ions during the hydrolysis of one ATP molecule).
Φ Ca 2+-transporting ATPases(Ca 2 + -ATPase) pump calcium ions out of the cytoplasm in exchange for protons against a significant electrochemical Ca 2+ gradient.
Φ Mitochondrial ATPase type F (F 0 F:) - ATP synthase of the inner membrane of mitochondria - catalyzes the final stage of ATP synthesis. Mitochondrial cristae contain ATP synthase, which couples oxidation in the Krebs cycle and phosphorylation of ADP to ATP. ATP is synthesized by the reverse flow of protons into the matrix through a channel in the ATP-synthesizing complex (the so-called chemiosmotic coupling).
Φ Lysosomal proton pumps[H+-ATPases type V (from Vesicular)], embedded in the membranes that surround lysosomes (also the Golgi complex and secretory vesicles), transport H+ from the cytosol to these membrane-bound organelles. As a result, their pH value decreases, which optimizes the functions of these structures.
Secondary active transport. There are two known forms of active secondary transport - combined (simport) and counter (antiport)(See Figure 2-5).
Φ Simport carry out integral membrane proteins. Transfer of substance X against its electrochemical
dient (μ x) in most cases occurs due to entry into the cytosol from the intercellular space along the diffusion gradient of sodium ions (i.e., due to Δμ Na)), and in some cases due to entry into the cytosol from the intercellular space along the diffusion gradient protons (i.e. due to Δμ H. As a result, both ions (Na+ or H+) and substance X (for example, glucose, amino acids, inorganic anions, potassium and chlorine ions) move from intercellular substance into the cytosol. Φ Antiport(counter or exchange transport) typically moves anions in exchange for anions and cations in exchange for cations. The driving force of the exchanger is formed due to the entry of Na+ into the cell.
Maintaining intracellular ion homeostasis
Selective permeability of biological membranes, carried out using passive transport, facilitated diffusion and active transport, is aimed at maintaining the parameters of ionic homeostasis, , and other ions, important for the functioning of cells, as well as pH () and water (Table 2-1) and many others chemical compounds.
HomeostasisAnd involves the maintenance of an asymmetric and significant transmembrane gradient of these cations, ensures electrical polarization of cell membranes, as well as the accumulation of energy for the transmembrane transport of various chemicals.
Φ Significant and asymmetric transmembrane gradient.
and are characterized by a significant and asymmetric transmembrane gradient of these cations: the extracellular one is about 10 times higher than the cytosol, while the intracellular one is about 30 times higher than the extracellular one. The maintenance of this gradient is almost entirely ensured by Na+-, K+-ATPase (see Fig. 2-8).
Φ Membrane polarization. The Na+-, K+-pump is electrogenic: its work helps maintain membrane potential (MP), i.e. a positive charge on the outer (extracellular) surface of the membrane and a negative charge on the inner (intracellular) surface of the membrane. The charge value (V m) measured on the inner surface of the membrane is approx. -60 mV.
Φ Transmembrane electrochemical Na+ gradient, directed into the cell, promotes the passive entry of Na + into the cytosol and - most importantly! - accumulation of energy. It is this energy that cells use to solve a number of problems. important tasks- ensuring secondary active transport and transcellular transfer, and in excitable cells - generation of action potential (AP).
♦ Transcellular transfer. IN epithelial cells, forming the wall of various tubes and cavities (for example, nephron tubules, small intestine, serous cavities, etc.), Na+ channels are located on the apical surface of the epithelium, and Na+ and K+ pumps are built into the plasmalemma of the basal surface of the cells. This asymmetric arrangement of Na+ channels and?+ pumps allows pump over sodium ions through the cell, i.e. from the lumen of the tubules and cavities in internal environment body.
♦ Action potential(PD). In electrically excitable cellular elements (neurons, cardiomyocytes, skeletal MVs, SMCs), passive entry into the cytosol through voltage-gated Na+ channels is critical for the generation of AP (for more details, see Chapter 5).
Homeostasis.Since cytosolic Ca 2+ acts as a second (intracellular) messenger that regulates many functions, then in the cytosol of the cell is in a state
rest is minimal (<100 нМ, или 10 -7 M). В то же время внеклеточная около 1 мМ (10 -3 M). Таким образом, разни- ца трансмембранного электрохимического градиента для Ca 2+ (Δμ^) гигантская - 4 порядка величины μ Ca ! Другими словами, между цитозолем и внеклеточной средой (а также между цитозолем и внутриклеточными депо кальция, в первую очередь цистернами эндоплазматической сети) существует весьма значительный трансмембранный градиент Ca 2+ . Именно поэтому поступление Ca 2+ в цитозоль происходит практически мгновенно: в виде «выброса» Ca 2 + из кальциевых депо или «вброса» Ca 2 + из межклеточного пространства. Поддержание столь низкой в цитозоле обеспечивают Са 2 +-АТФазы, Na+-Ca 2 +-обменники и Ca 2 +-буферные внутриклеточные системы (митохондрии и Ca 2 +-связывающие белки).
Homeostasis. In all cells, there is approximately 10 times less in the cytosol outside the cell. This situation is supported by anion channels (Cl - passively passes into the cytosol), Na-/K-/Cl-cotransporter and Cl-HCO^-exchanger (Cl - enters the cell), as well as K-/Cl-cotransporter (K+ output and Cl - from the cell).
pH. To maintain pH, [HCO-3] and PCO 2 are also essential. The extracellular pH is 7.4 (with [HCO - 3 ] about 24 mM and PCO 2 about 40 mm Hg). At the same time, the intracellular pH value is 7.2 (shifted to the acidic side, while being the same on both sides of the membrane, and the calculated value of [HCO - 3 ] should be about 16 mM, while in reality it is 10 mM). Consequently, the cell must have systems that release H + from it or capture HCO - 3. Such systems include Na + - ^ exchanger, Na + -Cl - -HCO - 3 exchanger and Na + -HCO - 3 - cotransporter. All of these transport systems are sensitive to changes in pH: they are activated when the cytosol is acidified and blocked when the intracellular pH shifts to the alkaline side.
Water transport and cell volume maintenance
By definition, a semipermeable membrane itself (which is what a biological membrane is) is impermeable to water. Moreover, transmembrane water transport is always passive
a process (simple water diffusion occurs through aquaporin channels, but no special pumps for active water transport have been found), carried out through transmembrane pores and channels as part of other transporters and pumps. Nevertheless, the distribution of water between cellular compartments, the cytosol and cell organelles, between the cell and the interstitial fluid and its transport through biological membranes are of great importance for cell homeostasis (including the regulation of their volume). Flow of water through biological membranes(osmosis) determines the difference between osmotic and hydrostatic pressure on both sides of the membrane.
Osmosis- the flow of water through a semi-permeable membrane from a compartment with a lower concentration of substances dissolved in water into a compartment with a higher concentration. In other words, water flows from where its chemical potential (Δμ a) is higher to where its chemical potential is lower, since the presence of substances dissolved in water reduces the chemical potential of water.
Osmotic pressure(Fig. 2-9) is defined as the pressure of a solution that stops dilution with water through a semi-permeable membrane. Numerically, the osmotic pressure at equilibrium (water has ceased to penetrate through the semi-permeable membrane) is equal to the hydrostatic pressure.
Osmotic coefficient(Φ). The Φ value for electrolytes in physiological concentrations is usually less than 1 and as the solution is diluted, Φ approaches 1.
Osmolality. The terms “osmolality” and “osmolality” are non-systemic units. Osmol(osm) is the molecular mass of a solute in grams, divided by the number of ions or particles into which it dissociates in solution. Osmolality(osmotic concentration) is the degree of concentration of the solution, expressed in osmoles, and osmolality of solution(F ic) are expressed in osmoles per liter.
Osmoticity of solutions. Depending on the osmolality, solutions can be isosmotic, hyper- and hypo-osmotic (sometimes the not entirely correct term “tonic” is used, which is valid for the simplest case - for electrolytes). Assessment of the osmoticity of solutions (or cy-
Rice. 2-9. Osmotic pressure . A semi-permeable membrane separates compartments A (solution) and B (water). The osmotic pressure of the solution is measured in compartment A. The solution in compartment A is subject to hydrostatic pressure. When the osmotic and hydrostatic pressures are equal, equilibrium is established (water does not penetrate through the semi-permeable membrane). Osmotic pressure (π) is described by the Van't Hoff equation.
cytosol and interstitial fluid) makes sense only when comparing two solutions (for example, A&B, cytosol and interstitial fluid, infusion solutions and blood). In particular, regardless of the osmolality of two solutions, osmotic movement of water occurs between them until an equilibrium state is reached. This osmoticity is known as effective osmoticity(tonicity for electrolyte solution).
Isoosmotic solution A: osmotic pressure of solutions A and B the same.
Hypoosmotic solution A: less osmotic pressure of solution B. Hyperosmotic solution A: osmotic pressure of solution A more osmotic pressure of solution B.
Kinetics of water transport through the membrane is linear, unsaturated and is a function of the sum of the driving forces of transport (Δμ water, sum), namely the difference in chemical potential on both sides of the membrane (Δμ water a) and the difference in hydrostatic pressure (Δμ water pressure) on both sides of the membrane.
Osmotic swelling and osmotic shrinkage of cells. The state of cells when the osmoticity of the electrolyte solution in which the cells are suspended changes is discussed in Fig. 2-10.
Rice. 2-10. State of erythrocytes suspended in NaCl solution . The abscissa is the concentration (C) of NaCl (mM), the ordinate is the cell volume (V). At a NaCl concentration of 154 mM (308 mM osmotically active particles), the volume of cells is the same as in blood plasma (a solution of NaCl, C0, V0, isotonic to red blood cells). As the concentration of NaCl increases (hypertonic NaCl solution), water leaves the red blood cells and they shrink. When the concentration of NaCl decreases (hypotonic NaCl solution), water enters the red blood cells and they swell. When the solution is hypotonic, approximately 1.4 times greater than the value of an isotonic solution, membrane destruction occurs (lysis).
Regulation of cell volume. In Fig. 2-10 the simplest case is considered - a suspension of red blood cells in a NaCl solution. In this model experiment in vitro the following results were obtained: if the osmotic pressure of the NaCl solution increases, then water leaves the cells by osmosis, and the cells shrink; if the osmotic pressure of the NaCl solution decreases, water enters the cells and the cells swell. But the situation in vivo more difficult. In particular, the cells are not in a solution of a single electrolyte (NaCl), but in a real environment
many ions and molecules with different physical and chemical characteristics. Thus, the plasma membrane of cells is impermeable to many extra- and intracellular substances (for example, proteins); In addition, in the case considered above, the charge of the membrane was not taken into account. Conclusion. Below we summarize the data on the regulation of water distribution between compartments separated by a semipermeable membrane (including between cells and extracellular substance).
Since the cell contains negatively charged proteins that do not pass through the membrane, Donnan forces cause the cell to swell.
The cell responds to extracellular hyperosmolality by accumulating organic solutes.
The tonicity gradient (effective osmolality) ensures the osmotic flow of water across the membrane.
Infusion of isotonic saline and salt-free solutions (5% glucose), as well as administration of NaCI (equivalent to isotonic saline) increases the volume of intercellular fluid, but has different effects on cell volume and extracellular osmolality. In the examples below, all calculations are based on the following initial values: total body water - 42 l (60% of the body of a man weighing 70 kg), intracellular water - 25 l (60% of total water), extracellular water - 17 l (40% of total water). The osmolality of extracellular fluid and intracellular water is 290 mOsm.
Φ Isotonic saline solutions. Infusion of isotonic saline (0.9% NaCI) increases the volume of interstitial fluid but does not affect the volume of intracellular fluid.
Φ Isotonic salt-free solutions. Taking 1.5 liters of water or infusion of an isotonic salt-free solution (5% glucose) increases the volume of both intercellular and intracellular fluid.
Φ Sodium chloride. Introduction of NaCI (equivalent to isotonic saline) into the body increases the volume of intercellular water, but reduces the volume of intracellular water.
Membrane electrogenesis
The different concentrations of ions on both sides of the plasmalemma of all cells (see Table 2-1) leads to a transmembrane difference in electrical potential - Δμ - membrane potential (MP, or V m).
Membrane potential
resting MP- the difference in electrical potential between the inner and outer surfaces of the membrane at rest, i.e. in the absence of an electrical or chemical stimulus (signal). In the resting state, the polarization of the inner surface of the cell membrane has a negative value, therefore the value of the resting MF is also negative.
MP valuedepends significantly on the type of cells and their size. Thus, the resting MP of the plasmalemma of nerve cells and cardiomyocytes varies from -60 to -90 mV, the plasmalemma of the skeletal MV - -90 mV, the SMC - about -55 mV, and the erythrocytes - about -10 mV. Changes in the magnitude of MP are described in special terms: hyperpolarization(increase in MP value), depolarization(decrease in MP value), repolarization(increase in MP value after depolarization).
Nature of MPdetermined by transmembrane ion gradients (formed directly due to the state of ion channels, the activity of transporters, and indirectly due to the activity of pumps, primarily Na + -/K + -ATPase) and membrane conductivity.
Transmembrane ion current. The strength of the current (I) flowing through the membrane depends on the concentration of ions on both sides of the membrane, the MP and the permeability of the membrane for each ion.
If the membrane is permeable to K+, Na+, Cl - and other ions, their total ionic current is the sum of the ionic current of each ion:
I total = I K + + I Na+ + + I CI- + I X + + I X1 +... +I Xn.
Action potential (PD) is discussed in Chapter 5.
Transport membrane vesicles
Transport processes of the cell occur not only through the semi-permeable membrane, but also with the help of transport membrane vesicles that separate from the plasmalemma or merge with it, as well as separate from various intracellular membranes and merge with them (Fig. 2-11). With the help of such membrane vesicles, the cell absorbs water, ions, molecules and particles from the extracellular environment (endocytosis), releases secretory products (exocytosis) and carries out transport between organelles within the cell. All these processes are based on the exceptional ease with which, in the aqueous phase, the phospholipid bilayer of membranes releases (“unties”) such vesicles (liposomes, collectively called endosomes) into the cytosol and drains into the cytosol.
Rice. 2-11. Endocytosis (A) and exocytosis (B) . During endocytosis, a section of the plasma membrane invaginates and closes. An endocytic vesicle containing the absorbed particles is formed. During exocytosis, the membrane of transport or secretory vesicles fuses with the plasma membrane and the contents of the vesicles are released into the extracellular space. Special proteins are involved in membrane fusion.
with them. In a number of cases, membrane proteins have been identified that promote the fusion of phospholipid bilayers.
Endocytosis(endo- internal, inside + Greek. kytos- cell + Greek osis- state, process) - absorption (internalization) by the cell of substances, particles and microorganisms (Fig. 2-11, A). The variants of endocytosis are pinocytosis, receptor-mediated endocytosis and phagocytosis.
Φ Pinocytosis(Greek pino- drink + Greek kytos- cell + Greek osis- state, process) - the process of absorption of liquid and dissolved substances with the formation of small bubbles. Pinocytotic vesicles form in specialized areas of the plasma membrane - bordered pits (Fig. 2-12).
Φ Receptor-mediated endocytosis(see Fig. 2-12) is characterized by the absorption of specific macromolecules from the extracellular fluid. Process progress: binding of ligand and membrane receptor - concentration of the complex ligand-receptor on the surface of the bordered pit - immersion into a cell inside a bordered vesicle. Similarly, the cell absorbs transferrin, cholesterol along with LDL, and many other molecules.
Φ Phagocytosis(Greek phagein- eat, devour + Greek. kytos- cell + Greek osis- state, process) - absorption
Rice. 2-12. Receptor-mediated endocytosis . Many extracellular macromolecules (transferrin, LDL, viral particles, etc.) bind to their receptors in the plasmalemma. Clathrin-bordered pits are formed, and then bordered vesicles containing the ligand-receptor complex are formed. Bordered vesicles after release from clathrin are endosomes. Inside endosomes, the ligand is cleaved from the receptor.
large particles (for example, microorganisms or cell debris). Phagocytosis (Fig. 2-13) is carried out by special cells - phagocytes (macrophages, neutrophil leukocytes). During phagocytosis, large endocytic vesicles are formed - phagosomes. Phagosomes fuse with lysosomes to form phagolysosomes. Phagocytosis is induced by signals acting on receptors in the plasmalemma of phagocytes. Similar signals are provided by antibodies (also complement component C3b), which opsonize the phagocytosed particle (such phagocytosis is known as immune). Exocytosis(exo- external, out + Greek. kytos- cell + Greek osis- state, process), or secretion, is a process in which intracellular secretory vesicles (for example, synaptic) and secretory vesicles and granules merge with the plasmalemma, and their contents are released from the cell (see Fig. 2-11, B). The secretion process can be spontaneous and regulated.
Rice. 2-13. Phagocytosis . A bacterium coated with IgG molecules is effectively phagocytosed by a macrophage or neutrophil. Fab fragments of IgG bind to antigenic determinants on the surface of the bacterium, after which the same IgG molecules, with their Fc fragments, interact with Fc fragment receptors located in the plasma membrane of the phagocyte and activate phagocytosis.
Chapter Summary
The plasma membrane consists of proteins located between two layers of phospholipids. Integral proteins are immersed in the thickness of the lipid bilayer or penetrate the membrane through. Peripheral proteins are attached to the outer surface of cells.
The passive movement of solutes through the membrane is determined by their gradient and reaches equilibrium at the moment when the movement of dissolved particles stops.
Simple diffusion is the passage of fat-soluble substances across the plasma membrane by diffusion between the lipid bilayer.
Facilitated diffusion is the passage of water-soluble substances and ions through hydrophilic pathways created by integral proteins built into the membrane. The passage of small ions is mediated by specific ion channel proteins.
Active transport is the use of metabolic energy to move dissolved particles against their concentration gradients.
The rapid passage of water across plasma membranes occurs through channel proteins, so-called aquaporins. Water movement is a passive process, activated by differences in osmotic pressure.
Cells regulate their volume by moving dissolved particles in or out, creating an osmotic pull for water to enter or exit, respectively.
The resting membrane potential is determined by the passive movement of ions through constantly open channels. In a muscle cell, for example, the permeability of the membrane for sodium ions is lower compared to potassium ions, and the resting membrane potential is created by the passive release of potassium ions from the cell.
Transport membrane vesicles are the main means of transporting proteins and lipids within the cell.
The most important functions of membranes: membranes control the composition of the intracellular environment, provide and facilitate intercellular and intracellular transmission of information, and ensure the formation of tissues through intercellular contacts.
Organelles (organelles) of a cell are permanent parts of the cell that have a specific structure and perform specific functions. There are membrane and non-membrane organelles. TO membrane organelles include the cytoplasmic reticulum (endoplasmic reticulum), lamellar complex (Golgi apparatus), mitochondria, lysosomes, peroxisomes. Non-membrane organelles represented by ribosomes (polyribosomes), the cell center and cytoskeletal elements: microtubules and fibrillar structures.
Rice. 8.Diagram of the ultramicroscopic structure of a cell:
1 – granular endoplasmic reticulum, on the membranes of which attached ribosomes are located; 2 – agranular endoplasmic reticulum; 3 – Golgi complex; 4 – mitochondria; 5 – developing phagosome; 6 – primary lysosome (storage granule); 7 – phagolysosome; 8 – endocytic vesicles; 9 – secondary lysosome; 10 – residual body; 11 – peroxisome; 12 – microtubules; 13 - microfilaments; 14 – centrioles; 15 – free ribosomes; 16 – transport bubbles; 17 – exocytotic vesicle; 18 – fatty inclusions (lipid drop); 19 - glycogen inclusions; 20 – karyolemma (nuclear membrane); 21 – nuclear pores; 22 – nucleolus; 23 – heterochromatin; 24 – euchromatin; 25 – basal body of the cilium; 26 - eyelash; 27 – special intercellular contact (desmosome); 28 – gap intercellular contact
2.5.2.1. Membrane organelles (organelles)
The endoplasmic reticulum (endoplasmic reticulum, cytoplasmic reticulum) is a set of interconnected tubules, vacuoles and “cisterns”, the wall of which is formed by elementary biological membranes. Opened by K.R. Porter in 1945. The discovery and description of the endoplasmic reticulum (ER) is due to the introduction of the electron microscope into the practice of cytological studies. The membranes that form the EPS differ from the plasmalemma of the cell in their smaller thickness (5-7 nm) and higher concentration of proteins, primarily those with enzymatic activity . There are two types of EPS(Fig. 8): rough (granular) and smooth (agranular). Rough XPS It is represented by flattened cisterns, on the surface of which ribosomes and polysomes are located. The membranes of granular ER contain proteins that promote the binding of ribosomes and flattening of the cisterns. The rough ER is especially well developed in cells specialized in protein synthesis. The smooth ER is formed by intertwining tubules, tubes and small vesicles. The channels and tanks of the EPS of these two types are not differentiated: membranes of one type pass into membranes of another type, forming the so-calledtransitional (transient) EPS.
Mainfunctions of granular EPS are:
1) synthesis of proteins on attached ribosomes(secreted proteins, proteins of cell membranes and specific proteins of the contents of membrane organelles); 2) hydroxylation, sulfation, phosphorylation and glycosylation of proteins; 3) transport of substances within the cytoplasm; 4) accumulation of both synthesized and transported substances; 5) regulation of biochemical reactions, associated with the orderly localization in the structures of EPS of substances that enter into reactions, as well as their catalysts - enzymes.
Smooth XPS It is distinguished by the absence of proteins (ribophorins) on the membranes that bind ribosomal subunits. It is assumed that smooth ER is formed as a result of the formation of outgrowths of rough ER, the membrane of which loses ribosomes.
Functions of smooth EPS are: 1) lipid synthesis, including membrane lipids; 2) synthesis of carbohydrates(glycogen, etc.); 3) cholesterol synthesis; 4) neutralization of toxic substances endogenous and exogenous origin; 5) accumulation of Ca ions 2+ ; 6) restoration of the karyolemma in telophase of mitosis; 7) transport of substances; 8) accumulation of substances.
As a rule, smooth ER is less developed in cells than rough ER, but it is much better developed in cells that produce steroids, triglycerides and cholesterol, as well as in liver cells that detoxify various substances.
Rice. 9. Golgi complex:
1 – stack of flattened tanks; 2 – bubbles; 3 – secretory vesicles (vacuoles)
Transitional (transient) EPS - this is the site of transition of granular ER into agranular ER, which is located at the forming surface of the Golgi complex. The tubes and tubules of the transitional ER disintegrate into fragments, from which vesicles are formed that transport material from the ER to the Golgi complex.
The lamellar complex (Golgi complex, Golgi apparatus) is a cell organelle involved in the final formation of its metabolic products.(secrets, collagen, glycogen, lipids and other products),as well as in the synthesis of glycoproteins. The organoid is named after the Italian histologist C. Golgi, who described it in 1898. Formed by three components(Fig. 9): 1) a stack of flattened tanks (sacs); 2) bubbles; 3) secretory vesicles (vacuoles). The zone of accumulation of these elements is called dictyosomes. There may be several such zones in a cell (sometimes several dozen or even hundreds). The Golgi complex is located near the cell nucleus, often near the centrioles, and less often scattered throughout the cytoplasm. In secretory cells, it is located in the apical part of the cell, through which secretion is released by exocytosis. From 3 to 30 cisterns in the form of curved disks with a diameter of 0.5-5 microns form a stack. Adjacent tanks are separated by spaces of 15-30 nm. Separate groups of cisternae within the dictyosome are distinguished by a special composition of enzymes that determine the nature of biochemical reactions, in particular protein processing, etc.
The second constituent element of the dictyosome is vesicles They are spherical formations with a diameter of 40-80 nm, the moderately dense contents of which are surrounded by a membrane. Bubbles are formed by splitting off from the tanks.
The third element of the dictyosome is secretory vesicles (vacuoles) They are relatively large (0.1-1.0 μm) spherical membrane formations containing a secretion of moderate density that undergoes condensation and compaction (condensation vacuoles).
The Golgi complex is clearly vertically polarized. It contains two surfaces (two poles):
1) cis-surface, or an immature surface that has a convex shape, faces the endoplasmic reticulum (nucleus) and is associated with small transport vesicles separating from it;
2) trans-surface, or the surface facing the concave plasmolemma (Fig. 8), on the side of which vacuoles (secretory granules) are separated from the cisterns of the Golgi complex.
Mainfunctions of the Golgi complex are: 1) synthesis of glycoproteins and polysaccharides; 2) modification of the primary secretion, its condensation and packaging into membrane vesicles (formation of secretory granules); 3) molecular processing(phosphorylation, sulfation, acylation, etc.); 4) accumulation of substances secreted by the cell; 5) formation of lysosomes; 6) sorting of proteins synthesized by the cell at the trans-surface before their final transport (produced through receptor proteins that recognize the signal regions of macromolecules and direct them to various vesicles); 7) transport of substances: From transport vesicles, substances penetrate into the stack of cisterns of the Golgi complex from the cis surface, and exit it in the form of vacuoles from the trans surface. The mechanism of transport is explained by two models: a) a model for the movement of vesicles budding from the previous cistern and merging with the subsequent cistern sequentially in the direction from the cis surface to the trans surface; b) a model of cisternae movement, based on the idea of continuous new formation of cisternae due to the fusion of vesicles on the cis surface and subsequent disintegration into vacuoles of cisternae moving toward the trans surface.
The above main functions allow us to state that the lamellar complex is the most important organelle of the eukaryotic cell, ensuring the organization and integration of intracellular metabolism. In this organelle, the final stages of formation, maturation, sorting and packaging of all products secreted by the cell, lysosome enzymes, as well as proteins and glycoproteins of the cell surface apparatus and other substances take place.
Organelles of intracellular digestion. Lysosomes are small vesicles bounded by an elementary membrane containing hydrolytic enzymes. The lysosome membrane, about 6 nm thick, performs passive compartmentalization, temporarily separating hydrolytic enzymes (more than 30 varieties) from the hyaloplasm. In an intact state, the membrane is resistant to the action of hydrolytic enzymes and prevents their leakage into the hyaloplasm. Corticosteroid hormones play an important role in membrane stabilization. Damage to lysosome membranes leads to self-digestion of the cell by hydrolytic enzymes.
The lysosome membrane contains an ATP-dependent proton pump, ensuring acidification of the environment inside the lysosomes. The latter promotes the activation of lysosome enzymes - acid hydrolases. Along with the the lysosome membrane contains receptors that determine the binding of lysosomes to transport vesicles and phagosomes. The membrane also ensures the diffusion of substances from lysosomes into the hyaloplasm. The binding of some hydrolase molecules to the lysosome membrane leads to their inactivation.
There are several types of lysosomes:primary lysosomes (hydrolase vesicles), secondary lysosomes (phagolysosomes, or digestive vacuoles), endosomes, phagosomes, autophagolysosomes, residual bodies(Fig. 8).
Endosomes are membrane vesicles that transport macromolecules from the cell surface to lysosomes by endocytosis. During the transfer process, the contents of endosomes may not change or undergo partial cleavage. In the latter case, hydrolases penetrate into the endosomes or the endosomes directly merge with hydrolase vesicles, as a result of which the medium gradually becomes acidified. Endosomes are divided into two groups: early (peripheral) And late (perinuclear) endosomes.
Early (peripheral) endosomes are formed in the early stages of endocytosis after the separation of vesicles with captured contents from the plasmalemma. They are located in the peripheral layers of the cytoplasm and characterized by a neutral or slightly alkaline environment. In them, ligands are separated from receptors, ligands are sorted, and, possibly, receptors are returned in special vesicles to the plasmalemma. Along with the in early endosomes, cleavage of com-
Rice. 10 (A). Scheme of the formation of lysosomes and their participation in intracellular digestion.(B)Electron micrograph of a section of secondary lysosomes (indicated by arrows):
1 – formation of small vesicles with enzymes from the granular endoplasmic reticulum; 2 – transfer of enzymes to the Golgi apparatus; 3 – formation of primary lysosomes; 4 – isolation and use of (5) hydrolases during extracellular cleavage; 6 - phagosomes; 7 – fusion of primary lysosomes with phagosomes; 8, 9 – formation of secondary lysosomes (phagolysosomes); 10 – excretion of residual bodies; 11 – fusion of primary lysosomes with collapsing cell structures; 12 – autophagolysosome
complexes “receptor-hormone”, “antigen-antibody”, limited cleavage of antigens, inactivation of individual molecules. Under acidic conditions (pH=6.0) the environment in early endosomes, partial breakdown of macromolecules may occur. Gradually, moving deeper into the cytoplasm, early endosomes turn into late (perinuclear) endosomes located in the deep layers of the cytoplasm, surrounding the core. They reach 0.6-0.8 microns in diameter and differ from early endosomes in their more acidic (pH = 5.5) contents and a higher level of enzymatic digestion of the contents.
Phagosomes (heterophagosomes) are membrane vesicles that contain material captured by the cell from outside, subject to intracellular digestion.
Primary lysosomes (hydrolase vesicles) - vesicles with a diameter of 0.2-0.5 microns containing inactive enzymes (Fig. 10). Their movement in the cytoplasm is controlled by microtubules. Hydrolase vesicles transport hydrolytic enzymes from the lamellar complex to the organelles of the endocytic pathway (phagosomes, endosomes, etc.).
Secondary lysosomes (phagolysosomes, digestive vacuoles) are vesicles in which intracellular digestion is actively carried out through hydrolases at pH≤5. Their diameter reaches 0.5-2 microns. Secondary lysosomes (phagolysosomes and autophagolysosomes) formed by fusion of a phagosome with an endosome or primary lysosome (phagolysosome) or by fusion of an autophagosome(membrane vesicle containing the cell's own components) with primary lysosome(Fig. 10) or late endosome (autophagolysosome). Autophagy ensures the digestion of areas of the cytoplasm, mitochondria, ribosomes, membrane fragments, etc. The loss of the latter in the cell is compensated by their new formation, which leads to renewal (“rejuvenation”) of cellular structures. Thus, in human nerve cells, which function for many decades, most organelles are renewed within 1 month.
A type of lysosome containing undigested substances (structures) is called residual bodies. The latter can remain in the cytoplasm for a long time or release their contents through exocytosis outside the cell.(Fig. 10). A common type of residual bodies in the body of animals are lipofuscin granules, which are membrane vesicles (0.3-3 µm) containing the sparingly soluble brown pigment lipofuscin.
Peroxisomes are membrane vesicles with a diameter of up to 1.5 µm, the matrix of which contains about 15 enzymes(Fig. 8). Among the latter, the most important catalase, which accounts for up to 40% of the total protein of the organelle, as well as peroxidase, amino acid oxidase, etc. Peroxisomes are formed in the endoplasmic reticulum and are renewed every 5-6 days. Along with mitochondria, peroxisomes are an important center for oxygen utilization in the cell. In particular, under the influence of catalase, hydrogen peroxide (H 2 O 2), formed during the oxidation of amino acids, carbohydrates and other cellular substances, breaks down. Thus, peroxisomes protect the cell from the damaging effects of hydrogen peroxide.
Organelles of energy metabolism. Mitochondria first described by R. Kölliker in 1850 in the muscles of insects called sarcosomes. They were later studied and described by R. Altman in 1894 as "bioplasts", and in 1897 by K. Benda called them mitochondria. Mitochondria are membrane-bound organelles that provide the cell (organism) with energy. The source of energy stored in the form of phosphate bonds of ATP is oxidation processes. Along with the Mitochondria are involved in the biosynthesis of steroids and nucleic acids, as well as in the oxidation of fatty acids.
M Rice. eleven.
Mitochondria structure diagram: 1 – outer membrane; 2 – internal membrane; 3 – cristae; 4 – matrix
Itochondria have elliptical, spherical, rod-shaped, thread-like and other shapes that can change over a certain time. Their dimensions are 0.2-2 microns in width and 2-10 microns in length. The number of mitochondria in different cells varies widely, reaching 500-1000 in the most active ones. In liver cells (hepatocytes), their number is about 800, and the volume they occupy is approximately 20% of the volume of the cytoplasm. In the cytoplasm, mitochondria can be located diffusely, but they are usually concentrated in areas of maximum energy consumption, for example, near ion pumps, contractile elements (myofibrils), and organelles of movement (sperm axoneme). Mitochondria consist of outer and inner membranes,
separated by intermembrane space,and contain a mitochondrial matrix into which folds of the inner membrane - cristae - face
(Fig. 11, 12).
N
Rice. 12.
Electron photograph of mitochondria (cross section)
The intermembrane space of mitochondria, 10-20 nm wide, contains a small amount of enzymes. It is limited from the inside by the inner mitochondrial membrane, which contains transport proteins, respiratory chain enzymes and succinate dehydrogenase, as well as an ATP synthetase complex. The inner membrane is characterized by low permeability to small ions. It forms folds 20 nm thick, which are most often located perpendicular to the longitudinal axis of mitochondria, and in some cases (muscle and other cells) - longitudinally. With increasing mitochondrial activity, the number of folds (their total area) increases. On the cristae areoxisomes - mushroom-shaped formations consisting of a rounded head with a diameter of 9 nm and a stalk 3 nm thick. ATP synthesis occurs in the head region. The processes of oxidation and ATP synthesis in mitochondria are separated, which is why not all the energy is accumulated in ATP, being partially dissipated in the form of heat. This separation is most pronounced, for example, in brown adipose tissue, which is used for the spring “warming up” of animals that were in a state of “hibernation.”
The inner chamber of the mitochondrion (the area between the inner membrane and the cristae) is filledmatrix (Fig. 11, 12), containing Krebs cycle enzymes, protein synthesis enzymes, fatty acid oxidation enzymes, mitochondrial DNA, ribosomes and mitochondrial granules.
Mitochondrial DNA represents the mitochondria's own genetic apparatus. It has the appearance of a circular double-stranded molecule, which contains about 37 genes. Mitochondrial DNA differs from nuclear DNA in its low content of non-coding sequences and the absence of connections with histones. Mitochondrial DNA encodes mRNA, tRNA and rRNA, but provides the synthesis of only 5-6% of mitochondrial proteins(enzymes of the ion transport system and some enzymes of ATP synthesis). The synthesis of all other proteins, as well as the duplication of mitochondria, is controlled by nuclear DNA. Most of the mitochondrial ribosomal proteins are synthesized in the cytoplasm and then transported to the mitochondria. Inheritance of mitochondrial DNA in many species of eukaryotes, including humans, occurs only through the maternal line: the paternal mitochondrial DNA disappears during gametogenesis and fertilization.
Mitochondria have a relatively short life cycle (about 10 days). Their destruction occurs through autophagy, and new formation occurs through division (ligation) preceding mitochondria. The latter is preceded by mitochondrial DNA replication, which occurs independently of nuclear DNA replication at any phase of the cell cycle.
Prokaryotes do not have mitochondria, and their functions are performed by the cell membrane. According to one hypothesis, mitochondria originated from aerobic bacteria as a result of symbiogenesis. There is an assumption about the participation of mitochondria in the transmission of hereditary information.
Cell. Structure of a plant cell
A cell is a living biological system that underlies the structure, development and functioning of all living organisms. This is a biologically autonomous system, which is characterized by all life processes: growth, development, nutrition, respiration, oxygen, reproduction, etc. The cellular structure of plants and animals was discovered in 1665 by the English scientist Robert Hooke. The shape and structure of cells are very diverse. There are:
1) parenchyma cells - their length is equal to width;
2) prosenchymal cells - the length of these cells exceeds the width.
Young plant cells are covered cytoplasmic membrane(CPM). It consists of a double layer of lipids and protein molecules. Some of the proteins lie mosaically on both sides of the membrane, forming enzyme systems. Other proteins penetrate the lipid layers to form pores. CPMs provide structure to all cell organelles and the nucleus; limit the cytoplasm from the cell membrane and vacuole; have selective permeability; ensure the exchange of substances and energy with the external environment.
Hyaloplasm is a colorless, optically transparent colloidal system that unites all cellular structures that perform various functions. Cytoplasm is the substrate of life for all cell organelles. This is the living contents of the cell. It is characterized by signs: movement, growth, nutrition, breathing, etc.
The composition of the cytoplasm includes: water 75-85%, proteins 10-20%, fats 2-3%, inorganic substances 1%.
Membrane organelles of plant cells
Membranes inside the cytoplasm form the endoplasmic reticulum (ER) - a system of small vacuoles and tubules connected to each other. The granular ER carries ribosomes, while the smooth ER lacks them. The ER ensures the transport of substances within the cell and between neighboring cells. Granular EPS is involved in protein synthesis. In the EPS channels, protein molecules acquire secondary, tertiary, quaternary structures, fats are synthesized, and ATP is transported.
Mitochondria- most often elliptical or round organelles up to 1 micron. Covered with a double membrane. The inner membrane forms projections - cristae. The mitochondrial matrix contains redox enzymes, ribosomes, RNA, and circular DNA. This is the respiratory and energy center of the cell. In the mitochondrial matrix, organic substances are broken down and energy is released, which is used for the synthesis of ATP (on the cristae).
Golgi complex is a system of flat, arched, parallel tanks, bounded by a central compressor station. Vesicles are separated from the edges of the cisterns, transporting polysaccharides formed in the Golgi complex. They are involved in the construction of the cell wall. Products of synthesis and breakdown of substances accumulate in the tanks, they are used by the cell or removed outside.
Plastids- depending on the presence of certain pigments, three types of plastids are distinguished: chloroplasts, chromoplasts, leucoplasts.
Chloroplasts are oval, 4-10 microns in size, double-membrane organelles of all green parts of the plant. The inner membrane forms projections - thylakoids, groups of which form grana (like a stack of coins). Thylakoids lie in the stroma and unite the grana with each other. On the inner surface of thylakoids there is a green pigment - chlorophyll. The stroma of chloroplasts contains enzymes, ribosomes, and its own DNA. The main function of chloroplasts is photosynthesis (the formation of carbohydrates from CO2 and H2O, minerals using solar energy), as well as the synthesis of ATP, ADP, the synthesis of assimilative starch, and its own proteins. In addition to chlorophyll, chloroplasts contain auxiliary pigments - carotenoids.
Chromoplasts - colored plastids - varied in shape; painted red, yellow, orange. Contain pigments - carotene (orange), xanthophyll (yellow). They give flower petals a color that attracts pollinating insects; color the fruits, facilitating their distribution by animals. They are rich in rose hips, currants, tomatoes, carrot roots, marigold petals, etc.
Leucoplasts are small plastids, round in shape, colorless. They serve as a site for the deposition of reserve nutrients: starch, proteins, forming starch and aleurone grains. Contained in fruits, roots, rhizomes. Plastids are capable of interconversion: leucoplasts turn into chloroplasts in the light (greening of potato tubers), chromoplasts turn into chloroplasts (greening of carrot roots in the light during growth).
Plants and fungi are composed of three main parts: the plasma membrane, the nucleus and the cytoplasm. Bacteria differ from them in that they do not have a nucleus, but they also have a membrane and cytoplasm.
How is the cytoplasm structured?
This is the inner part of the cell in which hyaloplasm (liquid medium), inclusions and inclusions can be distinguished - these are non-permanent formations in the cell, which are basically drops or crystals of reserve nutrients. Organelles are permanent structures. Just as in the body the main functional units are organs, so in a cell all the main functions are performed by organelles.
Membrane and non-membrane cell organelles
The former are divided into single-membrane and double-membrane. The last two are mitochondria and chloroplasts. Single-membrane cells include lysosomes, Golgi complex, reticulum), and vacuoles. We will talk more about non-membrane organelles in this article.
Cell organelles of non-membrane structure
These include ribosomes, the cell center, as well as the cytoskeleton formed by microtubules and microfilaments. Also included in this group are the organelles of movement possessed by unicellular organisms, as well as male reproductive cells of animals. Let's look in order at the non-membrane cell organelles, their structure and functions.
What are ribosomes?
These are cells that consist of ribonucleoproteins. Their structure includes two parts (subunits). One of them is small, one is large. In a calm state they are separated. They connect when the ribosome begins to function.
These non-membrane cell organelles are responsible for protein synthesis. Namely, for the process of translation - the connection of amino acids into a polypeptide chain in a certain order, information about which is copied from DNA and recorded on mRNA.
The size of ribosomes is twenty nanometers. The number of these organelles in a cell can reach up to several tens of thousands.
In eukaryotes, ribosomes are found both in the hyaloplasm and on the surface of the rough endoplasmic reticulum. They are also present inside double-membrane organelles: mitochondria and chloroplasts.
Cell center
This organelle consists of a centrosome, which is surrounded by a centrosphere. The centrosome is represented by two centrioles - empty inside cylinders consisting of microtubules. The centrosphere consists of microtubules extending radially from the cell center. It also contains intermediate filaments and microfibrils.
The cell center performs functions such as the formation of a division spindle. It is also the center of microtubule organization.
As for the chemical structure of this organelle, the main substance is the protein tubulin.
This organelle is located in the geometric center of the cell, which is why it has this name.
Microfilaments and microtubules
The first are filaments of the protein actin. Their diameter is 6 nanometers.
The diameter of microtubules is 24 nanometers. Their walls are made of the protein tubulin.
These nonmembrane cell organelles form a cytoskeleton that helps maintain a constant shape.
Another function of microtubules is transport; organelles and substances in the cell can move along them.
Locomotion organoids
They come in two types: cilia and flagella.
The first are unicellular organisms such as slipper ciliates.
Chlamydomonas have flagella, as well as animal sperm.
Locomotion organelles consist of contractile proteins.
Conclusion
As a conclusion, we provide generalized information.
| Organoid | Location in the cage | Structure | Functions |
| Ribosomes | They float freely in the hyaloplasm and are also located on the outer side of the walls of the rough endoplasmic reticulum | Consist of small and large parts. Chemical composition - ribonucleoproteins. | Protein synthesis |
| Cell center | Geometric center of the cell | Two centrioles (cylinders of microtubules) and a centrosphere - radially extending microtubules. | Spindle formation, microtubule organization |
| Microfilaments | In the cytoplasm of the cell | Thin filaments made from the contractile protein actin | Creating support, sometimes providing movement (for example, in amoebas) |
| Microtubules | In the cytoplasm | Hollow tubulin tubes | Creation of support, transport of cell elements |
| Cilia and flagella | From the outside of the plasma membrane | Made up of proteins | Movement of a single-celled organism in space |
So we looked at all the non-membrane organelles of plants, animals, fungi and bacteria, their structure and functions.
