Cell is the smallest and basic structural unit of living organisms, capable of self-renewal, self-regulation and self-reproduction.
Characteristic cell sizes: bacterial cells - from 0.1 to 15 microns, cells of other organisms - from 1 to 100 microns, sometimes reaching 1-10 mm; eggs of large birds - up to 10-20 cm, processes of nerve cells - up to 1 m.
Cell shape very diverse: there are spherical cells (cocci), chain (streptococci), elongated (rods or bacilli), curved (vibrios), crimped (spirilla), multifaceted, with motor flagella, etc.
Types of cells: prokaryotic(non-nuclear) and eukaryotic (having a formed nucleus).
Eukaryotic cells, in turn, are divided into cells animals, plants and fungi.
Structural organization of a eukaryotic cell
Protoplast- this is all the living contents of the cell. The protoplast of all eukaryotic cells consists of a cytoplasm (with all organelles) and a nucleus.
Cytoplasm- this is the internal contents of the cell, with the exception of the nucleus, consisting of hyaloplasm, organelles immersed in it and (in some types of cells) intracellular inclusions (reserve nutrients and/or end products of metabolism).
Hyaloplasma- basic plasma, cytoplasmic matrix, the main substance that is the internal environment of the cell and is a viscous colorless colloidal solution (water content up to 85%) of various substances: proteins (10%), sugars, organic and inorganic acids, amino acids, polysaccharides, RNA, lipids, mineral salts, etc.
■ Hyaloplasm is a medium for intracellular metabolic reactions and a connecting link between cell organelles; it is capable of reversible transitions from sol to gel; its composition determines the buffering and osmotic properties of the cell. The cytoplasm contains a cytoskeleton consisting of microtubules and contractile protein filaments.
■ The cytoskeleton determines the shape of the cell and is involved in the intracellular movement of organelles and individual substances. The nucleus is the largest organelle of a eukaryotic cell, containing chromosomes in which all hereditary information is stored (see below for more details).
Structural components of a eukaryotic cell:
■ plasmalemma (plasma membrane),
■ cell wall (only in plant and fungal cells),
■ biological (elementary) membranes,
■ core,
■ endoplasmic reticulum (endoplasmic reticulum),
■ mitochondria,
■ Golgi complex,
■ chloroplasts (only in plant cells),
■ lysosomes, s
■ ribosomes,
■ cell center,
■ vacuoles (only in plant and fungal cells),
■ microtubules,
■ cilia, flagella.
Schemes of the structure of animal and plant cells are given below:
Biological (elementary) membranes- These are active molecular complexes that separate intracellular organelles and cells. All membranes have a similar structure.
Structure and composition of membranes: thickness 6-10 nm; consist mainly of protein molecules and phospholipids.
■Phospholipids form a double (bimolecular) layer in which their molecules face their hydrophilic (water-soluble) ends outward and their hydrophobic (water-insoluble) ends inward of the membrane.
■ Protein molecules located on both surfaces of the lipid bilayer ( peripheral proteins), penetrate both layers of lipid molecules ( integral proteins, most of which are enzymes) or only one layer of them (semi-integral proteins).
Membrane properties: plasticity, asymmetry(the composition of the outer and inner layers of both lipids and proteins is different), polarity (the outer layer is positively charged, the inner one is negatively charged), the ability to self-close, selective permeability (in this case, hydrophobic substances pass through the lipid bilayer, and hydrophilic ones pass through the pores in integral proteins ).
Membrane functions: barrier (separates the contents of an organoid or cell from the environment), structural (provides a certain shape, size and stability of the organoid or cell), transport (ensures the transport of substances into and out of the organoid or cell), catalytic (ensures near-membrane biochemical processes), regulatory ( participates in the regulation of metabolism and energy between the organelle or cell and the external environment), participates in the conversion of energy and the maintenance of transmembrane electrical potential.
Plasma membrane (plasmalemma)
Plasma membrane, or plasmalemma, is a biological membrane or a complex of biological membranes tightly adjacent to each other, covering the cell from the outside.
The structure, properties and functions of the plasmalemma are basically the same as those of elementary biological membranes.
❖ Structural features:
■ the outer surface of the plasma membrane contains the glycocalyx - a polysaccharide layer of glycolipoid and glycoprotein molecules that serve as receptors for “recognition” of certain chemicals; in animal cells it can be covered with mucus or chitin, and in plant cells - with cellulose or pectin substances;
■ usually the plasmalemma forms projections, invaginations, folds, microvilli, etc., increasing the surface of the cell.
■ Additional functions: receptor (participates in the “recognition” of substances and in the perception of signals from the environment and transmitting them to the cell), ensuring communication between cells in the tissues of a multicellular organism, participation in the construction of special cell structures (flagella, cilia, etc.).
Cell wall (envelope)
Cell wall is a rigid structure located outside the plasmalemma and representing the outer cover of the cell. Present in prokaryotic cells and cells of fungi and plants.
❖Cell wall composition: cellulose in plant cells and chitin in fungal cells (structural components), proteins, pectins (which are involved in the formation of plates that hold together the walls of two neighboring cells), lignin (which holds cellulose fibers together into a very strong frame), suberin (deposited on the shell from the inside and makes it is practically impermeable to water and solutions), etc. The outer surface of the cell wall of epidermal plant cells contains a large amount of calcium carbonate and silica (mineralization) and is covered with hydrophobic substances, waxes and cuticle (a layer of the substance cutin, permeated with cellulose and pectins).
❖ Functions of the cell wall: serves as an external frame, maintains cell turgor, performs protective and transport functions.
Cell organelles
Organelles (or organelles)- These are permanent, highly specialized intracellular structures that have a specific structure and perform corresponding functions.
❖ By purpose
organelles are divided into:
■ general purpose organelles (mitochondria, Golgi complex, endoplasmic reticulum, ribosomes, centrioles, lysosomes, plastids) and
■ organelles for special purposes (myofibrils, flagella, cilia, vacuoles).
❖ By the presence of a membrane
organelles are divided into:
■ double-membrane (mitochondria, plastids, cell nucleus),
■ single-membrane (endoplasmic reticulum, Golgi complex, lysosomes, vacuoles) and
■ non-membrane (ribosomes, cell center).
The internal contents of membrane organelles always differ from the hyaloplasm surrounding them.
Mitochondria- double-membrane organelles of eukaryotic cells that carry out the oxidation of organic substances to final products with the release of energy stored in ATP molecules.
■ Structure: rod-shaped, spherical and thread-like shapes, thickness 0.5-1 µm, length 2-7 µm; double-membrane, the outer membrane is smooth and has high permeability, the inner membrane forms folds - cristae, on which there are spherical bodies - ATP-somes. Hydrogen ions 11, which are involved in oxygen respiration, accumulate in the space between the membranes.
■ Internal contents (matrix): ribosomes, circular DNA, RNA, amino acids, proteins, Krebs cycle enzymes, tissue respiration enzymes (located on the cristae).
■ Functions: oxidation of substances to CO 2 and H 2 O; synthesis of ATP and specific proteins; the formation of new mitochondria as a result of fission in two.
Plastids(available only in plant cells and autotrophic protists).
■ Types of plastids: chloroplasts (green), leucoplasts (colorless, round in shape), chromoplasts (yellow or orange); plastids can change from one type to another.
■Structure of chloroplasts: they are double-membrane, round or oval in shape, length 4-12 µm, thickness 1-4 µm. The outer membrane is smooth, the inner membrane has thylakoids - folds forming closed disc-shaped invaginations, between which there is stroma (see below). In higher plants, thylakoids are collected in stacks (like a column of coins) grains , which are connected to each other lamellae (single membranes).
■ Chloroplast composition: in the membranes of thylakoids and grana - grains of chlorophyll and other pigments; internal contents (stroma): proteins, lipids, ribosomes, circular DNA, RNA, enzymes involved in CO 2 fixation, storage substances.
■Functions of plastids: photosynthesis (chloroplasts contained in the green organs of plants), synthesis of specific proteins and accumulation of reserve nutrients: starch, proteins, fats (leukoplasts), imparting color to plant tissues in order to attract pollinating insects and distributors of fruits and seeds (chromoplasts).
Endoplasmic reticulum (EPS), or endoplasmic reticulum, found in all eukaryotic cells.
■Structure: is a system of interconnected tubules, tubes, cisterns and cavities of various shapes and sizes, the walls of which are formed by elementary (single) biological membranes. There are two types of EPS: granular (or rough), containing ribosomes on the surface of channels and cavities, and agranular (or smooth), not containing ribosomes.
■Functions: division of the cell cytoplasm into compartments that prevent the mixing of the chemical processes occurring in them; rough ER accumulates, isolates for maturation and transports proteins synthesized by ribosomes on its surface, synthesizes cell membranes; smooth EPS synthesizes and transports lipids, complex carbohydrates and steroid hormones, removes toxic substances from the cell.
Golgi complex (or apparatus) - a membrane organelle of a eukaryotic cell, located near the cell nucleus, which is a system of cisterns and vesicles and is involved in the accumulation, storage and transportation of substances, the construction of the cell membrane and the formation of lysosomes.
■Structure: the complex is a dictyosome - a stack of membrane-bound flat disc-shaped sacs (cisterns), from which vesicles bud, and a system of membrane tubules connecting the complex with the channels and cavities of the smooth ER.
■Functions: the formation of lysosomes, vacuoles, plasmalemma and the cell wall of a plant cell (after its division), the secretion of a number of complex organic substances (pectin substances, cellulose, etc. in plants; glycoproteins, glycolipids, collagen, milk proteins, bile, a number of hormones, etc. animals); accumulation and dehydration of lipids transported along the EPS (from smooth EPS), modification and accumulation of proteins (from granular EPS and free ribosomes of the cytoplasm) and carbohydrates, removal of substances from the cell.
Mature dictyosome cisternae lacing vesicles (Golgi vacuoles), filled with secretion, which is then either used by the cell itself or removed beyond its boundaries.
❖ Lysosomes- cellular organelles that ensure the breakdown of complex molecules of organic substances; are formed from vesicles separated from the Golgi complex or smooth ER and are present in all eukaryotic cells.
■ Structure and composition: lysosomes are small single-membrane round vesicles with a diameter of 0.2-2 µm; filled with hydrolytic (digestive) enzymes (~40), capable of breaking down proteins (to amino acids), lipids (to glycerol and higher carboxylic acids), polysaccharides (to monosaccharides) and nucleic acids (to nucleotides).
Merging with endocytic vesicles, lysosomes form a digestive vacuole (or secondary lysosome), where the breakdown of complex organic substances occurs; the resulting monomers enter the cell cytoplasm through the membrane of the secondary lysosome, and undigested (non-hydrolyzed) substances remain in the secondary lysosome and then, as a rule, are excreted outside the cell.
■ Functions: heterophagy- breakdown of foreign substances that enter the cell through endocytosis, autophagy - destruction of structures unnecessary for the cell; autolysis is the self-destruction of a cell that occurs as a result of the release of the contents of lysosomes during cell death or degeneration.
❖ Vacuoles- large vesicles or cavities in the cytoplasm that form in the cells of plants, fungi and many protists and bounded by an elementary membrane - the tonoplast.
■ Vacuoles protists are divided into digestive and contractile (having bundles of elastic fibers in the membranes and serving for osmotic regulation of the cell’s water balance).
■Vacuoles plant cells filled with cell sap - an aqueous solution of various organic and inorganic substances. They may also contain toxic and tannin substances and end products of cell activity.
■Vacuoles of plant cells can merge into a central vacuole, which occupies up to 70-90% of the cell volume and can be penetrated by strands of cytoplasm.
Functions: accumulation and isolation of reserve substances and substances intended for excretion; maintaining turgor pressure; ensuring cell growth due to stretching; regulation of cell water balance.
♦Ribosomes- cell organelles, present in all cells (in the amount of several tens of thousands), located on the membranes of granular EPS, in mitochondria, chloroplasts, cytoplasm and outer nuclear membrane and carrying out the biosynthesis of proteins; Ribosomal subunits are formed in the nucleoli.
■ Structure and composition: ribosomes are the smallest (15-35 nm) non-membrane granules of round and mushroom shape; have two active centers (aminoacyl and peptidyl); consist of two unequal subunits - a large one (in the form of a hemisphere with three protrusions and a channel), which contains three RNA molecules and a protein, and a small one (containing one RNA molecule and a protein); the subunits are connected using the Mg+ ion.
■ Function: synthesis of proteins from amino acids.
❖ Cell center- an organelle of most animal cells, some fungi, algae, mosses and ferns, located (in interphase) in the center of the cell near the nucleus and serving as the assembly initiation center microtubules .
■ Structure: The cell center consists of two centrioles and a centrosphere. Each centriole (Fig. 1.12) has the appearance of a cylinder 0.3-0.5 µm long and 0.15 µm in diameter, the walls of which are formed by nine triplets of microtubules, and the middle is filled with a homogeneous substance. The centrioles are located perpendicular to each other and are surrounded by a dense layer of cytoplasm with radiating microtubules forming a radiating centrosphere. During cell division, the centrioles move toward the poles.
■ Main functions: formation of cell division poles and achromatic filaments of the division spindle (or mitotic spindle), ensuring equal distribution of genetic material between daughter cells; in interphase, it directs the movement of organelles in the cytoplasm.
Cytosklst cells is a system microfilaments And microtubules , penetrating the cytoplasm of the cell, associated with the outer cytoplasmic membrane and nuclear envelope and maintaining the shape of the cell.
■Microflanges- thin, contractile filaments 5-10 nm thick and consisting of proteins ( actin, myosin and etc.). Found in the cytoplasm of all cells and pseudopods of motile cells.
Functions: microfilaments provide the motor activity of hyaloplasm, are directly involved in changing the shape of the cell during the spreading and amoeboid movement of protist cells, and participate in the formation of the constriction during the division of animal cells; one of the main elements of the cell cytoskeleton.
■ Microtubules- thin hollow cylinders (25 nm in diameter), consisting of tubulin protein molecules, arranged in spiral or straight rows in the cytoplasm of eukaryotic cells.
Functions: microtubules form spindle filaments, are part of centrioles, cilia, flagella, and participate in intracellular transport; one of the main elements of the cell cytoskeleton.
Organelles of movement — flagella and cilia , are present in many cells, but are more common in single-celled organisms.
■ Cilia- numerous cytoplasmic short (5-20 µm long) projections on the surface of the plasmalemma. Available on the surface of various types of animal cells and some plants.
■ Flagella- single cytoplasmic projections on the surface of the cells of many protists, zoospores and spermatozoa; ~10 times longer than cilia; are used for movement.
■ Structure: cilia and flagella (Fig. 1.14) consist of them microtubules, arranged according to the 9 × 2 + 2 system (nine double microtubules - doublets form a wall, in the middle there are two single microtubules). The doublets are able to slide past each other, which leads to the bending of the cilium or flagellum. At the base of flagella and cilia there are basal bodies, identical in structure to centrioles.
■ Functions: cilia and flagella ensure the movement of the cells themselves or the surrounding fluid and particles suspended in it.
Inclusions
Inclusions- non-permanent (temporarily existing) components of the cell cytoplasm, the content of which varies depending on the functional state of the cell. There are trophic, secretory and excretory inclusions.
■ Trophic inclusions- these are reserves of nutrients (fat, starch and protein grains, glycogen).
■ Secretory inclusions- these are waste products of the endocrine and exocrine glands (hormones, enzymes).
■ Excretory inclusions- These are metabolic products in the cell that must be excreted from the cell.
Nucleus and chromosomes
Core- the largest organelle; is an obligatory component of all eukaryotic cells (with the exception of phloem sieve tube cells of higher plants and mature erythrocytes of mammals). Most cells have a single nucleus, but there are bi- and multinucleated cells. There are two states of the nucleus: interphase and fissile
Interphase nucleus comprises nuclear envelope(separating the internal contents of the nucleus from the cytoplasm), nuclear matrix (karyoplasm), chromatin and nucleoli. The shape and size of the nucleus depend on the type of organism, type, age and functional state of the cell. It has a high content of DNA (15-30%) and RNA (12%).
■ Kernel functions: storage and transmission of hereditary information in the form of an unchanged DNA structure; regulation (through the protein synthesis system) of all cell vital processes.
❖ Nuclear envelope(or karyolemma) consists of outer and inner biological membranes, between which there is perinuclear space. The inner membrane has a protein lamina that gives shape to the nucleus. The outer membrane is connected to the ER and carries ribosomes. The shell is permeated with nuclear pores, through which the exchange of substances between the nucleus and the cytoplasm occurs. The number of pores is not constant and depends on the size of the nucleus and its functional activity.
■ Functions of the nuclear membrane: it separates the nucleus from the cytoplasm of the cell, regulates the transport of substances from the nucleus to the cytoplasm (RNA, ribosomal subunits) and from the cytoplasm to the nucleus (proteins, fats, carbohydrates, ATP, water, ions).
❖ Chromosome- the most important organelle of the nucleus, containing one DNA molecule in complex with specific histone proteins and some other substances, most of which are located on the surface of the chromosome.
Depending on the phase of the cell's life cycle, chromosomes may be in two states — despiralized and spiralized.
» In a despiralized state, chromosomes are in the period interphase cell cycle, forming threads invisible in an optical microscope that form the basis chromatin .
■ Spiralization, accompanied by shortening and compaction (100-500 times) of DNA strands, occurs in the process cell division ; while the chromosomes take on a compact shape and become visible under an optical microscope.
Chromatin- one of the components of nuclear matter during the interphase period, the basis of which is decoiled chromosomes in the form of a network of long thin strands of DNA molecules in complex with histones and other substances (RNA, DNA polymerase, lipids, minerals, etc.); stains well with dyes used in histological practice.
■ In chromatin, sections of the DNA molecule wrap around histones, forming nucleosomes (they look like beads).
Chromatid is a structural element of a chromosome, which is a strand of a DNA molecule in complex with histone proteins and other substances, repeatedly folded like a superhelix and packaged in the form of a rod-shaped body.
■ During helicalization and packaging, individual sections of DNA are arranged in a regular manner so that alternating transverse stripes are formed on the chromatids.
❖ Structure of a chromosome (Fig. 1.16). In the spiralized state, the chromosome is a rod-shaped structure about 0.2-20 µm in size, consisting of two chromatids and divided into two arms by a primary constriction called the centromere. Chromosomes may have a secondary constriction separating a region called a satellite. Some chromosomes have a section ( nucleolar organizer ), which encodes the structure of ribosomal RNA (rRNA).
Types of chromosomes depending on their shape: equal shoulders , unequal shoulders (centromere is displaced from the middle of the chromosome), rod-shaped (the centromere is close to the end of the chromosome).
After anaphase of mitosis and anaphase of meiosis II, chromosomes consist of one chromitid, and after DNA replication (doubling) at the synthetic (S) stage of interphase, they consist of two sister chromitids connected to each other at the centromere. During cell division, spindle microtubules are attached to the centromere.
❖ Functions of chromosomes:
■ contain genetic material
- DNA molecules;
■ carry out DNA synthesis
(during the doubling of chromosomes in the S-period of the cell cycle) and mRNA;
■ regulate protein synthesis;
■ control the vital activity of the cell.
Homologous chromosomes- chromosomes belonging to the same pair, identical in shape, size, location of centromeres, carrying the same genes and determining the development of the same characteristics. Homologous chromosomes can differ in the alleles of the genes they contain and exchange sections during meiosis (crossing over).
Autosomes chromosomes in the cells of dioecious organisms, identical in males and females of the same species (these are all chromosomes of a cell with the exception of sex chromosomes).
Sex chromosomes(or heterochromosomes ) are chromosomes that carry genes that determine the sex of a living organism.
Diploid set(designated 2p) - chromosome set somatic cells in which each chromosome has its paired homologous chromosome . The body receives one of the chromosomes of the diploid set from the father, the other from the mother.
■ Diploid set person consists of 46 chromosomes (of which 22 pairs of homologous chromosomes and two sex chromosomes: women have two X chromosomes, men have one X and Y chromosome each).
Haploid set(indicated by 1l) - single chromosome set sexual cells ( gametes ), in which the chromosomes do not have paired homologous chromosomes . The haploid set is formed during the formation of gametes as a result of meiosis, when from each pair of homologous chromosomes only one gets into the gamete.
Karyotype- this is a set of constant quantitative and qualitative morphological characteristics characteristic of the chromosomes of somatic cells of organisms of a given species (their number, size and shape), by which the diploid set of chromosomes can be unambiguously identified.
Nucleolus- round, highly compacted, not limited
membrane body 1-2 microns in size. The nucleus has one or more nucleoli. The nucleolus is formed around the nucleolar organizers of several chromosomes that attract each other. During nuclear division, the nucleoli are destroyed and re-formed at the end of division.
■ Composition: protein 70-80%, RNA 10-15%, DNA 2-10%.
■ Functions: synthesis of r-RNA and t-RNA; assembly of ribosomal subunits.
Karyoplasm (or nucleoplasm, karyolymph, nuclear juice ) is a structureless mass that fills the space between the structures of the nucleus, into which chromatin, nucleoli, and various intranuclear granules are immersed. Contains water, nucleotides, amino acids, ATP, RNA and enzyme proteins.
Functions: ensures the interconnection of nuclear structures; participates in the transport of substances from the nucleus to the cytoplasm and from the cytoplasm to the nucleus; regulates DNA synthesis during replication, mRNA synthesis during transcription.
Comparative characteristics of eukaryotic cells
Features of the structure of prokaryotic and eukaryotic cells
Transport of substances
Transport of substances- this is the process of transporting necessary substances throughout the body, to cells, inside the cell and within the cell, as well as removing waste substances from the cell and the body.
Intracellular transport of substances is ensured by the hyaloplasm and (in eukaryotic cells) the endoplasmic reticulum (ER), the Golgi complex and microtubules. The transport of substances will be described later on this site.
Methods of transport of substances through biological membranes:
■ passive transport (osmosis, diffusion, passive diffusion),
■ active transport,
■ endocytosis,
■ exocytosis.
Passive transport does not require energy expenditure and occurs along the gradient concentration, density or electrochemical potential.
Osmosis is the penetration of water (or other solvent) through a semi-permeable membrane from a less concentrated solution to a more concentrated one.
Diffusion- penetration substances through the membrane along the gradient concentration (from an area with a higher concentration of a substance to an area with a lower concentration).
Diffusion water and ions are carried out with the participation of integral membrane proteins that have pores (channels), diffusion of fat-soluble substances occurs with the participation of the lipid phase of the membrane.
Facilitated diffusion through the membrane occurs with the help of special membrane transport proteins, see the picture.
Active transport requires the expenditure of energy released during the breakdown of ATP, and serves to transport substances (ions, monosaccharides, amino acids, nucleotides) against gradient their concentration or electrochemical potential. Carried out by special carrier proteins permiases , having ion channels and forming ion pumps .
Endocytosis- capture and envelopment of macromolecules (proteins, nucleic acids, etc.) and microscopic solid food particles ( phagocytosis ) or droplets of liquid with substances dissolved in it ( pinocytosis ) and enclosing them in a membrane vacuole, which is drawn “into the cell. The vacuole then fuses with a lysosome, whose enzymes break down the molecules of the trapped substance into monomers.
Exocytosis- a process reverse to endocytosis. Through exocytosis, the cell removes intracellular products or undigested debris enclosed in vacuoles or vesicles.
Cell shapes are very diverse. In unicellular organisms, each cell is a separate organism. Its shape and structural features are associated with the environmental conditions in which this single-celled organism lives, with its way of life.
Differences in cell structure
The body of every multicellular animal and plant is composed of cells that differ in appearance, which is associated with their functions. Thus, in animals one can immediately distinguish a nerve cell from a muscle or epithelial cell (epithelium-integumentary tissue). Plants have different cell structures in leaves, stems, etc.
Cell sizes are just as variable. The smallest of them (some) do not exceed 0.5 microns. The size of the cells of multicellular organisms ranges from several micrometers (the diameter of human leukocytes is 3-4 microns, the diameter of red blood cells is 8 microns) to enormous sizes (the processes of one human nerve cell are more than 1 m long ). In most plant and animal cells, their diameter ranges from 10 to 100 microns.
Despite the diversity of structure, shapes and sizes, all living cells of any organism are similar in many features of their internal structure. Cell- a complex holistic physiological system in which all the basic processes of life are carried out: energy, irritability, growth and self-reproduction.
The main components of the cell structure
The main common components of a cell are the outer membrane, cytoplasm and nucleus. A cell can live and function normally only in the presence of all these components, which closely interact with each other and with the environment.
Drawing. 2. Cell structure: 1 - nucleus, 2 - nucleolus, 3 - nuclear membrane, 4 - cytoplasm, 5 - Golgi apparatus, 6 - mitochondria, 7 - lysosomes, 8 - endoplasmic reticulum, 9 - ribosomes, 10 - cell membrane
The structure of the outer membrane. It is a thin (about 7.5 nm2 thick) three-layer cell membrane, visible only in an electron microscope. The two outer layers of the membrane consist of proteins, and the middle one is formed by fat-like substances. The membrane has very small pores, thanks to which it easily allows some substances to pass through and retains others. The membrane takes part in phagocytosis (the cell captures solid particles) and pinocytosis (the cell captures droplets of liquid with substances dissolved in it). Thus, the membrane maintains the integrity of the cell and regulates the flow of substances from the environment into the cell and from the cell into its environment.
On its inner surface, the membrane forms invaginations and branches that penetrate deeply into the cell. Through them, the outer membrane is connected to the shell of the nucleus. On the other hand, the membranes of neighboring cells, forming mutually adjacent invaginations and folds, very closely and reliably connect cells into multicellular tissues.
Cytoplasm is a complex colloidal system. Its structure: transparent semi-liquid solution and structural formations. The structural formations of the cytoplasm common to all cells are: mitochondria, endoplasmic reticulum, Golgi complex and ribosomes (Figure 2). All of them, together with the nucleus, represent the centers of certain biochemical processes that collectively make up the cell. These processes are extremely diverse and occur simultaneously in a microscopically small volume of the cell. This is related to the general feature of the internal structure of all structural elements of the cell: despite their small size, they have a large surface on which biological catalysts (enzymes) are located and various biochemical reactions are carried out.
Mitochondria(Figure 2, 6) - energy centers of the cell. These are very small bodies, but clearly visible in a light microscope (length 0.2-7.0 microns). They are found in the cytoplasm and vary significantly in shape and number in different cells. The liquid contents of mitochondria are enclosed in two three-layer membranes, each of which has the same structure as the outer membrane of the cell. The inner membrane of the mitochondrion forms numerous invaginations and incomplete septa within the body of the mitochondrion (Figure 3). These invaginations are called cristae. Thanks to them, with a small volume, a sharp increase in the surface area is achieved on which biochemical reactions take place, and among them, first of all, the reactions of accumulation and release of energy through the enzymatic conversion of adenosine diphosphoric acid into adenosine triphosphoric acid and vice versa.
Drawing. 3. Scheme of the structure of mitochondria: 1 - outer shell. 2 - inner shell, 3 - shell ridges directed inside the mitochondria Endoplasmic reticulum(Figure 2, 8) is a multiply branched invagination of the outer cell membrane. The membranes of the endoplasmic reticulum are usually arranged in pairs, and tubules are formed between them, which can expand into larger cavities filled with biosynthesis products. Around the nucleus, the membranes that make up the endoplasmic reticulum directly pass into the outer membrane of the nucleus. Thus, the endoplasmic reticulum connects all parts of the cell together. In a light microscope, when examining the structure of a cell, the endoplasmic reticulum is not visible.
The structure of the cell is divided into rough And smooth endoplasmic reticulum. The rough endoplasmic reticulum is densely surrounded by ribosomes, where protein synthesis occurs. The smooth endoplasmic reticulum is devoid of ribosomes and synthesizes fats and carbohydrates. The tubules of the endoplasmic reticulum carry out intracellular exchange of substances synthesized in various parts of the cell, as well as exchange between cells. At the same time, the endoplasmic reticulum, as a denser structural formation, serves as the skeleton of the cell, giving its shape a certain stability.
Ribosomes(Figure 2, 9) are located both in the cytoplasm of the cell and in its nucleus. These are tiny grains with a diameter of about 15-20 nm, which makes them invisible in a light microscope. In the cytoplasm, the bulk of ribosomes are concentrated on the surface of the tubules of the rough endoplasmic reticulum. The function of ribosomes is the most important process for the life of the cell and the organism as a whole - the synthesis of proteins.
Golgi complex(Figure 2, 5) was first found only in animal cells. However, recently similar structures have been discovered in plant cells. The structure of the Golgi complex is close to the structural formations of the endoplasmic reticulum: these are tubules of various shapes, cavities and vesicles formed by three-layer membranes. In addition, the Golgi complex includes rather large vacuoles. Some synthesis products accumulate in them, primarily enzymes and hormones. During certain periods of a cell’s life, these reserved substances can be removed from a given cell through the endoplasmic reticulum and are involved in the metabolic processes of the body as a whole.
Cell center- formation, so far described only in the cells of animals and lower plants. It consists of two centrioles, the structure of each of which is a cylinder up to 1 micron in size. Centrioles play an important role in mitotic cell division. In addition to the described permanent structural formations, certain inclusions periodically appear in the cytoplasm of various cells. These are droplets of fat, starch grains, protein crystals of a special shape (aleurone grains), etc. Such inclusions are found in large quantities in the cells of storage tissues. However, in the cells of other tissues such inclusions can exist as a temporary reserve of nutrients.
Core(Figure 2, 1), like the cytoplasm with the outer membrane, is an essential component of the vast majority of cells. Only in some bacteria, when examining the structure of their cells, it was not possible to identify a structurally formed nucleus, but in their cells all the chemical substances inherent in the nuclei of other organisms were found. There are no nuclei in some specialized cells that have lost the ability to divide (red blood cells of mammals, sieve tubes of plant phloem). On the other hand, there are multinucleated cells. The nucleus plays a very important role in the synthesis of enzyme proteins, in the transmission of hereditary information from generation to generation, and in the processes of individual development of the body.
The nucleus of a non-dividing cell has a nuclear envelope. It consists of two three-layer membranes. The outer membrane is connected through the endoplasmic reticulum to the cell membrane. Through this entire system, there is a constant exchange of substances between the cytoplasm, the nucleus and the environment surrounding the cell. In addition, there are pores in the nuclear shell, through which the nucleus is also connected to the cytoplasm. Inside, the nucleus is filled with nuclear juice, which contains clumps of chromatin, a nucleolus and ribosomes. Chromatin is made up of protein and DNA. This is the material substrate that, before cell division, is formed into chromosomes, visible in a light microscope.
Chromosomes- formations that are constant in number and form, identical for all organisms of a given species. The functions of the nucleus listed above are primarily associated with chromosomes, or more precisely, with the DNA that is part of them.
Nucleolus(Figure 2.2) is present in one or more quantities in the nucleus of a non-dividing cell and is clearly visible in a light microscope. At the moment of cell division it disappears. Recently, the enormous role of the nucleolus has been elucidated: ribosomes are formed in it, which then enter the cytoplasm from the nucleus and carry out protein synthesis there.
All of the above applies equally to animal cells and plant cells. Due to the specificity of metabolism, growth and development of plants and animals, in the structure of the cells of both there are additional structural features that distinguish plant cells from animal cells. More information about this is written in the sections “Botany” and “Zoology”; Here we note only the most general differences.
Animal cells, in addition to the listed components, have special formations in the structure of the cell - lysosomes. These are ultramicroscopic vesicles in the cytoplasm filled with liquid digestive enzymes. Lysosomes carry out the function of breaking down food substances into simpler chemical substances. There are some indications that lysosomes are also found in plant cells.
The most characteristic structural elements of plant cells (except for those common ones that are inherent in all cells) - plastids. They exist in three forms: green chloroplasts, red-orange-yellow
chromoplasts and colorless leucoplasts. Under certain conditions, leukoplasts can turn into chloroplasts (greening of potato tubers), and chloroplasts, in turn, can become chromoplasts (autumn yellowing of leaves).
Drawing. 4. Scheme of the structure of a chloroplast: 1 - chloroplast shell, 2 - groups of plates in which the process of photosynthesis occurs Chloroplasts(Figure 4) represent a “factory” for the primary synthesis of organic substances from inorganic ones using solar energy. These are small bodies of quite varied shapes, always green in color due to the presence of chlorophyll. The structure of chloroplasts in a cell: they have an internal structure that ensures maximum development of free surfaces. These surfaces are created by numerous thin plates, clusters of which are located inside the chloroplast.
On the surface, the chloroplast, like other structural elements of the cytoplasm, is covered with a double membrane. Each of them, in turn, is three-layered, like the outer membrane of the cell.
The cell is the basic elementary unit of all living things, therefore it has all the properties of living organisms: a highly ordered structure, receiving energy from the outside and using it to perform work and maintain order, metabolism, an active response to irritations, growth, development, reproduction, duplication and transmission of biological information to descendants, regeneration (restoration of damaged structures), adaptation to the environment.
The German scientist T. Schwann in the middle of the 19th century created the cellular theory, the main provisions of which indicated that all tissues and organs consist of cells; cells of plants and animals are fundamentally similar to each other, they all arise in the same way; the activity of organisms is the sum of the vital activities of individual cells. The great German scientist R. Virchow had a great influence on the further development of cell theory and on the doctrine of the cell in general. He not only brought together all the numerous disparate facts, but also convincingly showed that cells are a permanent structure and arise only through reproduction.
Cell theory in its modern interpretation includes the following main provisions: the cell is a universal elementary unit of living things; the cells of all organisms are fundamentally similar in their structure, function and chemical composition; cells reproduce only by dividing the original cell; multicellular organisms are complex cellular assemblies that form integral systems.
Thanks to modern research methods, it was revealed two main cell types: more complexly organized, highly differentiated eukaryotic cells (plants, animals and some protozoa, algae, fungi and lichens) and less complexly organized prokaryotic cells (blue-green algae, actinomycetes, bacteria, spirochetes, mycoplasmas, rickettsia, chlamydia).
Unlike a prokaryotic cell, a eukaryotic cell has a nucleus bounded by a double nuclear membrane and a large number of membrane organelles.
ATTENTION!
The cell is the basic structural and functional unit of living organisms, carrying out growth, development, metabolism and energy, storing, processing and implementing genetic information. From a morphological point of view, a cell is a complex system of biopolymers, separated from the external environment by a plasma membrane (plasmolemma) and consisting of a nucleus and cytoplasm, in which organelles and inclusions (granules) are located.
What types of cells are there?
Cells are diverse in their shape, structure, chemical composition and nature of metabolism.
All cells are homologous, i.e. have a number of common structural features on which the performance of basic functions depends. Cells are characterized by unity of structure, metabolism (metabolism) and chemical composition.
At the same time, different cells also have specific structures. This is due to their performance of special functions.
Cell structure
Ultramicroscopic cell structure:
1 - cytolemma (plasma membrane); 2 - pinocytotic vesicles; 3 - centrosome, cell center (cytocenter); 4 - hyaloplasm; 5 - endoplasmic reticulum: a - membrane of the granular reticulum; b - ribosomes; 6 - connection of the perinuclear space with the cavities of the endoplasmic reticulum; 7 - core; 8 - nuclear pores; 9 - non-granular (smooth) endoplasmic reticulum; 10 - nucleolus; 11 - internal reticular apparatus (Golgi complex); 12 - secretory vacuoles; 13 - mitochondria; 14 - liposomes; 15 - three successive stages of phagocytosis; 16 - connection of the cell membrane (cytolemma) with the membranes of the endoplasmic reticulum.
Chemical composition of the cell
The cell contains more than 100 chemical elements, four of which account for about 98% of the mass; these are organogens: oxygen (65–75%), carbon (15–18%), hydrogen (8–10%) and nitrogen (1 .5–3.0%). The remaining elements are divided into three groups: macroelements - their content in the body exceeds 0.01%); microelements (0.00001–0.01%) and ultramicroelements (less than 0.00001).
Macroelements include sulfur, phosphorus, chlorine, potassium, sodium, magnesium, calcium.
Microelements include iron, zinc, copper, iodine, fluorine, aluminum, copper, manganese, cobalt, etc.
Ultramicroelements include selenium, vanadium, silicon, nickel, lithium, silver and more. Despite their very low content, microelements and ultramicroelements play a very important role. They mainly affect metabolism. Without them, the normal functioning of each cell and the organism as a whole is impossible.
The cell consists of inorganic and organic substances. Among inorganic substances, the largest amount of water is present. The relative amount of water in the cell is between 70 and 80%. Water is a universal solvent; all biochemical reactions in the cell take place in it. With the participation of water, thermoregulation is carried out. Substances that dissolve in water (salts, bases, acids, proteins, carbohydrates, alcohols, etc.) are called hydrophilic. Hydrophobic substances (fats and fat-like substances) do not dissolve in water. Other inorganic substances (salts, acids, bases, positive and negative ions) account for 1.0 to 1.5%.
Among organic substances, proteins (10–20%), fats or lipids (1–5%), carbohydrates (0.2–2.0%), and nucleic acids (1–2%) predominate. The content of low molecular weight substances does not exceed 0.5%.
A protein molecule is a polymer that consists of a large number of repeating units of monomers. Amino acid protein monomers (20 of them) are connected to each other by peptide bonds, forming a polypeptide chain (the primary structure of the protein). It twists into a spiral, forming, in turn, the secondary structure of the protein. Due to the specific spatial orientation of the polypeptide chain, the tertiary structure of the protein arises, which determines the specificity and biological activity of the protein molecule. Several tertiary structures combine with each other to form a quaternary structure.
Proteins perform essential functions. Enzymes - biological catalysts that increase the rate of chemical reactions in a cell hundreds of thousands of millions of times, are proteins. Proteins, being part of all cellular structures, perform a plastic (construction) function. Cell movements are also carried out by proteins. They provide transport of substances into the cell, out of the cell and within the cell. The protective function of proteins (antibodies) is important. Proteins are one of the sources of energy. Carbohydrates are divided into monosaccharides and polysaccharides. The latter are built from monosaccharides, which, like amino acids, are monomers. Among the monosaccharides in the cell, the most important are glucose, fructose (contains six carbon atoms) and pentose (five carbon atoms). Pentoses are part of nucleic acids. Monosaccharides are highly soluble in water. Polysaccharides are poorly soluble in water (glycogen in animal cells, starch and cellulose in plant cells). Carbohydrates are a source of energy; complex carbohydrates combined with proteins (glycoproteins), fats (glycolipids) are involved in the formation of cell surfaces and cell interactions.
Lipids include fats and fat-like substances. Fat molecules are built from glycerol and fatty acids. Fat-like substances include cholesterol, some hormones, and lecithin. Lipids, which are the main components of cell membranes, thereby perform a construction function. Lipids are the most important sources of energy. So, if with complete oxidation of 1 g of protein or carbohydrates 17.6 kJ of energy is released, then with complete oxidation of 1 g of fat - 38.9 kJ. Lipids carry out thermoregulation and protect organs (fat capsules).
DNA and RNA
Nucleic acids are polymer molecules formed by nucleotide monomers. A nucleotide consists of a purine or pyrimidine base, a sugar (pentose) and a phosphoric acid residue. In all cells, there are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which differ in the composition of bases and sugars.
Spatial structure of nucleic acids:
(according to B. Alberts et al., with modification). I - RNA; II - DNA; ribbons - sugar phosphate backbones; A, C, G, T, U are nitrogenous bases, the lattices between them are hydrogen bonds.
DNA molecule
A DNA molecule consists of two polynucleotide chains twisted around one another in the form of a double helix. The nitrogenous bases of both chains are connected to each other by complementary hydrogen bonds. Adenine combines only with thymine, and cytosine - with guanine (A - T, G - C). DNA contains genetic information that determines the specificity of the proteins synthesized by the cell, that is, the sequence of amino acids in the polypeptide chain. DNA transmits by inheritance all the properties of a cell. DNA is found in the nucleus and mitochondria.
RNA molecule
An RNA molecule is formed by one polynucleotide chain. There are three types of RNA in cells. Informational, or messenger RNA tRNA (from the English messenger - “intermediary”), which transfers information about the nucleotide sequence of DNA to ribosomes (see below). Transfer RNA (tRNA), which carries amino acids to ribosomes. Ribosomal RNA (rRNA), which is involved in the formation of ribosomes. RNA is found in the nucleus, ribosomes, cytoplasm, mitochondria, and chloroplasts.
Composition of nucleic acids.
Divides all cells (or alive organisms) into two types: prokaryotes And eukaryotes. Prokaryotes are nuclear-free cells or organisms, which include viruses, prokaryotic bacteria and blue-green algae, in which the cell consists directly of the cytoplasm, in which one chromosome is located - DNA molecule(sometimes RNA).
Eukaryotic cells have a core containing nucleoproteins (histone protein + DNA complex), as well as others organoids. Eukaryotes include the majority of modern unicellular and multicellular living organisms known to science (including plants).
The structure of eukaryotic granoids.
|
Organoid name |
Organoid structure |
Functions of the organoid |
|---|---|---|
|
Cytoplasm |
The internal environment of a cell in which the nucleus and other organelles are located. It has a semi-liquid, fine-grained structure. |
|
|
Ribosomes |
Small organoids of spherical or ellipsoidal shape with a diameter of 15 to 30 nanometers. |
They provide the process of synthesis of protein molecules and their assembly from amino acids. |
|
Mitochondria |
Organelles that have a wide variety of shapes - from spherical to filamentous. Inside the mitochondria there are folds from 0.2 to 0.7 µm. The outer shell of mitochondria has a double-membrane structure. The outer membrane is smooth, and on the inner there are cross-shaped outgrowths with respiratory enzymes. |
|
|
Endoplasmic reticulum (ER) |
A system of membranes in the cytoplasm that forms channels and cavities. There are two types: granular, which has ribosomes, and smooth. |
|
|
Plastids(organelles characteristic only of plant cells) are of three types: |
Double membrane organelles |
|
|
Leukoplasts |
Colorless plastids that are found in tubers, roots and bulbs of plants. |
They are an additional reservoir for storing nutrients. |
|
Chloroplasts |
Organelles are oval-shaped and green in color. They are separated from the cytoplasm by two three-layer membranes. Chloroplasts contain chlorophyll. |
They convert organic substances from inorganic ones using solar energy. |
|
Chromoplasts |
Organelles, yellow to brown in color, in which carotene accumulates. |
Promote the appearance of yellow, orange and red colored parts in plants. |
|
Lysosomes |
Organelles are round in shape with a diameter of about 1 micron, having a membrane on the surface and a complex of enzymes inside. |
Digestive function. They digest nutrient particles and eliminate dead parts of the cell. |
|
Golgi complex |
May be of different shapes. Consists of cavities delimited by membranes. Tubular formations with bubbles at the ends extend from the cavities. |
|
|
Cell center |
It consists of a centrosphere (a dense section of the cytoplasm) and centrioles - two small bodies. |
Performs an important function for cell division. |
|
Cellular inclusions |
Carbohydrates, fats and proteins, which are non-permanent components of the cell. |
Spare nutrients that are used for cell functioning. |
|
Organoids of movement |
Flagella and cilia (outgrowths and cells), myofibrils (thread-like formations) and pseudopodia (or pseudopods). |
They perform a motor function and also provide the process of muscle contraction. |
Cell nucleus is the main and most complex organelle of the cell, so we will consider it
The cells of animals and plants, both multicellular and unicellular, are in principle similar in structure. Differences in the details of cell structure are associated with their functional specialization.
The main elements of all cells are the nucleus and cytoplasm. The nucleus has a complex structure that changes at different phases of cell division, or cycle. The nucleus of a nondividing cell occupies approximately 10–20% of its total volume. It consists of karyoplasm (nucleoplasm), one or more nucleoli (nucleoli) and a nuclear membrane. Karyoplasm is a nuclear sap, or karyolymph, in which there are strands of chromatin that form chromosomes.
Basic properties of the cell:
- metabolism
- sensitivity
- reproductive capacity
The cell lives in the internal environment of the body - blood, lymph and tissue fluid. The main processes in the cell are oxidation and glycolysis - the breakdown of carbohydrates without oxygen. Cell permeability is selective. It is determined by the reaction to high or low salt concentrations, phago- and pinocytosis. Secretion is the formation and release by cells of mucus-like substances (mucin and mucoids), which protect against damage and participate in the formation of intercellular substance.
Types of cell movements:
- amoeboid (pseudopods) – leukocytes and macrophages.
- sliding – fibroblasts
- flagellar type – spermatozoa (cilia and flagella)
Cell division:
- indirect (mitosis, karyokinesis, meiosis)
- direct (amitosis)
During mitosis, the nuclear substance is distributed evenly between daughter cells, because Nuclear chromatin is concentrated in chromosomes, which split into two chromatids that separate into daughter cells.
Structures of a living cell
Chromosomes
Mandatory elements of the nucleus are chromosomes, which have a specific chemical and morphological structure. They take an active part in the metabolism in the cell and are directly related to the hereditary transmission of properties from one generation to another. It should, however, be borne in mind that although heredity is ensured by the entire cell as a single system, nuclear structures, namely chromosomes, occupy a special place in this. Chromosomes, unlike cell organelles, are unique structures characterized by constant qualitative and quantitative composition. They cannot replace each other. An imbalance in the chromosomal complement of a cell ultimately leads to its death.
Cytoplasm
The cytoplasm of the cell exhibits a very complex structure. The introduction of thin sectioning techniques and electron microscopy made it possible to see the fine structure of the underlying cytoplasm. It has been established that the latter consists of parallel complex structures in the form of plates and tubules, on the surface of which there are tiny granules with a diameter of 100–120 Å. These formations are called the endoplasmic complex. This complex includes various differentiated organelles: mitochondria, ribosomes, Golgi apparatus, in the cells of lower animals and plants - centrosome, in animals - lysosomes, in plants - plastids. In addition, the cytoplasm reveals a number of inclusions that take part in the cell’s metabolism: starch, fat droplets, urea crystals, etc.
Membrane
The cell is surrounded by a plasma membrane (from the Latin “membrane” - skin, film). Its functions are very diverse, but the main one is protective: it protects the internal contents of the cell from the influences of the external environment. Thanks to various outgrowths and folds on the surface of the membrane, the cells are firmly connected to each other. The membrane is permeated with special proteins through which certain substances needed by the cell or to be removed from it can move. Thus, metabolism occurs through the membrane. Moreover, what is very important, substances are passed through the membrane selectively, due to which the required set of substances is maintained in the cell.
In plants, the plasma membrane is covered on the outside with a dense membrane consisting of cellulose (fiber). The shell performs protective and supporting functions. It serves as the outer frame of the cell, giving it a certain shape and size, preventing excessive swelling.
Core
Located in the center of the cell and separated by a two-layer membrane. It has a spherical or elongated shape. The shell - karyolemma - has pores necessary for the exchange of substances between the nucleus and the cytoplasm. The contents of the nucleus are liquid - karyoplasm, which contains dense bodies - nucleoli. They secrete granules - ribosomes. The bulk of the nucleus is nuclear proteins - nucleoproteins, in the nucleoli - ribonucleoproteins, and in the karyoplasm - deoxyribonucleoproteins. The cell is covered with a cell membrane, which consists of protein and lipid molecules that have a mosaic structure. The membrane ensures the exchange of substances between the cell and the intercellular fluid.
EPS
This is a system of tubules and cavities on the walls of which there are ribosomes that provide protein synthesis. Ribosomes can be freely located in the cytoplasm. There are two types of EPS - rough and smooth: on the rough EPS (or granular) there are many ribosomes that carry out protein synthesis. Ribosomes give membranes their rough appearance. Smooth ER membranes do not carry ribosomes on their surface; they contain enzymes for the synthesis and breakdown of carbohydrates and lipids. Smooth EPS looks like a system of thin tubes and tanks.
Ribosomes
Small bodies with a diameter of 15–20 mm. They synthesize protein molecules and assemble them from amino acids.
Mitochondria
These are double-membrane organelles, the inner membrane of which has projections - cristae. The contents of the cavities are matrix. Mitochondria contain a large number of lipoproteins and enzymes. These are the energy stations of the cell.
Plastids (characteristic only of plant cells!)
Their content in the cell is the main feature of the plant organism. There are three main types of plastids: leucoplasts, chromoplasts and chloroplasts. They have different colors. Colorless leucoplasts are found in the cytoplasm of cells of uncolored parts of plants: stems, roots, tubers. For example, there are many of them in potato tubers, in which starch grains accumulate. Chromoplasts are found in the cytoplasm of flowers, fruits, stems, and leaves. Chromoplasts provide yellow, red, and orange colors to plants. Green chloroplasts are found in the cells of leaves, stems and other parts of the plant, as well as in a variety of algae. Chloroplasts are 4-6 microns in size and often have an oval shape. In higher plants, one cell contains several dozen chloroplasts.
Green chloroplasts are able to transform into chromoplasts - that’s why leaves turn yellow in the fall, and green tomatoes turn red when ripe. Leucoplasts can transform into chloroplasts (greening of potato tubers in the light). Thus, chloroplasts, chromoplasts and leucoplasts are capable of mutual transition.
The main function of chloroplasts is photosynthesis, i.e. In chloroplasts, in the light, organic substances are synthesized from inorganic ones due to the conversion of solar energy into the energy of ATP molecules. The chloroplasts of higher plants are 5-10 microns in size and resemble a biconvex lens in shape. Each chloroplast is surrounded by a double membrane that is selectively permeable. The outside is a smooth membrane, and the inside has a folded structure. The main structural unit of the chloroplast is the thylakoid, a flat double-membrane sac that plays a leading role in the process of photosynthesis. The thylakoid membrane contains proteins similar to mitochondrial proteins that participate in the electron transport chain. The thylakoids are arranged in stacks resembling stacks of coins (10 to 150) called grana. Grana has a complex structure: chlorophyll is located in the center, surrounded by a layer of protein; then there is a layer of lipoids, again protein and chlorophyll.
Golgi complex
This is a system of cavities delimited from the cytoplasm by a membrane and can have different shapes. The accumulation of proteins, fats and carbohydrates in them. Carrying out the synthesis of fats and carbohydrates on membranes. Forms lysosomes.
The main structural element of the Golgi apparatus is the membrane, which forms packets of flattened cisterns, large and small vesicles. The cisterns of the Golgi apparatus are connected to the channels of the endoplasmic reticulum. Proteins, polysaccharides, and fats produced on the membranes of the endoplasmic reticulum are transferred to the Golgi apparatus, accumulate inside its structures and are “packaged” in the form of a substance, ready either for release or for use in the cell itself during its life. Lysosomes are formed in the Golgi apparatus. In addition, it is involved in the growth of the cytoplasmic membrane, for example during cell division.
Lysosomes
Bodies delimited from the cytoplasm by a single membrane. The enzymes they contain accelerate the breakdown of complex molecules into simple ones: proteins into amino acids, complex carbohydrates into simple ones, lipids into glycerol and fatty acids, and also destroy dead parts of the cell and entire cells. Lysosomes contain more than 30 types of enzymes (protein substances that increase the rate of chemical reactions tens and hundreds of thousands of times) capable of breaking down proteins, nucleic acids, polysaccharides, fats and other substances. The breakdown of substances with the help of enzymes is called lysis, hence the name of the organelle. Lysosomes are formed either from the structures of the Golgi complex or from the endoplasmic reticulum. One of the main functions of lysosomes is participation in the intracellular digestion of nutrients. In addition, lysosomes can destroy the structures of the cell itself when it dies, during embryonic development, and in a number of other cases.
Vacuoles
They are cavities in the cytoplasm filled with cell sap, a place of accumulation of reserve nutrients and harmful substances; they regulate the water content in the cell.
Cell center
It consists of two small bodies - centrioles and centrosphere - a compacted section of the cytoplasm. Plays an important role in cell division
Cell movement organoids
- Flagella and cilia, which are cell outgrowths and have the same structure in animals and plants
- Myofibrils are thin filaments more than 1 cm long with a diameter of 1 micron, located in bundles along the muscle fiber
- Pseudopodia (perform the function of movement; due to them, muscle contraction occurs)
Similarities between plant and animal cells
The characteristics that are similar between plant and animal cells include the following:
- Similar structure of the structure system, i.e. presence of nucleus and cytoplasm.
- The metabolic process of substances and energy is similar in principle.
- Both animal and plant cells have a membrane structure.
- The chemical composition of the cells is very similar.
- Plant and animal cells undergo a similar process of cell division.
- Plant cells and animal cells have the same principle of transmitting the code of heredity.
Significant differences between plant and animal cells
In addition to the general features of the structure and vital activity of plant and animal cells, there are also special distinctive features of each of them.
Thus, we can say that plant and animal cells are similar to each other in the content of some important elements and some vital processes, and also have significant differences in structure and metabolic processes.
