Basic Biology Cell Structure and Function PDF

BIO 101 CELL STRUCTURE, ORGANISATION, AND FUNCTIONS OF CELLULAR COMPONENTS Cell the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed is the basic, structural, and functional unit of life...

BIO 101 CELL STRUCTURE, ORGANISATION, AND FUNCTIONS OF CELLULAR COMPONENTS Cell the basic membrane-bound unit that contains the fundamental molecules of life and of which all living things are composed is the basic, structural, and functional unit of life. Living things can be composed of one cell making them unicellular organisms or composed of two or more cells making them multicellular organisms. Organisms with a single cell are often complete organisms in themselves while organisms with more than one cell, have their cells cooperate within to carry out specialized functions in the body of the living organisms. Unicellular organisms include bacteria, yeasts, and archaea while multicellular organisms include all mammals, arthropods, poriferans, plants, etc. Although cells are much larger than atoms of elements, they are still very small. The smallest known cells are a group of tiny bacteria called mycoplasmas while the largest cell is an egg of ostrich. The largest cell is an unfertilized ostrich egg, it is about 15cm to 18 cm long and wide. The longest cell is the nerve cell and the largest cell in the human body is the female ovum. As an individual unit, the cell is capable of metabolizing its nutrients synthesizing its needed molecules including ATP, and replicating itself to replace worn-out individuals. Cells are closed compartments in which different chemical reactions take place simultaneously. These reactions are under very precise control so that they contribute to the life and procreation of the cell. In multicellular organisms, cells become specialized to perform distinct functions through the process of differentiation. To do this, cells involved in such distinct functions keep in constant communication with one another. As each of the cells receives and expels nutrients and wastes from and into its surroundings respectively, it adheres to and cooperates with other cells. Cooperative assemblies of similar cells to perform a distinct function within a multicellular organism form tissue; cooperation between tissues to perform a distinct function forms organs and assembling of different organs to carry out functions necessary to sustain the life of an organism forms an organ-system.  Types of Cells There are two types of cells based on the arrangement of the internal structure and components of the cells. These are; Prokaryotic cells These are cells found in some living organisms with distinct characteristics such as; 1. Lack of true nucleus. Instead of the nucleus, prokaryotes have a region called nucleoid in the cell cytoplasm where the genetic material is freely suspended. 2. Presence of single circular DNA material. 3. They are found among unicellular organisms only. 4. They reproduce by asexual reproduction (binary fission). 5. The cell size is small ranging from 0.1 to 0.5 µm in diameter. 6. The genetic material can either be DNA or RNA. 7. The prokaryotes are known to live and survive in extreme environmental conditions 8. Lack of membrane-bound organelles. 9. The cell structure and its cellular activities are simple. 10. The cell wall mostly composed of peptidoglycan is located inside the cell. 11. It has a capsule that protects the organism against external agents such as macrophages and antibiotic medications. 12. It has pilus which aids the adherence of the animal to the external environment. 13. It has flagella for the locomotion activities. 14. It has a plasmid a circular DNA different from the chromosomal DNA of the cell which carries genes that confer antibiotic resistance on the host cell when expressed and are used to transfer genetic material among the bacteria species. 15. It has inclusion which is a storehouse of nutrients and gas vacuole for the organism. 16. Examples of organisms that have prokaryotic cells are bacteria and archaea.  Diagram of a Typical Prokaryotic cell Eukaryotic cells The second type of cell is known as eukaryotic cells. They have the following characteristics; 1. This has of true nucleus separated from the cytoplasm and houses the genetic materials 2. Presence of double linear DNA structure. 3. They are found among multicellular organisms and some unicellular organisms (yeast, euglena, etc.). 4. They reproduce by sexual and asexual reproduction. 5. The cell size is larger than that of prokaryotes ranging from 10–100 µm in diameter. 6. The genetic material is mostly DNA. 7. The eukaryotes are known to live and survive in moderate environmental conditions. 8. Presence of membrane-bound organelles such as mitochondria, Golgi apparatus, etc. 9. The cell structure and its cellular activities are complex. 10. The cell wall if present mostly composed of cellulose, chitin, glucans, and glycoproteins and covers the outermost part of the cell. 11. Examples of organisms that have prokaryotic cells are fungi, protists, animals, and plants. Diagram of a Typical Plant cell (Eukaryotic cell) Diagram of a Typical Animal cell (Eukaryotic cell)  CELLULAR STRUCTURES AND FUNCTIONS Eukaryotic cells are more complex types of cells with a well-organized structure that enables them to carry out complex physiological activities in a well-organized mechanism. A cell has three main parts which are the cell membrane, the nucleus, and the cytoplasm. Cell Membrane: This is also known as the plasma membrane which encloses the cell and forms a selective barrier that allows nutrients to enter and waste products to leave the cell. The cell membrane is composed of; Phospholipid bilayer: The cell membrane is a phospholipid bilayer (has two phospholipid layers) and each of the phospholipid molecules has a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The hydrophilic heads are projected into the cytoplasm and to the eternal environment interacting with the watery environment outside and inside the cell respectively. The hydrophobic tails point inwards, away from the water, forming the core of the membrane. This arrangement of the cell membrane creates a barrier that allows some substances to pass through easily (like small, non-charged molecules) and restricts the passage of others (like large molecules and ions). Proteins: These are embedded in the phospholipid bilayer and perform specific functions. They include: ✓ Channel proteins: These act like pores, allowing specific ions or molecules to pass through the membrane. ✓ Carrier proteins: These bind to specific molecules and transport them across the membrane in a facilitated diffusion process. ✓ Receptor proteins: These proteins on the cell surface bind to signal molecules from the outside, triggering specific responses within the cell. Carbohydrates: This molecule is attached to some proteins and lipids on the outer surface of the membrane. These carbohydrates play a role in cell-to-cell recognition and communication. Functions of Cell Membrane 1. By structure, it is a selective porous permeable membrane that permits the movement of selective substances in and out of the cell. It allows essential nutrients, gases, and water to  enter while keeping out harmful substances or maintaining the concentration of ions necessary for various cellular processes. 2. It protects the cellular component from damage and leakage. 3. The cell membrane helps maintain the cell's shape and provides a degree of support. 4. Proteins on the cell surface act as receptors, allowing cells to communicate with each other and respond to external signals. 5. Carbohydrates on the cell membrane structure act as identification tags, allowing cells to recognize other cells of the same type or foreign particles. This is important for the immune system to distinguish self from non-self.  Cell Membrane Structures Cell Nucleus: This is a membrane-bound organelle located in the center of the cell and serves as the cell control unit. Each cell contains only one nucleus, and has the following components; Nuclear Envelope: This separates the cell’s DNA from the cytoplasmic environment. Nucleolus: This is a condensed region in the nucleus that is composed of RNA and proteins and aids ribosome synthesis which will be transported to the cytoplasm. Nuclear pore: this is located on the nuclear envelope and serves as a channel through which molecules produced in the nucleus are transported to the cytoplasm of the cell. Chromatin/Chromosome: This is the genetic material of the cell. It is the DNA molecule bounded by a protein molecule known as histone. Nucleoplasm: this is also called karyoplasm which is the gelatinous substance within the nuclear envelope. It is composed mainly of water with dissolved salts, enzymes, and organic molecules suspended within. The nucleolus and chromosomes are surrounded by nucleoplasm, which functions to cushion and protect the contents of the nucleus and serve as a medium by which materials, such as enzymes and nucleotides (DNA and RNA subunits), can be transported throughout the nucleus. It also supports the nucleus by helping to maintain its shape. Functions of Nucleus It contains the hereditary material of the cell, the DNA.  It sends signals to the cells to grow, mature, reproduce, or die. It protects the DNA from the external environment. It is also the site for RNA formation which Is necessary for protein synthesis. Structure of Nucleus Cytoplasm: The cytoplasm is a glass-forming liquid or traditionally a thick, clear, jelly-like substance (gelatinous liquid) that fills the inside of a cell. It is composed of cytosol (a part of cytoplasm that has no organelle), filaments, water, salts, and various organic molecules such as proteins and carbohydrates. It houses every other component of the cell in a suspended manner and most of the intracellular organelles, such as the nucleus and mitochondria, are enclosed by membranes that separate them from the cytoplasm. Cell protein production and most of the cell chemical reactions take place in this part of the cell including glycolysis. The cytoplasm in eukaryotic cells associates with the cell contents except for the nucleus (containing genetic material) but in prokaryotic cells, the cytoplasm associates with the genetic material of the cell as they do not possess a defined nuclear membrane that will separate them from the cytoplasm.  Function of Cytoplasm It enables cells to maintain their turgidity, which enables the cells to hold their shape. It aids chemical reactions required by the cell for its life processes as it contains molecules and enzymes that are crucial in cellular activities. Mitochondria: This is known as the powerhouse of the cell as it is a site for ATP production. It is a double membrane-bound rod-shaped organelle of size 0.5 to 1.0 micrometer in diameter that also has its unique DNA material which contains genes needed for ATP production, an important biological process. The structure comprises an outer membrane, an inner membrane, and a gel-like material called the matrix. The outer membrane and the inner membrane are made of proteins and phospholipid layers separated by the intermembrane space. The outer membrane covers the surface of the mitochondrion and has a large number of special proteins known as porins. The inner membrane of mitochondria is complex in structure and has many folds that form a layered structure called cristae. This helps in increasing the surface area inside the organelle and together with proteins of the inner membrane to enable efficient ATP production. The inner mitochondrial membrane is only permeable to oxygen and ATP molecules. The mitochondrial matrix is a viscous fluid that contains a mixture of enzymes, proteins, ribosomes, inorganic ions, mitochondrial DNA, nucleotide cofactors, and organic molecules. The enzymes present in the matrix play an important role in the synthesis of ATP molecules. Functions of Mitochondria 1. The most important function of mitochondria is to produce energy through the process of oxidative phosphorylation. 2. It promotes the growth of new cells and cell multiplication as it releases the energy needed for such a process. 3. It aids detoxification of ammonia in the liver cells. 4. It plays an important role in apoptosis or programmed cell death. 5. Helps in maintaining an adequate concentration of calcium ions within the compartments of the cell.  Structure of Mitochondrion Endoplasmic Reticulum: The is a membrane-bound organelle involved in protein synthesis, protein folding, lipid and steroid synthesis, carbohydrate metabolism, calcium storage, and the transportation of substances throughout the cell. Except for sperm cells and red blood cells, the endoplasmic reticulum is observed in every other type of eukaryotic cell and is the largest organelle in the cell. The endoplasmic reticulum can be rough or smooth depending on the presence or absence of ribosomes. The rough endoplasmic reticulum has ribosomes attached to its surface making it appear rough. It lies immediately adjacent to the cell nucleus, and its membrane is continuous with the outer membrane of the nucleus (nuclear envelope). It plays a primary role in aiding protein synthesis, folding, and sorting, therefore, is prominent in cells that are involved in protein synthesis such as hepatocytes (liver cells). The smooth endoplasmic reticulum on the other hand is involved in the synthesis of lipids including cholesterol and phospholipids which are used in the production of new cellular membranes and steroid hormones and transportation of the products of the rough endoplasmic reticulum. It regulates the calcium ion concentration in the cytoplasm of some cells such as striated muscle cells and contributes to the detoxification of drugs and harmful chemicals in the cells of livers. Golgi Apparatus: This also known as Golgi bodies or Golgi complex is a membrane-bound organelle found near the endoplasmic reticulum and nucleus. It receives, modifies, sorts, and packages proteins and lipids produced by the endoplasmic reticulum (ER), and transports them to their final destinations within the cell or outside of it. The Golgi body comprises a 5 to 8-cup- shaped, series of compartments known as cisternae which is a flattened, disk-shaped, stacked pouches that make up the Golgi apparatus. Animal cells generally contain around 10 to 20 Golgi stacks per cell, which are connected by tubular connections. Function of Golgi Apparatus It receives and modifies proteins from the rough endoplasmic reticulum. The protein received from the rough endoplasmic reticulum can be modified by glycosylation, phosphorylation, sulfation, and cleavage They also sort and package the modified proteins into membrane-bound vesicles, which are then transported to various destinations, such as lysosomes and plasma membranes. They also take part in the transport of lipids and the formation of lysosomes Golgi apparatus is the site for the synthesis of various glycolipids, sphingomyelin, etc. Diagram Showing the Endoplasmic Reticulum and Golgi Apparatus  Diagram Showing the Structural Components of the Golgi Apparatus Lysosome: This is a spherical-shaped membrane-bound organelle that functions as the digestive system of the cell degrading material taken up from outside the cell and digesting obsolete components of the cell itself. They are therefore known as cell suicide bags. They contain an array of hydrolytic enzymes capable of breaking down all types of biological macromolecules (e.g. protein, nucleic acids, carbohydrates, and lipids) and enable cells to engulf and digest foreign bodies entering the cell. The lumen of the organelle, with a Ph range of 4.5 and 5.0 (acidic) contains cellular debris and the hydrolytic enzymes (over 50 of them) produced in the rough endoplasmic reticulum. The enzymes once produced, are brought in from the Golgi apparatus in tiny vesicles or sacs, which then merge with bigger acidic vesicles. The enzymes produced by the endoplasmic reticulum for lysosomes are mixed with the molecule of mannose 6-phosphate to get them fixed appropriately up into acidified vesicles. The nuclear gene controls the production of lysosomal enzymes. Any mutations in these genes that encode these enzymes can result in more than 30 different human genetic diseases, which are called lysosomal storage diseases because undegraded material accumulates within the lysosomes of affected individuals. Most of these diseases result from deficiencies in single lysosomal enzymes. For example, Gaucher’s disease (the most common of these disorders) results from a mutation in the gene that encodes a lysosomal enzyme required for the breakdown of glycolipids. Besides breaking down biomolecules, lysosomes are also involved in various other cell processes such as energy metabolism, counting discharged materials, cell signaling, and restoration of the plasma membrane. Diagram of a Lysosome Peroxisome: Peroxisome is a membrane-bound organelle found in all eukaryotic cells. It is a major site of oxygen utilization apart from mitochondria and contains over 60 oxidative enzymes, including catalase and urate oxidase which aid oxidative reactions. Apart from catalase enzymes, other enzymes contained in peroxisome produce hydrogen peroxide during reaction with the target substrate. However, hydrogen peroxide is potentially toxic to the cell, because it can react with many other molecules. Therefore, the catalase also contained in the peroxisome will convert hydrogen peroxide to water and oxygen molecules thereby neutralizing the toxicity. Also, catalase utilizes the H2O2 generated by other enzymes in the organelle to oxidize a variety of other substrates including phenols, formic acid, formaldehyde, and alcohol, by the “peroxidative” reaction producing water as a harmless molecule. In that way, peroxisomes provide a safe location for the oxidative metabolism of certain molecules. This type of oxidative reaction is particularly important in liver and kidney cells, where the peroxisomes detoxify various toxic molecules that enter the bloodstream. About 25% of the ethanol, we drink is oxidized to acetaldehyde in this way. A major function of the oxidative reactions performed in peroxisomes is the breakdown of fatty acid and amino acid molecules. In a process called β oxidation, the alkyl chains of fatty acids are shortened sequentially by blocks of two carbon atoms at a time, thereby converting the fatty acids to acetyl CoA. The acetyl CoA is then exported from the peroxisomes to the cytosol for reuse in biosynthetic reactions. In mammalian cells, β oxidation occurs in both mitochondria and peroxisomes but in the yeast and plant cells, it occurs only in peroxisomes. An essential biosynthetic function of animal peroxisomes is to catalyze the first reactions in the formation of plasmalogens, which are the most abundant class of phospholipids in myelin. Deficiency of plasmalogens causes profound abnormalities in the myelination of nerve cells, which is one reason why many peroxisomal disorders lead to neurological disease. Peroxisomes are unusually diverse organelles, and even in the various cell types of a single organism, they may contain different sets of enzymes. They can also adapt remarkably to changing conditions. Yeast cells grown on sugar, for example, have small peroxisomes. But when some yeasts are grown on methanol, they develop large peroxisomes that oxidize methanol; and when grown on fatty acids, they develop large peroxisomes that break down fatty acids to acetyl CoA by β oxidation. Functions of Peroxisomes Peroxisomes are specialized for carrying out oxidative reactions using molecular oxygen. They generate hydrogen peroxide, which they use for oxidative purposes—destroying the excess using the catalase they contain. In animals it has an important role in the synthesis of specialized phospholipids- plasmalogens required for nerve cell myelination. They also contribute to the biosynthesis of membrane lipids known as plasmalogens. In plant cells, peroxisomes carry out additional functions, including the recycling of carbon from phosphoglycolate during photorespiration and converting fatty acids to carbohydrates during seed germination (an action that ensures that energy is available for the germinating seed).  Firefly peroxisomes contain the luciferase enzyme, which promotes bioluminescence and helps the flies find a mate or a meal. NOTE The difference between lysosomes and peroxisomes is that catalase is present in peroxisomes to detoxify the hydrogen peroxide produced by the peroxisome's beta-oxidation of lipids which lysosome does not do rather they function by endocytosis, phagocytosis, and autophagy. Also, whereas lysosomal proteins are made in the rough ER with the aid of nuclear genes, and vesicles with the right enzymes branch off from the Golgi apparatus to create the lysosome, peroxisomes derive their proteins from the cytosol. Diagram of a Peroxisome Vacuole: A vacuole is a membrane-bound cell organelle found in both animal and plant cells performing functions such as storage, ingestion, digestion, excretion, and expulsion of excess water. The membrane surrounding the vacuole is known as tonoplast. In animal cells, vacuoles are generally small and help sequester waste products. It gets rid of harmful toxins or clears the extracellular space of those harmful toxins by bringing them into the cell for conversion; for chemical conversion into safer molecules. In plant cells, vacuoles help maintain water balance. Sometimes a single vacuole can take up most of the interior space of the plant cell. In  some instances, the waste product to be handled by the vacuole is water, therefore a vacuole would have as its function to maintain the balance of water inside and outside a cell. Function of Vacuole A vacuole stores salts, minerals, pigments, and proteins within the cell. The solution that fills a vacuole is known as the cell sap. It is filled with protons from the cytosol that help in maintaining an acidic environment within the cell. A large number of lipids are also stored within the vacuoles. The vacuoles are filled with water and exert force on the cell wall. This is known as turgor pressure. It provides shape to the cell and helps it to withstand extreme conditions. They are also involved in endocytosis and exocytosis of unwanted particles in the cell just like lysosome therefore the liking of vacuole to lysosome. Diagram of Vacuole Plasmid: This is a double-membraned cell organelles that play a primary role in the manufacturing and storing of food. There are three types of plastids;  Chromoplasts: These are the color plastids, found in all flowers, and fruits and are mainly responsible for their distinctive colors. Leucoplasts: They are colorless plastids and are mainly used for the storage of starch, lipids, and proteins within the plant cell. Chloroplasts: These are green-colored plastids, usually oval or biconvex in shape which comprise green-colored pigments within the plant cell called chlorophyll that capture sunlight and convert it into useful energy, thereby, releasing oxygen from water. Other pigments, such as carotenoids, are also present in chloroplasts and serve as accessory pigments, trapping solar energy and passing it to chlorophyll. The size of the chloroplast usually varies between 4-6 µm in diameter and 1-3 µm in thickness and is found exclusively in green plants and algae cells. They are the food producers of plants mostly found in parenchyma cells of the mesophyll located in the leaves of the plants. They have circular DNA molecules just like mitochondria making them semiautonomous and with this, they produce proteins and lipids required for the production of chloroplast membranes. The structure of chloroplast consists of a chloroplast envelope which is a double membrane made of outer and inner membranes and between is the intermembrane space. The outer membrane is a membrane envelope that is made of inner and outer lipid bilayer membranes. A third, internal membrane, extensively folded and characterized by the presence of closed disks (or thylakoids), is known as the thylakoid membrane. The inner membrane separates the stroma from the intermembrane space. The inner membrane is made of grana and stroma. Grana is composed of stacks of disc-shaped structures known as thylakoids or lamellae which contain the chlorophyll pigments and are the functional units of chloroplasts. Each granum is made up of about 20-30 thylakoids and different protein complexes including photosystem I, photosystem II, and ATP synthase which are specialized for light-dependent photosynthesis. On the other hand, the stroma is the homogenous matrix that contains grana and is similar to the cytoplasm in cells in which all the organelles are embedded. Stroma also contains various enzymes, DNA, ribosomes, and other substances. Stroma lamellae function by connecting the stacks of thylakoid sacs or grana. The most important function of the chloroplast is to synthesize food by the process of photosynthesis. Diagram of a Chloroplast Cell Wall: A cell wall is a permeable non-living component of a cell, covering the outmost layer of a cell and separating the interior contents of the cell from the exterior environment. Generally, a cell wall provides shape, support, and protection to the cell and its organelles. The cell wall is present exclusively in eukaryotic plants, fungi, and a few prokaryotic organisms. Its composition varies according to the organism. Fungi possess cell walls, made up of chitin, also found in the exoskeletons of arthropods. Just like the cell walls in plants, they provide structural support and prevent desiccation. Bacteria and archaea also contain cell walls internally made of large polymers known as peptidoglycans and externally composed of lipoproteins and lipopolysaccharides. Cell walls in prokaryotes serve as a form of protection and prevent lysis (bursting of the cell and expulsion of cellular contents). The plant cell wall is generally arranged in 3 layers and composed of carbohydrates, like pectin, cellulose, hemicellulose, and other smaller amounts of minerals, which form a network along with structural proteins to form the cell wall. The three major layers are: 1. Primary Cell Wall: The primary cell is situated closest to the inside of the cell and is the first-formed cell wall. It is mainly made up of cellulose, allowing the wall to stretch for growth but can also be made of pectic polysaccharides and structural proteins. It is thinner and more permeable than other layers. 2. The Middle Lamella: This is the outermost layer of cell wall and it acts as an interface between the other neighboring cells gluing them together. 3. The Secondary Cell Wall: The secondary cell wall is formed inside the primary cell wall once the cell is completely grown. It consists of cellulose and lignin that provide additional rigidity and waterproofing. It is the thickest layer but permeable layer providing the characteristic rectangular or square shape of a cell. Functions of the Cell Wall The cell wall provides definite shape, strength, and rigidity to the organisms. It provides rigid building blocks from which stable structures of higher order, such as leaves and stems can be produced. It protects the organisms against mechanical stress and physical shocks. It helps to control cell expansion due to the intake of water. It helps in preventing water loss from the cell. It provides a porous medium for the circulation and distribution of water, minerals, and other small nutrient molecules. It provides a storage site for regulatory molecules that sense the presence of pathogenic microbes and controls the development of tissues. It acts as a barrier between the interior cellular components and the external environment.  Diagram of a Cell Wall Cytoskeleton: This is a non-membranous network of fibers made of filamentous proteins, that helps maintain cell shape and internal organization of the cell’s parts. It also provides mechanical support that enables cells to carry out essential functions like division and movement. All cytoskeletons consist of three major classes of elements that differ in size and protein composition. They are microtubules, actin filaments (microfilaments), and intermediate filaments. Microtubules are the largest type of filament, with a diameter of about 25 nanometers (nm), and they are composed of a protein called tubulin. They are hollow, straw-shaped filaments formed by thirteen protofilaments containing dimers of two polypeptide subunits and are ever-changing, with reactions constantly adding and subtracting tubulin dimers at both ends of the filament. The rates of change at either end are not balanced — one end grows more rapidly and is called the plus end, whereas the other end is known as the minus end. In cells, the minus ends of microtubules are anchored in structures called microtubule organizing centers. The primary MTOC in a cell is called the centrosome, and it is usually located adjacent to the nucleus. In non-dividing cells, microtubule networks radiate out from the centrosome to provide the basic organization of the cytoplasm, including the positioning of organelles.  Diagram of Microtubules The protein actin is abundant in all eukaryotic cells. Actin filaments are extremely dynamic and can rapidly form and disassemble. It was first discovered in skeletal muscle, where actin filaments slide along filaments of another protein called myosin to make the cells contract. Actin filaments are made up of identical actin proteins arranged in a long spiral chain. Like microtubules, actin filaments have plus and minus ends, with more ATP-powered growth occurring at a filament’s plus end. In many types of cells, networks of actin filaments are found beneath the cell cortex, which is the meshwork of membrane-associated proteins that supports and strengthens the plasma membrane. Such networks allow cells to hold — and move — specialized shapes, such as the brush border of microvilli. Actin filaments are also involved in cytokinesis and cell movement. Intermediate filaments come in several types, but they are generally non-polar strong, and ropelike. Their functions are primarily mechanical and, as a class, intermediate filaments are less dynamic than actin filaments or microtubules. Intermediate filaments commonly work in tandem with microtubules, providing strength and support for the fragile tubulin structures. Diagram of Actin Filament All cells have intermediate filaments, but the protein subunits of these structures vary. Some cells have multiple types of intermediate filaments, and some intermediate filaments are associated with specific cell types. For example, neurofilaments are found specifically in neurons (most prominently in the long axons of these cells), and keratins are found specifically in epithelial cells. Other intermediate filaments are distributed more widely. For example, vimentin filaments are found in a broad range of cell types and frequently colocalize with microtubules. Similarly, lamins are found in all cell types, where they form a meshwork that reinforces the inside of the nuclear membrane. Diagram of Intermediate Filaments Ribosome: This is a non-membranous organelle produced in the nucleolus of the cell nucleus and made of both RNA and protein therefore known as ribonucleoprotein. It is composed of smaller and larger subunits. The smaller subunit is where the mRNA binds and is decoded, and the larger subunit is where amino acids get added. Both of the subunits contain protein and ribonucleic acid components and are joined to each other by interactions between the rRNAs in one subunit and proteins in the other subunit. Ribosomes are the site of protein synthesis in the cell. They read the messenger RNA (mRNA) sequence and translate that genetic code into a specified string of amino acids, which grow into long chains that fold to form proteins. Generally, the proteins synthesized in the cytoplasm are utilized in the cytoplasm itself, the proteins synthesized by bound ribosomes are transported outside the cell. Diagram of Ribosome Attached to Endoplasmic Reticulum NOTE Generally, cells from different tissues or organisms will be composed of organelles in different proportions. Whereas all cells contain ribosomes, cytoplasm, and cell membranes, some animal cells do not have cell walls and chloroplast. Also, the cells of the liver (hepatocytes) and muscles (myocytes) will usually contain endoplasmic reticulum, peroxisomes, and mitochondria in different proportions based on the function of the cells. Mature red blood cells, blood platelets, keratinocytes, and lens fibre cells have a nucleus at the juvenile stage of the cell but lose the structure as the cell matures as an anatomical and physiological adaptation mechanism to enable them to carry out its functions. Furthermore, some cells keep dividing throughout their life duration whereas some stop at the maturity stage. Examples of cells that halt division at a mature stage are mature cardiomyocytes, neurons, myocytes, lens fiber cells, and red blood cells. They are known as terminally differentiated cells.