Basic Cell Biology PDF

26-09-2024 In biology nothing makes sense, except in the light of evol...

26-09-2024 In biology nothing makes sense, except in the light of evolution. —Theodosius Dobzhansky, Geneticist 1900–1975 Basic Cell Biology The first organisms were microorganisms, and they had a profound impact on Earth’s development. 1 2 Introduction to cell biology Introduction to Cell Biology - 2 Evolution has produced an immense diversity of life forms Biochemical studies demonstrated the underlying unity of the living world A study predicted that there are ~8.7 (+/- 1.3) million different life forms (all organisms has proteins, lipids, nucleic acids, inorganic compounds, Ancient philosophers Aristotle (~350 BCE) and Paracelsus (~1500 AD) genetic material, etc) concluded that “All animals and plants, however complicated, are Advent of Electron Microscope (EM) helped discover subcellular structures constituted of a few elements which are repeated in each of them.” (probably referring to the macroscopic structures of an organism, like roots, Cell fractionations helped isolate and study various subcellular organelle. leaves, and flowers common to different plants, or segments and organs that Living matter demonstrate series of integrated levels of organization are repeated in the animal kingdom). The rules/laws encountered at one level may not apply at a lower level Later, the invention of magnifying lenses (microscopes) led to the discovery of the microscopic world The diversity of the living world depends on the genetic program coded in the nucleic acids, executed through complex regulatory circuits Biochemical studies have demonstrated that living matter itself is composed of the same elements that comprise the inorganic world (although Characteristic of a cell type is a result of the expression of different genes fundamental differences do exists in their organization (Jacob, 1977) 3 4  26-09-2024 Miller-Urey Experiment Human Eye Limit of Light Microscope Electron Microscope 5 6 7 8  26-09-2024 9 10 Schwann claimed that "there is one universal principle of development for the elementary parts of organisms, however different, and this principle is the formation of cells Schwann supported this claim by examining adult animal tissues and showing that all tissues could be classified in terms of five types of highly differentiated cellular tissues. cells that are independent and separate, e.g. blood cells cells that are independent but compacted together in layers, e.g. skin, fingernails, feathers cells whose connecting walls have coalesced, e.g. cartilage, bones, and tooth enamel elongated cells forming fibers, e.g. tendons and ligaments cells formed by the fusion of walls and cavities, e.g. muscles, tendons and nerves 11 12  26-09-2024 The Universal Features of Cells on Earth All cells store their hereditary information in the same linear chemical code-DNA Replicate their hereditary information by templated polymerization Transcribe portion of their hereditary information into the same intermediary form- RNA All cells use proteins as a catalyst (some RNA molecule also act as catalyst) Translate protein in the same way Each protein is coded by the same specific gene Functions as biochemical factories dealing with the same basic molecular building blocks Enclosed in a plasma membrane across which nutrients and waste materials must pass 13 14 1.2 Tracing Biological Evolution Three domains of organisms are now Whittaker postulated division of living beings into five kingdoms: recognized: Monera, Protista, Fungi, Plantae, and Animalia Only Monera (e.g. bacteria and blue green algae) are prokaryotic Bacteria cells, while other kingdoms consists of eukaryotic cells Archaea – many live in extreme environments Eukarya – fungi, algae, protozoa, plants, animals 15 16  26-09-2024 Figure 1.6 Tree of Life 1.2 Tracing Biological Evolution Ribosomes most important in tracing evolution Ribosomes—sites of protein synthesis— occur in all organisms. Ribosomes consist of RNA and proteins. Ribosomal RNA (rRNA) has changed very slowly during evolution: highly conserved. rRNA is used to study relatedness among organisms, especially 16S and 18S. 17 18 The most difficult questions How did the first cell originate? What were its characteristics? Was the first cell a progenitor of all life? The questions still remain unanswered 19 20  26-09-2024 21 22 23 24  26-09-2024 Blue-green algae & mycoplasma 25 26 27 28  26-09-2024 29 30 Organelle Markers (predominantly marker enzymes) nuclei = DNA (intactness) mitochondria = matrix = fumarase (intactness) inner membrane = cyt c oxidase, succinate dehydrogenase plastids = rubisco (intactness) chloroplasts = thylakoid membranes = chlorophyll glyoxysomes = catalase, isocitrate lyase, malate synthase (all check intactness) peroxisomes = catalase (intactness) ER = antimycin A insensitive cyt c reductase RER = RNA, Mg++ shift Golgi apparatus = detergent-stimulated IDPase (intactness) plasma membrane = vanadate-sensitive ATPase tonoplast = nitrate-sensitive ATPase vacuoles = RNase, anthocyanins, betacyanins (all check intactness) protein bodies = storage proteins (intactness) lipid bodies = acid lipase (intactness) cytosol = alcohol dehydrogenase 31 32  26-09-2024 Example of marker enzyme distributions in differential centrifugation Nucleus Genetic information is stored in the nucleus. (some genes are also present in mitochondria and plastids) Usually ~5 µm in diameter Surrounded by double lipid membrane (two lipid bilayer) Each lipid bilayer (with associated proteins) separated by space of 20- 40nm Each membrane is perforated Pores has protein structures (pore complex), that play important role in cellular regulation by controlling entry and exit of RNA and proteins Mammalian RBCs have no nucleus, whereas osteoclast (involve in the repair of bone tissue) have many nuclei 33 34 Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array Within the nondividing nucleus is the nucleolus (plural, nucleoli), which appears through of protein filaments (in animal cells, called intermediate filaments) that maintains the shape of the the electron microscope as a mass of densely stained granules and fibers adjoining part nucleus by mechanically supporting the nuclear envelope. of the chromatin. There is also much evidence for a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic Here, ribosomal RNA (rRNA) is synthesized from genes in the DNA. material so it functions efficiently. Also in the nucleolus, proteins imported from the cytoplasm are assembled with rRNA Within nucleus, DNA is organized into chromosomes. into large and small subunits of ribosomes. These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit can assemble into Each chromosome is a long DNA chain associated with many proteins (including histones) a ribosome. DNA protein complex making up the chromosome is called chromatin Sometimes, there are two or more nucleoli; the number depends on the species and the When a cell is not dividing, stained chromatin appears as a diffuse mass in micrographs, and the stage in the cell’s reproductive cycle. chromosomes cannot be distinguished from one another, even though discrete chromosomes are present. As a cell prepares to divide, however, the chromosomes form loops and coil, condensing and becoming thick enough to be distinguished under a microscope as separate structures. Each species has a characteristic number of chromosomes 35 36  26-09-2024 The nuclear envelope. The double-membrane envelope is penetrated by pores in which nuclear pore complexes (not shown) are positioned. The outer nuclear membrane is continuous with the endoplasmic reticulum (ER). The ribosomes that are normally bound to the cytosolic surface of the ER membrane and outer nuclear membrane are not shown. The nuclear lamina is a fibrous protein meshwork underlying the inner membrane. 37 38 Ribosomes Comparison of Prokaryotic & Eukaryotic Ribosomes Ribosomes consists of rRNA and proteins and are the site of protein ribosome subunit rRNAs r-proteins synthesis 23S (2904 nt) Discovered by Palade in 1955 70S (Bacteria & 50S 31 (L1, L2,…..) 5S (120 nt) Archaea) Each ribosome consist of two subunits – a small subunit and a larger subunit 30S 16S (1542 nt) 21 (S1, S2,…..) Prokaryotes have the 70S (30S + 50S) type ribosomes and eukaryotes have ribosome subunit rRNAs r-proteins 80S (40S + 60S) type ribosomes. 28S (4718 nt) Smaller subunit has the decoding function and is bound to mRNA 80S 60S 5.8S (160 nt) 49 (L1, L2, L3,…..) Larger subunit mainly has catalytic function and is bound to aminoacylated (Eukaryotes) 5S (120 nt) tRNA 40S 18S (1874 nt) 33 (S1, S2, S3,….) Ribosomes are present free in the cytoplasm or are bound to the Endoplasmic Reticulum (rough ER) Although Archaeal ribosomes are similar to Bacterial Ribosomes, at the sequence level archaeal ribosomes are similar to Eukaryotes 39 40  26-09-2024 Plastoribosomes and Mitoribosomes The Endomembrane System In eukaryotes ribosomes are present in plastids and mitochondria also. Consists of the envelope, endoplasmic reticulum (ER), and golgi complex Ribosomes present in Plastids are known as Plastoribosomes (other membrane-bound organelles are also part of the endomembrane system) Ribosomes present in Mitochondria are known as Mitoribomes The nuclear envelope is made of flattened sacs or cisternae. Has two lipid Plastoribosomes and Mitoribosomes are 70S types (similar to prokaryotic bilayer ribosomes) The two membranes merge at pores, allowing transfer of material between Mitochondria and Plastids are considered to have originated from bacteria the nucleus and cytoplasm The mitochondrial genome contains 37 genes that encode 13 proteins, 22 ER forms the bulk of endomembrane system. Made up of tubules and tRNAs, and 2 rRNAs. The 13 mitochondrial gene-encoded proteins all flattened sacs. Two types: RER and SER instruct cells to produce protein subunits of the enzyme complexes of the oxidative phosphorylation system, which enables mitochondria to act as the Golgi complex is a differentiated part of the endomembrane system. ER and powerhouses of our cells. Golgi complex are involved in the formation of lysosomes and peroxisomes. 41 42 Endoplasmic Reticulum (ER) The ER is continuous with the outer layer of the nuclear envelope It forms the bulk of the endomembrane system It is of two types: Rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER) The outer surface of RER is covered with Ribosomes which synthesize proteins that are delivered into the lumen of the reticulum The SER (or agranular ER) is continuous with RER and is involved in the transport of products with their cavities. 43 44  26-09-2024 Smooth Endoplasmic Reticulum (SER) Rough Endoplasmic Reticulum (RER) Performs diverse metabolic processes which vary with the cell types, including synthesis of lipids, Proteins that are secreted are produced by ribosomes on the RER. For E.g. Insulin carbohydrate metabolism, detoxification of drugs and poisons, & storage of calcium ions. hormone is synthesized on the RER of pancreatic cells and secreted in the bloodstream Enzymes synthesize lipids, steroids, new membrane phospholipids. A lot of secreted proteins are glycoproteins (proteins with attached carbohydrates). The carbohydrate moiety is attached to the protein in the ER lumen In animals, steroids synthesized are sex hormones and various steroid hormones synthesized in the adrenal glands. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud The cells that synthesize and secrete these hormones—in the testes and ovaries, for example— like bubbles from a specialized region called transitional ER are rich in smooth ER, a structural feature that fits the function of these cells. Other enzymes of the smooth ER help detoxify drugs and poisons, especially in liver cells. Detoxification usually involves adding -OH groups to drug, making them more water-soluble and easier to flush out. Drugs like barbiturates, alcohol, and others induce the proliferation of smooth ER and its associated detoxification enzymes, thus increasing the rate of detoxification. This, in turn, increases tolerance to the drugs, meaning that higher doses are required to achieve a particular effect, such as sedation. Also, because some of the detoxification enzymes have relatively broad action, the proliferation of smooth ER in response to one drug can increase the need for higher dosages of other drugs as well. Barbiturate abuse, for instance, can decrease the effectiveness of certain antibiotics and other useful drugs. 45 46 Rough Endoplasmic Reticulum Golgi Apparatus After leaving the ER, many transport vesicles travel to the Golgi apparatus. Rough ER is a membrane factory for the cell; it grows in place by adding membrane Can consider Golgi as a warehouse for receiving, sorting, shipping, and even some proteins and phospholipids to its own membrane. manufacturing. As polypeptides destined to be membrane proteins grow from the ribosomes, they are Products of the ER, like proteins, are modified and stored and then sent to other inserted into the ER membrane itself and anchored there by their hydrophobic portions. destinations. Like the smooth ER, the rough ER also makes membrane phospholipids; enzymes built Golgi apparatus is especially extensive in cells specialized for secretion. into the ER membrane assemble phospholipids from precursors in the cytosol. Golgi apparatus consists of a group of associated, flattened membranous sacs— The ER membrane expands, and portions of it are transferred in the form of transport cisternae. Cells can have many, even hundreds, of these stacks. vesicles to other components of the endomembrane system. Golgi stack has a distinct structural directionality, with the membranes of cisternae on opposite sides of the stack differing in thickness and molecular composition It has two sides – cis (meaning on the same side) and trans face. Cis-face is usually located near ER. Cis-face acts as receiving department, while trans-face as shipping department 47 48  26-09-2024 Golgi Apparatus Golgi Apparatus Vesicle that buds from the ER can add its membrane and the contents of its lumen to Golgi is dynamic the cis face by fusing with a Golgi membrane on that side. The cisternal maturation model, the cisternae of the Golgi actually progress forward The trans face gives rise to vesicles that pinch off and travel to other sites. from the cis to the trans face, carrying and modifying their cargo as they move. Products from ER are usually modified while they transition from cis to trans face Before a Golgi stack dispatches its products by budding vesicles from the trans face, it sorts these products and targets them for various parts of the cell. E.g. glycoproteins from ER when reaches Golgi, some carbohydrate monomers are removed and new monomers are added Molecular identification tags, such as phosphate groups added to the Golgi products, aid in sorting by acting like zip codes on mailing labels. Membrane phospholipids are altered Finally, transport vesicles budded from the Golgi may have external molecules on their Golgi also manufactures molecules. membranes that recognize “docking sites” on the surface of specific organelles or on the Many polysaccharides secreted by cells are Golgi products. For example, pectins and plasma membrane, thus targeting the vesicles appropriately. certain other non-cellulose polysaccharides are made in the Golgi of plant cells and then incorporated along with cellulose into their cell walls. Golgi manufactures and refines its products in stages, with different cisternae containing unique teams of enzymes. 49 50 Lysosomes Lysosomes are membranous sacs filled with hydrolytic enzymes used to digest macromolecules Hydrolytic enzymes in lysosomes work best in acidic environment If lysosomes leaks (or break open), the enzymes will not work in the neutral cytoplasmic pH (although leakage from multiple lysosomes can destroy cell) Hydrolytic enzymes and lysosome membranes make in RER, and then goes to Golgi for further processing Lysosomes bud from the trans face of Golgi apparatus How are the proteins of the inner surface of the lysosomal membrane and the digestive enzymes themselves spared from destruction? 51 52  26-09-2024 Lysosomes Performs intracellular digestion in variety of circumstances: Unicellular protists, like amoeba, engulf food or smaller organism by phagocytosis. The food vacuole then fuses with lysosomes, which digest the food. Digestion products, including simple sugars, amino acids, and other monomers, pass into the cytosol and become nutrients for the cell Many human cells, like macrophage (a type of WBC) protects body by engulfing bacteria and other pathogens. Also use their hydrolytic enzymes to recycle cell’s own material by autophagy. Damaged organelle or small amount of cytosol becomes surrounded by a double membrane, and a lysosome fuses with the outer membrane of this vesicle. Genetic disorders associated with lysosome storage enzymes. e.g. Tay-Sachs disease, a lipid digesting enzyme is missing or inactive, and the brain becomes impaired by an accumulation of lipids in the cells. 53 54 Vacuoles Vacuoles Vacuoles are vesicles derived from the endoplasmic reticulum Some plant vacuoles contain pigments, such as the red and blue pigments of petals that Vacuolar membrane is selective in transporting solutes; as a result, the solution inside a help attract pollinating insects to flowers. vacuole differs in composition from the cytosol Mature plant cells generally contain a large central vacuole, which develops by the Performs different functions in different types of cells coalescence of smaller vacuoles. Food vacuoles formed by phagocytosis The solution inside the central vacuole, called cell sap, is the plant cell’s main repository Many unicellular protists living in freshwater have contractile vacuoles that pump excess of inorganic ions, including potassium and chloride. water out of the cell, thereby maintaining a suitable concentration of ions and molecules The central vacuole plays a major role in the growth of plant cells, which enlarge as the inside the cell vacuole absorbs water, enabling the cell to become larger with a minimal investment in In plants and fungi, certain vacuoles carry out enzymatic hydrolysis, a function shared by new cytoplasm lysosomes in animal cells In plants, small vacuoles can hold reserves of important organic compounds, such as the proteins stockpiled in the storage cells in seeds. Vacuoles may also help protect the plant against herbivores by storing compounds that are poisonous or unpalatable to animals. 55 56  26-09-2024 57 58 Mitochondria and chloroplasts change energy from one form to another The endosymbiont theory of the origins In eukaryotic cells, mitochondria and chloroplasts are the organelles that convert energy of mitochondria and chloroplasts in to forms that cells can use for work. eukaryotic cells. Mitochondria (singular, mitochondrion) are the sites of cellular respiration, the According to this theory, the proposed metabolic process that uses oxygen to drive the generation of ATP by extracting energy ancestors of mitochondria were oxygen from sugars, fats, and other fuels. using non-photosynthetic prokaryotes Chloroplasts, found in plants and algae, are the sites of photosynthesis. This process in that were taken into host cells, while the chloroplasts converts solar energy to chemical energy by absorbing sunlight and using it proposed ancestors of chloroplasts were to drive the synthesis of organic compounds such as sugars from carbon dioxide and photosynthetic prokaryotes. The large water. arrows represent change over Mitochondria and Chloroplast share a similar evolutionary origin. They display evolutionary time; the small arrows similarities with bacteria that led to the endosymbiont theory inside the cells show the process of the endosymbiont becoming an organelle, also over long periods of time. 59 60  26-09-2024 Support for the Endosymbiotic Theory Mitochondria: Chemical Energy Conversion Found in nearly all eukaryotes (Monocercomonoides sp. is a eukaryotic microorganism Rather than being bounded by a single membrane like organelles of the with no mitochondria) endomembrane system, mitochondria and typical chloroplasts have two membranes Some cells have single large mitochondrion, whereas some can have hundreds or even surrounding them. thousands of mitochondria. The number co-relates with cell’s metabolic activity Like prokaryotes, mitochondria and chloroplasts contain ribosomes, as well as circular The two membranes enclosing the mitochondrion is a phospholipid bilayer with a DNA molecules—like bacterial chromosomes—associated with their inner unique collection of embedded proteins membranes. The DNA in these organelles programs the synthesis of some organelle The outer membrane is smooth, but the inner membrane is convoluted, with infoldings proteins on ribosomes that have been synthesized and assembled there as well called cristae. Cristae gives mitochondria large surface area to carry out cellular Also consistent with their probable evolutionary origins as cells, mitochondria and respiration chloroplasts are autonomous (somewhat independent) organelles that grow and The inner membrane divides the mitochondrion into two internal compartments. The reproduce within the cell first is the intermembrane space, the narrow region between the inner and outer membranes. The second compartment, the mitochondrial matrix, is enclosed by the inner membrane. Human mitochondrial DNA is 16569 bp, and codes for 37 genes (13 protein-coding, 22 tRNA, and 2 rRNA genes) 61 62 Mitochondria Mitochondria are generally in the range of 1–10 μm long. Time-lapse films of living cells reveal mitochondria moving around, changing their shapes, and fusing or dividing into separate fragments, unlike the static structures seen in most diagrams and electron micrographs 63 64  26-09-2024 Chloroplasts: Capture of Light Energy Chloroplasts contain the green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar Are about 3–6 μm in length, are found in leaves and other green organs of plants and in algae Has an envelope consisting of two membranes separated by a very narrow intermembrane space. Inside the chloroplast is another membranous system in the form of flattened, interconnected sacs called thylakoids. In some regions, thylakoids are stacked like poker chips; each stack is called a granum (plural, grana). The fluid outside the thylakoids is the stroma, which contains the chloroplast DNA and ribosomes as well as many enzymes The membranes of the chloroplast divide the chloroplast space into three compartments: the intermembrane space, the stroma, and the thylakoid space. 65 66 Peroxisomes: Oxidation Peroxisome is a specialized metabolic compartment bounded by a single membrane. Chloroplasts are dynamic. Their shape is changeable, and they grow and occasionally Peroxisomes contain enzymes that remove hydrogen atoms from various substrates and pinch in two, reproducing themselves. They are mobile and, with mitochondria and other transfer them to oxygen (O2), producing hydrogen peroxide (H2O2) as a by-product (from organelles, move around the cell along tracks of the cytoskeleton which the organelle derives its name). The chloroplast is a specialized member of a family of closely related plant organelles Some peroxisomes use oxygen to break fatty acids down into smaller molecules that are called plastids. transported to mitochondria and used as fuel for cellular respiration. One type of plastid, the amyloplast, is a colorless organelle that stores starch (amylose), Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring particularly in roots and tubers. hydrogen from the poisonous compounds to oxygen. Another is the chromoplast, which has pigments that give fruits and flowers their orange The H2O2 formed by peroxisomes is itself toxic, but the organelle also contains an enzyme and yellow hues. that converts H2O2 to water. The enzymes that produce H2O2 and those that dispose of this toxic compound are sequestered away from other cellular components that could be damaged. Peroxisomes grow larger by incorporating proteins made in the cytosol and ER, as well as lipids made in the ER and within the peroxisome itself. 67 68  26-09-2024 Peroxisomes Cytoskeleton Specialized peroxisomes called Earlier it was thought that the organelles of a eukaryotic cell floated freely in the glyoxysomes are found in the fat- cytosol. storing tissues of plant seeds. Improvements in both light microscopy and electron microscopy have revealed Glyoxysomes contain enzymes the cytoskeleton, a network of fibers that initiate the conversion of fatty acids to sugar, which the emerging extending throughout the cytoplasm. seedling uses as a source of The eukaryotic cytoskeleton plays a major role in organizing the structures and energy and carbon until it can activities of the cell. produce its own sugar by photosynthesis. Peroxisomes are roughly spherical and often have a granular or crystalline core that is thought to be a dense collection of enzyme molecules. Chloroplasts and mitochondria cooperate with peroxisomes in certain metabolic functions (TEM). 69 70 Roles of Cytoskeleton: Support and Motility Cytoskeleton gives mechanical support to the cell and maintains its shape, especially important for animal cells, which lack walls. The remarkable strength and resilience of the cytoskeleton as a whole are based on its architecture. Like a dome tent, the cytoskeleton is stabilized by a balance between opposing forces exerted by its elements. Cytoskeleton provides anchorage for many organelles and even cytosolic enzyme molecules. Cytoskeleton is more dynamic than an animal skeleton. It can be quickly dismantled in one part of the cell and reassembled in a new location, changing the shape of the cell. Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move along fibers outside the cell 71 72  26-09-2024 Microtubules Centrosomes and Centrioles In animal cells, microtubules grow out from a centrosome, These microtubules function as compression-resisting girders of the Microtubules, hollow rods constructed from globular proteins called tubulins. cytoskeleton. Each tubulin protein is a dimer, consisting of α-tubulin and β-tubulin. Within the centrosome is a pair of centrioles, each composed of nine sets of triplet microtubules arranged in a ring (many other eukaryotic cells lack centrosomes with Microtubules grow in length by adding tubulin dimers; they can also be disassembled centrioles and instead organize microtubules by other means). and their tubulins used to build microtubules elsewhere in the cell. Cilia and Flagella Some eukaryotic cells have flagella that contains microtubules Because of the orientation of tubulin dimers, the two ends of a microtubule are Many unicellular protists are propelled through water by cilia or flagella that act as slightly different. locomotor appendages, and the sperm of animals, algae, and some plants have One end can accumulate or release tubulin dimers at a much higher rate than the flagella. other, thus growing and shrinking significantly during cellular activities. (This is called Motile cilia is present in large number, whereas flagella is limited to one or few per the “plus end,” not because it can only add tubulin proteins but because it’s the end cell. where both “on” and “off” rates are much higher.) Flagella usually longer than cilia and differs in their beating pattern. flagellum has an Shape and support cell. Serve as tracks along which organelles with motor proteins undulating motion like the tail of a fish. In contrast, cilia have alternating power and move. Microtubules guide vesicles from ER to Golgi and from Golgi to plasma recovery strokes, much like the oars of a racing crew boat membrane. Also involved in the separation of chromosomes during cell division. A cilium may also act as a signal-receiving “antenna” for the cell 73 74 Beating pattern, motile cilia and flagella share a common structure. Each motile cilium or flagellum has a group of microtubules sheathed in an extension of the plasma Membrane. Nine doublets of microtubules are arranged in a ring with two single microtubules in its center. This arrangement, referred to as the “9 + 2” pattern, is found in nearly all eukaryotic flagella and motile cilia. (Nonmotile primary cilia have a “9 + 0” pattern, lacking the central pair of microtubules.) The microtubule assembly of a cilium or flagellum is anchored in the cell by a basal body, which is structurally very similar to a centriole, with microtubule triplets in a “9 + 0” pattern. 75 76  26-09-2024 Microfilaments (Actin Filaments) Thin solid rods built from actin. Is a twisted double chain of actin subunits Present in all eukaryotic cells The structural role of microfilaments in the cytoskeleton is to bear tension (pulling forces) Network formed by microfilaments just to the inside of the plasma membrane (cortical microfilaments) helps support the cell’s shape Thousands of actin filaments and thicker filaments made of a protein called myosin interact to cause contraction of muscle cells localized contractions brought about by actin and myosin are involved in the amoeboid (crawling) movement of the cells. In plant cells, actin-protein interactions contribute to cytoplasmic streaming 77 78 Intermediate Filaments Intermediate filaments have a diameter larger than microfilaments but smaller than microtubules Found in only some animal cells Diverse class of cytoskeletal elements. Each type is constructed from a particular molecular subunit belonging to a family of proteins whose members include the keratins Intermediate filaments are more permanent fixtures of cells than are microfilaments and microtubules, which are often disassembled and reassembled in various parts of a cell. Even after cells die, intermediate filament networks often persist; for example, the outer layer of our skin consists of dead skin cells full of keratin filaments are especially sturdy and play an important role in reinforcing the shape of a cell and fixing the position of certain organelles. E.g. nucleus typically sits within a cage made of intermediate filaments. Other intermediate filaments make up the nuclear lamina, which lines the interior of the nuclear envelope 79 80  26-09-2024 Cell Walls of Plants Help plants maintain its shape and prevent excessive uptake of water. Some bacteria and protists also have a cell wall Plant cell walls are much thicker than the plasma membrane, ranging from 0.1 μm to several micrometers, and their exact composition varies from species to species and even from one cell type to other Plant cell first secretes a relatively thin and flexible wall called the primary cell wall. Between primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaccharides called pectins. The middle lamella glues adjacent cells together. Cells add a secondary cell wall between the plasma membrane and the primary wall Plant cell walls are perforated with plasmodesmata (singular, plasmodesma; from the Greek desma, bond), channels that connect cells. 81 82 83 84  26-09-2024 85