# Cell Biology/Cell types/Eukaryotes

Eukaryotes house a distinct nucleus, a structure in which the genetic material (DNA) is contained, surrounded by a membrane much like the outer cell membrane. Eukaryotic cells are found in most algae, protozoa, all multicellular organisms (plants and animals) including humans. The genetic material in the nucleus forms multiple chromosomes that are linear and complexed with proteins that help the DNA 'pack' and are involved in regulation of gene expression.
The cells of higher plants differ from animal cells in that they have large vacuoles, a cell wall, chloroplasts, and a lack of lysosomes, centrioles, pseudopods, and flagella or cilia. Animal cells do not have the chloroplasts, and may or may not have cilia, pseudopods or flagella, depending on the type of cell.

## Comparing Prokaryotic and Eukaryotic Cells

All cells have several basic features in common: they are all bounded by a selective barrier, plasma membrane. Cytosol is a jellylike substance that is semifluid. All cells contain chromosomes which carry genes in the form of DNA, and ribosomes that make proteins according to instructions from the gene. The major difference between prokaryotic and eukaryotic cells is the location of their DNA. In eukaryotic cell, DNA is found at the nucleus, which is bounded by a double membrane. (the word eukaryotic is from the Greek eu, true, and karyon, kernel, here referring to the nucleus).
Eukaryotic cells are much larger than prokaryotic cells; size is general aspect of cell structure that relates to function. The logistics of carrying out cellular metabolism sets limits on cell size. At the lower limit, the smallest cells, known are bacteria called mycoplasmas have diameters between 0.1 and 1.0mm. These are the smallest packages with enough DNA to program metabolism and enough enzymes and other cellular equipment to carry out the activities necessary for a cell to sustain itself and reproduce.
Metabolic requirements also impose theoretical upper limits on the size that is practical for a single cell. Plasma membrane functions as a selective barrier that allows sufficient passage of oxygen, nutrients, and wastes to service the entire cell. For each square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to volume is critical. As a cell increases in size, its volume grows proportionately more than its surface area. Area is proportional to a linear dimension squared, whereas volume is proportional to the linear dimension cubed. Therefore a smaller object has a greater ration of surface area to volume.
The need for a surface area sufficiently large to accommodate the volume helps explain the microscopic size of most cells, and the narrow, elongated shapes of others, such as nerve cells. Larger organisms has more cells compare to smaller cells. High ratio of surface area to volume is especially important in cells that exchange a lot of material with their surroundings such as intestinal cells. Such cells may have many long, thin projections from their surface called microvilli, which increase surface area without an appreciable increase in volume.

## Animal Cells

Flagellum: locomotion organelle present in some animal cells; composed of a cluster of microtubules within an extension of the plasma membrane.
Centrosome: region where the cell's microtubules are initiated contains a pair of centrioles which function is unknown.
Cytoskeleton: reinforces cell's shape, functions in cell movement components are made of protein. It includes microfilaments, intermediate filaments, and microtubules.
Microvilli: projections that increase the cell's surface area.
Peroxisome: organelle with carious specialized metabolic functions; produces hydrogen peroxide as a by-product, then converts it to water.
Mitochondrion: organelle where cellular respiration occurs and most ATP is generated.
Lysosome: digestive organelle where macromolecules are hydrolyzed.
Golgi apparatus: organelle active in synthesis, modification, sorting, and secretion of cell products.
Ribosomes: complexes (small brown dots) that make proteins; free in cytosol or bound to rough ER or nuclear envelope.
Plasma membrane: membrane enclosing the cell
Endoplasmic Reticulum (ER): network of membraneous sacs and tube; active in membrane synthesis and other synthetic and metabolic processes; has rough (ribosome-studded) and smooth regions. (Rough ER, and Smooth ER)
Nucleus: nucleus contains:
In animal cells, lysosomes, centrosomes with centrioles, and flagella are present but not in plant cells.

## Plant Cell

Cell Wall: outer layer that maintains cell's shape and protects cell from mechanical damage; made of cellulose, other polysaccharide, and protein.
Plasmodesmata: channels through cell walls that connect the cytoplasms of adjacent cells.
Chloroplast: photosynthetic organelle; converts energy of sunlight to chemical energy stored in sugar molecules.
Central vacuole: prominent organelle in older plant cells; functions include storage, breakdown of waste products, hydrolysis of macromolecules; enlargement of vacuole is a major mechanism of plant growth.
Nucleus: nucleus contains:
Golgi apparatus: organelle active in synthesis, modification, sorting, and secretion of cell products.
Endoplasmic Reticulum (ER): network of membraneous sacs and tube; active in membrane synthesis and other synthetic and metabolic processes; has rough (ribosome-studded) and smooth regions. (Rough ER, and Smooth ER)
Ribosomes: complexes (small brown dots) that make proteins; free in cytosol or bound to rough ER or nuclear envelope.
Cytoskeleton: reinforces cell's shape, functions in cell movement components are made of protein. It includes microfilaments, intermediate filaments, and microtubules.
In plant cell, chloroplasts, central vacuole, cell wall, and plasmodesmata are present but not in animal cells. Chromatin in the plant cell is a primary protein

## Nucleus

The nucleus contains most of the genes in the eukaryotic cell; some genes are located in mitochondria and chloroplast. It is generally the most conspicuous organelle in a eukaryotic cell. The nuclear envelope encloses the nucleus, sparating its contents from the cytoplasm. The nuclear envelope is a double membrane, each a lipid bilayer with associated proteins. The envelope is perforated by pore structure that are about 100nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. Pore complex lines each pore and regulates the entry and exit of most proteins and RNAs, as well as large complexes of macromolecules. Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. Also nuclear matrix, a framework of fibers extending throughout the nuclear interior, present.
Chromosomes are organized DNA units that carry the genetic information. Each chromosome is made up of material called chromatin, a complex of proteins and DNA. Stained chromatic usually appears as a diffuse mass, byt as a cell prepares to divide, the thin chromatin fibers coil up and condense thick enough to be distinguished as chromosomes. Each eukaryotic species has a characteristic number of chromosomes. For example human has 46 chromosomes.
Nucleolus is a prominent structure within the nondividing nucleus. Ribosomal RNA (rRNA) is synthesized from instructions in the DNA; in nucleolus, proteins imported from the cytoplasm are assembled with rRNA into large and small ribosomal subunits. Theses subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit can assemble into a ribosome. the number depends on the species and the stage in the cell's reproductive cycle.
The Nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA molecule reaches the cytoplasm, ribosomes translate the mRNA's genetic message into the primary structure of a specific poly peptide.

## Ribosomes

Ribosomes are complexes made of ribosomal RNA and protein; ribosomes are the cellular components that carry out proteins synthesis, also known as protein factories. Cells that have high rates of protein synthesis have particularly large number of ribosomes. Cells active in protein synthesis also have prominent nucleoli. Ribosomes build proteins in two cytoplasmic locales. Free ribosomes are suspended in the cytosol, while bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope. Bound and free ribosomes are structurally identical, and ribosomes can alternate between the two roles. Most of proteins are made on free ribosomes function within the cytosol. Bound ribosomes generally make proteins that are destined for insertion into membranes, for packaging within certain organelles such as lysosomes, or for export from the cell (secretion).

## The Endomembrane System

Endomembrane system carries out a variety of tasks in the cell. These tasks include synthesis of proteins and their transport into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membrane of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles. The various membranes are not identical in structure and function; the thickness, molecular composition, and types of chemical reactions carried out in a given membrane are not fixed but modified several times during the membrane's life. The endomembrane system includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, various kinds of vacuoles, and the plasma membrane.

## Endoplasmic Reticulum (ER)

Endoplasmic reticulum (ER) is an extensive network of membrane that it accounts for more than half the total membrane in many eukaryotic cells. The word endoplasmic means "within the cytoplasm", and reticulum is Latine for "little net". The ER consists of a network of membranous tubules and sacs called cisternae. The ER membrane separates the internal compartment of the ER, ER lumen (cavity) or cisternal space, from the cytosol. Since ER membrane is continuous with the nuclear envelope, the space between the two membranes of the envelope is continuous with the lumen of the ER. Smooth ER lacks ribosomes on its outer surface, and Rough ER has ribosomes on the outer surface of the membrane. Ribosomes are also attached to the cytoplasmic side of the nuclear envelope's outer membrane.
Smooth ER- The smooth ER functions in diverse metabolic processes, which vary with cell type. These processes include synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. Enzymes of the smooth ER are important in the synthesis of lipids, including oils, phospholipids, and steroids. Sex hormones of vertebrates and the various steroid hormones are produced by the smooth ER in animal cells. Other enzymes of the smooth ER help detoxify drugs and poisons in liver cells. Detoxification involves adding hydroxyl groups to drug molecules, making them more soluble and easier to flush from the body. For example, sedative phenobarbital and other barbiturates are the drugs that metabolized in this manner by smooth ER in liver cells. Barbiturates, alcohol, and many other drugs induce the proliferation of smooth ER and its associated detoxification enzymes, therefore, increasing tolerance to the drugs; in other words, higher doses are required to achieve a particular effect. Also, because some of the detoxification enzymes have relatively broad action, the proliferation of smooth ER in response to one drug can increase tolerance to other drugs as well. The smooth ER also stores calcium ions; in muscle cells, a specialized smooth ER membrane pumps calcium ions from the cytosol into the ER lumen. When a muscle cell is stimulated by a nerve impulse, calcium ions rush back across the ER membrane into the cytosol and trigger contraction of the muscle cell.
Rough ER- Many times of cells secrete proteins produced by ribosomes attached to rough ER. As a polypeptide chain grows from a bound ribosomes, it is threaded into the ER lumen through a pore formed by a protein complex in the ER membrane. As the new protein enters the ER lumen, it folds into its native shape. Most secretory proteins are glycoproteins, which have carbohydrates covalently bonded to them. After secretory proteins are formed, the ER membrane keeps them separate from proteins that are produced by free ribosomes and will remain in the cytosol. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud like bubbles from a specialized region called transitional ER. Transport vesicles are the vesicles in transit from one part of the cell to another. Rough ER is also a membrane factory for the cell; it grows in place by adding membrane proteins and phospholipids to its own membrane. As polypeptide destined to be membrane proteins grow from the ribosomes, they are inserted into the ER membrane and are anchored there by their hydrophobic portions. The rough ER makes its own membrane phospholipids; enzymes build into the ER membrane assemble phospholipids from precursors in the cytosol. The ER membrane expands and is transferred in the form of transport vesicles to other components of the endomembrane system.

## Golgi Apparatus

Golgi is a center of manufacturing, warehousing, sorting, and shipping. The products of the ER are modified and stored and then sent to other destinations. Golgi apparatus is extensive in cells specialized for secretion. The Golgi apparatus consists of flattened membranous sac, cisternae. The membrane of each cisterna in a stack separates ints internal space from the cytosol. Besicles concentrated in the vicinity of the Golgi apparatus are engaged in the transfer of material between parts of the Golgi and other structures. Golgi stack has a distinct structural polarity with the membrane of cisternae on opposite side of the stack different in thickness and molecular composition. The two poles of a Golgi stack are referred to as the cis face and the trans face; cis is the receiving and trans is shipping departments of the Golgi apparatus. The cis face is usually located near ER. Transport vesicles move material from the ER to the Golgi apparatus. A vesicle that buds from the ER can add its membrane and the contents of its lumen to the cis face by fusing with a Golgi membrane. The trans face give rise to vesicles, which pinch off and travel to other sites. The products of ER are usually modified during their transit from the cis region to the trans region of the Golgi. Various Golgi enzymes modify the carbohydrate portions of glycoproteins; carbohydrates are first added to proteins in the rough ER during the process of polypeptide synthesis. The carbohydrate on the resulting glycoprotein is then modified as it passes through the rest of the ER and the Golgi. The Golgi removes some sugar monomers and substitutes other, producing a large variety of carbohydrates. In addition, the Golgi apparatus manufactures certain macromolecules by itself. Many polysaccharides secreted by cells are Golgi products, including pectins and certain other non-cellulose polysaccharides made by plant cells and incorporated along with cellulose into their cell walls. Similar to secretory proteins, non-protein Golgi products will be secreted depart from the trans face of the Golgi inside transport vesicles that eventually fuse with the plasma membrane.
The Golgi manufactures and refines its products in stages, with different cisternae containing unique teams of enzymes. Recent research has give rise to a new model of the Golgi as a more dynamic structure; According to the cisternal maturation model, the cisternae of the Golgi actually progress forward from the cis to the tras face of the Golgi, carrying and modifying their cargo as they move. Before a Golgi stack dispatches its products by budding vesicles fromt he trans face, it sorts these products and targets them for various parts of the cell. Molecular identification tags, such as phosphate groups added to the Golgi products, aid in sorting. Transport vesicles budded fromt he Golgi may have external molecules on their membranes that recognize "docking site" on the surface of specific organelles or on the plasma membrane, therefore, targeting the vesicle appropriately.

## Lysosomes

Lysosome is a membranous sac of hydrolytic enzymes that an animal cell uses to digest macromolecules. Lysosomal enzymes work best in the acidic environment found in lysosomes. If a lysosome breaks open or leaks its contents, the released enzymes are not very active because the cytosol has a neutral pH. However, excessive leakage from a large number of lysosomes can destroy a cell by autodigestion. Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for further processing. Proteins of the inner surface of the lysosomal membrane and the digestive enzymes are spared from destruction by having three dimensional shapes that protect vulnerable bonds from enzymatic attack.
Phagocytosis is a process that amoebas and many other protists eat by engulfing smaller organisms or other food particles. The food vacuole formed , and then fuses with a lysosome and digests the food. Digestion products pass into cytosol and become nutrients for the cell. In human body, white blood cell helps defend the body by engulfing and destroying bacteria and other invaders.
Lysosome use their hydrolytic enzymes to recycle the cell's own organic material; this is called autophagy. During autophagy, damaged organelle or small amount of cytosol become surrounded by a double membranes, and lysosome fuses with the outer membrane of their vesicle. The lysosomal enzymes dismantle the enclosed material, and the organic monomers are returned to the cytosol for reuse. The lysosomes become engorged with indigestible substrates, which begin to interfere with other cellular activities.

## Vacuoles

Vacuoles are membrane-bounded vesicles whose functions vary in different kinds of cells. Food vacuoles are formed by phagocytosis. Many freshwater protists have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell. In plants and fungi, which lacks lysosomes, vacuoles carry out hydrolysis;
The central vacuole develops by the coalescence of smaller vacuoles, themselves derived from the endoplasmic reticulum and Golgi apparatus. The vacuolar membrane is selective in transportin solutes. As result, the solution inside the central vacuole is called cell sap, is different in composition from the cytosol. It can hold reserves of important organic compounds such as proteins stockpiled in the vacuoles of storage cells in seeds. Also it is the plant cell's main repository of inorganic ions, such as potassium and chloride. Many plant cells use their vacuoles contain pigments that color the cells. Vacuoles may also help protect the plant against predators by containing compounds that are poisonous or unpalatable to animals. The vacuole has a major role in the growth of plant cells, which enlarge as their vacuoles absorb water, enabling the cell to become larger with a minimal investment in new cytoplasm.

## Mitochondria and Chloroplasts

Mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work. Mitochondria are the site of cellular respiration, the metabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen. Chloroplasts, are found in plants and algae, and they are the sites of photosynthesis. They convert solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugar from carbon dioxide and water. Both of them are not part of endomembrane system. Mitochondria have two membrane separating their innermost space from the cytosol, and chloroplasts have three. The membrane proteins of mitochondria and chloroplasts are made not by ribosomes bound to the ER, but by free ribosomes in the cytosol and by ribosomes contained within these organelles themselves. They also contain small amount of DNA that programs the synthesis of the proteins made on the organelle's ribosomes. Mitochondria and chloroplasts are semiautonomous organelles that grow and reproduce within the cell.
Mitochondria Mitochondria are found in all eukaryotic cells; Some cells have a single large mitochondrion, but more often a cell has hundreds or thousands of mitochondria. The number correlates with the cell's level of metabolic activity. The mitochondrion is enclosed by two membranes, each a phospholipid bilayer with a unique collection of embedded proteins. The outer membrane is smooth, but the inner membrane is convoluted, with infolding called cristae. The inner membrane divides the mitochondrion into two internal compartments : the first is the inter-membrane space, the narrow region between the inner and outer membranes and the second compartment, the mitochondrial matrix, is enclosed by the inner membrane. the matrix contains many different enzymes as well as the mitochondrial DNA and ribosomes. Enzymes in the matrix catalyze some steps of cellular respiration. Other proteins that function in respiration, including the enzyme that makes ATP are built into the inner membrane, As highly folded surface, the cristae give the inner mitochondrial membrane a large surface ares, thus enhancing the productivity of cellular respiration.
Chloroplasts The chloroplast is a specialized member of related plant organelles called plastids. Chloroplasts contain the green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar. Its shape is lens-shaped and found in leaves. The contents of a chloroplast are partitioned from the cytosol by 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, inter-connected sacs called thylakoids. Thylakoids are stacked like poker ships, and each stack is called granum. 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.

## Cytoskeleton

Cytoskeleton is a network of fibers extending throughout the cytoplasm. It plays a major role in organizing the structure and activities of the cell. It is composed of three types of molecular structure: microtubules, microfilaments, and intermediate filaments. The main function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. Cytoskeleton is stabilized by balance between opposing forces exerted by its elements. The 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. Also several types of cell motility involve the cytoskeleton. the cell motility encompasses both changes in cell location and more limited movements of parts of the cell. Cell motility require the interaction of the cytoskeleton with motor proteins. Cytoskeletal elements and motor proteins work together with plasma membrane molecules to allow whole cells to move along fibers outside the cell. The cytoskeleton is also involved in regulating biochemical activities in the cell in response to mechanical stimulation forces exerted by extracellular molecules via cell-surface proteins are apparently transmitted into the cell by cytoskeletal elements, and the forces may even reach the nucleus.
Microtubules- thickest
All eukaryotic cells have microtubules; the wall of the hollow tube is constructed from a globular protein called tubulin. Each tubulin protein is a dimer, a molecule made up of two subunits. A tubulin dimer consists of two slightly different polypeptides, alpha-tublin, and beta-tubulin. Microtubules grow in length by adding tubulin dimers. Due to the architecture of a microtubules, its two ends are slightly different; one end can accumulate or release tubulin dimers at a much higher rate than the other, therefore, growing and shrinking significantly during cellular activities. This is called the "plus end", not because it can only add tubulin proteins but because it's the end where both "on" and "off" rates are much higher. Microtubules shape and support the cell and also serve as tracks along which organelles equipped with other proteins can move.
In animal cells, microtubules grow out from a centrosomes, a region that is often located near the nucleus and considered a "microtubule-organizing center". Theses microtubules function as compression-resisting girders of the cytoskeleton. Within the centrosome are a pair of centrioles, each composed of nine sets of triplet microtubules arrange in a ring. Before division, the centrioles replicate; although centrosomes with centrioles may help organize microtubule assembly in animal cell,s they are not essential for this function in all eukaryotes.
Also specialized arrangement of microtubules is responsible for the beating of flagella and cilia. There are microtubule containing extensions that project from some cells. When cilia or flagella extend from cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue. Flagella and cilia different in their beating patterns. A flagellum has an undulating motion that generates force in the same direction as the flagellum's axis. However cilia work more like oars, with alternating power and recovery strokes generating force in a direction perpendicular to the cilium's axis. A cillium may also act as a signal-receiving "antenna" for the cell. Cilia that have this function are nonmotile, and there is only one per cell. Membrane proteins on this kind of cilium transmit molecular signals from the cell's movement to its interior, triggering signaling pathways that may lead to changes in the cell's activities. Cillia-based signaling appears to be crucial to brain function and to embryonic development. Motile cilia and flagella share a common ultrastructure; each has a core of microtubules sheathed in an extension of the plasma membrane. Nine doublets of microtubules, the members of each parit sharing part of their walls, are arranged in a ring. This arrangement, referred to as the "9+2" pattern, is found in all eukaryotic flagella and motile cilia. Non-motile primary cilia have "9+0" pattern, lacking the central pari 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.
In flagella and motile cilia, flexible cross-linking proteins, evenly spaced along the length of the cilium or flagellum, connect the outer doublets to each other and to the two central microtubules Each outer doublet also has paris of protruding proteins spaced along its length and reaching toward the neighboring doublet; These are large motor proteins called dyneins, composed of several polypeptides. Dyneins are responsible for the bending movements of the organelle. A dynein molecule performs a complex cycle of movements cause by changes in these shape of the proteins, with ATP providing the energy for these changes. The mechanics of dynein-based bending involve a process that resembles walking. A typical dynein protein has two "feet" that "walk" along the microtubule of the adjacent doublet, one foot maintaining contact while the other releases and reattaches one step further along the microtubules. Without any restraints ont he movement of the microtubules doublets, one doublet would continue to "walk" along and slide past the surface of the other, elongating the cilium or flagellum rather than bending it.
Microfillaments- thinnest
Microfilaments are solid rods about 7nm in diameter. They are also called as actin filaments because they are build from molecules of actin, a globular protein. A microfilament si a twisted double chain of actin subunits. Microfilaments can form structural networks, due to the presence of proteins that bind along the side of an actin filament and allow a new filament to extend as a branch. The structure role of microfilaments in the cytoskeleton is to bear tension. A cortical microfilaments, a three-dimensional network formed by microfilaments just inside the plasma membrane, helps support the cell's shape. This network give the outer cytoplasmic layer of a cell called the cortex.
In animal cells specialized for transporting materials across the plasma membrane, such as intestinal cells, bundles of microfilaments make up the core of microvilli. Microfillaments are well known for their role in cell motility, particularly as part of the contractile apparatus of muscle cells (myosin). Localized contraction brought about by actin and myosin also plays a role in amoeboid movement, which a cell such as an amoeba crawls along a surface by extending and flowing into cellular extension called pseudopodia. pseudopodia extend and contract through the reversible assembly of actin subunits nto microfilaments and of microfilaments into networks that convert cytoplasm fro a sol to a gel. The pseudopodium extends until the actin reassembles into a network.
In plant cells, both actin-myosin interactions and sol-gel transformations brought about by actin may be involved in cytoplasmic streaming, a circular flow of cytoplasm within cells.
intermediate filaments- middle range intermediate filaments are larger than the diameter of microfilaments but smaller than that of microtubules. Specialized for bearing tension (like microfilaments), intermediate filaments are a diverse class of cytoskeletal elements. Each type is constructed from a different molecular subunit such as keratins. Intermediate filaments are more permanent fixture of cells than are microfilaments and microtubules. Even after the death of the cell, intermediate filament networks often persist. Intermediate filaments are important in reinforcing the shape of a cell and fixing the position of certain organelles. For instance, the nucleus commonly sits within a cage made of intermediate filaments, fixed in location by braches of the filaments that extend into the cytoplasm. Other intermediate filaments make p the nuclear lamina that lines the interior of the nuclear envelope. In case where the shape of the entire cell is correlated with function, intermediate filaments support that shape.

## Cell Wall

Cell wall is an extracellular structure of plant cell that distinguishes them from animal cells. The wall protects the plant cell, maintains its shape, and prevents excessive uptake of water. The strong walls of specialized cells hold the plant up against the force of gravity. Plant cell walls are much thicker than the plasma membrane, and the exact chemical composition of the wall varies from species to species and even from one cell type to another in the same plant, but basic design of the wall is consistent. Microfibrils made of the polysaccharide cellulose are synthesized by an enzyme called cellulose synthase and secreted to the extracellular space, where they become embedded in a matrix of other polysaccharides and proteins. This combination of materials, strong fibers in a "ground substance" (matrix), is the same basic architectural design found in steel-reinforced concrete and in fiberglass.
A young plant cell first secrets a thins and flexible wall called the primary cell wall; as the cell grows, the cellulose fibrils are oriented at right angles to the direction of cell expansion, possibly affecting the growth pattern. Between primary walls of adjacent cell is the middle lamella, which is a thin layer rich in sticky polysaccharides pectins. The middle lamella flues adjacent cells together. When the cell mature and stops growing, it strengthens its wall. Some plant cells do this simply by secreting hardening substances into the primary wall, but other cells add a secondary cell wall between the plasma membrane and the primary wall. Then secondary wall, often deposited in several laminated layer, has a strong and durable matrix that afford the cell protection and support.

## References

Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 7th ed. New York: W.H. Freeman, 2012. Print.
Reece, Campbell, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minosky, and Robert B. Jackson. Biology. 8th ed. San Francisco: Cummings, 2010. Print.

### See also

Prokaryotes