BIOS 1700 Exam 2: Chapter 10 Notes

What is a tissue?

A biological tissue is a collection of cells that work together to perform a specific function. Our own tissues and organs all are communities of cells that work together to perform highly specific and important tasks in response to cues from the environment.

Where might microfilaments be found?

At the cortex, microfilaments reinforce the plasma membrane and organize proteins associated with it. These cortical microfilaments are also important in maintaining the shape of a cell. For example, in absorptive epithelial cells such as those in the small intestine, bundles of microfilaments are found in microvilli, hairlike projections that extend from the surface of the cell. Longer bundles of microfilaments form a band that extends around the circumference of epithelial cells. This band is attached to a type of cell junction called an adherens junction, which connects a cell to its neighbors. As a result, the band provides a great deal of structural support not only to individual epithelial cells, but also to the entire epithelial layer of cells.

What are the three types of tissue in plants?

Plants are made up of just three types of tissue: dermal (the outer protective layer), ground (the bulk of the plant body), and vascular (the channels that transport nutrients and water throughout the plant).

What are plasmodesmata?

Plasmodesmata are passages through the cell walls of adjacent plant cells. They are similar to gap junctions in that they allow cells to exchange ions and small molecules directly, but the similarity ends there. In plasmodesmata, the plasma membranes of the two connected cells are actually continuous. The size of the opening is considerably larger than that of gap junctions, large enough for cells to transfer RNA molecules and proteins, a capability that is especially important during plant embryonic development. Plasmodesmata allow plant cells to send signals to one another despite being enclosed within rigid cell walls.

What are adherens junctions?

A long bundle of actin microfilaments forms a band that extends around the circumference of epithelial cells. This band of actin is attached to the plasma membrane by cadherins in a beltlike junctional complex called an adherens junction. The cadherins in the adherens junction of one cell attach to the cadherins in the adherens junction of adjacent cells. This arrangement establishes a physical connection among the actin cytoskeletons of all cells present in an epithelial layer of cells.

Why is the extracellular matrix important to cell travel?

A migrating cell must be able to attach to extracellular matrix proteins in order to move forward. These contacts are also made by means of integrins. Cell migration in animals is normal and necessary during embryonic development, wound healing, and the immune response to infection. However, inappropriate cell migration can have devastating consequences.

What do motor proteins do?

A motor is a device that imparts motion. Microtubules and microfilaments have some capacity to move by polymerization and depolymerization, but on their own their capacity for movement is fairly limited. However, when joined by small accessory proteins called motor proteins, microtubules and microfilaments are capable of causing amazing movements.

How do cytoskeletal elements support the endosymbiotic theory?

A number of studies have shown that many prokaryotes also have a system of proteins similar in structure to the cytoskeletal elements of eukaryotic cells and are involved in similar processes, including the separation of daughter cells during cell division. Interestingly, at least one of these prokaryotic cytoskeleton-like proteins is expressed in chloroplasts and mitochondria of some eukaryotic cells. The presence of this protein in these organelles lends support to the theory that chloroplasts and mitochondria were once independent prokaryotic cells that developed a symbiotic relationship with ancestral eukaryotic cells. This idea is called the endosymbiotic theory.

What is the structure of a tight junction?

A tight junction is a band of interconnected strands of integral membrane proteins that, like adherens junctions, encircles the epithelial cell. It also binds to tight-junction proteins on adjacent cells. Unlike adherens junctions, however, tight junctions are not connected to the cytoskeleton. Their function is to prevent passage of materials in between cells, not to anchor the cells together.

What are the four types of tissue in animals?

Animals and plants have tissues and organs that allow them to carry out the various processes necessary to sustain them. In animals, for example, four types of tissue—epithelial, connective, nervous, and muscle—combine to make up all the organs of the body.

How many genes are involved in cell attachment?

Between 5% and 10% of the genes in the human genome are involved in attaching cells to one another or to the extracellular matrix.

What kind of junctions are cadherins located in?

Cadherins are located in adherens junctions and desmosomes. These junctional complexes anchor cells to one another and are reinforced by the cytoskeleton.

What is the structure of cadherins?

Cadherins are transmembrane proteins. The extracellular domain of a cadherin molecule binds to the extracellular domain of a cadherin of the same type on an adjacent cell. The cytoplasmic portion of the protein is linked to the internal cytoskeleton. This arrangement provides structural continuity from the cytoskeleton of one cell to the cytoskeleton of another, increasing the strength of tissues and organs.

Why are lymph nodes important in cancer detection?

Cancer is characterized by uncontrolled cell division. Many types of cancer result in the formation of a mass of cells called a tumor. In most cases, the tumor itself is not the problem. The real concern is that tumor cells will metastasize, or spread to other parts of the body, where they can affect the function of distant organs. One way physicians check for metastasis is to look for intermediate filament proteins in places where they don't belong. Physicians may also examine lymph nodes for signs that a cancer has spread. Lymph nodes are part of the lymphatic system, which collects extracellular fluid from tissues in the body and returns it to the circulatory system. Each part of the body drains into a specific set of lymph nodes.

What are cell adhesion molecules and cellular junctions?

Cells are attached to one another and to the extracellular matrix by cell-surface proteins called cell adhesion molecules. The regions in the plasma membrane where cells make contact with and adhere to other cells or the extracellular matrix are called cellular junctions. Together, cell adhesion molecules, cellular junctions, and the extracellular matrix keep us all intact.

What are cadherins? What do they adhere?

Cells can sort themselves because of the presence of various cell adhesion molecules. While a number of cell adhesion molecules are now known, the cadherins are especially important in the adhesion of cells to other cells. There are many different kinds of cadherins, and a given cadherin may bind only to another cadherin of the same type. E-cadherin is present on the surface of embryonic epidermal cells, and N-cadherin is present on neuronal cells. The epidermal cells adhered to one another through E-cadherin, and the neuronal cells adhered to each other through N-cadherin.

How does the extracellular matrix influence cells?

Cells continue to interact with the extracellular matrix long after they have synthesized it or moved into it, and these interactions can have profound effects on the cell. For example, fibroblasts that secrete components of the extracellular matrix are in turn influenced by the extracellular matrix. The structure of the extracellular matrix can influence the shape of cells. The composition of the extracellular matrix can affect the shape of cells. In addition to influencing cell shape, the structure and composition of the extracellular matrix can influence gene expression of the cells that are grown in it.

Where does the lamellipodium form?

Cells do not have permanent front and back ends. Instead, the locations of the leading and trailing edges can change in an instant. This flexibility allows a migrating cell to change its direction of movement, toward a source of nutrients, for example. The formation of a lamellipodium is triggered when a cell-surface receptor binds to a nutrient molecule or other substance, so the lamellipodium forms in the region of the plasma membrane where the signal is strongest. In this way, cells can follow the concentration gradient of a nutrient to its source.

What are the two sides of a tight junction cell?

Cells that have tight junctions have two distinct sides because the tight junction divides the plasma membrane into two distinct regions. The portion of the plasma membrane in contact with the lumen, or the inside of any tubelike structure like the gut, is called the apical membrane. The apical membrane defines the "top" side of the cell. The rest of the plasma membrane is the basolateral membrane, which defines the bottom and sides of the cell. These two regions of the plasma membrane are structurally different because the tight junction prevents lipids and proteins in the membrane on one side of the junction from diffusing to the other side. As a result, the apical and basolateral membranes of a cell are likely to have different integral membrane proteins, which causes them to be functionally different as well.

What is the basal lamina?

Cellular junctions also connect the bottom layer of keratinocytes to a specialized form of extracellular matrix called the basal lamina, which underlies and supports all epithelial tissues.

Why is collagen important? What type of collagen is most abundant in us?

Collagen is the most abundant protein in the extracellular matrix of animals, and in fact is the most abundant animal protein on the planet. There are more than 20 different forms of collagen, and in humans collagen accounts for almost a quarter of the protein present in the body. Over 90% of this collagen is type I collagen, which is present in the dermis of your skin, where it provides strong, durable support for the overlying epidermis. The tendons that connect your muscles to bones and the ligaments that connect your bones to other bones are able to withstand the physical stress placed on the, because they are made up primarily of collagen.

What is the structure of collagen? Why is it important?

Collagen's strength is related to its structure. Collagen is composed of intertwined fibers that make it much stronger than if it were a single fiber of the same diameter. A collagen molecule consists of three polypeptides wound around one another in a triple helix. A bundle of collagen molecules forms a fibril, and the fibrils are assembled into fibers. The final structure is incredibly strong.

What are desmosomes?

Desmosomes are buttonlike points of adhesion that hold the plasma membrane of adjacent cells together. Cadherins are at work here, too, strengthening the connection between cells in a manner similar to adherens junctions. The cadherins in the desmosome of one cell bind to the cadherins in the desmosomes of adjacent cells. The cytoplasmic domain of these cadherins is linked to intermediate filaments in the cytoskeleton. This second type of physical connection among neighboring cells greatly enhances the structural integrity of epithelial cell layers. The structural support provided by desmosomes and their associated network of intermediate filaments is crucial to the function of several organs, such as the skin and heart. Mutations in several of the genes for desmosomal proteins are responsible for a number of serious diseases. In these instances, intercellular connections are profoundly weakened. The results rage from serious skin conditions due to fragile epidermis to a high risk of early heart failure due to severe weakening of the heart muscle.

Why is dynamic instability important?

Dynamic instability might seem like an undesirable feature for a component of the cytoskeleton, but it is actually very important for many functions of microtubules. For example, it allows microtubules to explore the space of the cell by growing into new areas and then shrinking back. This ability is especially important in the process of dividing chromosomes between the two daughter cells during cell division. Segregation of chromosomes requires that each chromosome be attached to two microtubules. The way that microtubules encounter chromosomes fast enough to form these attachments is by random exploration of the cytoplasm, which is driven by dynamic instability.

How are epithelial cells anchored to the basal lamina?

Epithelial cells are firmly anchored to the basal lamina by a version of the desmosome called a hemidesmosome. Integrins are the prominent cell adhesion molecules in hemidesmosomes. Their extracellular domains bind to the extracellular matrix proteins in the basal lamina, and their cytoplasmic domains are linked to intermediate filaments of the cytoskeleton. The intermediate filaments connected to hemidesmosomes are linked to those connected to the desmosomes in the lateral membrane. The result is a firmly anchored and tremendously reinforced layer of cells.

What do tight junctions do?

Epithelial cells, which form layers or sheets that cover or line other tissues and organs, define the boundaries of many of these tissues. Like any effective boundary, a layer of epithelial cells must limit or control the passage of material across it. Adherens junctions and desmosomes provide strong connections between cells, but they do not prevent the free passage of materials through the spaces between the cells they connect. Junctional complexes called tight junctions establish a seal between cells so that the only way a substance can travel from one side of a sheet of epithelial cells to the other is by moving through the cells by means of one of the cellular transport mechanisms.

What is the extracellular matrix? Why is it important?

Equally important to a strong, properly shaped tissue or organ is the ability of cells to adhere to a meshwork of proteins and polysaccharides outside the cell called the extracellular matrix. In plants, the extracellular matrix provides both a mechanism for cells to adhere to one another and structural support that maintains their shape.

How does a plant cell grow?

For a plant cell to grow, the cell wall must accommodate an increase in cell volume and surface area. Even though the matrix of cellulose and other polysaccharides of the primary cell wall is relatively flexible compared to the lignin-containing secondary cell wall, it is still very strong. When a plant cell grows, additional cell wall components must be synthesized to expand the area of the wall. The cellulose polymer is assembled outside the cell, on the extracellular surface of the plasma membrane. Both the glucose monomers that form the polymer and the enzymes that attach them are delivered to the cell surface by arrays of microtubules.

What are gap junctions?

Gap junctions are formed when a set of integral membrane proteins arranged in a ring connects to a similar ring of proteins in the membrane of another cell. Ions and signaling molecules pass through these junctions, allowing cells to communicate.

Why are genetic defects in intermediate filaments so severe?

Genetic defects that disrupt the intermediate filament network can have severe consequences. For example, individuals with epidermolysis bullosa, a rare genetic disease, have defective keratin genes inherited from their parents. Intermediate filaments do not polymerize properly in these individuals, weakening connections between the layers of cells that make up the epidermis. As a consequence, the outer layers can detach, resulting in extremely fragile skin that blisters in response to the slightest trauma.

What did H.V. Wilson discover?

In 1907, American embryologist H.V. Wilson discovered that if he pressed a live sponge through fine cloth he could break up the sponge into individual cells. Then if he swirled the cells together, they would coalesce back into a group resembling a sponge. If he swirled together the cells from sponges of two different species, he observed that the cells sorted themselves out.

What are integrins? What is their structure?

In addition to being stably connected to other cells, cells also attach to proteins of the extracellular matrix. This type of attachment provides structural reinforcement, especially to tissues under physical stress. The cell adhesion molecules that enable cells to adhere to the extracellular matrix are called integrins. Like the cadherins, integrins are transmembrane proteins, and their cytoplasmic domain is linked to the cytoskeleton. Also like the cadherins, integrins are of many different types, each with a specificity for a different extracellular matrix protein. Integrins are present on the surface of virtually every animal cell.

What is dynamic instability?

In addition, microtubules have an important property not shared by other cytoskeletal elements: their plus ends undergo seemingly random cycles of rapid shrinkage (depolymerization) followed by slower growth (polymerization). These cycles of shrinkage and growth are called dynamic instability. The dramatic shrinkage is often called microtubule catastrophe and takes place because the plus end of a microtubule is structurally unstable. Once depolymerization has occurred, tubulin dimers are added to reassemble the microtubule.

What does the extracellular matrix look like in animals?

In animals, the extracellular matrix is present in most tissues and is especially abundant in connective tissue, where it provides support and protection, and in the basal lamina found underneath all epithelial tissues. In both plants and animals, the extracellular matrix not only contributes structural support but also provides informational cues that determine the activity of the cells that are in contact with it.

What is the cell wall's structure and function?

In plants, the extracellular matrix forms the cell wall, and the main component of the plant cell wall is the polysaccharide cellulose. Cellulose is the most widespread organic macromolecule on Earth. The plant cell wall represents possibly one of the most complex examples of an extracellular matrix. Cell walls maintain the shape and turgor pressure of plant cells, allow plant cells to grow, and act as a barrier that prevents foreign materials and pathogens from reaching the plasma membrane. In many plants, cell walls collectively serve as a skeletal support structure for the entire plant.

What does the extracellular matrix look like in plants?

In plants, the extracellular matrix takes the form of the cell wall, which provides the support needed by individual cells and results in turgor pressure. Collectively, the interconnected cell walls of a plant act as an internal framework to support the entire organism. In both plants and animals, the extracellular matrix not only contributes structural support but also provides informational cues that determine the activity of the cells that are in contact with it.

How are integrins indicative of metastatic potential?

In some types of cancer, the number of specific integrins on the cell surface is an indicator of metastatic potential. Melanoma provides an example. A specific type of integrin is present in high amounts on metastatic melanoma cells but is absent on non-metastatic cells from the same tumor. In laboratory tests, blocking these integrins eliminates the melanoma cell's ability to cross an artificial basal lamina.

What are cellular junctions? Why are they important?

In turn, the adhesion of cells to one another depends on cell adhesion molecules and other cytosolic proteins that assemble into structures called cellular junctions.

What is the extracellular matrix? What is its structure and function?

It is the extracellular matrix that provides the molecular framework that ultimately determines the structural architecture of plants and animals. The extracellular matrix is synthesized, secreted, and modified by many different kinds of cells. It is an insoluble meshwork composed of proteins and polysaccharides. There are many different forms of extracellular matrix, which differ in the amount, type, and organization of the proteins and polysaccharides that make them up.

What do keratinocytes do?

Keratinocytes in the epidermis are specialized to protect underlying tissues and organs. They are able to perform this function in part because of their elaborate system of cytoskeletal filaments. These filaments are often connected to the cellular junctions that hold adjacent keratinocytes together.

What are malignant tumors?

Malignant tumors contain some cells that can metastasize, that is, break away from the main tumor and colonize distant sites in the body. Metastatic tumor cells have an enhanced ability to adhere to extracellular matrix proteins, especially those in the basal lamina. This is significant because for a cell to metastasize, it must enter and leave the bloodstream through capillaries. Since all blood vessels have a basal lamina, a metastatic tumor cell needs to cross a basal lamina at least twice—once on the way into the bloodstream and again on the way out.

What are lamellipodiums? How do they work?

Many cells move through their environment by crawling across a substrate or by squeezing between other cells and burrowing through connective tissues, rather than by using cilia or flagella. This type of movement relies on microfilaments. Commonly, new microfilaments are assembled and extended at one end of the cell and existing microfilaments are pulled together at the other end. The polymerization of actin into a microfilament exerts a considerable amount of force, enough to push the plasma membrane out into a thin, sheetlike structure called a lamellipodium. New points of adhesion are established between the lamellipodium and the substrate. Meanwhile, bundles of actin filaments at the other end of the cell contract by the interaction of myosin and actin, squeezing the cytoplasm and its contents forward toward the lamellipodium. The edge of the cell where the lamellipodium forms is called the leading edge, and the end where contraction takes place is called the trailing edge. Repeated cycles of actin polymerization at the leading edge and microfilament contraction at the trailing edge propel the cell forward. Cells are able to migrate considerable distances by this method.

What functions do intermediate filaments perform in the cell?

Once assembled, many intermediate filaments become attached at the cytoplasmic side of cellular junctions called desmosomes. The anchoring of intermediate filaments to desmosomes provides strong support for the cells. In the case of epithelial cells, this anchoring results in structural continuity from one cell to another that greatly strengthens the entire epithelial tissue, enhancing the ability of epithelial cell layers to withstand physical stress. This is especially important in a wide range of tissues that are regularly subject to such stress, including the skin and the lining of the intestine.

How do melanophores work?

Melanophores are similar to the melanocytes in our skin, but rather than hand off their melanin to other cells, as in humans, melanophores keep their pigment granules and move them around the cell in response to hormones or neuronal signals. This redistribution of melanin within the cell allows animals such as fish or amphibian embryos to change color. For example, at night the melanin granules in the skin of a zebrafish embryo are dispersed throughout the melanophores, making it darkly colored. As morning comes and the day brightens, the pigment granules are aggregated at the center of the cell around the centrosome, causing the embryo's color to lighten. The melanin granules in the melanophores move back and forth along microtubules, transported by kinesin and dynein. Kinesin moves the granules out toward the plus end of the microtubules during dispersal, and dynein moves them back toward the minus end during aggregation. The daytime and nighttime camouflage provided by this mechanism of color change keeps young, developing organisms from being spotted by hungry predators lurking below.

What functions do microfilaments perform in the cell?

Microfilaments are important for the transport of materials inside cells, especially plant cells. Furthermore, microfilaments are responsible for changes in the shape of many types of cells. An exceptionally dramatic example of such a shape change is the shortening of a muscle cell when it contracts. Microfilaments are found at the front edge of migrating cells, where they can move the plasma membrane forward. Finally, microfilaments arranged in a structure called the contractile ring separate the daughter cells at the end of animal cell division.

What is the structure of a microfilament?

Microfilaments are polymers of actin monomers, arranged to form a helix. They are the thinnest of the three cytoskeletal fibers and are present in various locations in the cytoplasm. They are relatively short and extensively branched in the cell cortex, the area of the cytoplasm just beneath the plasma membrane.

What motor proteins bind to microtubules? How do they work?

Microtubules also function as tracks for transport within the cell. Transport along microtubules takes place in a similar fashion to transport along microfilaments, except that the two motor proteins that transport cargo are kinesin and dynein. Kinesin is similar in structure to myosin and transports cargo toward the plus end of microtubules at the periphery of the cell. By contrast, dynein carries its load away from the plasma membrane towards the minus end. As with myosin, movement along microtubules by kinesin and dynein is driven by conformational changes in the motor proteins and is powered by energy harvested from ATP.

What are the plus and minus ends?

Microtubules and microfilaments are highly dynamic structures. They become longer by the addition of subunits to their ends, and shrink by the loss of subunits. The growth and shrinkage of microtubules and microfilaments are influenced by many factors, including the concentration of free tubulin and actin subunits and the activity of regulatory proteins that attach to the cytoskeleton. The rate at which protein subunits are added depends on the concentrations of tubulin and action in that region of the cell. At high concentrations of subunits, microtubules and microfilaments may grow at both ends, although these subunits are added more quickly to one end than to the other. The faster growing end is called the plus end and the slower growing end is called the minus end. The minus ends of microtubules in animal cells are positioned at the organizing center of the centrosome, and the plus ends project outward toward the plasma membrane.

What functions do microtubules perform in the cell?

Microtubules have diverse functions. In animal cells, microtubules radiate outward to the cell periphery from a microtubule-organizing center called the centrosome. This spokelike arrangement of microtubules helps cells withstand compression and thereby maintain their shape. Many organelles are tethered to microtubules, which guide the arrangement of organelles in the cell. Microtubules also provide tracks for the transport of material from one part of the cell to another. They are found specially arranged in cilia and flagella, organelles that propel the movement of cells or substances surrounding the cell. Finally, microtubules form the spindle apparatus that separates replicated chromosomes during eukaryotic cell division.

What is the structure of a microtubule?

Microtubules, which are hollow tube-like structures, have the largest diameter of the three cytoskeletal elements. They are polymers of protein dimers. Each protein dimer is made up of two slightly different tubulin proteins, called alpha and beta tubulin. One alpha tubulin and one beta tubulin combine to make a tubulin dimer. These tubulin dimers are assembled into microtubules.

What motor protein binds to microfilaments? How does it work?

Motor proteins cause muscle contraction by moving the actin microfilaments inside muscle cells. The cytoplasm of muscle cells is packed with actin microfilaments that are anchored to the ends of the cell. Myosin, a motor protein found in muscle cells, binds to actin and undergoes a conformational change. As a result, the actin microfilaments slide relative to myosin, causing the cell to shorten, or contract. The energy required for the conformational change in myosin and the movement of actin microfilaments comes from the energy stored in ATP. In other cells, myosin attached to various types of cellular cargo, such as transport vesicles, works by a similar mechanism to move materials from one part of the cell to another, using microfilaments as tracks.

What are benign tumors?

Nonmalignant, or benign, tumors are encapsulated masses of cells that divide continuously because regulation of cell division has gone awry. As the tumor grows, its border pushes outward against adjacent tissues. Benign tumors are rarely life threatening unless the tumor interferes with the function of a vital organ.

What is the secondary cell wall?

Once cell growth has stopped, the secondary cell wall is constructed in many, but not all, plant cells. It also is made largely of cellulose fibers but in addition contains a substance called lignin. Lignin hardens the cell wall and makes it water resistant. The rigid secondary cell wall permits the growth of woody plants to tremendous heights.

Why is the basal lamina important?

The basal lamina is a specialized layer of extracellular matrix that is present beneath all epithelial tissues. The role of the basal lamina is to provide structural foundation for these epithelial tissues. The basal lamina is made of several proteins, including a special type of collagen. The triple-helical structure of collagen provides flexible support to the epithelial sheet and also provides a scaffold on which other proteins are assembled.

What kind of cells and tissues are in the dermis?

The dermis is made up mostly of connective tissue, a type of tissue characterized by few cells and substantial amounts of extracellular matrix. The dermis is strong and flexible because it is composed of tough protein fibers of the extracellular matrix. The dermis also has many blood vessels and nerve endings. The main type of cell in the dermis is the fibroblast. Fibroblasts synthesize the extracellular matrix and repair wounds.

What kinds of cells are found in the epidermis?

The epidermal layer of skin is primarily composed of epithelial cells called keratinocytes. The epidermis also contains melanocytes that produce the pigment that gives skin its coloration. Different kinds of epithelial cells are specialized to carry out different functions, such as protection, secretion, and absorption.

What is epithelial tissue?

The epidermis is several cell layers thick. Cells arranged in one or more layers are called epithelial cells and together make up a type of animal tissue called epithelial tissue. Epithelial tissue covers the outside of the body and lines many internal structures.

In what type of animal tissue is the extracellular matrix most abundant?

The extracellular matrix can be found in abundance in animal connective tissue. Connective tissue underlies all epithelial tissues, as we have seen. A number of cell types, including the fibroblasts that synthesize most of the extracellular matrix proteins, reside in this tissue. Connective tissue is unusual compared to other tissue types in that it is dominated by the extracellular matrix and has a low cell density. Consequently, the extracellular matrix determines the properties of different types of connective tissue. Other more specialized types of animal connective tissue include bone, cartilage, and tendon.

What are major proteins in plant extracellular matrix? What is the matrix charge?

The extracellular matrix of animals, like that of plants, is secreted by cells and is a mixture of proteins and polysaccharides. The animal extracellular matrix is composed of large fibrous proteins, including collagen, elastinin, and laminin, which impart tremendous tensile strength. These fibrous proteins are embedded in a gel-like polysaccharide matrix. The matrix is negatively charged, attracting positively charged ions and water molecules that provide protection against compression and other physical stress.

What are the intercellular junctions of plant cells?

The gap junctions of animal cells and plasmodesmata of plant cells are connections between the plasma membranes of adjacent cells that permit materials to pass directly from the cytoplasm of one cell to the cytoplasm of another.

What is the structure of intermediate filaments?

The intermediate filaments of animal cells are so named because their diameter is intermediate between that of microtubules and microfilaments. They are polymers of intermediate filament proteins that combine to form strong, cable-like structures in the cell. As a result, they provide cells with mechanical strength. The proteins making up intermediate filaments differ from one cell type to another. For example, in epithelial cells, these protein subunits are keratins; in fibroblasts, they are vimentins; and in neurons, they are neurofilaments. Some intermediate filaments, called lamins, are even found inside the nucleus, where they provide support for the nuclear envelope. In fact, there are well over 100 different kinds of intermediate filaments.

What is the structure of cilia and flagella?

The microtubules in cilia and flagella are distributed in a characteristic "9+2" arrangement, that is, nine pairs of microtubules are located around the periphery of these organelles and two microtubules are at the center. The outer microtubules are connected to the center pair by cross-linking proteins and to their neighbors by dynein molecules. Energy harvested by the hydrolysis of ATP powers the motion of cilia and flagella. Dynein undergoes a conformational change that causes the pairs of microtubules to slide past each other. The sliding of the microtubules results in a whiplike motion in the case of flagella and an oarlike rowing motion in the case of cilia.

What is the middle lamella?

The middle lamella is made first, and is synthesized during the late stages of cell division. It is made of a gluelike complex carbohydrate, and it is the main mechanism by which plant cells adhere to one another.

What are the two layers of skin? What are their purposes?

The outer layer, the epidermis, serves as a water-resistant, protective barrier. The layer beneath the epidermis is the dermis. This layer of skin supports the epidermis, both physically and by supplying it with nutrients. It also provides a cushion surrounding the body.

What are the layers of the cell wall?

The plant cell wall is composed of as many as three layers: the outermost middle lamella, the primary cell wall, and, closest to the plasma membrane, the secondary cell wall.

What is the primary cell wall?

The primary cell wall is formed next and consists mainly of cellulose fibers, but it also contains a number of other molecules, including pectin and several proteins. The primary cell wall is laid down while the cells are still growing. It is assembled by enzymes on the surface of the cell and remains thin and flexible.

What are the three major cytoskeletal elements? Which is exclusive to animals?

The protein fibers of the cytoskeleton provide internal support for cells. All eukaryotic cells have at least two cytoskeletal elements, microtubules and microfilaments. Animal cells have a third element, intermediate filaments. All three of these cytoskeletal elements are long chains, or polymers, made up of protein subunits. In addition to providing structural support, microtubules and microfilaments enable the movement of many cells and the movement of substances within cells.

Are integrins in plants?

Though integrins have not been observed in plants, integrin-like proteins are present on the surface of some plant cells as well.

What determines cell shape? Why is it important?

Tissues and organs have distinctive shapes that reflect how they work and what they do. In the same way, the different cell types that make up these organs have distinct shapes based on what they do in the organ. In animals, the shape of cells is determined and maintained by structural protein networks in the cytoplasm called the cytoskeleton. The shape and structural integrity of tissues and organs depend on the ability of cells to adhere to one another.

What is an organ?

Two or more tissues often combine and function together as an organ. Our own tissues and organs all are communities of cells that work together to perform highly specific and important tasks in response to cues from the environment.

What are the three types of cellular movement?

We can think of cellular movement in three different ways. First, there is the movement of the cell itself, and many cells are capable of moving great distances on their own. The second kind of movement is the change in shape of a cell, as when a contracting muscle cell shortens. Changes in cell shape are also common in embryos and are an essential part of early development. The third kind of cellular movement is the movement of molecules and organelles within a cell. For example, melanocytes in the epidermis transport their packets of melanin along their branching dendrites to deliver them to neighboring keratinocytes. Similarly, in neurons, neurotransmitters are transported from the cell body to the end of the axon.