mod 10 chap 10 Flashcards
Tissues and Organs
multicellular organims consist of communities of cells with several notable features
first cells adhere to one another
most cells in multiclelular organisms are phsycially attached to one another or to usbstances in their surroundings
second, cells communicat with one another
cells respond to signals from neighbouring cells and the physical nevrinment
cells in tissues also hvae pathways for the movement of molecules from on cell to another
third, cells are speiclized to carry out diff functions as a result of their development and differentiation
the diff shapes of cells often reflect these specilized functions
Tissues and organs are communities of cells
a biolohgical tissue is a collection of cells that work together to perform a specific function
animals and plants have tissues that allow them to carry out the various processes necessary to sustain them
in animals for ex. four types of tissues - epithilial, connnectve, muscle and nervous - combine to mkae up all the organs of the body
two or more tissues often combine and function together as an organs like a heart or lung
tissues and organs have distinctive hsapes that reflect how they work and what they do
in the same way, the diff types of cells that makeup these ograns have distinctive shapes that reflect what they do in the organ
in animals the shape of cells is dteermined and maintained by strcutural protein netwroks in the cytoplasm called the cytoskeleton
the shape and intregtrity of organs and tisseusz depends on the ability of cells to connect to one another
in turn the connection of cells to one another depends on cell junctions, complexes of proteisn in the cell membrane hwere a cell makes contact with another cell or extracellular matrix
outside many cells and tissues is. ameshwork of proteisn and polysaccharides called extracellular matrix
the ability of cells to adhere to this meshwork is important to ensure a strong, porperly shaped tissue or organ
The tsructure of skin relates to its function
to understand the role of the cytoskeleton, cell junctions and extracellular matrix, we should consider skin
the structure of mammalian skin is tied to its function
the skin has two mian layers - the outer layer which is the epidermis which serves as a water resistnt proetctive barrier and the inner layer, the dermis whihc support the epidermis both physcially and by supplying it with nutrients - the dermis also provides a cushion surrounding the body
the epidermis is sevral cell layers thick
cells arranged in one or more layers are called epithelial cells and togetehr makeup a type of animal tissue called epithelial tissue
epithelial tissue covers the outside of the body and lines many internal strcutures like the digestive tract and vertebrate blood vessels
the epidermal layer of skin is composed of epithelial cells called keratinocytes
the epidemis also have melanocytes which prduce the pigment that gives us skin colour
keratincoytes in the epidermis are specilized to protect the underlying tissues and organs
they are able to perform this fucntion bc of their elaborate system of cytoskeletal filaments
these filaments are often connected to the cell junctions that hold adjascent kertinocutes together
cell junctions also connect the bottom layer of keratinocytes to a specilized form of extraclelular matrix called basal lamina (basement memrbane)
basal lamina udnerlies and supports all epithelial tissue
the dermis is made up of mostly connective tissues, a type of tissue characterized by few cells and susbtantial ammounts of extracellular matrix
the main type of cell in the dermis is fibroblast, hwihc synthesizes the extracellular matrix
the dermis is strong and felxible bc its extracellular matrix is composed of tough porteins
the dermis also contains many blood vessels and nerve endings
the cytoskeleton
Just as the bones of vertebrate skeletons provide internal support for the body, the protein fibers of the cytoskeleton provide internal support for cells (Fig. 10.2). All eukaryotic cells have at least two cytoskeletal elements, microfilaments and microtubules. Animal cells also 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, microfilaments and microtubules enable cells to change shape, move about, and transport substances.
Microfilaments, intermediate filaments, and microtubules are polymers of protein subunits
Microfilaments, intermediate filaments, and microtubules make up the cytoskeleton in animal cells (Fig. 10.2). They are all made up of protein subunits, but differ in diameter. Microfilaments have the smallest diameter of the three types of filaments, with a diameter of approximately 7 nm. Microfilaments are polymers of actin monomers, arranged to form a helix (Fig. 10.2a). Microfilaments 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 cell membrane. In the cortex, microfilaments reinforce the cell membrane and help to organize proteins associated with it.
These cortical microfilaments also play important roles in maintaining the shape of a cell, such as the biconcave shape of red blood cells (Fig. 3.2). In addition, long bundles of microfilaments form a band that extends around the circumference of epithelial cells. Microfilaments also maintain the shape of absorptive epithelial cells such as those in the small intestine. In these cells, bundles of microfilaments are found in microvilli, hairlike projections that extend from the surface of the cell
The intermediate filaments of animal cells have a diameter that is intermediate between those of microfilaments and microtubules, approximately 10 nm (Fig. 10.2b). They are polymers of intermediate filament proteins that combine to form strong, cable-like structures in the cell. In this way, they provide cells with mechanical strength
We have seen that different cell types all use actin monomers to form microfilaments. By contrast, 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 (Fig. 10.4). More than 100 different kinds of intermediate filaments have been identified to date.
Once assembled, many intermediate filaments become attached to cell junctions at their cytoplasmic side, providing strong support for the cells (Fig. 10.5). In the case of epithelial cells, this anchoring results in structural continuity from one cell to another that greatly strengthens the entire epithelial tissue. This is especially important for tissues that are regularly subject to physical stress, such as the skin and the lining of the intestine.
Genetic disorders that disrupt the intermediate filament network can have severe consequences. For example, some individuals with epidermolysis bullosa, a group of rare genetic diseases, have mutations in keratin genes. Intermediate filaments do not polymerize properly in these individuals, so they form weak connections between the layers of cells that make up the epidermis. As a result, the outer layers can detach, resulting in extremely fragile skin that blisters in response to the slightest trauma (Fig. 10.5). The sensitivity to physical stress is so extreme that infants with epidermolysis bullosa often suffer significant damage to the skin during childbirth. Therefore, cesarean section is sometimes recommended in cases where the disease is diagnosed during pregnancy.
Microtubules are the largest of the three cytoskeletal elements, with a diameter of approximately 25 nm. They form hollow tubelike structures in the cell (Fig. 10.2c). These structures are polymers of protein dimers. Each dimer is made up of two slightly different tubulin proteins, called (alpha) and (beta) tubulin. The tubulin dimers are assembled to form the microtubule.
Microtubules help maintain cell shape and internal structure. In animal cells, these structures radiate outward to the cell periphery from a microtubule organizing center called the centrosome. This spokelike arrangement of microtubules helps cells withstand compression. Many organelles are tethered to microtubules so that microtubules guide the arrangement of organelles in the cell
Microfilaments and microtubules are dynamic structures
microfilaments and microtubules are dynamic. They become longer by adding subunits to their ends and become shorter by losing subunits.
The rate at which protein subunits are added depends on the concentrations of tubulin and actin in that region of the cell. When high concentrations of subunits are present, microfilaments and microtubules can become longer at both ends, although subunits assemble, or polymerize, more quickly on one end than on the other. The faster-assembling end is called the plus end, and the slower-assembling end is called the minus end
The ability of microfilaments and microtubules to lengthen and shorten is important for some of their functions. For example, some forms of cell movement, such as a single-celled amoeba foraging for food or a mammalian white blood cell chasing down foreign bacteria, depend on actin polymerization and depolymerization. This ability is also required when a single cell divides in two during cytokinesis
Microtubules make up the spindles that attach to chromosomes during cell division. In this case, the ability of spindle microtubules to “explore” the space of the cell and encounter chromosomes is driven by a unique property of microtubules: their plus ends undergo seemingly random cycles of rapid depolymerization followed by slower polymerization. These cycles of depolymerization and polymerization, which are called dynamic instability (Fig. 10.7), allow spindle microtubules to quickly find and attach to chromosomes during cell division.
Motor proteins associate with microfilaments and microtubules to cause movemment
As noted earlier, microfilaments and microtubules have some capacity to lengthen and shorten by polymerization and depolymerization. When joined by small accessory proteins called motor proteins, microtubules and microfilaments are capable of causing amazing movements. A motor is a device that imparts motion, and motor proteins are molecular motors.
For example, microfilaments associate with the motor protein myosin to produce movement. Together, they are used to transport various types of cellular cargo, such as vesicles, within cells. Microfilaments associated with myosin are also responsible for changes in the shape of many types of cell. One of the most dramatic examples of cell shape change is the shortening, or contraction, of a muscle cell. Muscle contraction depends on the interaction of myosin with microfilaments and is powered by ATP
microtubules also associate with motor proteins. Microtubules function as tracks for transport within the cell. Two motor proteins that associate with these microtubule tracks are kinesin and dynein. Kinesin transports cargo toward the plus end of microtubules, located at the periphery of the cell (Fig. 10.8). By contrast, dynein carries its load away from the cell membrane toward the minus end, located at the centrosome in the interior of the cell. Movement along microtubules by kinesin and dynein is driven by conformational changes in the motor proteins and is powered by energy harvested from ATP.
Specialized skin cells called melanophores are present in some vertebrates. Melanophores are similar to the melanocytes in our own skin that produce the pigment melanin. However, rather than hand off their melanin to other cells (as occurs in humans), melanophores retain their pigment granules and move them around inside the cell in response to hormones or neuronal signals. This redistribution of melanin within the cell allows animals such as fishes and amphibians to change color. For example, at night the melanin granules in the skin of a zebrafish embryo are dispersed throughout the melanophores, darkening the skin color. As morning comes and the day brightens, the pigment granules aggregate at the center of the cell around the centrosome, causing the embryo’s color to lighten (Fig. 10.9). 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 microtubule during dispersal, and dynein moves them back toward the minus end during aggregation. The color change provides daytime and nighttime camouflage: hungry predators lurking in the water below are less likely to spot the young, developing zebrafish embryo.
In addition to providing tracks for the transport of material within the cell, microtubules are found in cilia (singular, cilium). Cilia are rodlike structures that extend from the surface of cells and are well conserved across eukaryotes. There are two types of cilia: those that don’t move (called nonmotile cilia) and those that move (called motile cilia). Nonmotile cilia are very common and can be found on many eukaryotic cells. In these cells, they often serve a sensory function, taking in environmental signals and transducing them to the cell interior. For example, nonmotile cilia are found in mammalian olfactory neurons in the nose and photoreceptors in the eyes.
Motile cilia propel the movement of cells or fluid surrounding the cell (Fig. 10.10). They can be found in single-celled eukaryotes, such as the green alga Chlamydomonas (Fig. 10.10a) and the protist Paramecium (Fig. 10.10b), where they propel the organisms through the water. They are also found in sperm cells (Fig. 10.10c). Long, elaborate cilia present in Chlamydomonas and sperm cells are sometimes called flagella, but they are similar in structure to the shorter cilia and are unrelated to the flagella found on bacterial cells, so most biologists now prefer the term “cilia.”
Motile cilia are also present on epithelial cells, such as those that line the upper respiratory tract (Fig. 10.10d). These cilia move the fluid above the surface of the cell layer, carrying away foreign particles and pathogens. In motile cilia, microtubules associate with the motor protein dynein. Because the microtubules are fixed in place, dynein causes the microtubules to bend, causing a wavelike motion of the cilia. In turn, the motion of the cilia leads to movement of the cell or surrounding fluid.
major functions of cytosjkeltal elements
micrfilaments: actin monomer subunits: functions include cell shape and support, cell movement by crawling, cell dividison (cytokinesis), vesicle trasnport, muscle contraction
intermeditae filaments: diverse subunits: major functions are cell shape and support
microtubules: tubulin dimer subunits: functions include cell shape and support, cell movement by flagella and cilia, cell division (chromsome segregation), vescle transport, and organelle arranegment
The cytoskeletan is an acinent feature of cells
Actin and tubulin are found in all eukaryotic cells, and their structure and function have remained relatively unchanged throughout the course of evolution. The amino acid sequences of yeast tubulin and human tubulin are 75% identical. In fact, a mixture of yeast and human actin monomers forms hybrid microfilaments that are able to function normally in the cell.
Not long ago, it was believed that cytoskeletal proteins were present only in eukaryotic cells. Recently, 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. These bacterial proteins are involved in processes similar to those in eukaryotic cells, including the separation of daughter cells during cell division. Interestingly, at least one of these prokaryotic cytoskeleton-like proteins is expressed in the mitochondria and chloroplasts of some eukaryotic cells. The presence of this protein in these organelles lends support to the theory that mitochondria and chloroplasts were once independent prokaryotic cells that developed a symbiotic relationship with another cell. This idea, called the endosymbiotic theory,
Cell Junctions
Tissues are held together and function as a unit because of cell junctions. Cell junctions physically connect one cell to the next and anchor cells to the extracellular matrix. Some tissues have cell junctions that perform roles other than adhesion. For example, cell junctions in the outer layer of the skin and the lining of intestine provide a seal so that the epithelial sheet can act as a selective barrier. Other cell junctions allow adjacent cells to communicate so that they work together as a unit.
Cell adhesion molecules allow cells to attach to other cells and to the extracellular matrix
Two key observations led to the discovery of proteins on the cell surface that allow cells to adhere to one another (Fig. 10.11). 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. If he then swirled the cells together, they would coalesce back into a group resembling a sponge. If he swirled the cells from sponges of two different species together, the cells sorted themselves out — that is, cells from one species of sponge associated only with cells from that same species (Fig. 10.11a). Fifty years later, German-born embryologist Johannes Holtfreter took neuronal cells and skin cells from an amphibian embryo and treated them the same way that Wilson had treated sponge cells. He observed that the embryonic cells sorted themselves according to tissue type
Cells are able to sort themselves because proteins on the surfaces of cells of the same type recognize each other. Various proteins on the cell surface called cell adhesion molecules attach cells to one another or to the extracellular matrix. While a number of cell adhesion molecules are now known, the cadherins (calcium-dependent adherence proteins) are especially important. Many different kinds of cadherins exist, but a given cadherin may bind only to another cadherin of the same type. This property explains Holtfreter’s observations that cells from amphibian embryos sorted themselves by tissue type. E-cadherin (“epidermal cadherin”) is present on the surface of embryonic epidermal cells, and N-cadherin (“neural cadherin”) is present on neuronal cells. The epidermal cells adhere to one another through E-cadherin, and the neuronal cells adhere to one another through N-cadherin.
Cadherins are transmembrane proteins (Fig. 10.12). 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, the intracellular domain, is linked to the cytoskeleton, including microfilaments and intermediate filaments (Fig. 10.12a). This arrangement essentially connects the cytoskeleton of one cell to the cytoskeleton of another, increasing the strength of tissues and organs.
As well as being stably connected to other cells, cells attach to proteins of the extracellular matrix through cell adhesion molecules called integrins. Like cadherins, integrins are transmembrane proteins, and their cytoplasmic domain is linked to microfilaments or intermediate filaments (Fig. 10.12b). Also like cadherins, integrins are of many different types, each binding to a specific extracellular matrix protein. Integrins are present on the surface of virtually every animal cell. In addition to their role in adhesion, integrins act as receptors that communicate information about the extracellular matrix to the interior of the cell
Anchoring junctions connect adjacent cells and are reinforced by the cytoskeleton
The cell adhesion molecules cadherins and integrins are often organized into cell junctions, complex structures in the cell membrane that allow cells to adhere to one another. Cell junctions formed by adhesion molecules are called anchoring cell junctions and are of two types: adherens junctions and desmosomes
As we saw in section 10.2, a long bundle of actin microfilaments forms a band that extends around the circumference of epithelial cells, such as those lining the intestine. This band of actin is attached to the cell membrane by cadherins in a beltlike structure called an adherens junction (Fig. 10.13). The cadherins in the adherens junction of one cell attach to the cadherins in the adherens junctions of adjacent cells. This arrangement establishes a physical connection among the actin cytoskeletons of all cells present in an epithelial layer.
Like adherens junctions, desmosomes are cell junctions that allow cells to adhere to one another. Whereas adherens junctions form a belt around the circumference of cells, desmosomes are buttonlike points of adhesion (Fig. 10.13). Cadherins are at work here, too, strengthening the connection between cells. Cadherins in the desmosome of one cell bind to cadherins in the desmosomes of adjacent cells. The cytoplasmic domain of these cadherins connects to intermediate filaments in the cytoskeleton. This second type of physical connection among neighboring cells greatly enhances the strength of epithelial cell layers.
Epithelial cells are attached not only to one another, but also to the underlying extracellular matrix (specifically, the basal lamina). In this case, the cells are firmly anchored to the extracellular matrix by a type of desmosome called a hemidesmosome (Fig. 10.13). Integrins are the most prominent cell adhesion molecules in hemidesmosomes. The extracellular domains bind extracellular matrix proteins (not shown in Fig. 10.13), and the cytoplasmic domains connect to intermediate filaments. These intermediate filaments connect to desmosomes in other parts of the cell membrane. The result is a firmly anchored and reinforced layer of cells.
Tight junctions prevent the movement of substances through the space b/w cells
Epithelial cells form sheets or boundaries that line tissues and organs, including the digestive tract, respiratory tract, and outer layer of the skin. 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 adhesion between cells, but they do not prevent materials from passing freely through the spaces between the cells. This function is provided by a different type of cell junction. In vertebrates, these are called tight junctions (Fig. 10.13). Tight junctions establish a seal between cells that prevents molecules from moving through this channel. For example, 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
A tight junction is a band of interconnected strands of integral membrane proteins, particularly proteins called claudins and occludins. Like adherens junctions, tight junctions encircle the epithelial cell. The proteins forming the tight junction in one cell bind to the proteins forming the tight junctions in adjacent cells. Also like adherens junctions, tight junctions connect to actin microfilaments.
A tight junction divides the cell membrane into two distinct regions (Fig. 10.13). As a result, cells that have tight junctions have two sides. The portion of the cell membrane in contact with the lumen, or the inside of any tubelike structure such as the gut, is called the apical membrane. The apical membrane defines the “top” side of the cell. The rest of the cell membrane is the basolateral membrane, which defines the bottom (“baso”) and sides (“lateral”) of the cell.
A 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. In the small intestine, for example, glucose is transported from the lumen into intestinal epithelial cells by transport proteins on the apical side of the cells, and is transported out of the cells into the circulation by facilitated diffusion through a different type of glucose transporter restricted to the basolateral sides of the cells
Molecules pass between cells through communictaing junctions
Not all cell junctions are involved in the adhesion of cells to each other or in sealing a layer of cells. Gap junctions of animal cells and plasmodesmata (singular, plasmodesma) of plant cells permit materials to pass directly from the cytoplasm of one cell to the cytoplasm of another, allowing cells to communicate with one another
Gap junctions are a complex of integral membrane proteins called connexins arranged in a ring. The ring of connexin proteins connects to a similar ring of proteins in the membrane of an adjacent cell. Together, the two connexin rings form a channel that connects the cytoplasm of adjacent cells. Ions and signaling molecules pass through these junctions, allowing cells to act in unison. In the heart, for example, ions pass though gap junctions connecting cardiac muscle cells. This rapid electrical communication allows the muscle cells to beat in a coordinated fashion
Plasmodesmata are passages through the cell walls of adjacent plant cells. Like gap junctions, they allow cells to exchange ions and small molecules, but the similarity ends there. In plasmodesmata, the cell membranes of the two connected cells are actually continuous. The size of the opening is considerably larger than that in gap junctions, large enough for cells to transfer RNA molecules and proteins, an ability that is especially important during embryonic development. Plant cells can send signals to one another through plasmodesmata despite being enclosed within rigid cell walls.
In summary, cell junctions interact to create stable communities of cells in the form of tissues and organs. These cell junctions are important for the functions of tissues, allowing cells to adhere to each other and the extracellular matrix, act as a barrier, and communicate rapidly.
types and functions of cell junctions
anchroing cell junctions:
- adherens junctions; major component is cadherins; cytoskeltal attachment is mcirofilaments; primary function is cell-cell adhesion
- demosome; major component is cadherins; cyctyskelatel attachemnt is intermediate filaments; cell to cell adhesion is the function
- hemidesmosome; integrins is the major component; cytoskeltal attachemnet is intermediate filaments; and primary function is ccell extracellular matrix adhesion
barier cell junction:
- tight junction; claudins and occludins are major componet; cytoskeltal atatchemnt is none; epithelial bounary is [priamry function
communicating:
- gap junction; connexins is major component; communcation between animal cells is the function
- plasmodesma; cell membrane is amjor componet; communication beween plant cells is the function
