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    gap junctions are a complex of integral membrane proteins called connexins arranged in a ring. in plasmodesmata, the plasma membranes of the two connected cells are actually continuous. gap junctions and plasmodesmata have what feature in common?

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    Cell Junctions

    Specialized cell junctions occur at points of cell-cell and cell-matrix contact in all tissues, and they are particularly plentiful in epithelia. Cell junctions are best visualized using either conventional or freeze-fracture electron microscopy (discussed in Chapter 9), which reveals that the interacting plasma membranes (and often the underlying cytoplasm and the intervening intercellular space as well) are highly specialized in these regions.

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    Molecular Biology of the Cell. 4th edition.

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    Cell Junctions

    Specialized cell junctions occur at points of cell-cell and cell-matrix contact in all tissues, and they are particularly plentiful in epithelia. Cell junctions are best visualized using either conventional or freeze-fracture electron microscopy (discussed in Chapter 9), which reveals that the interacting plasma membranes (and often the underlying cytoplasm and the intervening intercellular space as well) are highly specialized in these regions.

    Cell junctions can be classified into three functional groups:

    1.

    Occluding junctions seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other.

    2.

    Anchoring junctions mechanically attach cells (and their cytoskeletons) to their neighbors or to the extracellular matrix.

    3.

    Communicating junctions mediate the passage of chemical or electrical signals from one interacting cell to its partner.

    The major kinds of intercellular junctions within each group are listed in Table 19-1. We discuss each of them in turn, except for chemical synapses, which are formed exclusively by nerve cells and are considered in Chapters 11 and 15.

    Table 19-1

    A Functional Classification of Cell Junctions.

    Go to:

    Occluding Junctions Form a Selective Permeability Barrier Across Epithelial Cell Sheets

    All epithelia have at least one important function in common: they serve as selective permeability barriers, separating fluids on either side that have a different chemical composition. This function requires that the adjacent cells be sealed together by occluding junctions. Tight junctions have this barrier role in vertebrates, as we illustrate by considering the epithelium of the mammalian small intestine, or gut.

    The epithelial cells lining the small intestine form a barrier that keeps the gut contents in the gut cavity, the At the same time, however, the cells must transport selected nutrients across the epithelium from the lumen into the extracellular fluid that permeates the connective tissue on the other side (see Figure 19-1). From there, these nutrients diffuse into small blood vessels to provide nourishment to the organism. This depends on two sets of membrane-bound membrane transport proteins. One set is confined to the of the epithelial cell (the surface facing the lumen) and actively transports selected molecules into the cell from the gut. The other set is confined to the (basal and lateral) of the cell, and it allows the same molecules to leave the cell by facilitated diffusion into the extracellular fluid on the other side of the epithelium. To maintain this directional transport, the apical set of transport proteins must not be allowed to migrate to the basolateral surface of the cell, and the basolateral set must not be allowed to migrate to the apical surface. Furthermore, the spaces between epithelial cells must be tightly sealed, so that the transported molecules cannot diffuse back into the gut lumen through these spaces (Figure 19-2).

    Figure 19-2

    The role of tight junctions in transcellular transport. Transport proteins are confined to different regions of the plasma membrane in epithelial cells of the small intestine. This segregation permits a vectorial transfer of nutrients across the epithelium (more...)

    The tight junctions between epithelial cells are thought to have both of these roles. First, they function as barriers to the diffusion of some membrane proteins (and lipids) between apical and basolateral domains of the plasma membrane (see Figure 19-2). Mixing of such proteins and lipids occurs if tight junctions are disrupted, for example, by removing the extracellular Ca2+ that is required for tight junction integrity. Second, tight junctions seal neighboring cells together so that, if a low-molecular-weight tracer is added to one side of an epithelium, it will generally not pass beyond the tight junction (Figure 19-3). This seal is not absolute, however. Although all tight junctions are impermeable to macromolecules, their permeability to small molecules varies greatly in different epithelia. Tight junctions in the epithelium lining the small intestine, for example, are 10,000 times more permeable to inorganic ions, such as Na+, than the tight junctions in the epithelium lining the urinary bladder. These differences reflect differences in tight junction proteins that form the junctions.

    Figure 19-3

    The role of tight junctions in allowing epithelia to serve as barriers to solute diffusion. (A) The drawing shows how a small extracellular tracer molecule added on one side of an epithelium cannot traverse the tight junctions that seal adjacent cells (more...)

    Epithelial cells can transiently alter their tight junctions to permit an increased flow of solutes and water through breaches in the junctional barriers. Such is especially important in the absorption of amino acids and monosaccharides from the lumen of the intestine, where their concentration can increase enough after a meal to drive passive transport in the desired direction.

    Source : www.ncbi.nlm.nih.gov

    Cell

    Different types of intercellular junctions, including plasmodesmata, tight junctions, gap junctions, and desmosomes.

    Introduction

    If you were building a building, what kinds of connections might you want to put between the rooms? In some cases, you’d want people to be able to walk from one room to another, in which case you’d put in a door. In other cases, you’d want to hold two adjacent walls firmly together, in which case you might put in some strong bolts. And in still other cases, you might need to ensure that the walls were sealed very tightly together – for instance, to prevent water from dripping between them.

    As it turns out, cells face the same questions when they’re arranged in a tissue next to other cells. Should they put in doors that connect them directly to their neighbors? Do they need to spot-weld themselves to their neighbors to make a strong layer, or perhaps even form tight seals to prevent water from passing through the tissue? Junctions serving all of these functions can be found in cells of different types, and here, we’ll look at each of them in turn.

    Plasmodesmata

    Plant cells, surrounded as they are by cell walls, don’t contact one another through wide stretches of plasma membrane the way animal cells can. However, they do have specialized junctions called plasmodesmata (singular, plasmodesma), places where a hole is punched in the cell wall to allow direct cytoplasmic exchange between two cells.

    Image of two cells connected by a plasmodesma, showing how materials can travel from the cytoplasm of one cell to the next via the plasmodesma.

    Image credit: OpenStax Biology.

    Plasmodesmata are lined with plasma membrane that is continuous with the membranes of the two cells. Each plasmodesma has a thread of cytoplasm extending through it, containing an even thinner thread of endoplasmic reticulum (not shown in the diagram above).

    Molecules below a certain size (the size exclusion limit) move freely through the plasmodesmal channel by passive diffusion. The size exclusion limit varies among plants, and even among cell types within a plant. Plasmodesmata may selectively dilate (expand) to allow the passage of certain large molecules, such as proteins, although this process is poorly understood

    ^{1,2} 1,2

    start superscript, 1, comma, 2, end superscript

    .

    Gap junctions

    Functionally, gap junctions in animal cells are a lot like plasmodesmata in plant cells: they are channels between neighboring cells that allow for the transport of ions, water, and other substances

    ^3 3 cubed

    . Structurally, however, gap junctions and plasmodesmata are quite different.

    In vertebrates, gap junctions develop when a set of six membrane proteins called connexins form an elongated, donut-like structure called a connexon. When the pores, or “doughnut holes,” of connexons in adjacent animal cells align, a channel forms between the cells. (Invertebrates also form gap junctions in a similar way, but use a different set of proteins called innexins.)

    ^4 4

    start superscript, 4, end superscript

    Image of the plasma membranes of two cells held together by gap junctions. Where two connexons from the different cells meet, they can form a channel leading from one cell into the next.

    Image credit: OpenStax Biology. Modification of work by Mariana Ruiz Villareal.

    Gap junctions are particularly important in cardiac muscle: the electrical signal to contract spreads rapidly between heart muscle cells as ions pass through gap junctions, allowing the cells to contract in tandem.

    Tight junctions

    Not all junctions between cells produce cytoplasmic connections. Instead, tight junctions create a watertight seal between two adjacent animal cells.

    At the site of a tight junction, cells are held tightly against each other by many individual groups of tight junction proteins called claudins, each of which interacts with a partner group on the opposite cell membrane. The groups are arranged into strands that form a branching network, with larger numbers of strands making for a tighter seal

    ^5 5

    start superscript, 5, end superscript

    .

    Image of the membranes of two cells held together by tight junctions. The tight junctions are like rivets, and they are arranged in multiple strands that form lines and triangles.

    Image credit: OpenStax Biology. Modification of work by Mariana Ruiz Villareal.

    The purpose of tight junctions is to keep liquid from escaping between cells, allowing a layer of cells (for instance, those lining an organ) to act as an impermeable barrier. For example, the tight junctions between the epithelial cells lining your bladder prevent urine from leaking out into the extracellular space.

    Desmosomes

    Animal cells may also contain junctions called desmosomes, which act like spot welds between adjacent epithelial cells. A desmosome involves a complex of proteins. Some of these proteins extend across the membrane, while others anchor the junction within the cell.

    Source : www.khanacademy.org

    week 1 questions Flashcards

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    week 1 questions

    268 studiers in the last day

    Cadherins are: (Select all that apply.)

    proteins that provide a pathway for molecules to move between cells.

    cell adhesion molecules found in cell junctions.

    proteins involved in attachment of cells to the extracellular matrix.

    proteins involved in attachment of cells to neighboring cells.

    Click card to see definition 👆

    proteins involved in attachment of cells to neighboring cells.

    cell adhesion molecules found in cell junctions.

    Click again to see term 👆

    6. Gap junctions and plasmodesmata have what feature in common?

    They both are made up of protein subunits located in the plasma membrane.

    They both attach to the cytoskeleton.

    They are both found in plant cells.

    They both allow direct transport of materials between cells.

    Click card to see definition 👆

    They both allow direct transport of materials between cells.

    Click again to see term 👆

    1/83 Created by bravobelen24

    Terms in this set (83)

    Cadherins are: (Select all that apply.)

    proteins that provide a pathway for molecules to move between cells.

    cell adhesion molecules found in cell junctions.

    proteins involved in attachment of cells to the extracellular matrix.

    proteins involved in attachment of cells to neighboring cells.

    proteins involved in attachment of cells to neighboring cells.

    cell adhesion molecules found in cell junctions.

    6. Gap junctions and plasmodesmata have what feature in common?

    They both are made up of protein subunits located in the plasma membrane.

    They both attach to the cytoskeleton.

    They are both found in plant cells.

    They both allow direct transport of materials between cells.

    They both allow direct transport of materials between cells.

    5. Cadherins bind to which type of cytoskeletal element(s)?

    both intermediate filaments and microfilaments

    microfilaments

    intermediate filaments

    microtubules

    both intermediate filaments and microfilaments

    2. In skin, the cells of the epidermis connect to the basal lamina through cellular junctions known as hemidesmosomes. Select the cytoskeletal element that helps maintain the integrity of this connection.

    microtubules

    both intermediate filaments and microfilaments

    intermediate filaments

    microfilaments

    intermediate filaments

    3. Which of the following cell junctions is involved in creating a barrier between cells?

    adherens junction tight junction desmosome tight junction

    4. Some cell junctions allow materials to pass between adjacent cells so that they work together as a unit.

    False True True The basal lamina:

    All of these choices are correct.

    is a cell-rich layer.

    is a specialized extracellular matrix found beneath epithelial tissues.

    links to epithelial cells through tight junctions.

    is found on top of the epidermis of the skin.

    is a specialized extracellular matrix found beneath epithelial tissues.

    2. How do cells connect to the extracellular matrix?

    through integrins through cadherins

    through gap junctions

    through tight junctions

    through integrins

    3. The extracellular matrix is particularly important for which type of tissue?

    epithelial muscle connective nervous connective

    6. The strength of collagen comes from its:

    amino acid sequence.

    ability to bind to polysaccharide molecules.

    triple helical structure and bundling, as well as its amino acid sequence.

    triple helical structure and bundling.

    triple helical structure and bundling.

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