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Extracellular Interactions

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Kathleen Gilmour

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Topic 4: Extracellular Interactions -the ability for cells to recognize and bind to each other is important, especially in multicellular organisms (i.e.: animals, plants, fungi) -multicellular organisms have extracellular structures that are important for function -extra cellular structures are normally created by the cells they surround -the components of extra cellular structures are synthesized in the cell and exported to the extracellular environment Plant Cell Walls and Plasmodesmata -cell walls function to provide support, protection for the cell and regulates permeability, but limits the types of cell to cell connections that can occur -animal cells lack cell walls and so they can form a number of different types of cell to cell connections, depending on the function of the tissue -the cell wall is secreted by the cell -the cell wall is made up by 3 classes of molecules: 1) Structural Fibres -provide strength and rigidity to the cell wall -held in place by a hydrated, water-containing matrix -contain cellulose microfibrils -cellulose is a polysaccharide -consists of repeated units of B-D-glucose -linked by B-glycosidic linkages between C1 and C4 (stiff linkages) -cellulose is a relatively stiff, linear molecule -this makes it suitable for its function as reinforcing structural fibres in the cell wall -strings of cellulose molecules are lined up together to create a cellulose microfibril -cellulose is synthesized on the surface of the cell (the extracellular membrane) -enzymes in the cell membrane catalyze the synthesis of cellulose; these are integral transmembrane proteins known as rosettes -the rosette is an enzyme complex for cellulose synthesis -the extracellular surface of the protein (facing the external environment) has the catalytic activity for cellulose synthesis -the intracellular / cytosolic side of the rosettes link to microtubules by means of motor proteins -this creates a system that tow around the rosettes in the plasma membrane; they synthesize cellulose wherever they go -this system allows the cell to control the pattern of cellulose formation in the cell wall -in primary cell wall, you end up with a network / mesh of cellulose microfibrils in a variety of directions -in secondary cell walls, you end up with arrays of cellulose microfibrils -this is important in determining patterns of growth; where the cellulose microfibrils are rigid, the cell will not be able to grow in that direction -this is why the primary cell wall has a different organization than the secondary cell wall 2) Matrix -primarily made up of carbohydrates (90%) with some glycoprotein -main carbohydrates are hemicellulose and pectin -hemicellulose describes a collection of carbohydrates that all gave long side chains that link together to form a network -pectin is also a branched carbohydrate; the backbone of the molecule carries a lot of negative charges and tends to attract water molecules to keep the matrix hydrated, as well as to allow things to get embedded into it -pectin creates a gel-like mass (i.e.: jam) -serves as the glue / adhesive molecule that holds cell walls together and connects adjacent cell walls together -forms the bulk of the mass that surrounds the cells -cellulose microfibrils are inserted into the matrix of carbohydrates and provide the rigidity to the gel-like mass -the glycoprotein is extensin 3) Adhesive Molecules -involves pectin (see above) Primary and Secondary Cell Walls -the extracellular structures are synthesized by the cell on the external surface -in order from internal to external: 1) Middle Lamella -the first thing the cell does is lay down a layer of pectins; this is known as the middle lamella -middle lamella joins cell walls together 2) Primary Cell Walls -the cell secretes the primary cell wall after the middle lamella -the primary cell wall is the cell wall that has a network of cellulose microfibrils -this network is able to expand as the primary cell wall gets pushed further away from the cell 3) Secondary Cell Wall -layers of secondary cell wall are laid down after the primary cell wall -in each layer of secondary cell wall, the cellulose microfibrils have a different orientation -provides strength to the cell wall as a whole -the multiple layers are especially evident in woody plants and can be seen in their rigidity -secondary cell wall often contains lignin which contributes to the strength of the cell wall -the different patterns of cellulose microfibrils are achieved by the different movement of rosettes and their pattern of synthesis along microtubules and driven by motor proteins -the cell controls the pattern of microtubules and therefore the pattern of cellulose microfibrils Cell to Cell Connections in Plants 1) Plasmodesmata -cytoplasmic connections between cells -made up of a hole in the cell wall, lined with plasma membrane that allows the cytoplasm of the 2 cells to be in communication -as the cell wall is forming, there are bits of smooth endoplasmic reticulum that stretch across the location where the new cell wall is forming -the cell wall forms holes around those bits of tubule of the smooth ER (known as desmotubules which often remain in the plasmodesmata) -anything that can fit between the plasmodesmata can move from one cell to the next (i.e.: water, ions, larger molecules like plant hormones / RNA / signalling molecules, etc.) Cell to Cell Connections in Animals -animal cells have a variety of cell to cell connections because of the need to form cells into tissues -there are 3 broad types of connections: 1) Adhesive (Anchoring) Junctions: adherens, desmosomes -anchor cells together so that the tissue is integrated as a whole -hold animal cells within tissues together -adhesive junctions involve transmembrane proteins (i.e.: cadherins) -cadherins have large extracellular domains -extracellular domains contain repeated units of structure; these repeated units can be recognized by other cadherins of the same type -they are held together by non-covalent bonds -cadherins from 1 cell will bind with cadherins from the neighbouring cell and this holds the 2 cells together -the 2 sides have to match (recognize) and then link together -on the intracellular side, cadherins link to the cytoskeleton (i.e.: microfilaments or intermediate filaments) through clusters of linking proteins -when cadherins link to microfilaments, the junctions are called adherins junctions -they integrate the cytoskeleton into the junction and hold the cell together -bands of adherins junctions are usually found as a belt around the cell -adherins link to microfilaments intracellularly and cadherins extracellulary -punctate adherins junctions are spots of adherins junctions -the other type of anchoring junction is a desmosome -in this case, cadherins link to intermediate filaments -there are cadherins that are transmembrane proteins that link in the extracellular space and link to intermediate filaments on the intracellular side -they link through linking proteins; the linking proteins from dense plaques of proteins which make desmosomes strong connections -important in tissues that experience a lot of stress (i.e.: the skin, the heart, etc.) 2) Tight Junctions -seal up spaces between cells -are normally found in cell layers that separate 2 different compartments (i.e.: the gut, the kidney, etc.) -help to seal space between cells and prevent mixing of separate layers -the space between the cells is call
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