Biology (1) 1501143 Past Paper PDF 20241

Biology (1) 1501143 Dr Maysoun Qutob Applied Science Private University, 20241 1 Applied Science Private University 1  A Tour of the Cell Part 4 2 Applied Science Private University 2 Extracellular compo...

Biology (1) 1501143 Dr Maysoun Qutob Applied Science Private University, 20241 1 Applied Science Private University 1  A Tour of the Cell Part 4 2 Applied Science Private University 2 Extracellular components and connections between cells help coordinate cellular activities Cell Walls of Plants The cell wall is an extracellular structure of plant cells (distinguishes plant cells from animal cells). Functions: 1. The wall protects the plant cell 2. Maintains its shape 3. Prevents excessive uptake of water. Prokaryotes, fungi, and some unicellular eukaryotes also have cell walls.  Plant cell walls are much thicker than the plasma membrane. Plant cell walls are made of cellulose fibers embedded in other polysaccharides and proteins. Formation of cell wall: Young plant cell first secretes a relatively thin and flexible wall called the primary cell wall. Between primary walls of adjacent cells is the middle lamella, a thin layer rich in sticky polysaccharides called pectins. The middle lamella glues adjacent cells together. When the cell matures and stops growing, it strengthens its wall. 1. Some plant cells do this simply by secreting hardening substances into the primary wall. 2. Other cells add a secondary cell wall between the plasma membrane and the primary wall (woods). Plant cell walls are usually perforated by channels between adjacent cells called plasmodesmata. Figure 6.27 Plant cell walls. The drawing shows the relationship between primary and secondary cell walls in several mature plant cells. (Organelles aren’t shown because many cells with secondary walls, such as the water- conducting cells, lack organelles.) The TEM shows the cell walls where two cells come together. The multilayered partition between plant cells consists of adjoining walls individually secreted by the cells. Adjacent cells are glued together by a very thin layer called the middle lamella. The Extracellular Matrix (ECM) of Animal Cells Animal cells have extracellular matrix (ECM). The main ingredients of the ECM are glycoproteins and other carbohydrate-containing molecules secreted by the cells (mostly Collagen). Collagen, which forms strong fibers outside the cells, are embedded in a network woven out of proteoglycans secreted by cells (Figure 6.28). A proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached, so that it may be up to 95% carbohydrate.  Some cells are attached to the ECM by ECM glycoproteins, such as fibronectin, which bind to cell-surface receptor proteins called integrins that are built into the plasma membrane. Integrins span the membrane and bind on their cytoplasmic side to associated proteins attached to microfilaments of the cytoskeleton.  Integrins: transmit signals between the ECM and the cytoskeleton and thus to integrate changes occurring outside and inside the cell. By communicating with a cell through integrins, the ECM can regulate a cell’s behavior: a cell can influence the activity of genes in the nucleus. Cell Junctions Neighboring cells often adhere, interact, and communicate via sites of direct physical contact. Plasmodesmata in Plant Cells: cell walls are perforated with plasmodesmata: channels that connect cells. Cytosol passing through the plasmodesmata joins the internal chemical environments of adjacent cells. The plasma membranes of adjacent cells line the channel of each plasmodesma and thus are continuous. Water and small solutes can pass freely from cell to cell, and in some circumstances, certain proteins and RNA molecules can pass. Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells: In animals, there are three main types of cell junctions: 1. Tight junctions 2. Desmosomes 3. Gap junctions (Gap junctions are most like the plasmodesmata of plants, although gap junction pores are not lined with membrane). All three types of cell junctions are especially common in epithelial tissue, which lines the external and internal surfaces of the body. Type Tight Junctions Desmosomes Gap Junctions Features the plasma membranes of Also called: Anchoring junction Also called: Communicating neighboring cells are very Junctions. tightly pressed against each Intermediate filaments made of Gap junctions consist of other, bound together by sturdy keratin proteins anchor membrane proteins that specific proteins. desmosomes in the cytoplasm. surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Function a barrier that prevents fastening cells together into Provide cytoplasmic channels leakage of extracellular fluid strong sheets. from one cell to an adjacent across a layer of epithelial Desmosomes attach muscle cells cell cells. to each other in a muscle. Some Gap junctions are necessary for “muscle tears” involve the communication between cells in Example: skin cells rupture of desmosomes. many types of tissues, such as heart muscle, and in animal embryos.