Though lacking cell walls animal cells do have an elaborate extracellular

Though lacking cell walls animal cells do have an

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Though lacking cell walls, animal cells do have an elaborate extracellular matrix (ECM). The primary constituents of the extracellular matrix are glycoproteins, especially collagen fibers, embedded in a network of glycoprotein proteoglycans . In many cells, fibronectins in the ECM connect to integrins, intrinsic membrane proteins that span the membrane and bind on their cytoplasmic side to proteins attached to microfilaments of the cytoskeleton. The interconnections from the ECM to the cytoskeleton via the fibronectin-integrin link permit the integration of changes inside and outside the cell. The ECM can regulate cell behavior. Embryonic cells migrate along specific pathways by matching the orientation of their microfilaments to the “grain” of fibers in the extracellular matrix. The extracellular matrix can influence the activity of genes in the nucleus via a combination of chemical and mechanical signaling pathways. This may coordinate the behavior of all the cells within a tissue. Intercellular junctions help integrate cells into higher levels of structure and function. Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact. Plant cells are perforated with plasmodesmata, channels allowing cytosol to pass between cells. Water and small solutes can pass freely from cell to cell. In certain circumstances, proteins and RNA can be exchanged.
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Animals have 3 main types of intercellular links: tight junctions, desmosomes, and gap junctions. In tight junctions, membranes of adjacent cells are fused, forming continuous belts around cells. This prevents leakage of extracellular fluid. Desmosomes (or anchoring junctions) fasten cells together into strong sheets, much like rivets. Intermediate filaments of keratin reinforce desmosomes. Gap junctions (or communicating junctions) provide cytoplasmic channels between adjacent cells. Special membrane proteins surround these pores. Ions, sugars, amino acids, and other small molecules can pass. In embryos, gap junctions facilitate chemical communication during development. A cell is a living unit greater than the sum of its parts. While the cell has many structures with specific functions, all these structures must work together. For example, macrophages use actin filaments to move and extend pseudopodia to capture their bacterial prey. Food vacuoles are digested by lysosomes, a product of the endomembrane system of ER and Golgi. The enzymes of the lysosomes and proteins of the cytoskeleton are synthesized on the ribosomes. The information for the proteins comes from genetic messages sent by DNA in the nucleus. All of these processes require energy in the form of ATP, most of which is supplied by the mitochondria. A cell is a living unit greater than the sum of its parts.
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