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  • Cell differentiation is the process by which cells of an embryo become specialized structurally to augment specific cytoplasmic activities for functions at the level of tissues and organs.

  • Organelles are metabolically active structures or complexes, with or without membranes, in the cytoplasm of eukaryotic cells.

Plasma Membrane
  • The plasma membrane (cell membrane or plasmalemma) is the lipid bilayer with embedded proteins that surrounds a cell and is seen only with the TEM.

  • The lipid bilayer forms from amphipathic phospholipids, stabilized by cholesterol, and contains many embedded (integral) proteins and many peripheral proteins on its cytoplasmic surface.

  • Membrane proteins move laterally within the lipid bilayer, with less movement in areas referred to as lipid rafts, which have higher concentrations of cholesterol and saturated fatty acids.

  • Integral membrane proteins include receptors for external ligands, channels for passive or active movement of molecules across the membrane, and pumps for active membrane transport.

  • Endocytosis is cellular uptake of macromolecules or fluid by plasma membrane engulfment or invagination, followed by the “pinching off” of a filled membranous vesicle in the cytoplasm.

  • Major types of endocytosis include phagocytosis (uptake of particulate material), pinocytosis (uptake of dissolved substances), and receptor-mediated endocytosis (uptake of specific molecules bound to integral membrane receptor proteins).

  • Exocytosis is a type of cellular secretion in which cytoplasmic membrane vesicles fuse with the plasma membrane and release their contents to the extracellular space.

  • All types of cell signaling use membrane receptor proteins that are often linked to enzymes such as kinases or adenylyl cyclase whose activities initiate intracellular signaling pathways.

  • The two ribosomal subunits, each a complex of rRNA and many proteins, attach to mRNA and translate that message into protein.

  • Multiple ribosomes on the same mRNA make up a polyribosome (polysome), and an abundance of these produces basophilic cytoplasm after H&E staining.

Endoplasmic Reticulum
  • The ER is a convoluted network of membrane enclosing continuous spaces called cisternae and extending from the nucleus to the plasma membrane.

  • Rough ER has a granular, basophilic cytoplasmic surface due to the presence of polysomes making most membrane proteins, proteins in certain other organelles, or for exocytosis; RER is always well developed in cells actively secreting proteins.

  • Proteins to be processed through the RER contain initial signal peptides that bind receptors in the ER membrane, localizing them to that organelle.

  • After translocation across the membrane into the cisterna, the proteins undergo posttranslational modification and folding in a process monitored by RER molecular chaperones and enzymes.

  • Smooth ER (SER) lacks ribosomes but includes enzymes for lipid and glycogen metabolism, for detoxification reactions, and for temporary Ca2+ sequestration.

Golgi Apparatus
  • The Golgi apparatus is a dynamic organelle consisting of stacked membranous cisternae in which proteins made in RER are processed further and packaged for secretion or other roles.

  • Proteins in transport vesicles enter the cis or receiving face of the ...

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