Chapter 2: The Cytoplasm

The Cytoplasm SUMMARY OF KEY POINTS

• 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.

• 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.

• 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.

• 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 ...