witht-SNAREon the cell membrane and requires Rab GTPase-activity,
linking to tethering proteins, and eventually to a receptor protein in the cell
membrane.Receptor-mediated endocytosisis the process that permits
selective uptake of molecules into the cell using clathrin-coated pits and
vesicles.Molecules not recycled to the cell membrane enter early endo-
somesand subsequently late endosomes by way ofmultivesicular bod-
ies (MVBs). The late endosome is more acidic than the early endosome
and generally leads to degradation of the molecules in lysosomes. There are
several major pathways for shuttling of receptors and ligands.
- The internalized ligand-receptor complex dissociates in the early endo-
some with recycling of receptors [e.g., low density-lipoprotein (LDL)-
LDL-receptor complex]. - Receptor and ligand are recycled (e.g., iron-transferrin-transferrin receptor-
complex). - The internalized ligand-recepetor complex dissociates in the late endo-
some and is degraded in the lysosome (e.g., growth factors such as epi-
dermal growth factor). - Internalized ligand-receptor passes through the cell (transcytosis) and is
released at another surface (e.g., IgA uptake by small intestinal entero-
cytes).
Only the nucleus,which is the repository of genetic information
stored in deoxyribonucleic acid (DNA), and the mitochondria,which are
the storage sites of energy for cellular function in the form of adenosine
triphosphate(ATP),are enclosed in double membranes. Also included in
the cytoplasm are three classes of proteins that form the cytoskeletal infra-
structure: actin bundlesthat determine the shape of the cell; intermedi-
ate filamentsthat stabilize the cell membrane and cytoplasmic contents;
andmicrotubules (tubulin),which use molecular motors (i.e., dynein
and kinesin) to move organelles within the cell.
NUCLEUSThe nucleus consists of a nuclear envelopethat is continuous with the ER,
chromatin, matrix, and a nucleolusthe site of ribosomal ribonucleic acid
(rRNA) synthesis and initial ribosomal assembly. The nuclear envelope contains
pores for bidirectional transport and is supported by intermediate filament
proteins, the lamins. Chromatinconsists of euchromatin(eu= true),
16 Anatomy, Histology, and Cell Biology