Cytoplasm and Organelles, Cytoplasm is a dynamic fluid environment bounded by the cell membrane

Membrane & Cytoplasm

Cytoplasm is a dynamic fluid environment bounded by the cell membrane. It contains various membrane bound organelles, nonmembranous structures (such as lipid droplets, glycogen, and pigment granules), and structural or cytoskeletal proteins in either a soluble or insoluble form. The endoplasmic reticulum (ER) is a continuous tubular meshwork that may be either smooth (SER) or rough (RER) where studded with ribosomes.
Cytoplasm is a dynamic fluid environment bounded by the cell membrane. It contains various membrane bound organelles, nonmembranous structures (such as lipid droplets, glycogen, and pigment granules), and structural or cytoskeletal proteins in either a soluble or insoluble form. The endoplasmic reticulum (ER) is a continuous tubular meshwork that may be either smooth (SER) or rough (RER) where studded with ribosomes.
RER is involved in protein synthesis while the SER is involved in sterioid synthesis and detoxification. The discoid stacks (CGN, cis, medial, trans, and TGN as one moves from the RER-side to the secretory vesicle-side) of the Golgi apparatus are involved in packaging and routing proteins for export or delivery to other organelles, including lysosomes and peroxisomes.

Lysosomes degrade intracellular and imported debris, and peroxisomes oxidize a variety of substrates, through beta-oxidation and are the sole source of plasmalogens. Targeting sequences include KDEL, which targets ER proteins from the Golgi to the ER, and mannose 6-phosphate, which targets proteins to the lysosome. Mannose 6-phosphate receptors are found in the Golgi and in lysosomes. In the absence of mannose 6phosphate on lysosomal enzymes (I-cell disease) they follow the default pathway and are secreted from the cell. Lysosomal enzymes are specific for substrate; the absence of specific enzymes results in lysosomal storage diseases such as Tay-Sach’s. 

Secretory granules leave the TGN to dock with the plasma membrane. In that process, v-SNARE on the vesicle docks with t-SNARE on the cell membrane and requires Rab GTPase-activity, linking to tethering proteins, and eventually to a receptor protein in the cell membrane. Receptor-mediated endocytosis is 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 endosomes and subsequently late endosomes by way of multivesicular bodies (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 endosome with recycling of receptors [e.g., low density-lipoprotein (LDL)LDL-receptor complex].
  • Receptor and ligand are recycled (e.g., iron-transferrin-transferrin receptorcomplex).
  • The internalized ligand-recepetor complex dissociates in the late endosome and is degraded in the lysosome (e.g., growth factors such as epidermal growth factor).
  • Internalized ligand-receptor passes through the cell (transcytosis) and is released at another surface (e.g., IgA uptake by small intestinal enterocytes).

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 infrastructure: actin bundles that determine the shape of the cell; intermediate filaments that stabilize the cell membrane and cytoplasmic contents; and microtubules (tubulin), which use molecular motors (i.e., dynein and kinesin) to move organelles within the cell.

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