This is the fluid component inside the cell membrane and contains many specialized organelles. It contains a scaffolding or cytoskeleton that regulates the passage and direction in which the interior solutes and storage granules flow. The cytoplasm contains:

• Endoplasmic reticulum (ER). This consists of interconnecting tubules or flattened sacs (cisternae) of lipid bilayer membrane. It may contain ribosomes on the surface (termed rough endoplasmic reticulum (RER) when present, or smooth endoplasmic reticulum (SER) when absent). The ER is involved in the processing of proteins: the ribosomes translate mRNA into a primary sequence of amino acids of a protein peptide chain. This chain is synthesized into the ER where it is first folded and modified into mature peptides. ER is the major site of drug metabolism.
• Golgi apparatus. This consists of flattened cisternae similar to the ER. It is characterized as a stack of cisternae from which vesicles bud off from the thickened ends. The primary processed peptides of the ER are exported to the Golgi apparatus for maturation into functional proteins (e.g. glycosylation of proteins which are to be excreted occurs here) before packaging into secretory granules and cellular vesicles that bud off the end.
• Lysosomes. These are dense cellular vesicles containing acidic digestive enzymes. They fuse with phagocytotic vesicles from the outer cell membrane, digesting the contents into small biomolecules that can cross the lysosomal lipid bilayer into the cell cytoplasm. Lysosomal enzymes can also be released outside the cell by fusion of the lysosome with the plasma membrane. Lysosomal action is crucial to the function of macrophages and polymorphs in killing and digesting infective agents, tissue remodelling during development and osteoclast remodelling of bone. Not surprisingly, many metabolic disorders result from impaired lysosomal function.
• Peroxisomes. These are dense cellular vesicles so named because they contain enzymes that catalyse the breakdown of hydrogen peroxide. They are involved in the metabolism of bile and fatty acids, and are primarily concerned with detoxification, e.g. d-amino acid oxidase and H₂O₂ catalase. The inability of the peroxisomes to function correctly can lead to rare metabolic disorders such as Zellweger’s syndrome and rhizomelic dwarfism.
• Mitochondria. These organelles are the powerhouse of the cell. Each mitochondrion comprises two lipid bilayer membranes and a central matrix. It also possesses several copies of its own DNA in a circular genome. The outer membrane contains many gated receptors responsible for the import of raw materials like pyruvate and ADP, and the export of products such as oxaloacetate (precursor of amino acids and sugars) and ATP. An interesting caveat to our symbiotic relationship is that proteins of the Bcl-2/Bax family are incorporated in this outer membrane and can release mitochondrial enzymes that trigger apoptosis. The inner membrane is often highly infolded to form cristae to increase its effective surface area. It contains transmembrane enzyme complexes of the electron transport chain, which generate an H⁺ ion gradient. This gradient then drives the adjacent transmembrane ATPase complex to form ATP from ADP and P_i. The inner matrix contains the enzymes of the Krebs cycle that generate the substrates of both the electron transport chain (FADH₂ and NADH) and central metabolism.

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1. [...] You can see it on the right; it measures only 3 mm x 3 mm x 1 mm. Researchers from the University ofHealth Nova: Your Ultimate Medical Portal CYTOPLASM… from the outer cell membrane, digesting the contents into small biomolecules that can … in [...]

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