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CYTOPLASMIC ORGANELLES Inside the cell membrane the fluid cytoplasm (or cytosol) bathes metabolically active structures called organelles, which may be membranous (such as mitochondria) or nonmembranous protein complexes (such as ribosomes and proteasomes).Ribosomes are macromolecular machines, about 20 x 30 nm in size, which assemble polypeptides from amino acids on molecules of transfer RNA (tRNA) in a sequence specified by mRNA.Oxygen, CO2, electrolytic ions, low-molecular-weight substrates, metabolites, and waste products all diffuse through cytoplasm, either freely or bound to proteins, entering or leaving organelles where they are used or produced.Most organelles are positioned in the cytoplasm by movements along the polymers of the cytoskeleton, which also determines a cell's shape and motility.Cytosol also contains hundreds of enzymes, such as those of the glycolytic pathway, which produce building blocks for larger molecules and break down small molecules to liberate energy.The core of the small ribosomal subunit is a highly folded ribosomal RNA (rRNA) chain associated with more than 30 unique proteins; the core of the large subunit has three other rRNA molecules and nearly 50 other basic proteins.A functional ribosome has two subunits of different sizes bound to a strand of mRNA.

CYTOPLASMIC ORGANELLES Inside the cell membrane the fluid cytoplasm (or cytosol) bathes metabolically active structures called organelles, which may be membranous (such as mitochondria) or nonmembranous protein complexes (such as ribosomes and proteasomes). Most organelles are positioned in the cytoplasm by movements along the polymers of the cytoskeleton, which also determines a cell’s shape and motility. Cytosol also contains hundreds of enzymes, such as those of the glycolytic pathway, which produce building blocks for larger molecules and break down small molecules to liberate energy. Oxygen, CO2, electrolytic ions, low-molecular-weight substrates, metabolites, and waste products all diffuse through cytoplasm, either freely or bound to proteins, entering or leaving organelles where they are used or produced. Ribosomes are macromolecular machines, about 20 × 30 nm in size, which assemble polypeptides from amino acids on molecules of transfer RNA (tRNA) in a sequence specified by mRNA. A functional ribosome has two subunits of different sizes bound to a strand of mRNA. The core of the small ribosomal subunit is a highly folded ribosomal RNA (rRNA) chain associated with more than 30 unique proteins; the core of the large subunit has three other rRNA molecules and nearly 50 other basic proteins. The rRNA molecules in the ribosomal subunits not only provide structural support but also position transfer RNAs (tRNA) molecules bearing amino acids in the correct “reading frame” and catalyze the formation of the peptide bonds. The more peripheral proteins of the ribosome seem to function primarily to stabilize the catalytic RNA core. These ribosomal proteins are themselves synthesized in cytoplasmic ribosomes, but are then imported to the nucleus where they associate with newly synthesized rRNA. Thribosomal subunits thus formed then move from the nucleus to the cytoplasm where they are reused many times, for translation of any mRNA strand. During protein synthesis many ribosomes typically bind the same strand of mRNA to form larger complexes called polyribosomes, or polysomes (Figure 2–9). In stained preparations of cells polyribosomes are intensely basophilic because of the numerous phosphate groups of the constituent RNA molecules that act as polyanions. Thus, cytoplasmic regions that stain intensely with hematoxylin and basic dyes, such as methylene and toluidine blue, indicate sites of active protein synthesis. Proper folding of new proteins is guided by protein chaperones. Denatured proteins or those that cannot be refolded properly are conjugated to the protein ubiquitin that targets them for breakdown by proteasomes (discussed below). Proteins synthesized for use within the cytosol (eg,glycolytic enzymes) or for import into the nucleus and certain other organelles are synthesized on polyribosomes existing as isolated cytoplasmic clusters. Polyribosomes attached to membranes of the endoplasmic reticulum (ER) translate mRNAs coding for membrane proteins of the ER, the Golgi apparatus, or the cell membrane; enzymes to be stored in lysosomes; and proteins to undergo exocytosis from secretory vesicles.