Table 61

Continued.

Endoplasmic reticulum (ER) which maintains structural continuity with the cell membrane, is continuous with the nuclear envelope, and synthesizes and transports lipids and membrane proteins. There are two kinds of ER in the cell; the rough ER is a flattened sheet, studded with ribosomes carrying out protein synthesis, and the smooth ER is more tubular with no attached ribosomes, but with major lipid synthesis function and for the storage and release of calcium ions into the cytosol, where calcium catalyzes many processes. However, the function of smooth ER varies from tissue to tissue;

Golgi apparati (GA) play an active role in the synthesis, modification, storage, sorting, and secretion of the cell's chemical products. Lipids and proteins produced by the ER pass through the GA to the other organelles in the endomembrane system.

Lysosomes are made by the Golgi apparati, and are responsible for intracellular digestion of particles like other organelles that are no longer functioning properly, food molecules, foreign particles like bacteria, or antigens.

Endosomes are one of the bodies in the endocytic pathway. They carry newly endocytosed materials to the lysosomes for degradation.

Microbodies

Peroxisomes destroy hydrogen peroxide, a potentially dangerous byproduct of cell metabolism.

Glyoxysomes, in fat-storing tissues of germinating plant seeds, contain enzymes that start the conversion of fats into sugars.

Vacuoles (plants) store enzymes and waste products, as well as provide the turgor pressure that allows plants to stay upright. These organelles can comprise up to 90 percent of the volume of a plant cell.

Mitochondria, about the size of bacteria, are bilipid membrane bounded, and are the energy producing organelles found in all eukaryotic cells. The inner membrane usually contains many folds, or cristae, which give the organelle its characteristic appearance in cross-section. Communication between the mitochondrial matrix and the cytoplasm takes place through porins, junctions through the outer membrane. Mitochondria convert energy from the oxidation of foodstuffs into ATP, or adenosine 5'-triphosphate, a nucleotide present in every cell and the principal carrier of chemical energy. Mitochondria have their own genomes, which are circular chromosomes floating in the mitochondrial matrix, and they replicate by binary fission.

Ribosomes float free in the cytoplasm, or, in eukaryotes, some are attached to the endoplasmic reticulum and are responsible for the translation of mRNA into proteins. Mitochondria and chloroplasts have ribosomes for cDNA translation, but they resemble bacterial ribosomes more than they do those of eukaryotes.

Cytoskeleton, in eukaryotes is a highly flexible set of protein filaments that extends throughout the cytoplasm and is responsible for the shape of the cell, cell movement, cytokinesis (the division of the cytoplasm after the nucleus has split during cell division), and the organization of the organelles within the cell.

Chloroplasts (plants), like mitochondria, are energy producing organelles in plant cells. Unlike mitochondria, they convert sunlight, rather than food, into ATP. Again, as mitochondria, they are thought to have been introduced into cells early in the evolution of eukaryotes by endocytosis.

which is essential to bacteria that live in environments awash with natural antibiotics (that is, this predated by aeons the human development of antibiotics, in the battle by other organisms to protect themselves against bacterial attack).

The more complex types of cell, eukaryotes, are also bounded by a lipid bilayer membrane. The bulk of the genomic structures in eukaryotic cells are further sequestered within a nucleus encased in its own membrane that controls molecular transport in and out. Eukaryotic metabolic processes are carried out within and between membrane-bounded organelles floating in the cytosol (the contents of the cell minus the nucleus and organelles). Eukaryotic chromosomes are as a rule linear and vary greatly in size and number; as in prokaryotes, they are tightly wound but, like archaea but not bacteria, in association with periodically spaced protein complexes called histones, which package the DNA so that it will fit into the cell (see Figure 6-3).

Additional organelles in eukaryotic cells include the ribosomes, where protein synthesis takes place, as in prokaryotes, the Golgi apparati, involved in transport of macromolecules between organelles, the endoplasmic reticulum, which carries out the synthesis and transport of lipids and membrane proteins to the cell membrane for transport out of the cell, endosomes, lysosomes, and peroxisomes, which are involved in intracellular transport, digestion, and maintenance of the proper concentration of specific chemicals, the cytoskeleton, which consists of protein filaments and most importantly actin filaments, involved in movement of the cell, micro-tubules, which are most likely the primary organizers of the cytoskeleton, intermediate filaments, ropelike structures that are thought to be responsible for the

Linker DNA

Linker DNA

Nucleosome (200 bp DNA)

Figure 6-3. General characteristics of DNA packaging.

Nucleosome (200 bp DNA)

Figure 6-3. General characteristics of DNA packaging.

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