Figure 19.8d Chloroplasts in the plant cell

Ribosome. Ribosomes are organelles about 20 mm in diameter (figure 19.9). Each ribosomes consist of two subunits, one large and one small. Ribosomes are made of tRNA and protein. The translation of protein synthesis occurs at ribosomes. Also, the ribosomes found in the cytoplasm can be subdivided into two classes, those, which are free, and others, which are attached to the membranes. The two classes of ribosomes have different functions in the cell: the free synthesize the proteins, which remain free inside the cytoplasm, and the ribosomes attached to the membranes synthesize the proteins, which are finally secreted by the cell. For example, the pancreatic cells, which secrete enzymes that pour in the small intestine, are extremely rich with RRE. The SRE is the place where lipids are synthesized. The ribosomes are made up of two unequal sub-units: the big and the small sub-units. They are formed of proteins and of RNA.

Figure 19.9 Ribosome

The Centrosome. The centrosome is a cell centre made up of two small perpendicular cylinders: these are the two centrioles. The two centrioles of mammalian cells are called diplosome or paired allosomes. Each centriole is formed of 9 triplets of microtubules arranged along the wall of a cylinder (figure 19.10). Pericentriolar corpuscles, called massulae, are arranged around the centriole, from which the asteroid fibers branch. The centrosome plays the role of a kinetic center. That is a center of movement for chromosomes during mitosis.

Дата добавления: 2015-11-16