P(peptidyl)site A(aminoacyl)site 50S subunit E(exit)site mRNA-binding site 30S subunit (a) 50s Growing hain mRNA-binding site 30S mRNA (b) Figure 20-2 Important Binding Sites on the Prokaryotic Ribosome. mRNA This model of ribosome structure shows the A(aminoacyl)and P(peptidyl) sites as cavities on the ribosome where charged (amino acidcamying)tRNA molecules bind during polypeptide synthesis.The more recently postulated E (exit)site is the site from which discharged tRNAs leave the ribosome.The mRNA-binding site binds a particuar nucleotide sequence near the5end of the mRNA.placing the mRNA in the proper position for the translation of its irst(a)The diagrammatic representationofa ribsome that isused n this chapter.The pair of horizontal dashed lines indicate where the mRNA molecule lies.(b)A more realistic representation.The binding sites are al located ator near the interface between the large and small subunits
(3)内质网(endoplasmic eticulum,ER) 由封闭的内膜系统及其周围的腔形成的互相沟通的 网状结构。位于核附近。 根据其形态，分为粗面内质网和滑面内质网。 粗面内质网(rough endoplasmic eticulum,rER): 内质网膜上附有核糖体。浆细胞中丰富。其主要功能是 合成蛋白。 滑面内质网(smooth endoplasmicreticulums,sER) 内质网膜上无核糖体附着，胃腺壁细胞、睾丸间质细胞、 骨骼肌细胞中丰富。主要合成脂类物质。此外，生殖腺 内分泌细胞和肾上腺皮质细胞的sER能合成类固醇激素； 肝细胞的sER具有解毒功能；存在于肌细胞的sER能储存 钙离子
（3）内质网（endoplasmic eticulum，ER） 由封闭的内膜系统及其周围的腔形成的互相沟通的 网状结构。位于核附近。 根据其形态，分为粗面内质网和滑面内质网。 粗面内质网（rough endoplasmic eticulum，rER）： 内质网膜上附有核糖体。浆细胞中丰富。其主要功能是 合成蛋白。 滑面内质网（smooth endoplasmicreticulums，sER）： 内质网膜上无核糖体附着，胃腺壁细胞、睾丸间质细胞、 骨骼肌细胞中丰富。主要合成脂类物质。此外，生殖腺 内分泌细胞和肾上腺皮质细胞的sER能合成类固醇激素； 肝细胞的sER具有解毒功能；存在于肌细胞的sER 能储存 钙离子
内质网模式图 Plasma membrane Extracellular medi Endocytic vesicle Perinuclear space vesicles endoso Golgi Smooth- endoplasmic reticulum Rough reticulum Peroxisomes Cytosol Figure 12-1 Cellular Structures Discussed in This Chapter. This chapter focuses on the endoplasmic reticulum,the Golgi complex. The interior of the endoplasmic reticulum.in turn,is linked to the interiors of the Golgi complex.endosomes.and lysosomes by trans endosomes.lysosomes.and peroxisomes.Also relevant to the vesicles that shuttle from organelle to organelle.Together.the discussion are the nuclear envelope and plasma membrane.The surrounding membranes and internal spaces of these structures defn perinuclear space between the two membranes of the nuclear the endomembrane system of the eukaryotic cell:the remaining envelope is continuous with the interior of the endoplasmic reticulum portion of the cytoplasm is called the cytosol
内质网模式图