WJEC Biology for AS: Student Bk

The details of protein synthesis are given on p101. Link How Science Works To measure the size of ribosomes, scientists see how fast they sink through a solution spun very fast in an ultracentrifuge. Larger and denser structures sink faster. Sedimentation rate is measured in S units. (S stands for Svedburgh, the Swedish scientist who invented the ultracentrifuge.) Ribosomes Ribosomes are smaller in prokaryotic than eukaryotic cells. In prokaryotic cells they are 70S in size, whereas those in the cytoplasm of eukaryotic cells are 80S, where they occur singly or attached to membranes on the RER. Ribosomes have one large and one small subunit. They are assembled in the nucleolus from ribosomal RNA (rRNA) and protein. They are important in protein synthesis, as they are the site of translation, where mRNA is used to assemble the polypeptide chain. Ribosomes are much smaller than the nucleus and mitochondria. They often appear as black dots in electron micrographs. In diagrams, they are often shown like this: Diagram of a ribosome Golgi body / apparatus / complex The structure of the Golgi body resembles the structure of ER, but it is more compact. Vesicles containing polypeptides pinch off from the RER and fuse with the stack of membranes which constitute the Golgi body. Proteins are modified and packaged in the Golgi body. At the other end of the Golgi body, vesicles containing the modified proteins are pinched off. These may carry proteins elsewhere in the cell or move to and fuse with the cell membrane, secreting the modified proteins by exocytosis. The functions of the Golgi body include: ▪ ▪ Producing secretory enzymes, packaged into secretory vesicles. ▪ ▪ Secreting carbohydrates, e.g. for the formation of plant cell walls. ▪ ▪ Producing glycoprotein. ▪ ▪ Transporting and storing lipids. ▪ ▪ Forming lysosomes, containing digestive enzymes. Model of a section through the Golgi body Electron micrograph of section of Golgi body large subunit 2 tRNA attachment sites mRNA attachment site small subunit cisternae lumen secretory vesicle newly forming vesicle incoming transport vesicle Cell structure and organisation 35