The Endoplasmic Reticulum and Golgi Membranes

Although cytoplasm is fluid and in some organisms can undergo rapid streaming, the electron microscope has revealed that within the liquid portion, the cytosol, there is a complex network of membranes known as the endoplasmic reticulum (ER). The membranes of the ER form tubes, vesicles, and flattened sacs called cisternae. The intracisternal spaces appear to connect with the perinuclear space and to a series of 3–12
flattened, slightly curved disk-shaped membranes known as the Golgi apparatus. This organelle was first reported by Camillo Golgi in 1898. Its existence was long doubted, but it is known now to play a vital role in metabolism. The ER, the Golgi membranes, and secretion granules apparently represent an organized system for synthesis of secreted protein and formation of new membranes. Parts of the ER, the rough endoplasmic reticulum are lined with many ribosomes of 21–25 nm diameter. While resembling those of bacteria, these
eukaryotic ribosomes are about 50% heavier (4 x 106 Da). The smooth endoplasmic reticulum lacks ribosomes but proteins made in the rough ER may be modified in the smooth ER, e.g., by addition of carbohydrate chains. Small membrane vesicles break off rom the smooth ER and pass to the Golgi membranes which lie close to the smooth ER on the side toward the center of the cell. Here additional modification reactions occur (Chapter
20). At the outer edges the membranes of the Golgi apparatus pinch off to form vacuoles which are often densely packed with enzymes or other proteins. These secretion granules move to the surface and are released from the cell. In this process of exocytosis the membranes surrounding the granules fuse with the outer cell membrane. The rough ER appears to contribute membrane material to the smooth ER and Golgi apparatus, while material from Golgi membranes can become incorporated into the outer cell membrane and into lysosomes. Outer mitochondrial membranes and membranes around vacuoles in plant cells may also be derived directly from the ER. Outer membrane materials are probably “recycled” by endocytosis. The term microsome, frequently met in the biochemical literature, refers to small particles of 50–150 nm diameter which are mostly fragments of the ER together with some material from the plasma membrane. Microsomes are formed when cells are ground or homogenized. Upon centrifugation of the disrupted cells, nuclei and other large fragments sediment first, then the mitochondria. At very high speeds (e.g., at 100,000 times the force of gravity) the microsomes, whose masses are 108 –109 Da, settle. With the electron microscope we see that in the microsomes the membrane fragments have closed to give small sacs to the outside of which the ribosomes still cling: