Delphinidin

The outer membrane of the nucleus is connected and continuous with the endoplasmic reticulum. The endoplasmic reticulum facilitates cellular communication and channeling of materials. Many important activities, such as the synthesis of membranes for other organelles and modification of proteins from components assembled from elsewhere within the cell, occur either on the surface of the endoplasmic reticulum or within its compartments.

The endoplasmic reticulum (often referred to simply as ER) is an enclosed space consisting of a network of flattened sacs and tubes that form channels throughout the cytoplasm, the amount and form varying considerably from cell to cell. Transmission electron micrographs of sectioned ER give it the appearance of a series of parallel membranes that resemble long, narrow tubes or sacs, creating subcompartments within the cell.

Ribosomes may be distributed on the outer surface (i.e., the surface in contact with the cytoplasm) of the endoplasmic reticulum. Such endoplasmic reticulum is said to be rough and is primarily associphospholipids ated with the synthesis, secretion, or storage of proteins. This contrasts with smooth endoplasmic reticulum, which has few, if any, ribosomes lining the surface, and is associated with lipid secretion. Both types of endoplasmic reticulum can occur in the same cell and can be interconverted, depending on the demands of the cell.

Many enzymes involved in the process of cellular respiration aresynthesized on the surface of the endoplasmic reticulum. The enzymes, however, enter other organelles (primarily mitochondria, which are discussed later in this chapter) without passing through the endoplasmic reticulum. The endoplasmic reticulum also appears to be the primary site of membrane synthesis within the cell.