Biologia De La Celula

Chapter 8: Cytoplasmic membrane systems: structure, function and membraining trafficking
8.1 An Overview of the Endomembrane System

The endomembrane system includes organelles such as the endoplasmic reticulum, Golgi complex, endosomes, lysosomes, and vacuoles. The nuclear envelope and plasma membrane are also part of the endomembrane system.

Secretory activities
During constitutivesecretion, materials are transported in secretory vesicles from their sites of synthesis and discharged into the extracellular space in a continual manner. Most cells engage in constitutive secretion, a process that contributes not only to the formation of the extracellular matrix, but to the formation of the plasma membrane itself. Form by the extracellular matrix and plasma membrane.
Duringregulated secretion, materials are stored as membrane-bound packages and discharged only in response to an appropriate stimulus. Regulated secretion occurs, for example, in endocrine cells that release hormones, in pancreatic acinar cells that release digestive enzymes, and in nerve cells that release neurotransmitters. In some of these cells, materials to be secreted are stored in large, densely packedmembrane-bound secretory granules.
Example
A salivary gland cell requires that the proteins of salivary mucus, which are synthesized in the endoplasmic reticulum, are specifically targeted for secretory granules, while lysosomal enzymes, which are also manufactured in the endoplasmic reticulum, are specifically targeted to a lysosome. Different organelles also contain different integral membraneproteins. Consequently, membrane proteins must also be targeted to particular organelles, such as a lysosome or Golgi cisterna.
Biosynthetic pathway and the endocytic pathway

8.2 Approaches to the study of endomembrane

Autoradiography is a technique to where a particular isotope is located. A particle emitted from a radioactive atom activates a photographic emulsion. The location of theradioisotope in the specimen is determined by the positions of the overlying silver grains in a photographic emulsion.
GFP This technology utilizes a gene isolated from a jellyfish that encodes a small protein, called the green fluorescent protein (GFP), which emits a green fluorescent light.
Biochemical Analysis of Subcellular Fractions Techniques to homogenize cells and isolate some organelles,which can then be separated from one another through subcellular fractionation. Membrane vesicles derived from the endomembrane system form a collection of vesicles called microsomes, which can be characterized further through other techniques.
Cell-Free Systems Do not contain whole cells and have provided information about the roles of the proteins involved in membrane trafficking. Other type ofinformation identified includes: proteins that bind to the membrane to initiate vesicle formation, and proteins responsible for cargo selection.
Mutant Phenotypes Determining the precise nature of the deficiency provides information on the function of the normal protein. Study of the genetic basis of secretion has been carried out largely on yeast cells.

8.3 The endoplasmic reticulum

Theendoplasmic reticulum (ER) is divided into two sub compartments, the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). Both types of ER comprise a system of membranes that enclose a space or lumen that is separated from the surrounding cytosol. The two types of ER share many of the same proteins and engage in certain common activities, such as the synthesis of certainlipids and cholesterol. The rough ER is defined by the presence of ribosomes bound to its cytosolic surface, whereas the smooth ER lacks associated ribosomes. The RER is typically composed of a network of flattened sacs (cisternae). The membranous elements of the SER are highly curved and tubular, forming an interconnecting system of pipelines curving through the cytoplasm.

The Smooth...