Читать книгу Cell Biology - Stephen R. Bolsover - Страница 44

The endoplasmic reticulum (ER ) is a network of membrane‐enclosed tubes that run throughout the cell, forming a continuous mesh whose lumen (interior) is at all points separated from the cytosol by a single membrane. The membrane of the ER is continuous with the outer nuclear membrane (Figure 2.3). Two regions can be recognized in most cells, known as smooth ER and rough ER (Figure 1.2 on page 5). The basic difference is that the rough ER is covered in ribosomes, which gives it its rough appearance in the electron microscope.

The function of the smooth ER varies from tissue to tissue. In the ovaries, testes, and the adrenal gland it is where steroid hormones are made; in the liver it is the site of detoxication of foreign chemicals including drugs. Probably the most universal role of the smooth ER is the storage and sudden release of calcium ions. Calcium ions are pumped from the cytosol into the lumen of the smooth ER to more than 1000 times the concentration found in the cytosol. Many stimuli can cause this calcium to be released back into the cytosol, where it activates myriad cell processes (Chapter 10).

The rough ER is where cells make the proteins that will end up as integral membrane proteins in the plasma membrane, and the proteins that the cells will export (secrete) to the extracellular medium (such as the proteins of the extracellular matrix, page 8).