Membranes are constantly forming, transforming from one type to another, fusing with one another, and breaking down.
► Phospholipids in eukaryotes are synthesized on the surface of the smooth endoplasmic reticulum and rapidly dis tributed to membranes throughout the cell as vesicles form from the ER, move away, and fuse with other organelles.
► Membrane proteins are inserted into the rough endoplasmic reticulum as they form on ribosomes.
► Functioning membranes also move about within eukaryot-ic cells. Portions of the rough ER bud away as vesicles and join the cis face of the Golgi apparatus (see Figure 4.12). Rapidly—often in less than an hour—these segments of membrane find themselves in the trans regions of the Golgi, from which they bud away to join the plasma membrane.
► Membrane from vesicles is constantly merging with the plasma membrane by exocytosis, but this process is largely balanced by the removal of membrane in endo-cytosis, affording a recovery path by which internal membranes are replenished.
Because all membranes appear similar under the electron microscope, and because they interconvert readily, we might expect all subcellular membranes to be chemically identical. However, that is not the case, for there are major chemical differences among the membranes of even a single cell. Membranes are changed chemically when they form parts of certain organelles. In the Golgi apparatus, for example, the membranes of the cis face closely resemble those of the endoplasmic reticulum in chemical composition, but the transface membranes are more similar to the plasma membrane. As a vesicle is formed, the mix of proteins and lipids in its membrane is selected, just as its internal contents are selected, to correspond with the vesicle's target membrane.
In sum, there is a steady flux of membranes within the cell. Ceaselessly moving, functioning, changing their composition and roles, biological membranes are central to life.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.