Exocytosis performs two functions for cells: (1) It provides a way to replace portions of the plasma membrane that have been removed by endocytosis and, in the process, to add new membrane components as well, and (2) it provides a route by which membraneimpermeable molecules, such as protein hormones, synthesized by cells can be released (secreted) into the extracellular fluid.
How are substances that are to be secreted by exocytosis packaged into vesicles? The entry of newly formed proteins into the lumen of the endoplasmic reticulum and the protein's processing through the Golgi apparatus were described in Chapter 5. From the Golgi apparatus, the proteins to be secreted travel to the plasma membrane in vesicles from which they can
Vander et al.: Human I I. Basic Cell Functions I 6. Movement of Molecules I I © The McGraw-Hill
Physiology: The Across Cell Membranes Companies, 2001
Mechanism of Body Function, Eighth Edition
PART ONE Basic Cell Functions be released into the extracellular fluid by exocytosis. Very high concentrations of various organic molecules, such as neurotransmitters (Chapter 8), can be achieved within vesicles by a combination of mediated transport across the vesicle membrane followed by the binding of the transported substances to proteins within the vesicle.
The secretion of substances by exocytosis is triggered in most cells by stimuli that lead to an increase in cytosolic calcium concentration in the cell. As described in Chapter 7, these stimuli open calcium channels in either the plasma membrane and/or the membranes of intracellular organelles. The resulting increase in cytosolic calcium concentration activates proteins required for the vesicle membrane to fuse with the plasma membrane and release the vesicle contents into the extracellular fluid. Material stored in secretory vesicles is available for rapid secretion in response to a stimulus, without delays that might occur if the material had to be synthesized after the arrival of the stimulus.
lial cells occurs via the pathways (diffusion and mediated transport) already described for movement across membranes in general. However, the transport and permeability characteristics of the luminal and baso-lateral membranes are not the same. These two membranes contain different ion channels and different transporters for mediated transport. As a result of these differences, substances can undergo a net movement from a low concentration on one side of an epithelium to a higher concentration on the other side, or in other words, to undergo active transport across the overall epithelial layer. Examples of such transport are the absorption of material from the gastrointestinal tract into the blood, the movement of substances between the kidney tubules and the blood during urine formation, and the secretion of salts and fluid by glands.
Figures 6-23 and 6-24 illustrate two examples of active transport across an epithelium. Sodium is actively transported across most epithelia from lumen to blood side in absorptive processes, and from blood
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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.