The plasma (cell) membrane separates the intracellular environment from the extracellular environment. Molecules that move from the blood to the interstitial fluid, or molecules that move within the interstitial fluid between different cells, must eventually come into contact with the plasma membrane surrounding the cells. Some of these molecules may be able to penetrate the membrane, while others may not. Similarly, some intracellular molecules can penetrate, or "permeate," the plasma membrane and some cannot. The plasma membrane is thus said to be selectively permeable.
The plasma membrane is generally not permeable to proteins, nucleic acids, and other molecules needed for the structure and function of the cell. It is, however, permeable to many other molecules, permitting the two-way traffic of nutrients and wastes needed to sustain metabolism. The plasma membrane is also selectively permeable to certain ions; this permits electrochemical currents across the membrane used for production of impulses in nerve and muscle cells.
The mechanisms involved in the transport of molecules and ions through the cell membrane may be divided into two categories: (1) transport that requires the action of specific carrier proteins in the membrane, called carrier-mediated transport; and (2) transport through the membrane that is not carrier mediated. Carrier-mediated transport may be further subdivided into facilitated diffusion and active transport, both of which will be described later. Membrane transport that does not use carrier proteins involves the simple diffusion of ions, lipid-soluble molecules, and water through the membrane. Osmosis is the net diffusion of solvent (water) through a membrane.
Membrane transport processes may also be categorized by their energy requirements. Passive transport is the net movement of molecules and ions across a membrane from higher to lower concentration (down a concentration gradient); it does not require metabolic energy. Passive transport includes simple diffusion, osmosis, and facilitated diffusion. Active transport is net movement across a membrane that occurs against a concentration gradient (to the region of higher concentration). Active transport requires the expenditure of metabolic energy (ATP) and involves specific carrier proteins.
Test Yourself before You Continue
1. Describe the distribution of fluid in the body.
2. Describe the composition of the extracellular matrix and explain the importance of the matrix metalloproteinases.
3. List the subcategories of passive transport and distinguish between passive transport and active transport.
Fox: Human Physiology, I 6. Interactions Between I Text I I © The McGraw-Hill
Eighth Edition Cells and the Extracellular Companies, 2003
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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.