OO Reconnect to chapter 2 Bonding of Atoms page

Because the interior of the cell membrane consists largely of the fatty acid portions of the phospholipid molecules, it is oily. Molecules that are soluble in lipids, such as oxygen, carbon dioxide, and steroid hormones, can pass through this layer easily; however, the layer is impermeable to water-soluble molecules, such as amino acids, sugars, proteins, nucleic acids, and various ions. Many cholesterol molecules embedded in the interior of the membrane also help make it impermeable to water-soluble substances. In addition, the relatively rigid structure of the cholesterol molecules helps stabilize the cell membrane.

A cell membrane includes only a few types of lipid molecules but many kinds of proteins (fig. 3.7), which provide the specialized functions of the membrane. The membrane proteins can be classified according to their shapes. One group of proteins, for example, consists of tightly coiled, rodlike molecules embedded in the phos-pholipid bilayer. Some such fibrous proteins completely span the membrane; that is, they extend outward from its surface on one end, while their opposite ends communicate with the cell's interior. These proteins often function as receptors that are specialized to combine with specific kinds of molecules, such as hormones (see chapter 13, p. 504).

Another group of cell membrane proteins are more compact and globular. Some of these proteins, called integral proteins, are embedded in the interior of the phos-pholipid bilayer. Typically, they span the membrane and provide mechanisms by which small molecules and ions can cross the otherwise impermeable phospholipid bi-layer. For example, some of these integral proteins form "pores" in the membrane that allow water molecules to pass through. Other integral proteins are highly selective and form channels that allow only particular ions to enter. In nerve cells, for example, selective channels control the movements of sodium and potassium ions, which are important in nerve impulse conduction (see chapter 10, p. 374). Clinical Application 3.1 discusses how abnormal ion channels can cause disease.

Yet other globular proteins, called peripheral proteins, associate with the surface of the cell membrane. These proteins function as enzymes (see chapter 4, p. 110), and many are part of signal transduction. Carbo-

Faulty Ion Channels Cause Disease

What do collapsing horses, irregular heartbeats in teenagers, and cystic fibrosis have in common? All result from abnormal ion channels in cell membranes.

Ion channels are tunnels through the lipid bilayer of a biological membrane that consist of protein (see fig. 10.10). These passageways permit electrical signals to pass in and out of membranes in the form of ions. An ion channel functions as a gate, opening or closing to a specific ion in response to certain conditions. Ten million ions can pass through an ion channel in one second. Events that can trigger an ion channel to open or close include a change in voltage across the membrane, binding of a ligand (a molecule that binds specifically to a membrane receptor) to the cell membrane, or receiving biochemical messages from within the cell.

Abundant ion channels include those specific for calcium (Ca+2), chloride (Cl-), sodium (Na+), or potassium (K+). A cell may have a few thousand ion channels specific for each ion.

Many drugs act by affecting ion channels (table 3A). The distribution of specific ion channels on particular cell types explains the symptoms of illnesses that result from abnormal channels. Following are descriptions of three illnesses caused by malfunctioning ion channels.

hyperkalemic periodic Paralysis and Sodium Channels

The quarter horse was originally bred to run the quarter mile in the 1600s. Four particularly fast stallions were used to establish much of the current population of nearly 3 million animals. Unfortunately, one of the original stallions had an inherited condition called hy-perkalemic periodic paralysis. The horse was indeed a champion, but the disease brought on symptoms undesirable in a racehorse — attacks of weakness and paralysis that caused sudden collapse.

Hyperkalemic periodic paralysis results from abnormal sodium channels in the cell membranes of muscle cells. But the trigger for the temporary paralysis is another ion: potassium. When the blood potassium level rises, as it may following

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Essentials of Human Physiology

Essentials of Human Physiology

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.

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