Pump transports sodium-potassium

Ordinarily, when the current pulse is over, the excess charges will be drained through the passive transport channels, and by operation of the sodium-potassium pumps the original values of membrane potential and of the concentration gradients will be reestablished. However, when in the case of depolarization the negative value of cp has dropped below a certain threshold value, which is about -50 mV, the picture changes drastically Excitation of the membrane occurs. When the current is turned off, the membrane potential not only fails to be restored but continues to... 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function. 

ATP is used not only to power muscle contraction, but also to re-establish the resting state of the cell. At the end of the contraction cycle, calcium must be transported back into the sarcoplasmic reticulum, a process which is ATP driven by an active pump mechanism. Additionally, an active sodium-potassium ATPase pump is required to reset the membrane potential by extruding sodium from the sarcoplasm after each wave of depolarization. When cytoplasmic Ca2- falls, tropomyosin takes up its original position on the actin and prevents myosin binding and the muscle relaxes. Once back in the sarcoplasmic reticulum, calcium binds with a protein called calsequestrin, where it remains until the muscle is again stimulated by a neural impulse leading to calcium release into the cytosol and the cycle repeats. 

Palytoxin targets the sodium-potassium pump protein by binding to the molecule in such a way that the molecule is locked in a position where it allows passive transport of both the sodium and potassium ions, thereby destroying the ion gradient that is essential for most cells. 

Inorganic ions, such as sodium and potassium, move through the cell membrane by active transport. Unlike diffusion, energy is required for active transport as the chemical is moving from a lower concentration to a higher one. One example is the sodium-potassium ATPase pump, which transports sodium [Na ] ions out of the cell and potassium [K ] into the cell. 

This must obviously be the opposite of passive transport. Active transport does require energy, usually in the form of the consumption of ATP or GTP, because the molecules are moving against the concentration gradient from an area of lower concentration to an area of higher concentration. The most well known active transport system is the Sodium-Potassium-ATPase Pump (Na" "- K+ZATPase) which maintains an imbalance of sodium and potassium ions inside and outside the membrane, respectively. See Figure 3. 

Sodium-Potassium ATPase Pump An Active Transport System... 

The most universal transport systems are those involved in the transport of the ubiquitous inorganic ions, sodium, potassium and calcium1. The sodium pump counteracts passive water movement across the cell membrane by removing sodium ions together with chloride or other anions from the cytoplasm to lower its content of osmotically active substances. In most cells, however, the elimination of sodium ions is connected with an accumulation of potassium ions6. For three sodium ions leaving the cell two potassium ions are taken up9,10). The resulting concentration... 

One example of molecular transport requiring energy is the reuptake of neurotransmitter into its presynaptic neuron, as already mentioned above. In this case, the energy comes from linkage to an enzyme known as sodium-potassium ATPase (Fig. 2—9). An active transport pump is the term for this type of organization of two neurotransmitters, namely a transport carrier and an energy-providing system, which function as a team to accomplish transport of a molecule into the cell (Fig. 2—11). 

Reduced salt intake also negatively effects the all important sodium-potassium pump. This is the mechanism the body uses to shuttle many nutrients into cells like those that all muscle fibers are composed of. (Gee, ya think ) This would therefore inhibit creatine and some amino acid structures from adequately transporting, as well as inhibit glycogen synthesis. 

Digitalis exerts its primary effect by inhibiting the sodium-potassium pump on the myocardial cell membrane.66 The sodium-potassium pump is an active transport system that normally... 

Mineralocorticoids are believed to increase sodium reabsorption by affecting sodium channels and sodium pumps on the epithelial cells lining the renal tubules.18,58 Mineralocorticoids ability to increase the expression of sodium channels is illustrated in Figure 29-5. These hormones enter the tubular epithelial cell, bind to receptors in the cell, and create an activated hormone-receptor complex.18 This complex then travels to the nucleus to initiate transcription of messenger RNA units, which are translated into specific membrane-related proteins.27,58 These proteins in some way either create or help open sodium pores on the cell membrane, thus allowing sodium to leave the tubule and enter the epithelial cell by passive diffusion.27,83 Sodium is then actively transported out of the cell and reabsorbed into the bloodstream. Water reabsorption is increased as water follows the sodium movement back into the bloodstream. As sodium is reabsorbed, potassium is secreted by a sodium-potassium exchange, thus increasing potassium excretion (see Fig. 29-5). 

Many substances can be transported into the cell (and vice versa) against a concentration gradient. This is an active transport process, and it requires energy in the form of ATR It is to be distinguished from a passive transport process, which is simple diffusion across membranes. One of the better understood systems of this type is the sodium-potassium ATPase (or Na/K) pump, which maintains high potassium and low sodium levels in the cell. These are up to 160 meq/L for K+ and about 10 meq/L of Na+ inside the cell. Extracellular fluid contains about 145 meq/L of Na+ and 4 meq/L of K+. The simultaneous movement of one substance out of the cell and another into the cell is an antiport. A substantial percentage of the basal metabolic rate (see Chapter 21) is accounted for by the activity of the Na/K pump. ATPase (Na/K pump) is lo-... 

Sodium/Potassium pump - ion transport through membranes and ion channels... 

Biological membranes show anisotropy, as their molecules are preferentially ordered in a definite direction in the plane of the membrane, and the coupling between chemical reactions (scalar) and diffusion flow (vectorial) can take place. Almost all outer and inner membranes of the cell have the ability to undergo active transport. Sodium and potassium pumps operate in almost all cells, especially nerve cells, while the active transport of calcium takes place in muscle cells. The proton pumps operate in mitochondrial membranes, chloroplasts, and the retina. 

Na+K+ ATPases, so-called sodium-potassium pumps (Figure 4.6), embedded in the basolat-eral membrane. The sodium-potassium pump is a highly conserved integral membrane protein, expressed in virtually all animal cells. The transport of sodium creates both an electrical and a chemical gradient across the plasma membrane. In turn this provides ... 

The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between its two sides. Sodium-potassium ATPase pumps maintain this unequal concentration by actively transporting ions against their concentration gradients sodium in, potassium out. The membrane is positive outside and negative inside. This charge difference is referred to as the resting potential and is measured in millivolts (=—65 mV). 

When an ion is attached to a particular transporter, a similar ion (of the same or a different solute) competing for the same binding site cannot also be bound. For example, the similar monovalent cations K+ and Rb+ appear to bind in a competitive fashion to the same site on a transporter. For some cells, the same carrier might transport Na+ out of the cell and K+ in—such as the sodium-potassium pump alluded to previously. Ca2+ and Sr2+ apparently compete with each other for binding sites on another common carrier. The halides (Cl-, I-, and Br-) may also be transported by a single carrier. 

Alternatively, A and B may refer to the concentrations of an ion or molecule on the outside and inside of a cell, as in the active transport of a nutrient. The active transport of Na+ and K+ across membranes is driven by the phosphorylation of the sodium-potassium pump by ATP and its subsequent dephosphorylation (Section 13.2.1). 

Cystic fibrosis—a fatal, hereditary disease characterized by a heavy mucus buildup in the lungs—is caused by a defective plasma membrane protein. In persons with cystic fibrosis this transport protein, known as the sodium-potassium pump, abnormally transports sodium ions across the membrane without carrying the chloride ions that usually accompany them. Research is currently underway to correct through genetic engineering the faulty gene that codes for the plasma membrane protein. 

Sodium-potassium pump— A special transport protein in the membrane of cells that moves sodium ions out and potassium ions into the cell against their concentration gradients. 

The transport of food into cells involves the action of the sodium-potassium pump and coupled channels. 

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