Figure 614

Cotransport and countertransport during secondary active transport driven by sodium. Sodium ions always move down their concentration gradient into a cell, and the transported solute always moves up its gradient. Both sodium and the transported solute X move in the same direction during cotransport but in opposite directions during countertransport.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

A variety of organic molecules and a few ions are moved across membranes by sodium-coupled secondary active transport. For example, in most cells, amino acids are actively transported into the cell by cotransport with sodium ions, attaining intracellular concentrations 2 to 20 times higher than in the extracellular fluid.

An example of the secondary active transport of ions is provided by calcium. In addition to the previously described primary active transport of calcium from cytosol to extracellular fluid and organelle interior via Ca-ATPase, in many membranes there are also Na-Ca countertransporters (or Na-Ca "exchangers") that use the downhill movement of sodium ions into a cell to pump calcium ions out. Figure 6-15 illustrates the dependence of cytosolic calcium concentration on the several pathways that can move calcium ions into or out of the cytosol: calcium channels, Ca-ATPase pumps, and Na-Ca countertransport. Alterations in the movement of calcium through any of these pathways will lead to a change in cytosolic calcium concentra tion and, as a result, alter the cellular activities that are dependent on cytosolic calcium, as will be described in subsequent chapters.

For example, the mechanism of action of a group of drugs, including digitalis, that are used to strengthen the contraction of the heart (Chapter 14) involves several of these transport processes. These drugs inhibit the Na,K-ATPase pumps in the plasma membranes of the heart muscle, leading to an increase in cytosolic sodium concentration. This decreases the gradient for sodium diffusion into the cell, thereby decreasing calcium exit from the cell via sodium-calcium exchange and increasing cytosolic calcium concentration, which acts in muscle cells on the mechanisms that increase the force of contraction.

A large number of genetic diseases result from defects in the various proteins that form ion channels and transport proteins. These mutations can produce malfunctioning of the electrical properties of nerve and muscle cells, and the absorptive and secretory properties

Muscle And Calcium Concentration

FIGURE 6-15

Pathways affecting cytosolic calcium concentration. The active transport of calcium, both by primary Ca-ATPase pumps and by secondary active calcium countertransport with sodium, moves calcium ions out of the cytosol. Calcium channels allow net diffusion of calcium into the cytosol from both the extracellular fluid and cell organelles. Cytosolic calcium concentration is the resultant of all these processes. The symbols in this diagram will be used throughout this book to represent primary active transport and secondary active transport. The red arrow indicates the direction of the actively transported solute. Black arrows denote downhill movement. Diffusion of ions through channels will use the channel symbol.

FIGURE 6-15

Pathways affecting cytosolic calcium concentration. The active transport of calcium, both by primary Ca-ATPase pumps and by secondary active calcium countertransport with sodium, moves calcium ions out of the cytosol. Calcium channels allow net diffusion of calcium into the cytosol from both the extracellular fluid and cell organelles. Cytosolic calcium concentration is the resultant of all these processes. The symbols in this diagram will be used throughout this book to represent primary active transport and secondary active transport. The red arrow indicates the direction of the actively transported solute. Black arrows denote downhill movement. Diffusion of ions through channels will use the channel symbol.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Movement of Molecules Across Cell Membranes CHAPTER SIX

TABLE 6-1 Composition of Extracellular and Intracellular Fluids

Extracellular

Intracellular

Concentration,

Concentratiion,

mM

mM

Na+

145

15

K+

4

150

Ca2+

1

1.5

Mg2+

1.5

12

Cr

110

10

HCO3

24

10

Pi

2

40

Amino acids

2

8

Glucose

5.6

1

ATP

0

4

Protein

0.2

4

*The intracellular concentrations differ slightly from one tissue to another, but the concentrations shown above are typical of most cells. The intracellular concentrations listed above may not reflect the free concentration of the substance in the cytosol since some may be bound to proteins or confined within cell organelles. For example, the free cytosolic concentration of calcium is only about 0.0001 mM.

*The intracellular concentrations differ slightly from one tissue to another, but the concentrations shown above are typical of most cells. The intracellular concentrations listed above may not reflect the free concentration of the substance in the cytosol since some may be bound to proteins or confined within cell organelles. For example, the free cytosolic concentration of calcium is only about 0.0001 mM.

of epithelial cells lining the intestinal tract, kidney, and lung airways. Cystic fibrosis provides one example; others will be discussed in later chapters. Cystic fibrosis, as mentioned earlier, is the result of a defective membrane channel through which chloride ions move from cells into the extracellular fluid. Failure to secrete adequate amounts of chloride ions decreases the fluid secreted by the epithelial cells that is necessary to prevent the build up of mucus, which if allowed to thicken, leads to the eventual obstruction of the airways, pancreatic ducts, and male genital ducts.

In summary the distribution of substances between the intracellular and extracellular fluid is often unequal (Table 6-1) due to the presence in the plasma membrane of primary and secondary active transporters, ion channels, and the membrane potential.

Table 6-2 provides a summary of the major characteristics of the different pathways by which substances move through cell membranes, while Figure 6-16 illustrates the variety of commonly encountered channels and transporters associated with the movement of substances across a typical plasma membrane.

TABLE 6-2 Major Characteristics of Pathways by which Substances Cross Membranes

DIFFUSION

MEDIATED TRANSPORT

Through Lipid Bilayer

Through Protein Channel

Facilitated Diffusion

Primary Active Transport

Secondary Active Transport

Direction of net flux

High to low concentration

High to low concentration

High to low concentration

Low to high concentration

Low to high concentration

Equilibrium or steady state

C

C = C *

c

Co * C

Co * C

Use of integral membrane protein

No

Yes

Yes

Yes

Yes

Maximal flux at high concentration (saturation)

No

No

Yes

Yes

Yes

Chemical specificity

No

Yes

Yes

Yes

Yes

Use of energy and source

No

No

ion gradient (often Na)

Typical molecules using pathway

Nonpolar: O2, CO2, fatty acids

Ions:

Na+, K+, Ca2+

amino acids, glucose, some ions

*In the presence of a membrane potential, the intracellular and extracellular ion concentrations will not be equal at equilibrium.

*In the presence of a membrane potential, the intracellular and extracellular ion concentrations will not be equal at equilibrium.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

Intracellular Mechanisms Hyperplasy

Glucose

FIGURE 6-16

Movements of solutes across a typical plasma membrane involving membrane proteins. A specialized cell may contain additional transporters and channels not shown in this figure. Many of these membrane proteins can be modulated by various signals leading to a controlled increase or decrease in specific solute fluxes across the membrane.

Glucose

FIGURE 6-16

Movements of solutes across a typical plasma membrane involving membrane proteins. A specialized cell may contain additional transporters and channels not shown in this figure. Many of these membrane proteins can be modulated by various signals leading to a controlled increase or decrease in specific solute fluxes across the membrane.

The addition of a solute to water lowers the concentration of water in the solution compared to the concentration of pure water. For example, if a solute such as glucose is dissolved in water, the concentration of water in the resulting solution is less than that of pure water. A given volume of a glucose solution contains fewer water molecules than an equal volume of pure water since each glucose molecule occupies space formerly occupied by a water molecule (Figure 6-17). In quantitative terms, a liter of pure water weighs about 1000 g, and the molecular weight of water is 18. Thus, the concentration of water in pure water is 1000/18 = 55.5 M. The decrease in water concentration in a solution is approximately equal to the concentration of added solute. In other words, one solute molecule will displace one water molecule. The water concentration in a 1 M glucose solution is therefore approximately 54.5 M rather than 55.5 M. Just as adding water to a solution will dilute the solute, adding solute to a solution will "dilute" the water. The greater the solute concentration, the lower the water concentration.

It is essential to recognize that the degree to which the water concentration is decreased by the addition of solute depends upon the number of particles (molecules or ions) of solute in solution (the solute concentration) and not upon the chemical nature of the solute.

Essentials of Human Physiology

Essentials of Human Physiology

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  • karita uotila
    How does calcium move into the muscle cell?
    6 years ago

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