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The transport functions of the plasma membrane

4.1 Introduction

As explained in Chapter 3, each cell is bounded by a plasma membrane. The region outside the cell {the extracellular com­partment) is thereby separated from the inside of the cell {the intracellular compartment). This separation allows each cell to regulate its internal composition independently of other cells.

Chemical analysis has shown that the composition of the intracellular fluid is very different to that of the extracellular fluid (Table 4.1). It is rich in potassium ions (K+) but relatively poor in both sodium ions (Na+) and chloride ions (Cl~). It is also rich in proteins (enzymes and structural proteins) and the small organic molecules that are involved in metabolism and signaling (amino acids, ATP, fatty acids, etc.). The first part of this chapter is concerned with the mechanisms responsible for establishing and maintaining the difference in ionic composition between the intracellular and extracellular compartments. It then discusses the ways in which cells utilize ionic gradients to perform their essential physiological roles and concludes with a discussion of the mechanisms by which proteins and other large molecules cross the cell membrane—secretion and endocytosis.

Table 4.1 The approximate ionic composition of the intracellular and extracellular fluid of mammalian muscle

Ionic species







(mmol H)

(mmol I"1)





+ 53 mV




-97 mV



c. 2 X lO"4

+ 120 mV




-97 mV




-30 mV

Values are given in mmol 1 ' of cell water and the equilibrium potentials were calculated from rhe Nernst equation (see Box 4.1). Note that the resting membrane potential is about —90 mV, close to the equilibrium potential for potassium (the principal intracellular cation) and chloride.

^ 4.2 The permeability of cell membranes to ions and uncharged molecules

The plasma membrane consists of a lipid bilayer in which various proteins are embedded. Pure lipid membranes are permeable to gases and lipid-soluble molecules {hydrophobic molecules). They also have a significant permeability to water but they are very impermeable to ions (Na+, K+, Ca2+, CI-, etc.) and to water-soluble molecules {hydrophilic or polar molecules) such as glucose (Chapter 3). Natural membranes have a higher per­meability to hydrophilic molecules and ions but show great selectivity for the transport of particular substances. For example, the plasma membrane of a skeletal muscle fiber at rest is about 20 times as permeable to potassium as it is to sodium. This difference in permeability can be asctibed to the presence of specific membrane proteins, known as ion channels (see below).

What determines the direction in which

molecules and ions move across the cell


The direction in which uncharged molecules, such as urea, move is simply determined by the concentration gradient. There is net movement from a region of high concentration to one of lower concentration. Diffusion against the prevailing concentration gradient requires the expenditure of energy in active transport (see Section 4.3). When a molecule has a positive or a negative charge its movement will be affected by the electrical potential that exists across the cell membrane (the membrane potential).

The membrane potential of cells can be measured with fine glass electrodes (microelectrodes) that can puncture the cell membrane without destroying the cell. The magnitude of the resting membrane potential vaties from one type of cell to another but is a few tens of millivolts (1 mV = 1/1000 volt). It is greatest in nerve and muscle cells (excitable cells) where it is generally —70 to —90 mV (the - sign indicates that the inside of the cell is negative with respect to the outside). In nonexcitable cells the membrane potential may be somewhat lower. For example, the membrane potential of the hepatocytes of the liver is about -40 mV. The factors involved in establishing and maintaining the membrane potential are discussed on p. 41.


The transport functions of the plasma membrane

As the resting membrane potential is negative, the inward movement of positively charged ions such as sodium and calcium is favored and they are able to diffuse down their respective concentration gradients into the cell. In contrast, the negative value of the membrane potential opposes the inward movement of negatively charged ions such as chloride, even though their concentration gradient favors net inward move­ment (Table 4.1). Thus the direction in which ions and charged molecules move across the cell membrane is influenced by:

  1. the membrane potential;

  2. the charge of the molecule or ion; and

  3. the concentration gradient.

These influences are collectively known as the electrochemical gradient.

How do polar molecules cross the cell membrane?

The permeability of natural membranes to polar molecules can be attributed to two kinds of membrane transport protein: (1) channel proteins and (2) carrier proteins.

^ Channel proteins have a water-filled pore that spans the mem­brane and provides a route for a particular solute to diffuse down its electrochemical gradient (Fig. 4.1). Ion channel proteins (or ion channels) are a very important group of proteins that control the movement of inorganic ions across the plasma membrane. They can have a very high capacity for transport—for example, some potassium channels permit as many as 108 ions to cross the membrane each second. Ion channels show selectivity with respect to the kind of ion that can pass through the pore and they are named after the principal ion translocated. Thus potas­sium crosses the membrane via potassium channels, sodium via sodium channels, and so on. Many channel proteins have now been identified and their peptide sequence determined with the aid of the techniques of molecular biology. This has revealed that they can be grouped into large families according to certain fea­tures of their structure. For example, the peptide chains of many types of potassium channel have a similar sequence of amino acids in certain regions and make the same number of loops across the plasma membrane.

^ Carrier proteins (carriers) bind specific substances—usually small organic molecules such as glucose or inorganic ions (Na+, K+, etc.)—and then undergo a change of shape (known as a con­formational change) to move the solute from one side of the mem­brane to the other (Fig. 4.1). The capacity of a cell to transport a molecule is limited both by the number of carrier molecules and by the number of molecules each carrier is able to translocate in a given period of time (the 'turnover number'). Carriers tend to transport fewer ions or molecules than channels. The fastest car­riers transport about 10*4 molecules each second but more typical values are between 102 and 103 molecules a second. Carriers are selective for a particular type of molecule and can even dis­criminate between optical isomers. Thus the natural form of

Fig. 4.1 The differing modes of action of a membrane carrier (top) and an ion channel (bottom). Molecules that are transported across biological membranes by carrier proteins first bind to a specific site. The carrier then undergoes a conformational change and the bound molecule is able to leave the carrier on the other side of the membrane. Channels operate quite differently. When they are activated, an aqueous pore is opened and ions are able to diffuse from one side of the membrane to the other in a continuous stream.

glucose (D-glucose) is readily transported by carrier proteins but the synthetic L-isomer is not transported. Like our left and right hands, the D- and L-isomers of glucose are mirror images. This proves that the carrier can distinguish between optical isomers solely by their shape as both isomers have the same chemical constitution. This property is known as stereoselectivity.


  1. While lipid-soluble molecules can cross pure lipid membranes
    relatively easily, water-soluble molecules cross only with difficulty.
    Cells have therefore evolved two groups of proteins to facilitate
    the translocation of water-soluble molecules from one side of the
    membrane to the other: the carrier proteins and the ion channels.

  2. Ion channels permit the passage of ions from one side of the
    membrane to the other via a water-filled pore.

  3. Carrier molecules translocate a molecule from one side of the
    membrane to the other by binding the molecule and undergoing a
    conformational change.

^ 4.3 Active and passive transport


^ 4.3 The transport of ions and other molecules

across cell membranes: active and passive


When molecules and ions diffuse across the plasma membrane down their electrochemical gradients they do so by passive trans­port. Most passive transport occurs via ion channels or carrier proteins and is sometimes called facilitated diffusion.

Cells also transport ions and molecules 'uphill' against their prevailing electrochemical gradients. This uphill transport requires a cell to expend metabolic energy either directly or indi­rectly and is called active transport. All active transport involves carrier proteins and, in many cases, the activity of a carrier protein is directly dependent on metabolic energy derived from the hydrolysis of ATP (e.g. the sodium pump discussed below). In other cases the transport of a substance (e.g. glucose) can occur against its electrochemical gradient by coupling its 'uphill' movement to the 'downhill' movement of sodium into the cell. This type of active transport is known as secondary active transport. It depends on the ability of the sodium pump to keep the intra­cellular concentration of sodium significantly lower than that of the extracellular fluid.

Some carrier proteins bind a specific molecule on one face of the membrane and then transfer it to the other side. Transporters of this type are called uniports. Many substances are, however, transported across the membrane only in association with a second molecule or ion. Carriers of this type are called co-transporters and the transport itself is called cotransport or coupled transport. When both molecules move in the same direction across the membrane the carrier is called a symport; when the movement of a molecule into a cell is coupled to the movement of a second molecule out of the cell the carrier is called

Fig. 4.2 The main types of carrier proteins employed by mammalian cells. Unidirectional transport and counter transport may be linked to hydrolysis of ATP (and thus play a role in active transport). Symports exploit existing ionic gradients for secondary active transport.

an antiport. Figure 4.2 shows schematic representations of the different types of transport proteins.

The sodium pump is present in all cells

and exchanges intracellular sodium for

extracellular potassium against their

concentration gradients; this uphill

transport requires ATP

As shown in Table 4.1 the intracellular concentration of sodium, calcium, and chloride in muscle cells is much lower than the extracellular concentration. Conversely, the intracellular con­centration of potassium is much greater than that of the extra­cellular fluid. These differences in composition are common to all healthy mammalian cells although the precise values for the concentrations of intracellular ions vary from one kind of cell to another. What mechanisms are responsible for these differences in ionic composition?

The first clues about the mechanisms by which cells regulate intracellular sodium came from the problems associated with the storage of blood for transfusion. Like other cells, the red cells of human blood have a high intracellular potassium concentration and a low intracellular sodium. When blood is stored at a low temperature in a blood bank the red cells lose potassium and gain sodium over a period of time—a trend that can be reversed by warming the blood to body temperature (37 °C). If red cells that have lost their potassium during storage are incubated at 37 °C in an artificial solution similar in ionic composition to that of plasma, they only reaccumulate potassium if glucose is present. As the uptake of potassium and the extrusion of sodium by the red cells occurs against the concentration gradients for these ions, and as glucose is required for this exchange to occur, it is clear that the movement of these ions is dependent on the activity of a membrane pump driven by the energy liberated by the metabolic breakdown of glucose—in this case the sodium pump.

The sodium pump is found in all mammalian cells and plays a central role in regulating the intracellular environment. How does it work? Important clues were provided by experiments on the giant axon of the squid. This preparation was chosen for these experiments because it has a large diameter (1-2 mm) for a single cell and this made direct measurements of ionic move­ments across the plasma membrane possible. By injecting radioactive sodium into the axon, the rate at which the axon pumped out sodium could be followed by measuring the appear­ance of radioactive sodium in the bathing solution. When ATP generation was blocked by the metabolic inhibitor, cyanide, the rate of pumping declined. When ATP was subsequently injected into an axon that had been poisoned by cyanide the sodium pumping was restored close to normal levels (Fig. 4.3). This experiment showed that ATP was required for sodium to be pumped out of the axon against its electrochemical gradient. In the same series of experiments it was found that the sodium efflux was also inhibited if potassium was removed from the


4 The transport functions of the plasma membrane

Fig. 4.3 The sodium pump is driven by intracellular ATP. In this experiment, which was conducted on a large axon isolated from a squid, normal oxidative metabolism was blocked by cyanide. As ATP levels fell, the axon became unable to pump sodium until ATP was injected. Removal of the cyanide gradually restored the ability of the axon to pump sodium. The first part of the experiment, shown on the left, demonstrates that sodium can only be actively pumped out of the axon if it can be exchanged for potassium.

extracellular fluid, thus demonstrating that the uptake of potas­sium is closely coupled to the efflux of sodium.

Subsequent studies on red blood cells showed that the hydrol­ysis of ATP is tightly coupled to the efflux of sodium and to the influx of potassium in such a way that for each ATP molecule hydrolyzed a cell pumps out three sodium ions in exchange for two potassium ions. For this reason the sodium pump is also called the Na+, K+-ATPase. The sodium pump can be inhibited

Fig. 4.4 The sodium pump. Note that for every molecule of ATP hydrolyzed, three sodium ions are pumped out of the cell and two potassium ions are pumped into the cell.

by a glycoside called ouabain that binds to the extracellular face of the protein. A schematic diagram of the operation of the sodium pump is given in Fig. 4.4.

The sodium pump stabilizes cell volume by maintaining a low intracellular sodium

The osmotic pressure of a solution is determined by the total number of particles present in that solution. ^ Outside the cell the osmolarity is due chiefly to the large number of small inorganic-ions present in the extracellular fluid (Na+, K+, CI", etc.). Inside the cell the osmolarity is due to inorganic ions and to a large number of membrane impermeant molecules such as ATP and proteins. Over time, there is a tendency for the intracellular con­centration of ions to become equal to that of the extracellular fluid as the ions diffuse down their concentration gradients. If this tendency were not countered, the total osmolarity of the intracellular fluid would then be greater than that of the extra­cellular fluid as cells contain impermeant molecules as well as the diffusible ions. Water would move in by osmosis and cause cells to swell. By keeping the intracellular sodium concentration low, the sodium pump helps to maintain the total osmolarity of the intracellular compartment equal to that of the extracellular fluid and cell volume is kept relatively constant.

Cells use a number of transport proteins to regulate intracellular [H+]

All respiring cells continually produce metabolic acids (e.g. CO, and carboxylic acids) which are capable of altering the intracellu­lar concentration of hydrogen ions [H+]. Measurement of intra­cellular [H+] has shown that cells maintain the hydrogen ion concentration of the cytoplasm close to 10-, M. Although this is higher than that of the extracellular fluid (which is usually about 4 X 10~8M) it is about one-tenth of that expected if it were simply at electrochemical equilibrium.

The hydrogen ion concentration of a solution is usually expressed on the pH scale. This scale is somewhat confusing as a low pH value corresponds to a high value for hydrogen ion con­centration and a high pH value corresponds to a low hydrogen ion concentration. Thus an extracellular hydrogen ion con­centration of 4 X 10"8M corresponds to a pH value of 7.4, and an intracellular hydrogen ion concentration of 10_7M cor­responds to a pH value of 7.0 (see Chapter 29 for further details of the pH scale).

Since changes in intracellular hydrogen ion concentration can have major consequences for cellular activity, it is perhaps not surprising that cells have evolved at least three regulatory mechanisms to keep intracellular [H+] relatively constant:

  1. exchange of intracellular hydrogen ions for extracellular
    sodium ions;

  2. cotransport of sodium and bicarbonate into cells to
    increase intracellular bicarbonate; and

^ 4.3 Active and passive transport
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