Passive solute transport

Objectives

1.
Explain how the distribution of lipids and proteins in the cell membrane influences the membrane permeability to hydrophobic and hydrophilic solutes and ions.
2.
Differentiate the following mechanisms based on the source of energy driving the process and the necessity for an integral cell membrane protein: diffusion, mediated (facilitated) transport, and secondary active transport.
3.
Explain how the transport rates of certain molecules and ions are accelerated by specific integral membrane proteins (“carrier” and “channel” molecules).
4.
Explain how coupling of solute transport enables one solute to be transported against its electrochemical gradient by using energy stored in the electrochemical gradient of the coupled solute.
5.
Explain the two-step process involved in the net transport of certain solutes across epithelia.

Diffusion across biological membranes is limited by lipid solubility

We have just learned that all transport processes are driven by electrical or chemical gradients ( Chapter 9 ). The question is, “How are substances (solutes and the solvent, water) actually transported across biological membranes?” Bear in mind that some substances must be concentrated in cells, or rapidly taken up, because they are essential for cell function. Other substances must be excluded or extruded from cells. For example, too much Na ⁺ (or any other solute) in cells would be an excessive osmotic burden and cause cells to swell (see the discussion of the Donnan effect in Chapter 4 ).

Biological membranes are primarily lipid bilayers ( Chapter 1 ), which are poorly permeable to polar and hydrophilic solutes; these substances bear a net charge or have internal charge separation (i.e., they are uncharged molecules that behave like electric dipoles in which the positive and negative charge are separated). The ability of polar substances to pass across lipid membranes tends to be inversely proportional to molecular weight: the larger the molecule, the lower the permeability. Also, pure phospholipid bilayers are modestly permeable to water, a polar solvent ( Chapter 1 ), but the presence of cholesterol greatly reduces water permeability. Consequently, the phospholipid-cholesterol bilayer that constitutes the PM in many types of cells is not sufficiently permeable to water for physiological needs.

Consider the problem of moving a polar solute, such as glucose, across a biological membrane. Glucose crosses lipid bilayers extremely slowly ( Table 2.1 ). In other words, the bilayer is an effective barrier to the transport of these substances. Yet glucose is an essential fuel for cells.

Channel, carrier, and pump proteins mediate transport across biological membranes

To mediate and regulate the transfer of water and polar solutes, biological membranes contain integral proteins called channels, carriers, and “pumps” ( Fig. 10.1 ). More than one third of all the genes in the human genome code for membrane proteins, and approximately half of these genes (i.e., one sixth of the genome) code for transport proteins.

Fig. 10.1, Models of a channel or pore (A) , a gated channel (B) , and a carrier (C) . The gated channel is shown in closed ( a ) and open ( b ) configurations. The carrier is shown in the exofacial configuration ( a ) with the solute binding site open to the extracellular fluid, the “occluded” configuration with the bound solute inaccessible to either fluid ( b ), and the endofacial configuration with the solute binding site open to the cytosol ( c ). Note that the “simple” carrier must also be able to switch between the exofacial and endofacial configurations in the absence of bound solute to effect net transport of the solute down its electrochemical gradient.

Transport through channels is relatively fast

Channels in biological membranes ( Chapter 8 ) are proteins with central pores that open to both the extracellular fluid and the cytoplasm simultaneously ( Fig. 10.1 A). The narrowest region within the pore, called the selectivity filter, determines which substance(s) (ions, water, etc.) may pass through the channel. For example, channels in the family called aquaporins ^a

a Peter Agre shared the 2003 Nobel Prize in Chemistry for his discovery of aquaporins and his determination of their structure and function.

are selectively permeable to water. Some other channels ( Chapters 7 and 8 ) are highly selective for Na ⁺ ions, for K ⁺ ions, or for Ca ²⁺ ions, whereas still others are less selective and may, for example, be permeable to both Na ⁺ and K ⁺ (e.g., the nicotinic acetylcholine channel; Chapter 13 ).

Channels characteristically have high permeability. Typical turnover numbers for ion channels (i.e., the maximum number of ions that can pass through a channel in 1 second) are approximately 10 ⁶ to 10 ⁸ . For example, approximately 30 million K ⁺ ions can pass through a single K ⁺ channel in 1 second ( Table 10.1 ).

TABLE 10.1

Relative Transport Rates for Various Types of Transporters

Transporter	Turnover Number ^a (Per Sec)
K ⁺ channel	30,000,000
Valinomycin (carrier)	30,000
Glucose carrier (GLUT-1)	3,000
Na ⁺ /Ca ²⁺ exchanger	2,000
Ca ²⁺ pump (SERCA)	200
Na ⁺ pump	150

a Rate of cycling of the transporter molecule, except for ion channels such as the K ⁺ channel, where the turnover number indicates the maximum number of ions transported in 1 second under physiological conditions. Thus each Na ⁺ pump molecule cycles 150 times per second and transports 450 Na ⁺ (and 300 K ⁺ ) per second.

Channel density controls the membrane permeability to a substance

Aquaporins facilitate water flow between the extracellular fluid and the cytoplasm to maintain osmotic balance (equilibrium) in cells that require high water permeability (e.g., skeletal muscle and some epithelia). The relative permeability to water depends on the number of channels (aquaporin tetramers) present, per unit area, in the PM (i.e., the channel density), and hormones may regulate this. The hormone vasopressin increases water permeability in, for example, the principal cells of the renal cortical collecting duct, by stimulating cAMP production. In turn, cAMP promotes the insertion of aquaporin-2 (AQP-2) molecules into the apical membrane of the epithelial cells. As a result, the osmotic driving force can speed water reabsorption from the renal tubule lumen. Defects in this hormonal mechanism, such as loss-of-function mutations in the vasopressin receptor or the AQP-2 molecule, lead to diabetes insipidus (excretion of a high volume of dilute urine). This occurs because AQP-2 molecules cannot be inserted into the apical membrane of the renal cortical collecting duct. Therefore even though the osmotic force may favor water reabsorption, the intrinsically low water permeability in the kidney cortical collecting ducts minimizes water reabsorption.

The rate of transport through open channels depends on the net driving force

In contrast to the aforementioned situation, impaired insulin secretion or reduced sensitivity to insulin leads to high glucose content and high osmotic pressure in the renal proximal tubule fluid. Water reabsorption is thereby reduced even though the epithelial cell apical membranes may contain a large number of AQP-2 molecules. This causes diabetes mellitus (excretion of a large volume of sweet urine with an osmolality approaching that of plasma). In this case the abnormally high osmotic pressure in the kidney tubule lumen reduces the osmotic driving force for water reabsorption from the lumen even though apical membrane water permeability may be high ( Chapter 11 ).

Transport of substances through some channels is controlled by “gating” the opening and closing of the channels

Another mechanism of regulating channel permeability is by channel gating ( Fig. 10.1 B), which may be regulated by voltage or by ligands such as Ca ²⁺ or ATP ( Chapters 7 and 8 ). Still, other channels are gated by pressure and membrane deformation, as exemplified by the mechanosensitive monovalent cation channels in some sensory nerve terminals.

Carriers are integral membrane proteins that open to only one side of the membrane at a time

In contrast to channels, the solute binding sites in carriers undergo spontaneous conformational changes and thus have alternating access to the two sides of the membrane. A solute binds to the carrier at one side of the membrane; then, as a result of a conformational change, a “gate” closes and the solute is transiently occluded (the transition state ; Fig. 10.1 C). Then, through a further conformational change in the protein, the “gate” on the opposite side opens so that the solute can dissociate from the carrier at this side of the membrane ( Fig. 10.1 C, c). The terms exofacial and endofacial denote the transporter conformations in which the solute binding sites face the extracellular fluid and cytoplasm, respectively.

Carriers facilitate transport through membranes

Simple carriers are integral membrane proteins that bind and transport a single solute species across a membrane. They move the bound solute down its electrochemical gradient (i.e., down a concentration gradient and, in the case of charged solutes, a voltage gradient). The rate of transport is substantially slower than that mediated by channels ( Table 10.1 ) because substrate binding and carrier conformation changes take time. Nevertheless, transport is much faster than would be expected for diffusion in the absence of such carriers. In other words, carriers simply speed up (facilitate) processes that would normally occur, albeit much more slowly, by diffusion; hence this process is sometimes called facilitated diffusion ( Box 10.1 ). Thus transport by the simple carrier cannot be used to generate a steady-state electrochemical gradient, because the generation of such a gradient requires the expenditure of energy. This distinguishes passive transport from active transport , which is discussed later and in Chapter 11 .

BOX 10.1

The Mechanism of Carrier-Mediated Transport as Exemplified by the GLUT-1 Glucose Transporter

Carriers such as the GLUT-1 sugar transporter spontaneously change conformation, whether or not glucose is bound, so that the glucose binding site is open (accessible) either to the extracellular fluid or to the cytoplasm. Consider, for example, the situation in which the extracellular fluid glucose concentration is high and its concentration in the cytoplasm is low. Glucose molecules will then bind to the GLUT-1 binding sites with higher probability (because of the higher glucose concentration) when the sites are facing the extracellular fluid than when the sites are facing the cytoplasm. Glucose that is bound at the outside will tend to dissociate when the carrier conformation changes and the sites face the cytoplasm with its low glucose concentration. The result will be a net transport of glucose from the extracellular fluid to the cytoplasm. When the concentrations of glucose are equal on the two sides of the membrane, no net transport will occur, because the probability of glucose binding at the internal and at the external faces of the membrane will then be the same. Large movements of glucose (fluxes) in both directions may take place under the latter circumstances, but they will be equal in magnitude. Thus in the steady state, the simple carrier cannot be used to transport a solute against a concentration gradient.

Sugars are transported by carriers

Several solutes are transported by solute-selective simple carriers. A good example is the glucose carrier that mediates glucose transport across the human red blood cell (RBC) PM. This protein, GLUT-1, belongs to a family of sugar transporters ( Box 10.2 ). GLUT-1 has 12 membrane-spanning helices, 5 of which are amphiphilic : each has a hydrophobic and a hydrophilic surface. The hydrophobic regions interact with the surrounding lipids in the bilayer. In contrast, the hydrophilic surfaces of the five helices face one another and form a central, water-filled transmembrane channel or pore ( Fig. 10.2 ). This is the general structure of many transporter molecules.

BOX 10.2

The Family of Sugar Transporters

GLUT-1 is one of five homologous human sugar transporters (GLUT-1 to GLUT-5). Each is the product of a different gene, and each has a different tissue distribution and is regulated differently. Moreover, these transporters are members of one family of a superfamily of solute transporters, the major facilitator superfamily , with more than 1000 members and some well-conserved sequence motifs. The superfamily already includes as many as 34 separate families. Each family has a family-specific signature sequence and specificity for a single class of substrates. Some other families within this superfamily include the monocarboxylic acid transporters that transport pyruvate and lactate, for example, and the anion/cation cotransporters that transport sodium and phosphate simultaneously.

Fig. 10.2, Model of a Simple Glucose Carrier (e.g., GLUT-1).

The need for carrier-mediated glucose transport is exemplified by the consequences of mutations in the human GLUT-1 gene. GLUT-1 also is expressed in the brain, where it mediates glucose transport across the blood-brain barrier and glucose uptake by glial cells. Individuals with a mutant GLUT-1 gene have a cerebrospinal fluid glucose concentration that is approximately 50% of normal despite a normal blood glucose level. The low brain glucose causes a devastating neurological syndrome ( Box 10.3 ).

BOX 10.3

The Defective Glucose Transporter Protein Syndrome

Patients with a rare defect in GLUT-1 have infantile seizures (convulsions), starting at age 3–4 months. They also have microcephaly (small head size) and developmental delays. Laboratory examination reveals hypoglycorrhachia (i.e., a very low glucose concentration, approximately half normal) in the cerebrospinal fluid (CSF) and a low CSF/blood glucose ratio (< 0.5). The rate of glucose uptake by the red blood cells (RBCs) from these patients is also less than 50% of normal; this can be used as a diagnostic test. These manifestations can be explained by mutations in the GLUT-1 gene that cause malfunction of the expressed protein (a reduced turnover rate or maximum velocity of transport). GLUT-1 normally is expressed in many cells, including RBCs and epithelial cells of the choroid plexus and ependyma, as well as in blood vessel endothelial cells; it facilitates the transport of glucose from the blood to the CSF. GLUT-1 is also expressed in glia, where the transporter is concentrated in foot processes that surround neuronal synapses. Thus in patients with defective GLUT-1, the glucose concentration within glial cells may be particularly low. This may be rate limiting for cellular energy metabolism and brain function.

Transport by carriers exhibits kinetic properties similar to those of enzyme catalysis

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