Chemiosmotic Cycles

A simple chemiosmotic cycle couples a cation transporting pump to solute transport by a carrier (Fig. 11-1). The membrane could be a plasma membrane or an organelle membrane. The driving reaction is called the primary transport step, indicating an input of energy and, in most cases, some chemical reaction. Other reactions are called secondary transport reactions, indicating that they depend on ion gradients. The transported substrate is the same chemically on both sides of the mem-brane. Although they are simple in concept, the importance and power of chemiosmotic cycles should not be underestimated. They operate in every membrane of every cell.

Figure 11-1 a model chemiosmotic cycle in a membrane surrounding a closed space. An ATP-driven pump transports a cation C⁺ out of the compartment. The energy derived from ATP is stored as a concentration gradient of C⁺ (red triangle) and a membrane potential (yellow arrow) across the membrane. The carrier uses the electrochemical gradient of C⁺ to drive the transport of both C⁺ and a solute up a concentration gradient (green triangle) across the membrane.

Pumps use energy derived from ATP hydrolysis, light absorption, or another chemical reaction (see Table 8-1) to move ions in one direction across a membrane. This raises the concentration of a cation (C⁺) on one side and depletes it on the other side of a membrane-bounded compartment. An ion gradient is characterized by both a chemical term, the concentration gradient, and an electrical gradient (the membrane potential explained in Fig. 10-17). The electro-chemical potential across a membrane represents a reservoir of power and a capacity to do work, also known as an ion-motive force. A mechanical analog would be using a pump to fill an elevated reservoir with fluid.

Carriers and other membrane proteins use the potential energy of ion gradients to drive other processes. This is analogous to using fluid flow out of a reservoir to drive a turbine, which puts the energy to use for other types of work. Many carriers use energy derived from the downhill passage of one substrate to transport one or more other substances up their concentration gradients across the same membrane barrier. In Figure 11-1, the carrier links the transport of solute S to the movement of cation C⁺ down its gradient. Recirculation of cations allows a cell to accumulate solute against its concentration gradient. In addition to the osmotic work illustrated in the figure, chemiosmotic cycles can do chemical work. During both oxidative and photosynthetic phosphorylation, proton cycles drive ATP synthesis by F₀F₁-ATP synthases (see Fig. 19-5). Chemiosmotic cycles can also perform mechanical work. The electrochemical gradient of protons across the plasma membrane drives rotation of bacterial flagella (see Fig. 38-24).

Chemiosmotic cycles using protons dominate the biological world. Most bacterial cycles involve proton pumps, proton-linked carriers, or other proton-linked events. The same is true of lower eukaryotes, fungi, and plants. Plasma membranes of plant cells have a powerful proton pump and a collection of proton carriers. Proton chemiosmotic cycles are also characteristic of most eukaryotic organelles, including the Golgi apparatus, endosomes, lysosomes, mitochondria, and chloroplasts. Animal cell plasma membranes are a major exception, because they use predominantly sodium ions for their chemiosmotic cycles.

Epithelial Transport

Net transport across an epithelium depends on tight junctions (see Fig. 31-2) that seal the extracellular space between the cells (Fig. 11-2). These junctions separate two extracellular compartments. The apical compartment is the free surface (e.g., the skin) or the lumen of the organ (e.g., the intestine, respiratory tract, or kidney tubules). The basolateral compartment lies between epithelial cells and is continuous with the underlying connective tissue and its blood vessels. Tight junctions seal the extracellular space, inhibiting diffusion of solutes, between the apical and basolateral compartments of the extracellular space. The extent of this seal varies from very tight to leaky. Tight junctions also separate the plasma membrane into apical and basolateral domains, restricting the movement of integral membrane proteins between these domains.

Figure 11-2 glucose transport by the intestinal epithelium. Tight junctions seal the epithelium of polarized epithelial cells. Na⁺K⁺-ATPase pumps (space-filling model) in the basolateral plasma membrane drive Na⁺/glucose symporters in the apical plasma membrane (upper inset) and glucose uniporters in the basolateral plasma membrane (left icon in lower inset) to move glucose from the lumen of the intestine to the blood. Basolateral K⁺ channels (middle icon) recycle K⁺ pumped into the cell.

Salt and Water Transport in the Kidney

In a section of the kidney tubule called the loop of Henle, the epithelium uses Na⁺K⁺-ATPase pumps and Na⁺/K⁺/2Cl⁻ symporters to reabsorb NaCl that is filtered from blood into the excretory pathway (Fig. 11-3). Without this provision, salt would be lost in urine. Tight junctions seal this epithelium, so that salt must pass through the cells to return to the blood. Na⁺/K⁺/2Cl⁻ symporters in the apical plasma membrane provide a way for NaCl to enter the cell down its concentration gradient. Abundant Na⁺K⁺-ATPases in the basolateral plasma membrane (5000/mm²) create a Na⁺ gradient to drive the symporter and to clear the cytoplasm of Na⁺ accumulated from the tubule lumen. KCl that enters with Na⁺ through the Na⁺/K⁺/2Cl⁻ symporter leaves the cell through channels: K⁺ channels in apical and basolateral membranes and Cl⁻ channels in basolateral membranes.

Figure 11-3 sodium chloride transport by the epithelium of the kidney tubule. Tight junctions seal the space between these polarized epithelial cells of the thick ascending limb of the loop of Henle. Na⁺K⁺-ATPase pumps (space-filling model) in the basolateral plasma membrane drive Na⁺/K⁺/2Cl⁻ symporters in the apical plasma membrane. K⁺ channels in the apical plasma membrane and K⁺ channels and Cl⁻ channels in the basolateral plasma membrane provide paths for K⁺ to circulate and for Cl⁻ to follow Na⁺ across the cell from the lumen of the tubule to the blood compartment.

A drug that is used to treat congestive heart failure—furosemide—inhibits the Na⁺/K⁺/2Cl⁻ symporter in the loop of Henle. A weak heart leads to accumulation of fluid in the lungs (causing shortness of breath) and other tissues (causing swelling of the ankles). Inhibition of the Na⁺/K⁺/2Cl⁻ symporter reduces NaCl reabsorption, so the kidney produces large quantities of urine, clearing excess fluid from the body and relieving symptoms.

Cystic Fibrosis as a Transporter Disease

Normally, cells in the lung and gastrointestinal tract use a complicated selection of familiar pumps and carriers to secrete salt and water at their apical sur-faces (Fig. 11-4). Na⁺K⁺-ATPases in the basolateral membrane set up an electrochemical gradient of Na⁺, which is exploited by basolateral membrane Na⁺/K⁺/2Cl⁻ symporters to take in Na⁺, along with K⁺ and Cl⁻ anions. The inward movement of Na⁺ down its electrochemical gradient drives the entry of K⁺ and Cl⁻ up their gradients. This brings excess potassium chloride into the cell. (The K⁺ brought in by both the Na⁺K⁺-ATPase and the Na⁺/K⁺/2Cl⁻ symporter recycles after exiting from the cell by way of channels in the basolateral plasma membrane. Thus, K⁺ is merely catalytic.) Excess Cl⁻ is left inside the cell. The cystic fibrosis transmembrane regulator (CFTR) protein, an ABC pump, in the apical plasma membrane acts as a Cl⁻ channel. When protein kinase phosphorylates the regulatory domain and ATP binds to the cytoplasmic domains, a conformational change opens a Cl⁻ channel across the membrane. Cl⁻ moves down its electrochemical gradient out of the cell, carrying charge to the outside. The whole epithelium becomes polarized, with the lumen electrically negative relative to the extracellular fluid compartment. This electrical driving force allows Na⁺ to move between cells, from the extracellular fluid compartment through leaky tight junctions to the surface of the epithelium. Sodium chloride on the apical surface creates an osmotic force that draws water down its concentration gradient across the cells to the outside through water channels (see Fig. 10-15). CFTR also appears to inhibit the transport mechanisms that reabsorb fluid from the lumen of the epithelium. A balance between this fluid secretion and fluid reabsorption normally keeps the surface of the epithelium properly hydrated, allowing the cilia to clear the lung of bacteria and secretions and the ducts of the pancreas to secrete digestive enzymes.

Figure 11-4 salt and water transport across the epithelium lining the respiratory tract. Leaky tight junctions partially seal the space between these polarized epithelial cells. Na⁺K⁺-ATPase pumps in the basolateral plasma membrane drive Na⁺/K⁺/2Cl⁻ symporters in the basolateral plasma membrane. CFTR Cl⁻ channels in the apical plasma membrane allow Cl⁻ to move into the lumen, creating a negative electrical potential that pulls Na⁺ between the cells into the lumen. CFTR also releases ATP, which activates additional Cl⁻ channels. Water follows sodium chloride into the lumen through water channels and between the cells. Basolateral K⁺ channels allow K⁺ to circulate.

Patients with cystic fibrosis have mutations in CFTR that result in defects in apical Cl⁻ transport and secretion. Their lungs are too dry as a result of this imbalance in fluid secretion and reabsorption on the surface of epithelia. This situation is life-threatening because cilia in the respiratory tract cannot move sticky, dry, mucus containing bacteria and viruses out of the lungs, thereby predisposing to respiratory infections. Sticky secretions in the pancreatic ducts also interfere with the secretion of digestive enzymes by the pancreas. Many different mutations in the CFTR gene cause the disease. The most common mutation (67% of cases) deletes the codon for phenylalanine 508 (F508). The resulting protein is temperature-sensitive, not folding properly at 37°C and failing to negotiate the secretory pathway to the plasma membrane. Patients with two copies of this mutation on chromosome 7 have classic cystic fibrosis. Heterozygotes with one normal gene (about 5% of the population) have no symptoms. Patients who have a combination of this DF508 mutation with a number of other mutations in the other copy of the gene vary in the severity of their pancreatic problems but still have typical lung disease.

Cellular Volume Regulation

Cells employ both short- and long-term strategies involving pumps, carriers, and channels to maintain a constant volume (Fig. 11-5). These compensatory mechanisms are required because water moves across the plasma membrane through water channels and slowly through lipid bilayers if the osmotic strength of the environment differs even slightly from that inside the cell. Water moves to maintain an osmotic equilibrium, as is illustrated for red blood cells in Figure 7-6. In a hypotonic medium, water moves into a cell to dilute the cytoplasm. In a hypertonic medium, water moves out to concentrate the cytoplasm. Mechanisms that are employed to compensate for these volume changes are well defined, but the mechanisms that sense volume changes and trigger these responses are still being investigated.

Figure 11-5 acute cellular volume control. A, Cell is placed in hypertonic medium. B, Cell is placed in hypotonic medium. Cells compensate for volume changes by activating channels and carriers to move inorganic ions into or out of the cell. Water follows passively through channels and across the lipid bilayer.

Animal cells respond acutely to loss of water by activating Na⁺/H⁺ antiporters, Cl⁻/HCO₃⁻ antiporters, and/or Na⁺/K⁺/2Cl⁻ symporters that bring potassium chloride and sodium chloride into the cell. Water follows, returning the cell to its original volume in minutes. The acute response to swelling activates K⁺ channels, Cl⁻ channels (ClC-3), and/or a K⁺/Cl⁻ symporter, taking potassium chloride and water out of the cell. Compensation by moving inorganic ions works in the short run but is not an acceptable long-term solution because changes in the internal concentrations of K⁺, Na⁺, and Cl⁻ affect the membrane potential and other physiological processes.

In the long term, cells use small organic molecules called osmolytes to adjust the osmotic strength of cytoplasm and to maintain their volume. Osmolytes include amino acids, polyalcohols (sorbitol and inositol), and methylamines that do not interfere with cellular biochemistry or membrane excitability. Adjustment of osmolyte concentrations takes longer than that for ions, as it requires synthesis or degradation of transport proteins. In response to swelling, cells immediately activate channels that allow osmolytes and Cl⁻ to escape from cytoplasm; this is followed later by a reduction in the number of plasma membrane Na⁺/osmolyte symporters. In response to shrinking, cells close the osmolyte channels and synthesize additional Na⁺/osmolyte sym-porters.

Excitable Membranes

Regulation of membrane potential is particularly important in higher organisms, which use electrical signals generated by membrane channels for communication in their nervous and muscular systems. For example, reading and understanding this page depend on rapid creation and processing of electrical and chemical signals by cells in the visual system and brain. Ion channels produce the key event, a transient change in electrical potential of the plasma membrane, called an action potential. These energy-efficient electrical signals are the fastest means of communication in the body, spreading over the plasma membrane at tens of meters per second. Similarly, action potentials trigger skeletal muscle contraction, control the timing of the heartbeat, and coordinate the peristaltic motions of the gut and contractions of the uterus.

Electrical excitability is not limited to nerves and muscles. Eggs use a form of action potential as an early step in blocking fertilization by more than one sperm. Chemotaxis by macrophages and secretion of insulin and other hormones both depend on electrical excitability. The reader should be familiar with the appendixes in Chapter 10 to appreciate the following material.

Description of an Action Potential

If a microelectrode (see Fig. 10-16A) drives a small positive or negative current into a cell, a second microelectrode a short distance away detects a small voltage response. These electrotonic potentials decline rapidly with distance if the cell in nonexcitable.

In striking contrast to these small local currents, when the plasma membrane of an excitable cell, such as neuron or muscle, is depolarized beyond a certain level, called a threshold, the membrane responds over a few milliseconds with a large, stereotyped change in membrane potential, called an action potential (Fig. 11-6). Voltage-gated ion channels (see Fig. 10-7) generate this powerful electrical signal that spreads rapidly (10 m/sec) over the entire plasma membrane. During an action potential, the membrane potential can reach a peak of +40 to 50 mV before repolarizing to the resting potential. Because action potentials are self-triggering, they travel without dissipation over long distances. This high-speed transmission is very efficient, requiring movement of very few ions across the membrane.

Figure 11-6 the time course of an action potential passing a measuring electrode inserted through the plasma membrane of a squid giant axon. Spread of an action potential from an adjacent area of the membrane brings the membrane potential E_m, to threshold, triggering the action potential at this point on the membrane. The other curves show the conductance of the membrane at this point for Na⁺ and K⁺

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