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3.3e
| Question | Answer |
|---|---|
| The processes of membrane transport described up to this point don’t necessarily require a cell membrane; they can occur as well through artificial membranes | Now, however, we come to processes for which a cell membrane is necessary, because they employ transport proteins, or carriers. Thus, the next three processes are classified as carrier-mediated transport. |
| In these cases, a solute binds to a carrier in the plasma membrane, | which then changes shape and releases the solute to the other side. |
| Carriers can move substances into or out of a cell, and into or out of organelles within the cell. | The process is very rapid; for example, one carrier can transport 1,000 glucose molecules per second across the membrane. |
| Carriers act like enzymes in some ways: | The solute is a ligand that binds to a specific receptor site on the carrier, like a substrate binding to the active site of an enzyme. |
| The carrier exhibits ----- for its ligand, just as an enzyme does for its substrate. A glucose carrier, for example, can’t transport fructose. | specificity |
| Carriers also exhibit saturation; as the solute concentration rises, its rate of transport increases, but only up to a point. | When every carrier molecule is occupied, adding more solute can’t make the process go any faster. |
| The carriers are saturated—no more are available to handle the increased demand, and transport levels off at a rate called the --- | transport maximum (Tm) |
| You could think of carriers as analogous to buses. | If all the buses on a given line are full (“saturated”), they can’t carry any more passengers, regardless of how many people are waiting at the bus stop. |
| An important difference between a carrier and an enzyme is that ---- don’t chemically change their ligands; they simply pick them up on one side of the membrane and release them, unchanged, on the other. | carriers |
| There are three kinds of carriers: | uniports, symports, and antiports. |
| A --- carries only one type of solute. For example, most cells pump out calcium by means of a uniport, maintaining a low intracellular concentration so calcium salts don’t crystallize in the cytoplasm. | uniport |
| Some carriers move two or more solutes through a membrane simultaneously in the same direction; | this process is called cotransport and the carrier protein that performs it is called a symport. |
| For example, ----- of the small intestine and kidneys have a symport that takes up sodium and glucose simultaneously. | absorptive cells |
| Other carriers move two or more solutes in opposite directions; this process is called | For example, nearly all cells have an antiport called the sodium–potassium pump that continually removes Na+ from the cell and brings in K+ |
| There are three mechanisms of carrier-mediated transport: facilitated diffusion, primary active transport, and secondary active transport. | Facilitated diffusion (fig. 3.17) is the carrier-mediated transport of a solute through a membrane down its concentration gradient. It requires no expenditure of metabolic energy (ATP) by the cell. |
| Facilitated diffusion | It transports solutes such as glucose that cannot pass through the membrane unaided. |
| Facilitated diffusion | The solute attaches to a binding site on the carrier, then the carrier changes conformation and releases the solute on the other side of the membrane. |
| Primary active transport is a process in which a carrier moves a substance through a cell membrane up its concentration gradient using energy provided by ATP. | Just as rolling a ball up a ramp would require you to push it (an energy input), this mechanism requires energy to move material up its concentration gradient |
| ATP supplies this energy by transferring a phosphate group to the transport protein. The calcium pump mentioned previously uses this mechanism. Even though Ca2+ | is already more concentrated in the ECF than within the cell, this carrier pumps still more of it out |
| Active transport | also enables cells to absorb amino acids that are already more concentrated in the cytoplasm than in the ECF. |
| Secondary active transport also requires an energy input, but depends only indirectly on ATP. For example, certain kidney tubules have proteins called sodium-glucose transporters (SGLTs) that simultaneously bind sodium (Na+) | and glucose and transport them into the tubule cells, saving glucose from being lost in the urine |
| . An SGLT itself doesn’t use ATP. However, it depends on the fact that the cell actively maintains a low internal Na+ concentration, so Na+ diffuses down its gradient into the cell. Glucose “hitches a ride” with the incoming Na+ | But what keeps the intracellular Na+ concentration low is that the basal membrane of the cell has an ATP-driven sodium–potassium pump that constantly removes Na+ from the cell. |
| If not for this, the Na+ and glucose inflow via the SGLT would soon cease. Therefore, the SGLT doesn’t use ATP directly, but depends on ATP to drive the Na+ -K pump; | it is therefore a secondary active-transport protein. (Secondary active transport is an unfortunate name for this, as the SGLT is actually carrying out facilitated diffusion, but its dependence on a primary active-transport pump has led to this name.) |
| Secondary Active Transport. | In this example, the sodium–glucose transporter (SGLT) at the apical cell surface carries out facilitated diffusion, but depends on active transport by the Na+-K+ pump at the base of the cell to keep it running. |
| The sodium-potassium pump itself (fig. 3.19) is a good example of primary active transport. It is also known as Na+-K + ATPase because it is an enzyme that hydrolyzes ATP. The Na+ -K pump binds three Na + | simultaneously on the cytoplasmic side of the membrane, releases these to the ECF, binds two K+ simultaneously from the ECF, and releases these into the cell. |
| This keeps the K+ concentration higher and the Na+ concentration lower within the cell than they are in the ECF. | These ions continually leak through the membrane, and the Na+ -K pump compensates like bailing out a leaky boat. |
| Lest you question the importance of the Na+ -K pump, consider that half of the calories you use each day go to this purpose alone. | The pump typically operates at about 10 cycles/s, but under certain conditions it can achieve 100 cycles/s. |
| Various types of cells have from just a few hundred Na+ -K pumps (red blood cells) to millions of them (nerve cells), | so an average cell may exchange 30 million Na+ ions and 10 million ATPs per second |
| Beyond compensating for a leaky plasma membrane, the Na+ -K+ pump has at least four functions: | Secondary active transport, Regulation of cell volume, Maintenance of a membrane potential, Heat production |
| ------, Regulation of cell volume, Maintenance of a membrane potential, Heat production | Secondary active transport |
| Secondary active transport, -------, Maintenance of a membrane potential, Heat production | Regulation of cell volume |
| Secondary active transport, Regulation of cell volume, -------, Heat production | Maintenance of a membrane potential |
| Secondary active transport, Regulation of cell volume, Maintenance of a membrane potential, ------- | Heat production |
| Secondary active transport. | It maintains a steep Na+ concentration gradient across the membrane. Like water behind a dam, this gradient is a source of potential energy that can be tapped to do other work. The SGLT described previously is an example of this. |
| Regulation of cell volume. | Certain anions are confined to the cell and can’t penetrate the plasma membrane. These “fixed anions,” such as proteins and phosphates, attract and retain cations. |
| Regulation of cell volume. | If there were nothing to correct for it, the retention of these ions would cause osmotic swelling and possibly lysis of the cell. |
| Cellular swelling, however, elevates activity of the Na+-K+ pumps. Since each cycle of the pump removes one ion more than it brings in, | the pumps are part of a negative feedback loop that reduces intracellular ion concentration, controls osmolarity, and prevents cellular swelling. |
| Maintenance of a membrane potential. | All living cells have an electrical charge difference called the resting membrane potential across the plasma membrane. |
| Maintenance of a membrane potential. | Like the two poles of a battery, the inside of the membrane is negatively charged and the outside is positively charged. |
| The membrane potential is essential to the excitability of nerve and muscle cells. | This difference stems from the unequal distribution of ions on the two sides of the membrane, maintained by the Na+ -K+ pump. |
| Heat production. When the weather turns chilly, we turn up not only the furnace in our home but also the “furnace” in our body. Thyroid hormone stimulates cells to produce more Na+ -K+ pumps. | As these pumps consume ATP, they release heat from it, compensating for the body heat we lose to the cold air around us. |
| An important characteristic of proteins is their ability to change shape in response to the binding or dissociation of a ligand. Explain how this is essential to carrier-mediated transport. | Proteins help move substances across cell membranes by changing shape when they attach to specific molecules, allowing quick and efficient transport. |
| In summary, carrier-mediated transport is any process in which solute particles move through a membrane by means of a transport protein. | The protein is a uniport if it transports only one solute, a symport if it carries two types of solutes at once in the same direction, and an antiport if it carries two or more solutes in opposite directions. |
| If the carrier doesn’t depend on ATP at all and it moves solutes down their concentration gradient, the process is called facilitated diffusion. If the carrier itself consumes ATP and moves solutes up their concentration gradient, the process is called | primary active transport. If the carrier doesn’t directly use ATP, but depends on a concentration gradient produced by ATP-consuming Na+ -K pumps elsewhere in the plasma membrane, the process is called secondary active transport. |
Created by:
Russells3709