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Why is facilitated diffusion considered passive transport?

Despite the help of transport proteins, facilitated diffusion is considered passive transport because the solute is moving down its concentration gradient, a process that requires no energy.

Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane, but it does not alter the direction of transport. Some transport proteins, however, can move solutes against their concentration gradients, across the membrane from the side where they are less concentrated (whether inside or outside) to the side where they are more concentrated.

How does active transport work?

To pump a solute across a membrane against its gradient require work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins.

This makes sense because when channel proteins are open, they merely allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradients.

Active transport enables a cell to maintain internal concentrations of small solutes that differ from concentrations in its environment. For example, compared with it surroundings, an animal cell has a much higher concentration of potassium (K+) and a much lower concentration of sodium ions (Na+). The plasma membrane helps maintain these steep gradients by pumping Na+ out of the cell and K+ into the cell.

What is the energy source that supplies the energy for most active transport?

As in other types of cellular work, ATP supplies the energy for most active transport. One way ATP can power active transport is by transferring its terminal phosphate group directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane.

One transport system that works this way is the sodium-potassium pump, which exchanges Na+ for K+ across the plasma membrane of animal cells.

What is the sodium-potassium pump? (Fig 7.18)

This transport system pumps ions against steep concentration gradients: Sodium ion concentration [Na+] is high outside the cell and low inside, while potassium ion concentration [K+] is low outside the cell and high inside.

The pump oscillates between two shapes in a cycle that moves 3 Na+ out of the cell for every 2 K+ pumped into the cell. The two shapes have different affinities for Na+ and K+. ATP powers the shape change by transferring a phosphate group to the transport protein (phosphorylating the protein).

Fig 7.18 - Describe the steps that occur in the operation of the sodium-potassium pump, which is an example of active transport.

1. Cytoplasmic Na+ binds to the sodium-potassium pump. The affinity for Na+ is high when the protein has this shape.

2. Na+ binding stimulates phosphorylation (the transfer of a phosphate group to the transport protein) by ATP.

3. Phosphorylation leads to a change in protein shape, reducing its affinity for Na+, which is released outside.

4. The new shape has a high affinity for K+, which binds on the extracellular side and triggers release of the phosphate group.

5. Loss of the phosphate group restores the protein's original shape, which has a lower affinity for K+.

6. K+ is released; affinity for Na+ is high again, and the cycle repeats.

Do cells have voltages across their membranes?

Yes, all cells have voltages across their plasma membranes. Voltage is electrical potential energy - separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides. The voltage across a membrane, called a membrane potential, ranges from about -50 to -200 millivolts (mV). (The minus sign indicates that the inside of the cell is negative relative to the outside.

How does the membrane potential act like a battery?

The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Because the inside of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell.

Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement). This combination of forces acting on an ion is called the electrochemical gradient.

What is the electrochemical gradient?

It is the diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force).

How does the concept of passive transport change in the case of ions.

In the case of ions, we must refine our concept of passive transport: An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient.

For example, the concentration of Na+ inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions then "fall" down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of these cations to the negative side (inside) of the membrane.

In this example, both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so. In cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport may be necessary.

In Chapter 48, you will learn about the importance of electrochemical gradients and membrane potentials in the transmission of nerve impulses.

How do some membrane proteins that actively transport ions contribute to the membrane potential?

An example is the sodium potassium pump. Notice in Figure 7.18 that the pump does not translocate Na+ and K+ one for one, but pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell.

With each "crank" of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy as voltage.

What is an electrogenic pump?

It is a transport protein that generates voltage across a membrane. The sodium-potassium pump appears to be the major electrogenic pump of animal cells. The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports protons (hydrogen ions, H+) out of the cell.

The pumping of H+ transfers positive charge from the cytoplasm to the extracellular solution. By generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work. One important use of proton gradients in the cell is for ATP synthesis during cellular respiration (CR).

What is a proton pump?

Proton pumps are electrogenic pumps that store energy by generating voltage (charge separation) across membranes. A proton pump translocates positive charge in the form of hydrogen ions.

The voltage and H+ concentration gradient represent a dual energy source that can drive other processes, such as the uptake of nutrients. Most proton pumps are powered by ATP.

What is cotransport?

It is a single ATP-powered pump that transports a specific solute that can indirectly drive the active transport of several other solutes in a mechanism. The substance that has been pumped across a membrane can do work as it moves across the membrane by diffusion, analogous to water that has been pumped uphill and performs work as it flows back down.

Another transport protein, a cotransporter separate from the pump, can couple the "downhill" diffusion of this substance to the "uphill" transport of a second substance against its own concentration gradient (or electrochemical) gradient.

Give me an example of a cotransporter in action.

For example, a plant cell uses the gradient of H+ generated by its proton pumps to drive the active transport of amino acids, sugars, and several other nutrients into the cell. One transport protein couples the return of H+ to the transport of sucrose into the cell. (Figure 7.21)

This protein can translocate sucrose into the cell against its concentration gradient (more sugar inside the cell than outside), but only if the sucrose molecule travels in the company of a hydrogen ion. The hydrogen ion uses the transport protein as an avenue to diffuse down the electrochemical gradient maintained by the proton pump.

Plants use sucrose-H+ cotransport to load sucrose produced by photosynthesis into cells in the veins of leaves. The vascular tissue of the plant can then distribute the sugar to nonphotosynthetic organs, such as roots.

How has our knowledge about cotransport proteins in animal cells helped us to find more effective treatments for diarrhea, a serious problem in developing countries?

Normally, sodium in waste is reabsorbed in the colon, maintaining, constant levels in the body, but diarrhea expels waste so rapidly that reabsorption is not possible, and sodium levels fall precipitously.

To treat this life-threatening condition, patients are give a solution to drink containing high concentrations of salt (NaCI) and glucose. The solutes are taken up by the sodium-glucose cotransporters on the surface of intestinal cells and passed through the cells into the blood. This simple treatment has lowered infant mortality worldwide. (Fking amazing wow - simple knowledge - profound change in the world).

Sodium-potassium pumps help nerve cells establish a voltage across their plasma membranes. Do these use ATP or produce ATP? Explain.

I believe these pumps use ATP in a process called phosphorylation (where a phosphate that is broken off from the ATP molecule is used by the pump in order to change its confirmation and allow for transport of hydrogen ions across the membrane, from the cytoplasmic fluid to the extracellular fluid.)

The pump uses ATP. To establish a voltage, ions have to be pumped against their gradients, which requires energy.

Given the internal environment of a lysosome, what transport protein might you expect to see in its membrane?

The internal environment of a lysosome is acidic, so it has a higher concentration of H+ than does the cytoplasm. Therefore, you might expect the membrane of the lysosome to have a proton pump such as that shown in Figure 7.20 to pump H+ into the lysosome.

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