SmartWork5 Chapter 8

Given the type of transporter as determined in Part 1, choose all of the correct statements below that relate to the function of the Na+/Ca2+ transporter.

-Ca2+ is transported against its electrochemical gradient. -The transporter uses the Na+ electrochemical gradient as an energy source for transporting ions. An antiport transports one substrate down its concentration gradient while a second substrate is transported against its gradient. The energy from the movement of the first substrate is used to drive the transport of the second substrate against its gradient. An antiport moves the two substrates in opposite directions across the membrane. The Na+/Ca2+ antiport uses the energy in the Na+ gradient to transport Ca2+ out of the cell against its gradient as Na+ comes into the cell down its gradient.

Which of the following characteristics of K+ channels are important for the selectivity for K+ rather than other ions?

-Four rigid protein loops line the narrowest part of the pore. -Carbonyl groups line the wall of the pore. The K+ channel, like other channels, is selective for one solute only—in this case, K+ ions. The pore contains four rigid protein loops at the narrowest part of the pore that form the selectivity filter for K+ ions. The protein loops lead to a channel the correct size for K+ ions to pass through. The protein loops also contain carbonyl groups on the side chains of amino acids that are spaced to selectively interact with K+ ions and not other positive ions.

Intracellular Ca2+ levels are important in cardiac muscle. Increasing intracellular Ca2+ levels in heart muscle cells leads to an increase in muscle contraction. Lowering the intracellular Ca2+ levels decreases the strength of cardiac muscle contraction. Congestive heart failure can occur when the heart's pumping of blood is weaker than normal, which leads to fluid collecting around organs, including the heart. One treatment method is to give the patient drugs that increase the strength of the heart muscle contraction. Which of the following might function as an effective treatment of congestive heart failure by increasing the strength of heart muscle contraction?

-a drug that decreases the activity of the Na+/Ca2+ transporter -a drug that inhibits the Na+-K+ pump from establishing a strong Na+ gradient -a diuretic drug that triggers removal of excess Na+ from the body

Our hearing relies upon several cellular structures that go through a set of steps to signal the receipt of sound to the nerves. Put the following cellular steps involved in response to sound in the proper order.

1) The basilar membrane vibrates 2) The basilar membrane pushes stereocilia against the tectorial membrane 3) The stereocilia tilt, triggering an electrical response in the hair cell 4) The activated hair cell triggers the auditory nerve to fire When sound vibrations enter the ear, the basilar membrane vibrates in response. As the extracellular matrix membrane vibrates, it pushes the stereocilia on a hair cell up against a second extracellular membrane, the tectorial membrane. The stereocilia are tilted by the pressure against the tectorial membrane. Stretch-activated ion channels on the stereocilia open in response to the tilt, triggering an electrical response in the hair cell. The activated hair cell releases neurotransmitters that activate the auditory nerve cell, which passes the signal to the brain that a sound was heard. When exposed to extremely loud sounds, the stereocilia of the hair cells can be pushed against the tectorial membrane so hard that damage can occur. Hair cells do not regenerate, and so the damage from loud sounds leads to permanent hearing loss.

A sodium-potassium antiport maintains the extracellular concentration of sodium at levels that are about 20-30 times higher than inside the cells. What directly supplies the energy for maintaining this gradient?

ATP hydrolysis drives the function of the pump.

Cardiac muscle cells contain a Na+/Ca2+ transporter responsible for maintaining a low cytosolic Ca2+ concentration, which helps regulate cardiac muscle contraction. Ca2+ is transported out of the cell as Na+ is brought into the cell. What type of transporter is this protein?

Antiport- Antiports transport two substrates in opposite directions across the membrane. The Na+/Ca2+ antiport transports three Na+ molecules into the cell for every Ca2+ transported out of the cell. The Na+ is moving into the cell down its electrochemical gradient. The energy from Na+ moving down its gradient is used to drive the movement of Ca2+ against its electrochemical gradient as Ca2+ is pumped out of the cell. This keeps the concentration of Ca2+ very low inside the cytosol of the cell. The strength of cardiac muscle contraction is regulated in part by the concentration of Ca2+ in the cytosol.

Ions in solution are found in a hydration shell of water. This shell must be removed for an ion to pass through the channel. How does the K+ channel accomplish removal of the water from the shell around the ion?

Carbonyl groups lining the wall of the pore can interact with the unsolvated K+ ion, balancing the energy needed to remove the hydration shell. The K+ channel is formed from four identical subunits that together form a pore. The four rigid protein loops each contain amino acids with carbonyl side chains. These carbonyl groups are spaced at precisely the right distance to interact only with an unsolvated K+ ion stripped of its water molecules. The binding of a K+ ion to the carbonyl groups balances the energy needed to remove the hydration shell of water, allowing passage of the K+ ion through the channel.

Channels vs. Transporters

Channels are multipass transmembrane proteins that form a pore that discriminates loosely between ions when open, allowing their passive flow across the membrane and down the concentration gradient. Transporters are highly selective and active transporters can move solutes against the concentration gradient with the input of energy.

How do transporters and channels select which solutes they help move across the membrane?

Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. A transporter transfers only those molecules or ions that fit into specific binding sites on the protein. Transporters bind their solutes with great specificity, in the same way an enzyme binds its substrate, and it is this requirement for specific binding that gives transporters their selectivity.

Which of the following form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion?

Channels- Membrane channels form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion. Solutes that are small enough to pass through the channel will diffuse through, while those that are too large will not. Most channels only permit passage of ions and are therefore also referred to as ion channels. Because ions are electrically charged, their movements can create a powerful electric force—or voltage—across the membrane.

Lipid bilayers are highly impermeable to:

Charged ions, and Na+ and Cl- are common examples of ions that are excluded from the hydrophobic interior of a lipid bilayer.

Determine whether the following statement is true or false: The glucose-Na+ symport protein uses the electrochemical Na+ gradient to drive the active transport of glucose into the cell. Once this transporter has bound both Na+ and glucose, it preferentially opens toward the cytosol, where it releases both solutes.

False- A glucose-Na+ symporter uses the electrochemical Na+ gradient to drive the active import of glucose. The pump oscillates randomly between alternate states. Because conformational transitions are reversible, once the transporter has bound both solutes, two things can happen: (1) it can flip into the inward-open state, allowing both solutes to enter the cytosol, or (2) it can also remain in the outward-open state. In this case, the solutes would dissociate into the extracellular space and nothing would be gained. Even though Na+ and glucose can each bind to the pump in either of these "open" states, the pump can transition between them only through an "occluded" state in which both glucose and Na+ are bound ("occluded-occupied") or neither is bound ("occluded-empty"). Because conformational transitions are reversible, once the transporter has bound both solutes, two things can happen: (1) it can flip into the inward-open state, allowing both solutes to enter the cytosol, or (2) it can also remain in the outward-open state. In this case, the solutes would dissociate into the extracellular space and nothing would be gained.

Determine whether the following statement is true or false: A symport protein would function as an antiport protein if its orientation in the membrane were reversed.

False- A symport protein binds two different solutes on the same side of the membrane; an antiport protein binds two different solutes on opposite sides of the membrane. Thus, if a symport were "upside down" in the membrane, then both solutes likely would be moved in the opposite direction compared to normal. Just inverting a protein does not alter the mechanics of the protein from symport to antiport or vice versa.

Determine whether the following statement is true or false: Most ion channels undergo conformational changes with each ion that passes through.

False- Unlike transporters, ion channels do not undergo conformational changes as each ion passes through, which allows them to transport ions at a very rapid rate. This is a large advantage over a transporter with respect to its maximum rate of transport. More than a million ions can pass through an open channel each second, which is 1000 times greater than the fastest rate of transfer known for any transporter.

The glucose-Na+ symport transports glucose into the epithelial cells lining the gut. How would import of glucose into the cells be affected by addition of a leaky Na+ channel to their plasma membrane?

Glucose transport would slow because the Na+ gradient is dissipated by the Na+ channel. The glucose-Na+ symport transports Na+ down its concentration gradient while transporting glucose into the cell against its gradient. The Na+ gradient supplies the energy to transport glucose. The Na+ concentration must be higher outside the cell than inside to transport glucose. A leaky Na+ channel would dissipate the Na+ gradient. With a smaller Na+ gradient, the transport of glucose across the membrane would slow.

In one experiment, investigators create a liposome—a vesicle made of phospholipids—that contains a solution of 1 mM glucose and 1 mM sodium chloride. If this vesicle were placed in a beaker of distilled water, what would happen the fastest?

H2O would diffuse in. In this experiment, the possible molecules that can move across the membrane are water, glucose, and ionized sodium chloride. Glucose requires a transporter to move across a lipid membrane because of its relatively large size and the experimental design does not include these transporter proteins in the liposome. Therefore, glucose will not leave the liposome. Sodium chloride will ionize into Na+ and Cl- when dissolved into solution. While small in size, the charge of these molecules likewise means that they cannot diffuse across the nonpolar liposome membrane without a channel protein. On the other hand, water is small enough, as well as noncharged, meaning that it can cross the membrane. Distilled water outside of the cell lacks dissolved solutes (i.e., high water concentration), whereas the interior of the liposome has a relatively high concentration of two different solutes (i.e., low water concentration). Water will follow its concentration gradient and move into the liposome.

Which of the following statements is true?

Inside the cell, the quantity of positively charged ions is almost equal to the quantity of negatively charged ions. For the cytoplasm of a cell to avoid being significantly disrupted by electrical forces, the quantity of positively charged ions must be balanced by an almost exactly equal quantity of negatively charged ions. The high concentration of K+ inside is balanced by a variety of negatively charged inorganic and organic ions (anions), including nucleic acids, proteins, and many cell metabolites. Although the electrical charges inside and outside the cell are generally kept in balance, tiny excesses of positive or negative charge, concentrated in the neighborhood of the plasma membrane, do occur. Such electrical imbalances generate a voltage difference across the membrane called the membrane potential. The resting membrane potential is integral to many activities that occur across the plasma membrane.

When Na+ channels are opened in an animal cell, what happens to the membrane potential?

It becomes less negative inside the cell. Na+ channels of cells are usually opened in response to stimulation; the resting membrane potential is associated with an unstimulated cell. When stimulated, Na+ channels open in an animal cell and the membrane potential changes; it becomes less negative inside the cell compared to the resting membrane potential. This is because when Na+ channels are opened, Na+ rushes into the cell. This rapid entry of positive ions makes the membrane potential less negative inside. If this depolarization is sufficiently large, it will cause voltage-gated Na+ channels in the membrane to open transiently at the site.

What is true of the inside of a cell?

It is slightly more negative than the outside of a cell. This uneven charge distribution tends to pull positively charged solutes into the cell and drive negatively charged ones out. In animal cells, for example, the resting membrane potential can be anywhere between -20 and -200 millivolts (mV), depending on the organism and cell type. The value is expressed as a negative number because the interior of the cell is more negatively charged than the exterior.

Which of the following accurately describes the role of the Na+-K+ pump?

It maintains a higher Na+ concentration outside the cell.

To pass through the pore of an ion channel, what must be true of an ion?

It must interact with polar groups in the narrowest part of the channel. Ions must shed their water shells before entering the selectivity filter in the narrowest part of the channel. Ion channels are narrow enough in places to force ions into contact with the channel wall so that only those ions of the appropriate size and charge can pass. For the correct ion to pass through the pore of an ion channel, the ion must interact with polar groups in the narrowest part of the channel. These polar groups can include the regularly spaced oxygen atoms that are part of the polypeptide backbone.

In passive transport, the net movement of a charged solute across the membrane is determined by which of the following?

Its electrochemical gradient, which is a composite of two forces: one due to the concentration gradient and the other due to the membrane potential.

Which ion is generally maintained at a high concentration inside the cell and a low concentration outside the cell?

K+

In most animal cells, which ion can move through "leak" channels?

K+: When leak channels are open, they allow K+ to move freely out of the cell. In a resting cell, these are the main ion channels open in the plasma membrane, rendering the membrane much more permeable to K+ than to other ions. When K+ flows out of the cell—down the concentration gradient generated by the ceaseless operation of the Na+ pump—the loss of positive charge inside the cell creates a voltage difference, or membrane potential.

An extracellular molecule binds to a channel and triggers it to move more often to the open conformation than the closed conformation, as shown in the figure. This is referred to as a ___________ channel.

Ligand

Auditory hair cells in the ear depend on what type of ion channel to detect sound vibrations?

Mechanically-gated: the auditory hair cells in the ear depend on mechanically-gated ion channels to detect sound vibrations. Sound vibrations pull the mechanically-gated channels open, allowing ions to flow into the hair cells. This ion flow sets up an electrical signal that is transmitted from the hair cell to the auditory nerve, which then conveys the signal to the brain.

When the glucose-Na+ symport protein is in its outward-open state, which is more likely to occur?

Na+ binds to its binding site. A glucose-Na+ symport uses the electrochemical Na+ gradient to drive the active import of glucose. When the glucose-Na+ symport protein is in its outward-open state, Na+ binding to its binding site is more likely to occur. Because Na+ concentrations are high outside the cell, Na+ readily binds to the transporter in its outward-open state. The transporter must then wait for a rare glucose molecule to bind. In the outward-open state, the transporter binds to solutes in the extracellular space. The pump oscillates randomly between alternate states. In one state (outward-open), the pump is open to the extracellular space; in another state (inward-open), it is open to the cytosol.

The Na+ pump in the plasma membrane of animal cells uses energy from ATP hydrolysis to pump sodium and potassium ions against their electrochemical gradients. In which direction are the ions pumped across the membrane?

Na+ out and K+ in

Most sports drinks contain both carbohydrates and salts. The carbohydrates replace glucose burned during exercise and the salts replace salts lost in sweat. The salt also helps the small intestine absorb glucose. Pick the answer that accurately describes which salt is most beneficial for glucose absorption.

NaCl, because Na+ is needed for glucose entry. The Na+-K+ ATPase, located in the basal membrane, keeps intracellular sodium low. Glucose is brought into the epithelial cells from the gut lumen, against its concentration gradient, by the action of a glucose-Na+ symport protein.

All other factors (concentration, solute size, etc.) being equal, which type of solute does a cell tend to pull inside?

Positively-charged solutes- The excess of negative charge on the cytosolic side of the plasma membrane tends to pull positively charged solutes into the cell. Specifically, a variety of negatively charged inorganic and organic ions (anions), including nucleic acids, proteins, and many cell metabolites, maintain this relatively constant "positive pull" at the surface of the cell.

What diffuses across lipid bilayers most rapidly?

Small, nonpolar molecules, such as molecular oxygen (O2, molecular mass 32 daltons) and carbon dioxide (CO2, 44 daltons), diffuse rapidly across lipid bilayers. Indeed, cells depend on this permeability to gases for the cell respiration processes.

Which of the following statements is not true regarding active transport by transmembrane pumps?

Some solutes are transported across the membrane in tandem with other molecules, both moving from lower concentration to higher concentration. It is true that some solutes are transported across the membrane in tandem with other molecules, but both do not move from lower concentration to higher concentration. Coupled pumps can link the uphill transport of one solute across a membrane to the downhill transport of another. In this manner, the concentration gradient for one solute is used to drive the mechanism of the pump that moves another solute against its concentration gradient. In terms of other energy sources that drive membrane pumps, some solutes are moved across a membrane against their concentration gradient using energy from sunlight; such movement is facilitated by light-driven pumps. Additionally, some solutes are moved against their concentration gradients, from one side of a membrane to the other, using energy from ATP hydrolysis; such movement is facilitated by ATP-driven pumps.

Which term describes a coupled transporter that moves both solutes in the same direction across a membrane?

Symport

Sodium potassium pump

The Na+-K+ pump binds to Na+ ions on the inside of the liposome and is then phosphorylated by a phosphate from the ATP, which is also inside the liposome. The pump changes conformation in response to phosphorylation, releasing the Na+ on the outside. K+ from the outside of the liposome can then bind to the pump and trigger dephosphorylation. Dephosphorylation of the pump triggers switching of conformation back to the original conformation, leading to a release of K+ inside the liposome. The pump is then ready to repeat the cycle of pumping Na+ out and K+ into the liposome. The Na+ and ATP must be inside the liposome and K+ outside the liposome for pumping of ions to occur. Cl- and GTP are not used or pumped by this transporter.

The epithelial cells that line the gut have glucose-Na+ symport proteins that actively take up glucose from the lumen of the gut after a meal, creating a high glucose concentration in the cytosol. How do these cells release that glucose for use by other tissues in the body?

The cells have glucose uniports in their plasma membrane. Two types of glucose transporters enable gut epithelial cells to transfer glucose across the epithelial lining of the gut. Epithelial cells that have absorbed intestinal glucose release that glucose for use by other tissues in the body through glucose uniports in their plasma membrane. These passive glucose uniports allow glucose to move down its concentration gradient, out of the cell. The image shows this movement of glucose into the connective tissue. The glucose uniport is only found on the basal and lateral regions of the plasma membrane, ensuring it doesn't flow back into the gut lumen.

Your friend now has the pumps successfully pumping ions. She added an equal concentration of both ions to the correct sides of the liposomes along with an excess of the energy source. She is surprised when the pumps stop working after a short time. Which of the following could explain why the transporter stopped pumping ions?

The pump ran out of Na+ to pump because it pumps 3 Na+ out for every 2 K+ pumped in. Three Na+ bind to the pump on the inside of the liposome. This triggers pump phosphorylation and a change in conformation so that three Na+ are released outside the liposome. Two K+ bind the pump from outside the liposome. The pump is dephosphorylated and returns to the beginning conformation and releases the K+ inside the liposome. Since the experiment started with an equal concentration of Na+ inside the liposome to the K+ outside the liposome, the Na+ inside the liposome will run out first. In this case, an excess of ATP was added, so that is not the limiting factor.

What specific event triggers activation of the stereocilia before they activate the auditory neuron?

The stereocilia tilt when pushed against the tectorial membrane and stretch-activated ion channels open, releasing positive ions into the hair cell. The stereocilia found on hair cells contain stretch-activated ion channels on the plasma membrane. The channels are generally closed when the stereocilia are not pushed against the tectorial membrane and tilted. When the stereocilia are tilted, a linking filament on the channel on the first stereocilium pulls on a channel on neighboring stereocilia and opens the ion channel. Positive ions flow into the cell, depolarizing the membrane. This activates the hair cell, which then goes on to activate the auditory nerve.

In bacteria, the transport of many nutrients, including sugars and amino acids, is driven by the electrochemical H+ gradient across the plasma membrane. In E. coli, for example, an H+-lactose symporter mediates the active transport of the sugar lactose into the cell. Given what you know about coupled transport, which is likely true of the H+-lactose symporter?

The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. For a transporter to move solutes in the same direction across a membrane, the solutes must bind to the same conformation of the transporter. The H+-lactose symporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol. In one state, the transporter is open to the extracellular space; in the other, it is open to the cytosol. To transition from one state to the other, the transporter must pass through an "occluded" state in which the transporter is either empty or both solutes are bound. Coupled transporters use the flow of one solute down its electrochemical gradient to drive the transport of a second solute against its electrochemical gradient; therefore, no ATP hydrolysis is needed—the energy source is, in this case, the ion gradient.

Which type of membrane transport protein can perform either passive or active transport?

Transporters- Only transporters can move a solute against its concentration gradient. Some transporters also allow passive transport of molecules down their concentration gradient. A key aspect of active transport is the specificity of the molecule to be transported for its transporter protein. Transporters that accomplish active transport are called pumps.

Determine whether the following statement is true or false: The glucose-Na+ symporter in epithelial cells uses the electrochemical gradient of Na+ to draw glucose into the cell.

True- Because the electrochemical gradient for Na+ is so steep, as Na+ enters the cell, glucose is brought into the cell with it, even when glucose concentrations in the cell are high. For the glucose uniport, conformational changes in a transporter mediate the passive transport of a solute such as glucose. However, if glucose concentrations in the cell are high, then the uniport would not be a sufficient mechanism to keep glucose entering the cell. The glucose-Na+ symporter solves this problem because it will create a constant flow of glucose into the intestinal epithelial cells.

When glucose moves across a phospholipid bilayer by passive transport, which factor determines the direction of its transport?

When glucose moves across a phospholipid bilayer by passive transport, the concentrations of glucose on either side of the membrane determines the direction of its transport. Unlike ions, which move across membranes according to their concentration and membrane potential, glucose is uncharged, so the direction it moves is determined by its concentration gradient alone.

Which of the following would be able to cross a protein-free lipid bilayer most rapidly?

a steroid hormone (nonpolar, large) The lipid bilayer allows nonpolar molecules to pass because they interact favorably with the hydrophobic tails, and it excludes ions and charged molecules from passing through because of their interaction with water. Polar molecules without charge can pass through at a low rate, but are further limited by size constraints.

Which factors determine the force driving the passive transport of charged solutes across the membrane?

electrochemical gradient- Passive transport of charged solutes depends not only on concentration gradient, but also on the charge distribution across the membrane. The combined contributions of concentration gradient and membrane potential are referred to as "electrochemical gradient."

What is responsible for moving glucose from the gut lumen into intestinal epithelial cells?

glucose-sodium symport Glucose is found at higher concentrations inside the cell, so it must be actively transported into the cell. The sodium gradient outside the cell powers the symport that co-transports glucose, along with sodium, into the cell.

What condition must exist for glucose to be transported into a cell using the glucose-Na+ symport?

high Na+ concentration outside the cell- The glucose-Na+ symport transports both Na+ and glucose into the cell. The Na+ is transported down its concentration gradient, releasing energy that is used to transport glucose into the cell against its concentration gradient. The Na+ concentration must be higher outside the cell than inside to transport glucose. The glucose will be transported with Na+ whether the glucose concentration is high or low inside the cell. ATP is not used by the glucose-Na+ symport, although it is used by the Na+-K+ pump, which establishes the Na+ gradient across the membrane.

Ion channels contain a selectivity filter that

selects for ions based on size and charge due to the width of the channel and charge of amino acids lining the channel. Ion channels allow the influx of ions through a pore that contains a region called the selectivity filter. The selectivity filter's width imparts a size restriction on the ions that can pass through, and the charged amino acid residues in this region repel any ions of the wrong charge.

Cells, compared with the extracellular fluid are

slightly negatively charged.

Which of the following inhibits inorganic ions, such as Na+ and Cl-, from passing through a lipid bilayer?

the hydrophobic interior of the lipid bilayer, because ions are charged and the interior of the membrane is very nonpolar. The watery environment on either side of the lipid bilayer is ideal for the solubility of ions because ions will dissolve well in water due to its polar nature. In contrast, ions are repelled by the nonpolar, hydrophobic hydrocarbon tails of the phospholipids that compose the interior of the plasma membrane.