Cell Biology Final Exam Study Set

How is pyruvate imported into the mitochondrial matrix for use in the citric acid cycle? Choose one: A. ATP-driven pyruvate pump B. proton gradient-driven symport C. diffusion through porin complexes in the membrane D. sodium gradient-driven antiport

B. proton gradient-driven symport Pyruvate is transported across the mitochondrial inner membrane using the proton gradient. The transport protein is a symporter, moving protons in the same direction as the pyruvate molecule.

How does an action potential spread along the cell membrane? Choose one: A. Potassium leak channels quickly reverse the action potential to move the membrane depolarization away from the original site. B. The ions entering the cell upon triggering an action potential travel laterally along the membrane to carry the charge. C. A change in membrane potential triggers the opening of nearby voltage-gated sodium channels in a one-way direction. D. Voltage-gated Ca2+ channels are activated by the action potential and the calcium diffuses along the membrane.

C. A change in membrane potential triggers the opening of nearby voltage-gated sodium channels in a one-way direction. An action potential spreads along the cell membrane because the change in membrane potential triggers the opening of nearby Na+ channels, which then change the membrane potential in their local vicinity. This continues down the cell membrane in the direction away from the location of the originally stimulated channels.

Suppose the shaft of ATP synthase that is attached to the rotor were truncated (shortened) such that it no longer extended into the F1 ATPase head. What would be the consequence of this mutation? Choose one: A. ATP synthase would work in reverse, breaking down ATP and pumping protons against their gradient. B. Protons would not cross the membrane using the rotor. C. ATP would not be produced because the conformation of the F1 ATPase head would not be changed. D. The rotor in the membrane would no longer turn.

C. ATP would not be produced because the conformation of the F1 ATPase head would not be changed. As protons flow down their electrochemical gradient through the rotor in the membrane, the gradient energy is converted into a mechanical energy of rotation of the rotor. Rotation of the rotor drives rotation of the attached shaft, which projects into the head of the F1 ATPase subunit. The rotation of the shaft causes conformational changes in the F1 ATPase, driving the synthesis of ATP. If the shaft is truncated and does not reach into the F1 ATPase head, it will not drive the conformational changes and ATP will no longer be synthesized.

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? Choose one: A. Sodium supplies the energy, as it is moving along its concentration gradient. B. Potassium supplies the energy, as it is moving along its concentration gradient. C. A proton gradient in the mitochondria drives the antiport. D. ATP hydrolysis drives the function of the pump.

D. ATP hydrolysis drives the function of the pump. The sodium-potassium pump is an antiport that pumps sodium out and potassium into the cell. This pump moves both ions against their concentration gradient, and ATP supplies the energy for this active transport.

Each molecule of acetyl-CoA entering the citric acid cycle produces two ___________ and four ___________. Choose one: ATP; GTP activated carriers; H2O NADH; ATP CO2; activated carriers

CO2; activated carriers Acetyl CoA is a two-carbon molecule. In the citric acid cycle those carbons are oxidized to two CO2molecules. In the process of acetyl CoA oxidization, four activated carriers are converted to their reduced forms: 3 NADH and 1 FADH2.

Fatty acids can be used to produce energy by conversion to ___________ in the ___________ of the cell. Choose one: NADH; cytosol acetyl CoA; mitochondria lipids; plasma membrane pyruvate; endoplasmic reticulum

acetyl CoA; mitochondria Fatty acids are broken down to acetyl CoA in the mitochondria. This acetyl CoA can then enter the citric acid cycle to produce energy.

Which of the following processes generates the largest number of ATP molecules? Choose one: gluconeogenesis electron transport chain fermentation citric acid cycle glycolysis

electron transport chain For most animal and plant cells, the breakdown of glucose by glycolysis is only a prelude to the third and final stage of the oxidation of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phosphorylation,a process that requires the consumption of oxygen.Glycolysis and the citric acid cycle, which break down glucose into CO2 and water, each produce a small amount of ATP. More importantly, they generate activated carriers, such as NADH and FADH2. These activated carriers donate their high-energy electrons to the electron transport chain in the inner mitochondrial membrane. The movement of these electrons along the chain ultimately drives the synthesis of large amounts of ATP.Fermentation reactions break down sugar molecules in the absence of oxygen. In the process, they produce a small amount of ATP. Aerobic respiration, which requires molecular oxygen, produces a much larger yield of ATP.Gluconeogenesis consumes ATP, using its energy to produce glucose.

After an overnight fast, most of the acetyl CoA entering the citric acid cycle is derived from what type of molecule? Choose one: pyruvate glycogen amino acids glucose fatty acids

fatty acids Fats serve as a major reservoir of fuel in the body. Most animal species possess specialized fat-storing cells called adipocytes. In response to hormonal signals, fatty acids can be released from these depots into the bloodstream for other cells to use as required. Such a need arises after a period of not eating. Even a normal overnight fast results in the mobilization of fat: in the morning, most of the acetyl CoA that enters the citric acid cycle is derived from fatty acids rather than from glucose. After a meal, however, most of the acetyl CoA entering the citric acid cycle comes from glucose derived from food.Fasting cells can also mobilize glucose that has been stored in the form of glycogen. However, fat is a far more important storage material than glycogen, in part because the oxidation of a gram of fat releases about twice as much energy as the oxidation of a gram of glycogen.Amino acids are not used as a form of energy storage. A cell will metabolize its own proteins only under desperate circumstances, not after a regular overnight fast.

Which type of movement is the least common for lipids in a bilayer? 1. rotation 2. lateral diffusion 3. flip-flop 4. flexion

flip-flop Lipids rarely flip-flop between the different faces of the bilayer because the polar heads would have to contact the hydrophobic interior of the membrane. This only happens when catalyzed by transporter proteins.

When food is plentiful, animals can store glucose as what? Choose one: glycogen or starch glucose 6-phosphate starch acetyl CoA glycogen

glycogen In animal cells, glucose is stored in the form of glycogen, a branched polymer of glucose. This large polysaccharide is stored as small granules in the cytoplasm of many animal cells, but mainly in liver and muscle cells. The synthesis and degradation of glycogen occur by separate metabolic pathways, which can be rapidly and coordinately regulated to suit an organism's needs.Plants convert some of the sugars they make through photosynthesis during daylight into fats and into starch, a branched polymer of glucose very similar to animal glycogen.Acetyl CoA is an activated carrier, not a storage molecule. Glucose 6-phosphate is a glycolytic intermediate that, when abundant, can trigger the storage of glucose, but it is not itself a stored form of glucose.

Which portion of a membrane phospholipid faces the outside of the membrane? Choose one: amphipathic portion 1. tail 2. none, because phospholipids are 3. confined to the interior of the membrane 4. head 5. fatty acids

head Phospholipid heads are hydrophilic, so they face the outside of the membrane where water is present (both inside and outside the cell).

What provides the fuel to convert CO2 into sugars in chloroplasts? Choose one: 1. ATP and NADPH generated in the photosynthetic light reactions 2. a proton gradient across a membrane 3. Nothing; the reactions do not require energy. 4. oxidation of food molecules 5. ATP generated by cell respiration

1. ATP and NADPH generated in the photosynthetic light reactions The reactions of the carbon-fixation cycle take place in the chloroplast stroma and do not require a membrane. The energy and reducing power needed for these reactions come from the ATP and NADPH produced by the photosynthetic light reactions.Because the chloroplast's inner membrane is impermeable to ATP and NADPH, these carriers cannot actually leave the chloroplast. Instead, they are funneled into the carbon-fixation cycle, where they are used to make sugars. The ATP produced by oxidative phosphorylation in mitochondria cannot enter chloroplasts but is used instead to provide fuel for other cellular reactions.

How do fermentation reactions in oxygen-starved muscle cells and anaerobically grown yeast cells differ? Choose one: 1. Fermentation in muscle cells produces lactate and in yeast produces ethanol plus CO2. 2. Fermentation in muscle cells produces ethanol plus CO2 and in yeast produces lactate. 3. Fermentation in muscle cells regenerates NAD+ and in yeast regenerates glucose. 4. Fermentation in muscle cells generates NAD+ and in yeast generates NADH. 5. Fermentation in yeast cells includes glycolysis, whereas in muscle cells it bypasses glycolysis.

1. Fermentation in muscle cells produces lactate and in yeast produces ethanol plus CO2. For most animal and plant cells, glycolysis is only a prelude to the third and final stage of the breakdown of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phosphorylation, a process that requires oxygen. However, for many anaerobic microorganisms, which can grow and divide in the absence of oxygen, glycolysis is the principal source of ATP. Certain animal cells also rely on ATP produced by glycolysis when oxygen levels fall, such as in skeletal muscle cells during vigorous exercise.Such energy-yielding pathways, which break down sugar in the absence of oxygen, are called fermentations. In these anaerobic conditions, fermentation converts the pyruvate made by glycolysis into products that can be excreted from the cell: lactate in muscle cells, for example, or ethanol and CO2 in the yeast cells used in brewing and breadmaking. In both processes, the NADH gives up its electrons and is converted back to the NAD+ required to maintain the reactions of glycolysis.

Most of the energy for the synthesis of ATP comes from which molecule? Choose one: 1. NADH produced by the citric acid cycle 2. NADH produced by the conversion of pyruvate to acetyl CoA 3. GTP produced by the citric acid cycle 4. FADH2 produced by the citric acid cycle 5. NADH produced by glycolysis

1. NADH produced by the citric acid cycle Much of the energy carried by NADH and FADH2 is ultimately converted into the bond energy of ATP. The six NADH molecules produced in the mitochondrial matrix during the citric acid cycle (three per turn; two turns per original molecule of glucose oxidized) pass their high-energy electrons to the NADH dehydrogenase complex—the first complex in the chain. As the electrons pass from one enzyme complex to the next, they promote the pumping of protons across the inner mitochondrial membrane. In this way, each NADH molecule provides enough net energy to generate about 2.5 molecules of ATP--for a total of 15.FADH2 molecules, of which two are produced by two turns of the citric acid cycle, pass their electrons to a membrane-embedded mobile carrier further down the respiratory chain than do NADH; they thus promote the pumping of fewer protons and provide enough energy for only 1.5 molecules of ATP each.

What does it mean for a bond to be "high energy," such as the bonds between phosphate groups in ATP? Choose one: 1. The hydrolysis of the bond is energetically favorable. 2. The bond is strong and difficult to break. 3. The hydrolysis of the bond is energetically unfavorable. 4. The bond involves extra electrons. 5. The bond is strong but breaks easily.

1. The hydrolysis of the bond is energetically favorable. The bond is not particularly strong or hard to break, and it is chemically identical to other phosphate bonds. However, due to the chemical configuration of the molecule, and conditions inside the cell, hydrolysis of a high-energy phosphate bond releases a large amount of energy.The two outermost phosphate groups in ATP are held to the rest of the molecule by "high-energy" phosphoanhydride bonds and are readily transferred to other organic molecules—or to water. A hydrolysis reaction is essentially the transfer of the phosphate group to water, yielding ADP and inorganic phosphate (Pi).The large negative ΔGº of the ATP hydrolysis reaction arises from a number of factors. Release of the terminal phosphate group removes an unfavorable repulsion between adjacent negative charges; in addition, the inorganic phosphate ion (Pi) that is released is stabilized by favorable hydrogen-bond formation with water.Although the ΔGº of this reaction is -30.5 kJ/mole, its ΔG inside cells is much more negative because the ratio of ATP to the products ADP and Pi is kept so high. Inside a cell, this hydrolysis of the terminal phosphate of ATP yields between 46 and54 kJ/mole of usable energy.

The electron-transport chain pumps protons in which direction? Choose one: 1. from the matrix to the intermembrane space 2. from the matrix to the cytosol 3. from the intermembrane space to the cytosol 4. from the cytosol to the intermembrane space 5. from the intermembrane space to the matrix

1. from the matrix to the intermembrane space Each of the respiratory enzyme complexes in the electron-transport chain uses the energy released by the energetically favorable transfer of electrons from NADH to O2 to pump protons across the inner mitochondrial membrane, from the matrix into the intermembrane space. This proton pumping generates an H+ gradient—or pH gradient—across the inner membrane. As a result, the pH in the matrix (around 7.9) is about 0.7 units higher than it is in the intermembrane space (which is 7.2, the same pH as the cytosol). It also produces a voltage gradient—or membrane potential—across the inner membrane. This combined electrochemical gradient represents a form of stored energy.

When ATP and food molecules such as fatty acids are abundant, which will occur? Choose one: 1. When food molecules are plentiful, neither glycolysis nor gluconeogenesis will occur. 2. Enzymes involved in gluconeogenesis will use energy to produce glucose. 3. Enzymes involved in glycolysis will break down glucose to generate pyruvate. 4. When food and ATP are plentiful, both glycolysis and gluconeogenesis will occur. 5. Enzymes involved in glycolysis will operate in the reverse direction, using pyruvate to produce glucose.

2. Enzymes involved in gluconeogenesis will use energy to produce glucose. When energy and food molecules are plentiful, glycolysis is shut down. Under these conditions, the cell does not need to continue to break down glucose to generate energy. Thus, key enzymes in the glycolytic pathway tend to be inhibited by ATP and other metabolites.However, enzymes in the gluconeogenesis pathway tend to be regulated in the opposite direction. For example, the products of ATP hydrolysis—ADP and AMP—indicate that energy reserves are low. These molecules stimulate glycolysis. At the same time, AMP inhibits gluconeogenesis. Thus, when ATP is abundant, gluconeogenesis is favored.Under these conditions, cells produce glucose that can be converted into glycogen and stored for later use, when energy is depleted and food is scarce.Neither glycolysis nor gluconeogenesis can operate in reverse. Both pathways include enzymes whose reactions proceed in one direction so strongly that they are effectively irreversible.Because the activities of these enzymes are regulated by similar metabolites but in opposite directions, both pathways would not be inactivated under the same conditions. Nor will they ever proceed simultaneously. To do so would be hugely wasteful, as it would allow cells to synthesize and break down metabolites in a futile cycle, consuming energy for no reason.

What happens when ATP synthase operates "in reverse" and pumps H+ across a membrane against its electrochemical proton gradient? Choose one: 1. Nutrients are transported in the opposite direction across the membrane. 2. ATP is synthesized from ADP and Pi. 3. ATP is hydrolyzed to form ADP and Pi. 4. ATP is co-transported across the membrane. 5. Na+ is transported in the opposite direction across the membrane.

3. ATP is hydrolyzed to form ADP and Pi. ATP synthase can either synthesize ATP by harnessing the electrochemical H+ gradient or pump protons against this gradient by hydrolyzing ATP. The direction of operation (and of rotation) at any given instant depends on the net free-energy change (ΔG) for the coupled processes of H+ translocation across the membrane and the synthesis of ATP from ADP and Pi. For example, if the electrochemical proton gradient falls below a certain level, the ΔG for H+ transport into the matrix will no longer be large enough to drive ATP production; instead, ATP will be hydrolyzed by the ATP synthase to rebuild the proton gradient.

Which of the following is true for eukaryotic cells? Choose one: 1. Fats are converted to acetyl CoA, but sugars are not. 2. Sugars are converted to acetyl CoA in the mitochondria; fats are converted to acetyl CoA in the cytosol. 3. Fats are converted to acetyl CoA in the mitochondria; sugars are converted to acetyl CoA in the cytosol. 4. Sugars and fats are both converted to acetyl CoA in the mitochondria. 5. Sugars are converted to acetyl CoA, but fats are not.

4. Sugars and fats are both converted to acetyl CoA in the mitochondria. In aerobic metabolism in eukaryotic cells, the pyruvate produced by glycolysis is actively pumped into the mitochondrial matrix. There, it is rapidly decarboxylated by a giant complex of enzymes, called the pyruvate dehydrogenase complex. The products of this set of reactions are CO2 (a waste product), NADH, and acetyl CoA.In addition to sugar, which is broken down during glycolysis, fat is a major source of energy for most nonphotosynthetic organisms, including humans. Like the pyruvate derived from glycolysis, the fatty acids derived from fat are also converted into acetyl CoA in the mitochondrial matrix.Some amino acids are also transported from the cytosol into the mitochondrial matrix, where they can be converted into acetyl CoA or one of the other intermediates of the citric acid cycle.Thus, in the eukaryotic cell, the mitochondrion represents the center toward which all energy-yielding catabolic processes lead, whether they begin with sugars, fats, or proteins.

When scientists were first studying the fluidity of membranes, they did an experiment using hybrid cells. Certain membrane proteins in a human cell and a mouse cell were labeled using antibodies coupled with differently colored fluorescent tags. The two cells were then coaxed into fusing, resulting in the formation of a single, double-sized hybrid cell. Using fluorescence microscopy, the scientists then tracked the distribution of the labeled proteins in the hybrid cell.Which best describes the results they saw and what they ultimately concluded?Choose one: 1. Initially, the mouse and human proteins intermixed, but over time, they were able to resegregate into distinct membrane domains. This suggests that cells can restrict the movement of membrane proteins. 2. Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins recombined such that they all fluoresced with a single, intermediate color. 3. Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer. 4. The mouse and human proteins remained confined to the portion of the plasma membrane that derived from their original cell type. This suggests that cells can restrict the movement of their membrane proteins to establish cell-specific functional domains. 5. The mouse and human proteins began to intermix and spread across the surface of the hybrid cell, but over time, one set of proteins became dominant and the other set was lost. This suggests that cells can ingest and destroy foreign proteins. 6. At first, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became divided such that half faced the cytosol and half faced the hybrid cell exterior. This suggests that flippases are activated by cell fusion.

3. Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer. Because a membrane is a two-dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the bilayer. This lateral diffusion was initially demonstrated by experimentally fusing a mouse cell with a human cell to form a large, hybrid cell and then monitoring the distribution of certain mouse and human plasma membrane proteins. At first, the mouse and human proteins are confined to their own halves of the newly formed hybrid cell, but within half an hour or so, the two sets of proteins become evenly mixed over the entire cell surface, as shown below. To monitor the movement of the selected proteins, the cells were labeled with antibodies that bind to either human or mouse proteins; these antibodies are coupled to two different fluorescent tags—shown here in red and blue—so that the proteins to which the antibodies bind can be distinguished in a fluorescence microscope.

About how many molecules of ATP are produced by the complete oxidation of glucose to H2O and CO2? Choose one: 2 ATPs from glycolysis + 1 GTP from the citric acid cycle 2 4 30 3

30 Glucose is oxidized by glycolysis and the citric acid cycle. A net of two molecules of ATP are produced by glycolysis for every molecule of glucose that enters that pathway. (Although four molecules are produced, two are consumed during the early steps of the pathway to prepare the sugars for the energy-releasing reactions to come.) Additionally, a related molecule, GTP, is produced by the citric acid cycle.However, both of these pathways produce, in addition, activated carriers of electrons, including NADH and FADH2.These activated carriers then donate their electrons to the electron transport chain. The movement of electrons along the electron transport chain releases energy that is harnessed to produce additional ATP. Thus, the operation of the electron transport chain ultimately powers the production of approximately 30 ATP molecules for every molecule of glucose that is oxidized.

The drug 2,4-dinitrophenol (DNP) makes the mitochondrial inner membrane permeable to H+. The resulting disruption of the proton gradient inhibits the mitochondrial production of ATP.What additional effect would DNP have on the transport of ATP out of the mitochondrial matrix? Choose one: 1. ATP transport will increase because ATP synthase will be forced to operate in the "reverse" direction. 2. ATP transport will decrease because less ATP will be available to diffuse across the inner membrane. 3. None, because ATP export is not coupled to the movement of protons across the inner membrane. 4. ATP export will decrease because its carrier exploits the difference in voltage across the inner membrane. 5. None, because the inner membrane is permeable to ATP.

4. ATP export will decrease because its carrier exploits the difference in voltage across the inner membrane. The synthesis of ATP is not the only process driven by the proton gradient in mitochondria. Some molecules, such as pyruvate and Pi, are transported into the mitochondrial matrix along with the protons as they move down their electrochemical gradient.Although the transport of ATP out of the matrix is not directly coupled to the flow of protons, it does rely on an intact proton gradient. In this case, the special antiport carrier protein that exports ATP out of the matrix and brings ADP exploits the voltage difference generated by the electrochemical proton gradient. Because the matrix side of the membrane is more negatively charged than the side that faces the intermembrane space, the movement of ATP (which is more negatively charged than ADP) out of the matrix is energetically favorable.

For what reason is cytochrome c oxidase able to pump protons across the inner mitochondrial membrane? Choose one: 1. The complex has a high affinity for protons. 2. Cytochrome c binding alters the conformation of the complex. 3. The complex has a low affinity for protons. 4. Electron transport drives a conformational change in the protein complex. 5. Oxygen is co-transported along with the protons.

4. Electron transport drives a conformational change in the protein complex. Proton pumping occurs because the transfer of electrons drives allosteric changes in the conformation of cytochrome c oxidase that cause protons to be ejected from the mitochondrial matrix.The complex cycles through a series of three conformations. In one of these conformations, the protein has a high affinity for H+, causing it to pick up an H+ on the matrix side of the membrane. In another conformation, the protein has a low affinity for H+, causing it to release an H+ on the other side of the membrane. The cycle proceeds only in one direction—releasing the proton into the intermembrane space—because one of the steps is driven by allosteric changes in conformation that are driven by the energetically favorable transport of electrons.The same mechanism is thought to be used by the NADH dehydrogenase complex and by many other proton pumps.

What is true of the antenna complex of a photosystem? Choose one: 1. It converts light energy into chemical energy. 2. It includes a special pair of chlorophylls. 3. It extracts electrons from water. 4. It captures light energy. 5. It passes electrons to a photosynthetic electron transport chain.

4. It captures light energy. Each photosystem consists of a set of antenna complexes, which capture light energy, and a reaction center, which converts that light energy into chemical energy. In an antenna complex, hundreds of chlorophyll molecules are arranged so that the light energy captured by one chlorophyll molecule can be transferred to a neighboring chlorophyll molecule in the network. In this way, energy jumps randomly from one chlorophyll molecule to the next—either within the same antenna or in an adjacent antenna. At some point, this wandering energy will be passed to a chlorophyll dimer called the special pair, located in the reaction center. The special pair holds its electrons at a slightly lower energy than do the other chlorophyll molecules, so when energy is accepted by this special pair, it becomes effectively trapped there.

How do enzymes maximize the energy harvested from the oxidation of food molecules? Choose one: 1. They allow what would otherwise be an energetically unfavorable oxidation reaction to occur. 2. They allow oxidation reactions to take place without an input of activation energy. 3. They allow a larger amount of energy to be released from food molecules such as glucose. 4. They allow the stepwise oxidation of food molecules, which releases energy in small amounts. 5. They guarantee that each reaction involved in the oxidation of food molecules proceeds in just one direction.

4. They allow the stepwise oxidation of food molecules, which releases energy in small amounts. If food molecules, such as glucose, were oxidized by burning in a fire, the energy contained in those molecules would be released all at once. This quantity is too large to be captured by any carrier molecule, so all of this energy would be released as heat. In a cell, enzymes catalyze the breakdown of sugars via a series of small steps, in which a portion of the free energy released is captured by the formation of activated carriers—most often ATP and NADH. These activated carriers can then be made available to do useful work for the cell.Enzymes lower the activation energy barrier that must be surmounted by the random collision of molecules to allow these reactions. However, an initial input of energy is still required even for enzyme-catalyzed reactions.The complete oxidation of glucose to CO2 and H2O is energetically highly favorable. The ΔG° of the reaction is -2880 kJ/mole. Thus, the total free energy released by the complete oxidative breakdown of glucose is 2880 kJ/mole. This amount of energy is exactly the same for the enzyme-catalyzed reactions or for the direct burning of the sugar in a fire.Although the overall oxidation of glucose is energetically favorable, many of the reactions involved in the process are readily reversible and can occur in either direction, depending on the relative concentrations of the participating molecules.

Following an action potential, a nerve cell goes through a brief refractory period during which it cannot be stimulated. What is true during this refractory period? Choose one: 1. Voltage-gated Ca2+ channels are open. 2. Voltage-gated Na+ channels in the nerve cell membrane are open. 3. The membrane potential remains unchanged. 4. Voltage-gated Na+ channels in the nerve cell membrane are inactivated. 5. Voltage-gated K+ channels in the nerve cell membrane are inactivated.

4. Voltage-gated Na+ channels in the nerve cell membrane are inactivated. Following an action potential, a nerve cell goes through a brief refractory period during which it cannot be stimulated. In this refractory period, voltage-gated Na+ channels in the nerve cell membrane are inactivated. This temporary inactivation prevents the advancing front of membrane depolarization from spreading backward along the nerve cell axon. During the refractory period, membrane potential begins to return to the negative resting membrane potential and voltage-gated K+ channels stay open as long as the membrane is depolarized. The figure below provides a graphical representation of how the refractory period ensures unidirectional movement of an action potential.

In photosynthesis, what drives the generation of ATP by ATP synthase? Choose one: 1. the phosphorylation of ATP synthase 2. the transfer of high-energy electrons to ATP synthase 3. the generation of a charge separation in the photosynthetic reaction center 4. a proton gradient across the thylakoid membrane 5. the absorption of light by a photosynthetic reaction center

4. a proton gradient across the thylakoid membrane Chemiosmotic coupling drives ATP production during oxidative phosphorylation and photosynthesis. Both membrane-based processes make use of the energy stored in a proton gradient to synthesize ATP.During photosynthesis, energy captured by a chlorophyll-containing photosystem excites electrons, which are then passed on to a photosynthetic electron-transport chain. One of the electron carriers in this chain, called the cytochrome b6-f complex, uses some of this energy to pump protons across the thylakoid membrane. This carrier resembles the cytochrome c reductase complex of mitochondria.As in mitochondria, an ATP synthase embedded in the membrane then uses the energy of the electrochemical proton gradient to produce ATP.

Once excited by sunlight, chlorophylls in the antenna complex do which of the following? Choose one: 1. release that energy as heat 2. reflect the energy as light 3. transfer electrons to the chlorophyll special pair 4. transfer the energy to the chlorophyll special pair 5. transfer protons to the chlorophyll special pair

4. transfer the energy to the chlorophyll special pair In an antenna complex, hundreds of chlorophyll molecules are arranged so that the light energy captured by one chlorophyll molecule can be transferred to a neighboring chlorophyll molecule in the network. In this way, energy jumps randomly from one chlorophyll molecule to the next—either within the same antenna or in an adjacent antenna. When this energy is passed to the chlorophyll special pair in the reaction center, which holds its electrons at a slightly lower energy than do the other chlorophyll, it becomes effectively trapped there.The trapped energy excites an electron within the special pair, and this high-energy electron is passed to a set of electron carriers that ferry it to the electron-transport chain. This electron transfer converts the light energy that entered the special pair into the chemical energy of a transferable electron.

What does the pyruvate dehydrogenase complex do? Choose one: 1. It converts pyruvate into acetyl CoA in the cytosol. 2. It produces pyruvate in the cytosol. 3. It completes the oxidation of pyruvate to CO2 in the mitochondrial matrix. 4. It regenerates NAD+ in the mitochondrial matrix. 5. It converts pyruvate into acetyl CoA in the mitochondrial matrix.

5. It converts pyruvate into acetyl CoA in the mitochondrial matrix. In aerobic metabolism in eukaryotic cells, the pyruvate produced by glycolysis is actively pumped into the mitochondrial matrix. There, it is rapidly decarboxylated by a giant complex of three enzymes, called the pyruvate dehydrogenase complex. The pyruvate dehydrogenase complex contains multiple copies of three enzymes—pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. This enzyme complex removes a CO2 from pyruvate to generate NADH and acetyl CoA. The latter is produced when the acetyl group derived from pyruvate is linked to coenzyme A (CoA).In the next stage in cell respiration, the acetyl group in acetyl CoA is donated to the citric acid cycle in the mitochondrial matrix, where it will be fully oxidized to CO2.The action of the pyruvate dehydrogenase complex produces NADH; thus, it consumes NAD+rather than regenerating it. The enzyme complex, which operates in the mitochondrial matrix, oxidizes pyruvate, producing acetyl CoA and CO2. The complete oxidation of the remaining acetyl carbons to CO2 takes place during the citric acid cycle.

In eukaryotic cells, why must metabolism be tightly regulated? Choose one: 1. Anabolic and catabolic pathways must compete for oxygen. 2. The substrates involved in metabolic reactions are each recognized by only a single, unique enzyme. 3. Anabolic and catabolic pathways must compete for scarce resources. 4. All metabolic reactions require energy. 5. The substrates involved in metabolic reactions can be used by a number of different enzymes.

5. The substrates involved in metabolic reactions can be used by a number of different enzymes. The various products and intermediates of metabolic reactions are often substrates for multiple enzymes. Recall, for example, the number of intermediates from the citric acid cycle that can act as precursors for many important organic molecules, including amino acids, nucleic acids, sugars, and lipids.Thus, depending on conditions, a cell must decide whether to route key metabolites into anabolic or catabolic pathways—in other words, whether to use them to build other molecules or burn them to provide immediate energy.To balance the activities of these competing yet interrelated reactions—and to allow organisms to adapt swiftly to changes in food availability or energy expenditure—an elaborate network of control mechanisms regulates and coordinates the activity of the enzymes that catalyze the myriad metabolic reactions that go on in a cell.

How much of the energy that could, in theory, be derived from the breakdown of glucose or fatty acids to H2O and CO2 is captured and used to drive the energetically unfavorable synthesis of ATP? Choose one: 20% 2% 50% 98%

50% Through the production of ATP, the energy derived from the breakdown of sugars and fats is redistributed into packets of chemical energy in a form convenient for use in the cell. In total, nearly 50% of the energy that could, in theory, be derived from the breakdown of glucose or fatty acids to H2O and CO2 is captured and used to drive the energetically unfavorable reaction ADP + Pi → ATP. The remaining energy is released as heat, which in animals helps to keep the body warm. By contrast, a modern combustion engine, such as that of a car, can convert no more than 20% of the available energy in its fuel into useful work.

The pH of the mitochondrial matrix is ___________, which is ___________ than that of the intermembrane space. Choose one: 7.9; higher 7.2; higher 7.2; lower 7.9; lower

7.9; higher The pH of the mitochondrial matrix is 7.9, whereas the pH of the intermembrane space is pH 7.2. The higher pH of the matrix is due to the protons being pumped out into the intermembrane space.

Which of the following characteristics of aquaporins ensure that the channel selectively transports only water molecules and not other solutes? Choose one or more: A. The channel has a narrow pore that is only wide enough for a single water molecule to pass through. B. The channel undergoes conformational changes to push water through the channel. C. A glutamate at the entrance to the channel prevents positive ions from entering the channel. D. Two asparagines in the center of the pore prevent protons from passing through the channel.

A and D Aquaporin, like other channels, is specific for one substrate: water. The aquaporin channel is narrow, so only a single water molecule can pass through at one time. The channel is also lined with two asparagine side chains that function as selectivity filters to block protons from moving through the channel. Together, the size of the channel and the two asparagines function to make the channel specific for water. Water will move through the channel down the concentration gradient.

How do transporters and channels select which solutes they help move across the membrane? A. 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. B. Channels allow the passage of solutes that are electrically charged; transporters facilitate the passage of molecules that are uncharged. C. Both channels and transporters discriminate between solutes mainly on the basis of size and electric charge. D. Channels will allow the passage of any solute as long as it has an electrical charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. E. Transporters discriminate between solutes mainly on the basis of size and electric charge; channels bind their solutes with great specificity in the same way an enzyme binds its substrate.

A. 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. Channels discriminate mainly on the basis of size and electric charge: when the channel is open, only ions of an appropriate size and charge can pass through. 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.

When Na+ channels are opened in an animal cell, what happens to the membrane potential? Choose one: A. It becomes less negative inside the cell. B. It disappears, and membrane potential stabilizes at 0 mV. C. It stays the same. D. It rapidly reaches the resting membrane potential. E. It becomes more negative inside the cell.

A. 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.

Antimycin A is a piscicide (fish poison) used to manage fisheries and kill invasive species. Antimycin A blocks the transfer of electrons through the cytochrome b-c1 complex. What components of the electron transport chain are bound to high-energy electrons after treating a mitochondrion with antimycin A? Choose one: A. NADH and the NADH dehydrogenase complex are bound to high-energy electrons while O2 and the cytochrome c oxidase complex are not. B. None of the complexes are bound to high-energy electrons. C. O2 and the cytochrome c oxidase complex are bound to high-energy electrons while NADH and the NADH dehydrogenase complex are not. D. All three complexes and NADH are bound to high-energy electrons.

A. NADH and the NADH dehydrogenase complex are bound to high-energy electrons while O2 and the cytochrome c oxidase complex are not. The electron transport chain will be blocked at complex III (cytochrome b-c1), so any complexes after this point (cytochrome c oxidase) will not become reduced or bound to the high-energy electrons. O2 will also remain oxidized as O2 and not reduced to H2O. The complex before complex III (NADH dehydrogenase) will still be reduced and bound to high-energy electrons. The NADH will remain as reduced NADH since it will no longer be able to continue to pass electrons to complex I because complex I is already bound to electrons.

What is typically true of ion channels? Choose one: A. They are gated. B. They operate by active transport. C. They hydrolyze ATP. D. They are open all the time. E. They are nonselective.

A. They are gated. Selective ion channels are not open continuously; they open briefly and then close again. This is referred to as a "gated" channel because the flow of ions can happen only when the channel is in the proper conformation. For most of these ion channels, a specific stimulus triggers them to open. When open, these ion channels facilitate passive diffusion, allowing solutes to move down their electrochemical gradient. This process does not require an input of energy.

Why do cells lack membrane transport proteins that are specific for the movement of O2? A. because oxygen dissolves readily in lipid bilayers B. because oxygen, dissolved in water, can enter cells via aquaporins C. because transport of oxygen across cell membranes is energetically unfavorable D. because oxygen concentrations must be kept low inside cells to avoid creating reactive superoxide radicals that can damage DNA and proteins E. because oxygen is transported in and out of the cell by special oxygen-binding proteins such as hemoglobin

A. because oxygen dissolves readily in lipid bilayers Cells lack membrane transport proteins that are specific for the movement of O2 because oxygen dissolves readily in lipid bilayers. This small, nonpolar molecule can diffuse across the cell membrane without the need for a membrane transport protein. The channels of aquaporins are lined with amino acids that provide an environment for the formation of transient hydrogen bonds that facilitate the passage of water molecules, which line up in single file. The channels of aquaporins exclude ions and most other molecules, including O2.

Which of the following form tiny hydrophilic pores in the membrane through which solutes can pass by diffusion? Choose one: A. channels B. transporters C. liposomes D. anions E. pumps

A. 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.

What condition must exist for glucose to be transported into a cell using the glucose-Na+ symport? Choose one: A. high Na+ concentration outside the cell B. high glucose concentration outside the cell C. high Na+ concentration inside the cell D. high ATP concentration inside the cell for phosphorylation of the glucose-Na+ symport

A. 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.

A group of researchers wanted to sort different white blood cell types (monocytes, lymphocytes, and granulocytes) apart from each other based on size differences and to remove unwanted contaminating red blood cells. After a particular manipulation, the red blood cells lysed. The remaining white blood cells increased in size and, more importantly, the size differences among cells increased, allowing for size-based sorting (which requires minimum size differences among cells). What manipulation did the researchers use to increase cell size? A. placing cells in an environment with a lower solute concentration than that in the cells B. placing cells in an environment with lower temperatures than the cells were previously exposed to C. patch-clamp recording to monitor ion channel activity A. placing cells in an environment with a lower solute concentration than that in the cellsD. placing cells in an environment with a higher solute concentration than that in the cells

A. placing cells in an environment with a lower solute concentration than that in the cells A difference in solute concentrations on either side of a membrane leads to osmosis, the passive movement of water across a membrane from a region of low solute concentration (where the water concentration is high) to a region of high solute concentration (where the water concentration is low). In their article "Exploiting osmosis for blood cell sorting," the researchers suggest that after exposure to deionized water, different cell populations swell at different rates due to the relative abundance of aquaporins.

Which of the following activities helps restore the ion gradients across the plasma membrane of an axon after an action potential has occurred? Choose one: A. the action of Na+ pumps B. the activity of K+ leak channels C. the closing of voltage-gated K+ channels D. the opening of voltage-gated Na+ channels

A. the action of Na+ pumps The opening of voltage-gated Na+ channels allows Na+ to rush into the cell during an action potential. The opposite is needed for the restoration of the resting membrane potential. That is, the action of Na+ pumps helps to restore the ion gradients across the plasma membrane of an axon after an action potential has occurred. About 20% of the energy generated by the metabolism of food is used to operate the Na+ pumps that restore the balance of ions following action potentials.

What is gluconeogenesis? Choose one: A. the synthesis of glucose from pyruvate B. the mobilization of glucose from glycogen stores C. the blockage of glycolysis at the first step D. the production of glucose from starch

A. the synthesis of glucose from pyruvate Gluconeogenesis is the reversal of glycolysis—the synthesis of glucose from pyruvate. This process allows glucose to be remade if it becomes scarce.

Most of the energy released by oxidizing glucose is saved in the high-energy bonds of what molecules? Choose one: ADP and other activated carriers H2O and CO2 GDP and other activated carriers O2 ATP and other activated carriers

ATP and other activated carriers Much of the energy released by an energetically favorable reaction, such as the oxidation of a food molecule, must be stored temporarily before it can be used by cells to fuel energetically unfavorable reactions, such as the synthesis of all the other molecules needed by the cell. In most cases, the energy is stored as chemical bond energy in a set of activated carriers, small organic molecules that contain one or more energy-rich covalent bonds. Activated carriers store energy in an easily exchangeable form, either as a readily transferable chemical group or as readily transferable ("high-energy") electrons. The most important activated carriers are ATP and two molecules that are close chemical cousins, NADH and NADPH.

Why would a cell express the aquaporin protein if water can cross the membrane in the absence of aquaporin? Choose one: A. Aquaporin moves a positively charged ion along with water across the membrane. B. Aquaporin facilitates the faster movement of water molecules across the membrane. C. Water molecules cannot cross the membrane in the absence of a pore like aquaporin. D. Aquaporin limits the movement of water molecules so they do not move too quickly across the membrane.

B. Aquaporin facilitates the faster movement of water molecules across the membrane. Only small numbers of water molecules can diffuse across a membrane in a given amount of time. Adding aquaporins to a membrane facilitates quicker movement of water across a membrane. Aquaporins contribute to cellular function in certain tissues, such as the epithelial cells of the kidney where the flow of water into cells is particularly important.

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? Choose one: A. A leaky Na+ channel would not affect glucose transport because these two transporters are unrelated. B. Glucose transport would slow because the Na+ gradient is dissipated by the Na+ channel. C. Na+ transport would slow, but glucose transport would remain high because glucose could still be transported by the glucose-Na+ symport. D. Glucose transport would increase because the Na+ gradient is strengthened by the Na+ channel.

B. 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.

When voltage-gated Na+ channels in a nerve cell open, what happens to the axonal membrane? Choose one: A. The membrane potential rises to 0 mV and stays there. B. It depolarizes. C. The nerve cell becomes more negatively charged inside than outside. D. It becomes electrically charged. E. No change in the membrane potential occurs.

B. It depolarizes. Even when the cell is resting, its membrane is "electrically charged" in that there is a voltage difference across the membrane—this is referred to as the resting membrane potential. The entry of Na+ ions makes the nerve cell less negatively charged inside and ultimately positively charged. This is called depolarization and it takes place along the axonal membrane when voltage-gated Na+ channels in the nerve cell open. The wave of depolarization is also called an action potential and it travels very quickly, allowing the nervous system to respond as quickly as it does.

Which of the following accurately describes the role of the Na+-K+ pump? Choose one: A. It equilibrates the concentrations of Na+ and K+ across the plasma membrane. B. It maintains a higher Na+ concentration outside the cell. C. It maintains a lower Na+ concentration outside the cell. D. It maintains a higher K+ concentration outside the cell.

B. It maintains a higher Na+ concentration outside the cell. The Na+-K+ pump uses energy from the hydrolysis of ATP to establish a strong electrochemical gradient of Na+ and K+ ions, with Na+ ions pumped to the extracellular space and K+ ions pumped to the cytosol. Establishing these electrochemical gradients is such a fundamental process in cells that about one-third of the ATP consumed by the cell is used by this pump. In addition to ATP hydrolysis, the function of the pump requires a carefully orchestrated set of conformational changes to operate and ensures that only Na+ and K+ ions are moved across the membrane by the pump.

During the activation of a neuron, the action potential propagates in only one direction. How is this achieved in the neuron? Choose one: A. The Na+ channel becomes permanently inactivated after the action potential passes. B. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes. C. The Na+ channel remains open during the action potential and then rapidly returns to the closed state after the action potential passes. D. The Na+ channel closes during the action potential and then rapidly returns to the open state after the action potential passes.

B. The Na+ channel becomes inactivated and refractory to reopening for a short time after the action potential passes. During an action potential, the Na+ channels along the axon open in response to membrane depolarization. As Na+ rushes into the cell, the membrane is further depolarized, causing the Na+ channel to close and become inactive. In the inactive state, the Na+ channel is refractory to opening and therefore remains closed. The channels return to the closed state after a few milliseconds. They are then able to respond to another action potential. The closed, inactive state of the Na+ channel prevents the action potential from moving backward toward the cell body of the neuron.

Which of the following requires an input of energy to occur? Choose one: A. the movement of a solute from a region of higher concentration on one side of a membrane to a region of lower concentration on the other side B. The movement of a solute from a region of lower concentration on one side of a membrane to a region of higher concentration on the other side C. Both of these options require energy investment because diffusion is a change in a system, and any change requires energy.

B. The movement of a solute from a region of lower concentration on one side of a membrane to a region of higher concentration on the other side Active transport occurs when a solute from a region of lower concentration on one side of a membrane moves to a region of higher concentration on the other side of the membrane. This action requires an input of energy to occur. Passive transport describes the movement of a solute from a region of higher concentration on one side of a membrane to a region of lower concentration on the other side, and does not require an energy investment.

How do the high-energy electrons of activated carriers contribute to forming the high-energy phosphate bonds of ATP? Choose one: A. They are passed to ATP synthase to power ATP synthesis. B. They are used by the electron-transport chain to make a proton gradient. C. They are pumped across the membrane to form an electron gradient. D. They are transferred directly to ADP to form ATP.

B. They are used by the electron-transport chain to make a proton gradient. Activated carriers transfer their high-energy electrons to the electron-transport chain to form a proton gradient across the mitochondrial inner membrane. The proton gradient is then used to power ATP synthesis as they flow down their electrochemical gradient through ATP synthase.

Auditory hair cells in the ear depend on what type of ion channel to detect sound vibrations? Choose one: A. voltage-gated B. mechanically-gated C. ligand-gated

B. mechanically-gated As shown in the figure, 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.

The ethanol in wine and beer is produced from metabolic reactions carried out by the yeast Saccharomyces cerevisiae. Since it is of great commercial value, researchers have studied factors that influence ethanol production. To maximize ethanol yield, which environmental factor should be limiting? Choose one: A. glucose B. oxygen C. sunlight D. carbon dioxide

B. oxygen During glycolysis, NAD+ is reduced to NADH. In the presence of oxygen, NADH donates electrons to the electron-transport chain (ETC) in the inner mitochondrial membrane. Oxygen serves as the final electron acceptor in the ETC (see image).If oxygen is not available, yeast carry out fermentation, and NAD+ is regenerated when NADH donates electrons to pyruvate, leading to ethanol production.

Which type of ion channel plays the major role in propagating electrical signals in nerve cells? Choose one: A. ligand-gated B. voltage-gated C. mechanically-gated

B. voltage-gated Voltage-gated ion channels play a major role in propagating electrical signals in nerve cells. The opening of voltage-gated channels changes the membrane potential, which opens additional voltage-gated channels, thus amplifying and propagating the electrical signal. Voltage-gated ion channels have domains called voltage sensors that are extremely sensitive to changes in the membrane potential: changes above a certain threshold value exert sufficient electrical force on these domains to encourage the channel to switch from its closed to its open conformation.

The major products of the citric acid cycle are Choose one: NADH and ATP. CO2 and NADH. H2O and ATP. pyruvate and ATP.

CO2 and NADH. The citric acid cycle produces carbon dioxide and activated carriers from the oxidation of acetyl CoA.

Mutation in the hemoglobin gene can cause sickle-cell anemia. The defective protein found in sickle-cell anemia causes red blood cells to "sickle"—become a misshapen C shape. These misshapen cells abnormally stick to each other and can become trapped by leukocytes (white blood cells) that are rolling or paused on the endothelial cells lining the vessel. This causes blockages of small blood vessels, causing severe pain and strokes called vaso-occlusive crisis. A new drug that binds and blocks selectin proteins is in phase III clinical trials to test for improvement in patients' symptoms. Why might this be an effective treatment for vaso-occlusive crisis? Choose one: A. Blocking selectins would block the ability of selectin to bind carbohydrates on the surface of red blood cells, preventing the blockage. B. Blocking selectins would reduce activation of pain sensors in the blood vessels. C. Blocking selectins would block the ability of selectin to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells. D. Blocking selectins on red blood cells would prevent the red blood cells from binding to the blood vessel endothelial cells, preventing the blockage of red blood cells.

C. Blocking selectins would block the ability of selectin to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells. Selectins are expressed by the endothelial cells lining veins. The selectins bind to carbohydrates on the surface of leukocytes (white blood cells) to slow the movement of the leukocytes through the vein. The leukocytes roll along the vessel wall before squeezing between endothelial cells into the surrounding tissue. A drug that can bind and block selectin proteins would lessen the number of leukocytes bound to the vessel wall. There would then be fewer leukocytes to trap the deformed red blood cells and the red blood cells should continue to move through the blood vessels. Fewer blockages would lead to less pain and a reduced risk of strokes that occur in vaso-occlusive crisis.

The citric acid cycle produces which activated carriers that transfer high-energy electrons to the electron-transport chain? Choose one: A. NADP and FAD B. NAD+ and FAD C. NADH and FADH2 D. NADPH and NADH

C. NADH and FADH2 The citric acid cycle produces reduced forms of two activated carriers, NADH and FADH2. These electron carriers transfer a hydride ion, two electrons, and one proton to the electron-transport chain complexes.

What specific event triggers activation of the stereocilia before they activate the auditory neuron? Choose one: A. The basilar membrane vibrates and stretch-activated ion channels on stereocilia close, depolarizing the membrane. B. The tectorial membrane vibrates and stretch-activated ion channels open on the stereocilia, polarizing the membrane. C. The stereocilia tilt when pushed against the tectorial membrane and stretch-activated ion channels open, releasing positive ions into the hair cell. D. Stretch-activated ion channels on the tectorial membrane open, both depolarizing and activating the stereocilia.

C. 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.

You have joined a lab that studies the metabolic pathway shown below. Just recently, an inhibitor for one of the enzymes, enzyme 3, has become available and the head of the lab wants you to use this inhibitor to determine whether the pathway is linear (as shown) or circular, with another enzyme (not shown) converting F back to A. You have access to all the molecules. What result suggests that the pathway may be circular rather than linear? Choose one: A. Inhibition of enzyme 3 and addition of extra metabolite D leads to buildup of F. B. Inhibition of enzyme 3 and addition of extra metabolite A leads to buildup of C. C. Inhibition of enzyme 3 and addition of extra metabolite A leads to buildup of F. D. Inhibition of enzyme 3 and addition of extra metabolite D leads to buildup of C.

D. Inhibition of enzyme 3 and addition of extra metabolite D leads to buildup of C.

Tetrodotoxin is a potent toxin found in a variety of organisms including the pufferfish. The toxin binds to the extracellular side of the Na+ channel and prevents channel opening. This leads to paralysis of muscles, including the diaphragm. Death from respiratory failure can occur after ingestion of as little as 1 mg of the toxin. Why does this toxin cause paralysis? Choose one: A. The Na+ channel does not open wide enough to allow enough Na+ through the channel. B. The Na+ channels remain in the inactive, refractory state. C. The axon membranes become over-depolarized. D. The membrane depolarization is not amplified along the axon.

D. The membrane depolarization is not amplified along the axon. The tetrodotoxin binds to the extracellular side of the Na+ channel and prevents the channel from opening. This prevents Na+ from entering the cytosol of the cell and the subsequent depolarization of the membrane. If the membrane does not depolarize, the signal is not propagated along the axon. The muscle at the axon terminus does not receive the proper signal and remains in a relaxed state. When this occurs in the muscles required for breathing, such as the diaphragm, the victim is unable to breathe and can die from respiratory failure.

Your friends are on a low-fat, high-carbohydrate diet, which they claim will prevent fat accumulation within their bodies. They eat tons of pasta and bread without worrying about calorie count. What can you correctly say to your friends about their potential to accumulate lipids on their low-fat diet? Choose one: A. They will not accumulate fats because carbohydrates have less energy per gram than fats. B. They will not accumulate fats because cells have no way of storing fats. C. They will accumulate fats because cells have no way of storing carbohydrates. D. They will accumulate fats because cells can convert glycolytic metabolites into lipids.

D. They will accumulate fats because cells can convert glycolytic metabolites into lipids. After a meal, any excess glucose is stored as glycogen or fat. Cells can readily convert sugars to fats through glycolytic and citric acid cycle intermediates.

When glucose moves across a phospholipid bilayer by passive transport, which factor determines the direction of its transport? Choose one: A. the amount of energy available to fuel the transport process B. the charge difference across the membrane C. whether the cell is metabolically active or not D. the concentrations of glucose on either side of the membrane

D. the concentrations of glucose on either side of the membrane 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.

A toxin present in scorpion venom prolongs the duration of action potentials in nerve cells. Which of these actions would best explain how this toxin exerts its effect? Choose one: A. It inhibits the opening of voltage-gated Na+ channels. B. It prolongs the inactivation of voltage-gated Na+ channels. C. It accelerates the opening of voltage-gated K+ channels. D. It slows the inactivation of voltage-gated K+ channels. E. It slows the inactivation of voltage-gated Na+ channels.

E. It slows the inactivation of voltage-gated Na+ channels. The toxin in scorpion venom exerts its effect by slowing the inactivation of voltage-gated Na+channels, causing them to be stuck in the open conformation and prolonging the action potential. In contrast, prolonged inactivation of voltage-gated Na+ channels would delay subsequent action potentials and inhibition of voltage-gated Na+ channels would prevent depolarization of the membrane.

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? Choose one: A. The cells have a glucose pump that expels the glucose needed by other tissues. B. Glucose diffuses down its concentration gradient through the lipid bilayer of the plasma membrane. C. The cells have glucose channels in their plasma membrane. D. The cells run the glucose-Na+ symport proteins in reverse. E. The cells have glucose uniports in their plasma membrane.

E. 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.

Determine whether the following statement is true or false: The oxygen consumed during the oxidation of glucose in animal cells is returned to the atmosphere as CO2.

False A common misconception about the citric acid cycle is that the oxygen atoms required to make CO2 from the acetyl groups entering the citric acid cycle are supplied by atmospheric O2. In fact, these oxygen atoms come from water (H2O). Threemolecules of H2O are split as they enter each turn of the citric acid cycle, and their oxygen atoms are ultimately used to make CO2.The O2 that we breathe is actually reduced to H2O by the electron transport chain; it does not form the CO2 that we exhale.

In the electron transport chain, the oxygen atoms in O2 become part of which of the following molecules? Choose one: glucose (C6H12O6) NADH H2O ATP CO2

H2O The electron transport chain donates electrons to O2. This action reduces the oxygen, which combines with protons to produce H2O.CO2 is produced by the citric acid cycle, not by the electron transport chain. Its oxygen comes from water, not O2. Likewise, NADH is generated during the citric acid cycle and donates its electrons to the electron transport chain. It does not interact directly or combine with O2.Although the movement of electrons along the electron transport chain yields energy that is ultimately used to drive the synthesis of ATP, ATP does not interact with the electron transport chain or with molecular oxygen.Glucose is a food molecule whose oxidative breakdown ultimately fuels the synthesis of ATP. In the process, glucose is oxidized to CO2. It is not directly involved in electron transport and does not receive oxygen from the electron transport chain.

When an electron is removed from the reaction center of photosystem II, the missing electron is replaced by an electron from which of the following? Choose one: manganese H2O H+ sunlight photosystem I

H2O When a mobile electron carrier removes an electron from a photosynthetic reaction center, it leaves behind a positively charged chlorophyll special pair. To reset the system and allow photosynthesis to proceed, this missing electron must be replaced.For photosystem II, the missing electron is replaced by a special manganese-containing protein complex that removes electrons from water. The cluster of manganese atoms in this water-splitting enzyme holds onto two water molecules from which electrons are extracted one at a time. Once four electrons have been removed from these two water molecules—and used to replace the electrons lost by four excited chlorophyll special pairs—O2 is released.

How is the protein shown in the diagram associated with the plasma membrane? 1. It is linked to lipids in the bilayer. 2. It is attached to another protein that is a transmembrane protein. 3. It is a membrane-spanning protein (transmembrane) 4. It is associated with one layer of the lipid bilayer.

It is associated with one layer of the lipid bilayer. Proteins can be directly associated with the membrane by being fully inserted into the membrane (transmembrane) or they can be inserted into one of the layers of the bilayer (monolayer associated). The protein shown has one alpha helix that interacts with one face of the lipid bilayer, making it monolayer associated.

What is true of the inside of a cell? Choose one: It has the same charge as the outside of the cell. It is slightly more positive than the outside of a cell. It is slightly more negative than the outside of a cell.

It is slightly more negative than the outside of a cell. The inside of a cell 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.

How does the inclusion of cholesterol affect animal cell membranes? 1. It makes the lipid bilayer more permeable. 2. It tends to make the lipid bilayer less fluid. 3. It has little effect on the properties of the lipid bilayer. 4. It tends to make the lipid bilayer more fluid. 5. It makes the lipid bilayer wider.

It tends to make the lipid bilayer less fluid. In animal cells, membrane fluidity is modulated in part by the inclusion of cholesterol, which constitutes up to 20% (by weight) of the lipids in the plasma membrane. Because cholesterol molecules are short and rigid, they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated tails. In this space-filling model, cholesterol is shown in green to distinguish it from the hydrocarbon tails of the surrounding membrane phospholipids. The intercalation of cholesterol within the membrane phospholipids makes the bilayer stiffer and less flexible, as well as less permeable. It does not, however, alter the thickness of the membrane.

The electron-transport chain in mitochondria accepts high-energy electrons directly from which molecule? Choose one: pyruvate H2O NADH ATP acetyl CoA

NADH The generation of ATP is powered by the flow of electrons that are derived from the burning of carbohydrates, fats, and other foodstuffs during glycolysis and the citric acid cycle. These "high-energy" electrons are provided by activated carriers generated during these two sets of catabolic reactions, with the majority being churned out by the citric acid cycle that operates in the mitochondrial matrix. NADH produced during glycolysis can also donate electrons to the electron-transport chain, but it must first be imported into the mitochondrial matrix. Because the mitochondrial inner membrane is impermeable to NADH, this transport requires energy. Thus, NADH produced during glycolysis ultimately yields less net energy than NADH generated by the citric acid cycle, which takes place in the mitochondrial matrix.

In the electron-transport chain in chloroplasts, which molecule serves as the final electron acceptor? Choose one: NAD+ O2 NADP+ ADP H2O

NADP+ In the first stage of photosynthesis, an electron-transport chain in the thylakoid membrane harnesses the energy of electron transport to pump protons into the thylakoid space; the resulting proton gradient then drives the synthesis of ATP by ATP synthase.This electron-transport process differs from that of oxidative phosphorylation in terms of where its high-energy electrons come from—and where they wind up. During oxidative phosphorylation, high-energy electrons are donated by NADH produced during glycolysis and the citric acid cycle and they are ultimately passed along to molecular oxygen. In photosynthesis, the high-energy electrons donated to the photosynthetic electron-transport chain come from a molecule of chlorophyll that has absorbed energy from sunlight. And they are donated not to O2 but to NADP+, to produce NADPH.

Lipid bilayers are highly impermeable to which molecule(s)? Choose one: water carbon dioxide Na+ and Cl- steroid hormones oxygen

Na+ and Cl- Lipid bilayers are highly impermeable to many charged ions, and Na+ and Cl- are common examples of ions that are excluded from the hydrophobic interior of a lipid bilayer. For ion transport, cell membranes contain channel proteins that permit passage of ions. In contrast, small, nonpolar molecules, such as CO2 and O2, and hydrophobic steroid hormones easily pass through the lipid bilayer. Water, even though it is polar, is small enough that it does enter the membrane at a measurable rate. However, water moves much more quickly though through membrane proteins called aquaporins.

Which of the following statements is true? Choose one: K+ is the most plentiful positively charged ion outside the cell, while Na+ is the most plentiful inside. K+ and Na+ are both excluded from cells. Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. K+ and Na+ are present in the same concentration on both sides of the plasma membrane. K+ and Na+ are both maintained at high concentrations inside the cell compared to out.

Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. Because lipid bilayers are impermeable to inorganic ions, living cells are able to maintain internal ion concentrations that are very different from the concentrations of ions in the medium that surrounds them. Na+ is the most plentiful positively charged ion outside the cell, while K+ is the most plentiful inside. The intracellular concentration of sodium ions is approximately 5-15 mM, whereas the extracellular concentration of sodium ions is approximately 145 mM. Additionally, the intracellular concentration of potassium ions is approximately 140 mM, whereas the extracellular concentration of potassium ions is approximately 5 mM. Cells expend a great deal of energy to maintain this chemical imbalance, and such electrical imbalances generate a voltage difference across the membrane called the membrane potential.

In 1925, scientists exploring how lipids are arranged within cell membranes performed a key experiment using red blood cells. Using benzene, they extracted the lipids from a purified sample of red blood cells. Because these cells have no nucleus and no internal membranes, any lipids they obtained were guaranteed to come from the plasma membrane alone.The extracted lipids were floated on the surface of a trough filled with water, where they formed a thin film. Using a movable barrier, the researchers then pushed the lipids together until the lipids formed a continuous sheet only one molecule thick.The researchers then made an observation that led them to conclude that the plasma membrane is a lipid bilayer. Which of the following would have allowed the scientists to come to this conclusion? 1. When pushed together, the extracted lipids dissolved in water. 2. The extracted lipids covered twice the surface area of the intact red blood cells. 3. The extracted lipids covered half the surface area of the intact red blood cells. 4. The extracted lipids covered the same surface area as the intact red blood cells.

The extracted lipids covered twice the surface area of the intact red blood cells. When the extracted lipids were pushed together into one continuous monolayer, the researchers found that they occupied twice the area of the original, intact cells. Additional experiments showed that lipids can spontaneously form bilayers when mixed with water. Together, these observations suggest that in an intact cell membrane, the lipid molecules are doubled up to form a bilayer—an arrangement that has a profound influence on cell biology.Lipid molecules are not very soluble in water because part of the molecule is hydrophobic. Nudging them closer together with a movable barrier (imagine the edge of a ruler) would not change their solubility.

Why do phospholipids form bilayers in water? 1. The hydrophobic head is attracted to water, while the hydrophilic tail shuns water. 2. The hydrophilic head is attracted to water, while the hydrophobic tail shuns water. 3. The hydrophilic head is insoluble in water. 4. The hydrophobic tail is attracted to water, while the hydrophilic head shuns water. 5. The hydrophobic head shuns water, while the hydrophilic tail is attracted to water.

The hydrophilic head is attracted to water, while the hydrophobic tail shuns water. Correct. The hydrophilic head can form electrostatic attractions and hydrogen bonds with water, while the hydrophobic tails are insoluble in water.

When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol? 1. It depends on whether the vesicle is coming from the endoplasmic reticulum or the Golgi apparatus. 2. The blue monolayer will face the cytosol. 3. The orange monolayer will face the cytosol. 4. Half the time the orange monolayer will face the cytosol, and half the time the blue monolayer will face the cytosol. 5. It depends on the cargo the vesicle is carrying.

The orange monolayer will face the cytosol. Most cell membranes are asymmetric, as the two halves of the bilayer often include strikingly different sets of phospholipids. This asymmetry is preserved as membranes bud from one organelle and fuse with another, or with the plasma membrane. This means that all cell membranes have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or the interior space (lumen) of an organelle. Maintaining this asymmetric organization is essential for preserving the asymmetric distribution of phospholipids and glycolipids, which may be confined to one or another monolayer to carry out their physiological function.

Which of the following drives the production of ATP from ADP and Pi by ATP synthase? Choose one: hydrolysis phosphorylation sunlight a proton (H+) gradient a sodium (Na+) gradient

a proton (H+) gradient Membrane-based systems use the energy stored in an electrochemical proton (H+) gradient to synthesize ATP. This proton gradient is generated by the movement of electrons along a membrane. When the protons flow back across the membrane, down their electrochemical gradient, their energy can be harnessed to do work. ATP synthase uses the energy stored in the proton gradient to produce ATP.

Animals exploit the phospholipid asymmetry of their plasma membrane to distinguish between live cells and dead ones. When animal cells undergo a form of programmed cell death called apoptosis, phosphatidylserine—a phospholipid that is normally confined to the cytosolic monolayer of the plasma membrane—rapidly translocates to the extracellular, outer monolayer. The presence of phosphatidylserine on the cell surface serves as a signal that helps direct the rapid removal of the dead cell. How might a cell actively engineer this phospholipid redistribution? 1. by inactivating both a flippase and a scramblase in the plasma membrane 2. by boosting the activity of a flippase in the plasma membrane 3. by inverting the existing plasma membrane 4. by activating a scramblase and inactivating a flippase in the plasma membrane 5. by inactivating a scramblase in the plasma membrane

by activating a scramblase and inactivating a flippase in the plasma membrane All cells are separated from the extracellular environment by the plasma membrane. This cell membrane plays a key role in cell communication, presenting signals that relate information about the state of the cell, including its relative health. In healthy cells, the distribution of phospholipids in the plasma membrane is asymmetric. Some phospholipids, such as phosphatidylcholine and sphingomyelin, are confined to the noncytosolic half of the plasma membrane, while others such as phosphatidylserine and phosphatidylethanolamine are present only in the membrane's cytosolic monolayer.When cells are no longer needed or are damaged beyond repair, they can activate a form of programmed cell death called apoptosis. A cell undergoing apoptosis actively destroys itself from within, digesting its proteins and degrading its DNA. It also displays signals that direct circulating phagocytic cells to engulf its remains.One of these signals involves the relocation of phosphatidylserine. An apoptotic cell displays phosphatidylserine—normally confined to the cytosolic monolayer of the plasma membrane—on its surface.This reversal involves manipulating the activity of both flippases and scramblases in the plasma membrane. First, the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other must be activated. This scrambling causes phosphatidylserine—initially deposited in the cytosolic monolayer—to become distributed to both halves of the bilayer. At the same time, the flippase that would normally transfer phosphatidylserine from the extracellular monolayer to the cytosolic monolayer must be inactivated. Together, these actions cause phosphatidylserine to rapidly accumulate at the cell surface.Boosting the activity of flippases causes phosphatidylserine to be selectively transferred to the cytosolic half of the membrane. This distribution is the sign of a healthy cell—the opposite of what happens when cells undergo programmed cell death.If flippases were inactivated, any phosphatidylserines that had already made it to the extracellular side of the plasma membrane (through the random action of scramblases) would, indeed, remain there. But if scramblase were also inactivated, any newly synthesized phosphatidylserines would remain trapped in the cytosolic half of the bilayer. So, a limited number of phosphatidylserines would be exposed at the cell surface.

Which of the following describes a breakdown process in which enzymes degrade complex molecules into simpler ones? Choose one: anabolism metabolism catabolism Enzymes are not needed to break down complex molecules into simpler ones.

catabolism Most of the chemical reactions that cells perform normally occur only at temperatures that are much higher than those inside a cell. Each reaction therefore requires a major boost in chemical reactivity to enable it to proceed rapidly within the cell. This boost is provided by a large set of specialized proteins called enzymes, each of which accelerates, or catalyzes, just one of the many possible reactions that a particular molecule could, in principle, undergo.Rather than being an inconvenience, the necessity for catalysis is a benefit, as it allows the cell to precisely control its metabolism—the sum total of all the chemical reactions it needs to carry out to survive, grow, and reproduce. This control is central to the chemistry of life. Two opposing streams of chemical reactions occur in cells: the catabolic pathways and the anabolic pathways. The catabolic pathways (catabolism) break down foodstuffs into smaller molecules, thereby generatingboth a useful form of energy for the cell and some of the small molecules that the cell needs as building blocks. The anabolic, or biosynthetic, pathways (anabolism) use the energy harnessed by catabolism to drive the synthesis of the many molecules that form the cell. Together, these two sets of reactions constitute the metabolism of the cell.

The shape of a cell and the mechanical properties of its plasma membrane are determined by a meshwork of fibrous proteins called what? 1. basal lamina 2. tight junction 3. cell cortex 4. glycocalyx 5. lamellipodium

cell cortex The plasma membrane of animal cells is stabilized by a meshwork of filamentous proteins, called the cell cortex, that is attached to the underside of the membrane. The cortex has been extensively studied in red blood cells, where it helps the cells maintain their distinctive biconcave shape as they squeeze through narrow blood vessels. The main component of the cell cortex in red blood cells is flexible protein lattice made from long, thin fibers of spectrin. Animals or humans that produce a structurally abnormal form of spectrin tend to be anemic—they have fewer and more fragile red blood cells.

In plants, fats and starch are stored in which part of the cell? Choose one: cytosol endoplasmic reticulum mitochondrion chloroplast cell wall

chloroplast Plants convert some of the sugars they make through photosynthesis during daylight into fats and into starch, a branched polymer of glucose very similar to animal glycogen. The fats in plants are triacylglycerols, as they are in animals, and they differ only in the types of fatty acids that predominate.In plant cells, fats and starch are both stored in chloroplasts—specialized organelles that carry out photosynthesis. These energy-rich molecules serve as food reservoirs that are mobilized by the cell during periods of darkness. At night, stored starch and fat can be broken down to sugars and fatty acids, which are exported to the cytosol to help support the metabolic needs of the plant.Mitochondria are organelles in which sugars and other food-derived molecules are oxidized to produce ATP.The endoplasmic reticulum in plant cells serves the same function as it does in animal cells—as a site of protein production and folding.Plant cell walls are supportive structures reinforced with cellulose, not starch.

In an electron transport chain, electrons are passed from one transmembrane electron carrier to another, driving proton movement across a membrane (see image below). The protons then flow through ATP synthase (not shown) to generate ATP.In a 2018 article (Budin, et al., Science vol. 362) researchers probed how membrane fluidity affects electron transport chain activity and ATP production in E. coli by manipulating membrane fluidity and measuring respiration. How could researchers have increased membrane fluidity? A. increase the proportion of phospholipids with unsaturated fatty acids B. increase the amount of cholesterol present in the bacterial membranes C. decrease the temperature of the media the E. coli were grown in D. increase the length of the fatty acid tails in phospholipids

increase the proportion of phospholipids with unsaturated fatty acids The fluidity of a bilayer depends on the composition of the bilayer, with shorter chain lengths and unsaturated fatty acids decreasing interactions between adjacent phospholipids and thereby increasing membrane fluidity. The researchers found that more fluid membranes increased cellular respiration due to increased diffusion of the electron transport chain carriers (link to article). Higher temperatures also increase membrane fluidity, so cells produce longer chains with fewer double bonds (less unsaturated) at higher temperatures to maintain proper fluidity.

In an animal cell, where are the proteins of the electron-transport chain located? Choose one: mitochondrial matrix ER membrane inner mitochondrial membrane outer mitochondrial membrane plasma membrane

inner mitochondrial membrane In animal cells, oxidative phosphorylation takes place on the inner mitochondrial membrane, which contains both the proteins that form an electron-transport chain and ATP synthase.In this process, high-energy electrons—derived from the oxidation of food molecules—are transferred along the series of electron carriers that form the electron-transport chain. These electron transfers release energy that is used to pump protons across the inner mitochondrial membrane, thus generating an electrochemical proton gradient.These protons subsequently flow back down this electrochemical gradient, into the mitochondrial matrix, through the enzyme ATP synthase. ATP synthase couples the movement of these protons to the production of ATP.

Protons are pumped across the mitochondrial inner membrane to accumulate in the Choose one: intermembrane space. electron-transport chain. cytosol. mitochondrial matrix.

intermembrane space. The electron-transport chain pumps electrons from the mitochondrial matrix into the intermembrane space. This produces a higher concentration of protons in the intermembrane space, which is used to power ATP synthase.

In passive transport, the net movement of a charged solute across the membrane is determined by which of the following? Choose one: the membrane potential aloneits electrochemical gradient its osmotic gradient alone its concentration gradient alone

its electrochemical gradient In passive transport, the net movement of a charged solute across the membrane is determined by its electrochemical gradient, which is a composite of two forces: one due to the concentration gradient and the other due to the membrane potential.

What is the voltage difference across a membrane of a cell called? Choose one: membrane potential gradient establishment potential balance electrical current

membrane potential 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. 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.

In plant cells, where does the citric acid cycle take place? Choose one: The citric acid cycle does not occur in plant cells. chloroplasts mitochondria chloroplasts and mitochondria cytosol

mitochondria To be able to grow, divide, and carry out day-to-day activities, cells require a constant supply of energy. This energy comes from the chemical-bond energy in food molecules, which thereby serve as fuel for cells. Perhaps the most important fuel molecules are the sugars. While animals obtain sugars and other organic molecules by eating, plants make their own sugars from CO2 by photosynthesis in the chloroplast.Regardless of how these sugars are obtained, the process by which they are broken down to generate energy is very similar in both animals and plants. In both cases, the organism's cells harvest useful energy from the chemical-bond energy locked in sugars as the sugar molecule is broken down and oxidized to carbon dioxide (CO2) and water (H2O). The energy released during these reactions is captured in activated carriers such as ATP and NADH.In all eukaryotic cells, the complete oxidative breakdown of sugars such as glucose begins with glycolysis in the cytosol and continues in the mitochondria with the citric acid cycle and, ultimately, the generation of ATP via oxidative phosphorylation. These processes occur in the same compartments in the cells of both animals and plants.

In mitochondria, what is the final electron acceptor in the electron-transport chain? Choose one: oxygen (O2) carbon dioxide (CO2) ADP NADH and FADH2 water (H2O)

oxygen (O2) Like any other chemical reaction, the tendency of oxidation-reduction reactions to proceed spontaneously depends on the free-energy change (ΔG) for the electron transfer, which in turn depends on the relative electron affinities of the participating molecules. Electrons will pass spontaneously from molecules that have a relatively low affinity for some of their electrons, and thus lose them easily, to molecules that have a higher affinity for electrons. With its low electron affinity, NADH is an excellent molecule to donate electrons to the respiratory chain, while O2, with its high affinity for electrons, is well suited to act as the electron "sink" at the end of the pathway.

When grown at higher temperatures, bacteria and yeast maintain an optimal membrane fluidity by doing which of the following? 1. producing membrane lipids with tails that are shorter and contain fewer double bonds 2. producing membrane lipids with tails that are longer and contain more double bonds 3. producing membrane lipids with tails that are longer and contain fewer double bonds 4. adding cholesterol to their membranes 5. producing membrane lipids with tails that are shorter and contain more double bonds

producing membrane lipids with tails that are longer and contain fewer double bonds How fluid a lipid bilayer is at a given temperature depends on its phospholipid composition—particularly the nature of the hydrocarbon tails. The closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be. In bacterial and yeast cells, which have to adapt to varying temperatures, both the lengths and the degree of saturation of the hydrocarbon tails in the bilayer are adjusted constantly to maintain a membrane with a relatively consistent fluidity. At higher temperatures, for example, the cell makes membrane lipids with tails that are longer and that contain fewer double bonds. This allows the membrane lipids to maximize their interactions and thus to pack more tightly, which keeps the membranes from becoming too fluid. Although animal cells do not generally have to cope with large ranges of temperature, they can modulate membrane fluidity by the inclusion of the sterol cholesterol. This option is not available to bacteria and yeast, which do not produce cholesterol.

In the electron-transport chain, as electrons move along a series of carriers, they release energy that is used to do what? Choose one: oxidize food molecules hydrolyze ATP split water into protons and oxygen phosphorylate ADP to form ATP pump protons across a membrane

pump protons across a membrane In oxidative phosphorylation, which occurs in mitochondria, an electron-transport system receives high-energy electrons derived from the oxidation of food. The movement of these electrons along the electron-transport chain releases energy that is used to pump protons across the membrane. The flow of electrons back down this electrochemical proton (H+) gradient can then be harnessed to generate ATP.

The movement of electrons through the electron-transport chain in mitochondria does which of the following? Choose one: pumps protons out of the mitochondrial matrix pumps ATP across the inner mitochondrial membrane produces NADH consumes ATP produces oxygen

pumps protons out of the mitochondrial matrix The stepwise movement of these electrons through the components of the electron-transport chain releases energy that can then be used to pump protons across the inner mitochondrial membrane. This action represents stage 1 of chemiosmotic coupling. The electrons are ultimately passed to oxygen (the final electron acceptor) and the proton gradient is used to drive the synthesis of ATP. This chemiosmotic mechanism for ATP synthesis is called oxidative phosphorylation because it involves both the consumption of O2 and the addition of a phosphate group to ADP to form ATP.

In cells that cannot carry out fermentation, which products derived from glycolysis will accumulate under anaerobic conditions? Choose one: glucose 6-phosphate and NADH pyruvate and NADH glucose and NADH pyruvate and NAD+ lactate and NAD+

pyruvate and NADH In cells that can carry out fermentation, the pyruvate generated by glycolysis is converted into products that can be excreted from the cell: lactate in muscle cells, for example, or ethanol and CO2 in the yeast cells used in brewing and breadmaking. In the process, NADH gives up its electrons and is converted back to the NAD+ required to maintain the reactions of glycolysis.Without fermentation, NADH produced during glycolysis would accumulate. Because oxygen is the ultimate acceptor of electrons donated by NADH, in the absence of oxygen, NADH would be forced to keep hold of its electrons. As a result, the NAD+ needed for glycolysis to continue would no longer be regenerated.The enzyme complex that converts pyruvate into acetyl CoA—and the citric acid cycle that completes the oxidation of these carbons—also requires a source of NAD+. Without fermentation reactions to regenerate this NAD+, cells lacking oxygen would be unable to further oxidize pyruvate. Thus, this intermediate, too, would accumulate.

When nutrients are plentiful, plants can store glucose as what? Choose one: glucose 6-phosphate fats glycogen and starch glycogen starch

starch Plants convert some of the sugars they make through photosynthesis during daylight into fats and into starch, a branched polymer of glucose very similar to glycogen, which is the form in which glucose is stored in animal cells.An abundance of glucose 6-phosphate can trigger the storage of glucose, as it indicates a surplus of food molecules in the cell. It, however, is not a form of storage for glucose.Glucose is a sugar, so it is not stored as a fat.

When a vesicle fuses with the plasma membrane, which way will the monolayer that was exposed to the interior of the vesicle face? 1. It depends on where, along the plasma membrane, the vesicle fuses. 2. the cell cytoplasm 3. The direction the monolayer will face will be established randomly. 4. the endomembrane system 5. the cell exterior

the cell exterior Most cell membranes are asymmetric and have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or the interior space (lumen) of an organelle. This asymmetry is preserved as membranes, in the form of vesicles, which bud from one organelle and fuse with another or with the plasma membrane.

The NADH generated during glycolysis and the citric acid cycle feeds its high-energy electrons to which of the following? Choose one: H2O ADP the citric acid cycle the electron transport chain FAD

the electron transport chain The NADH produced during glycolysis (and the NADH and FADH2 generated by the citric acid cycle) transfers its high-energy electrons to the electron transport chain. This series of electron carriers is embedded in the inner mitochondrial membrane in eukaryotic cells (and in the plasma membrane of aerobic prokaryotes). As the electrons pass through the series of electron acceptor and donor molecules that form the chain, they fall to successively lower energy states. At specific sites in the chain, the energy released is used to drive protons (H+) across the inner membrane, from the mitochondrial matrix to the intermembrane space. This movement generates a proton gradient across the inner membrane, which serves as a source of energy (like a battery) that can be tapped to drive a variety of energy-requiring reactions. The most prominent of these reactions is the phosphorylation of ADP to generate ATP.At the end of the transport chain, the electrons are added to molecules of O2, and the resulting reduced oxygen molecules immediately combine with protons from the surrounding solution to produce H2O. The electrons have now reached their lowest energy level.

On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located? 1. both sides 2. the inside 3. the cytosolic side 4. the underside 5. the extracellular side

the extracellular side Like some of the lipids in the outer layer of the plasma membrane, most of the proteins in the plasma membrane have sugars covalently attached to them. The great majority of these proteins have short chains of sugars, called oligosaccharides, linked to them; they are called glycoproteins. Other membrane proteins, the proteoglycans, contain one or more long polysaccharide chains. All of the carbohydrates on the glycoproteins, proteoglycans, and glycolipids are located on the outside of the plasma membrane, where they form a sugar coating called the carbohydrate layer or glycocalyx.

Multipass transmembrane proteins can form pores across the lipid bilayer. The structure of one such channel is shown in the diagram. In this figure, what do the areas shown in red represent? 1. the hydrophobic side chains of the transmembrane β barrel 2. the hydrophilic side chains of the transmembrane β barrel 3. the hydrophilic side chains of the transmembrane α helices 4. the hydrophobic lipid tails of the bilayer 5. the amphipathic side chains of the transmembrane α helices 6. the hydrophobic side chains of the transmembrane α helices

the hydrophilic side chains of the transmembrane α helices Multiple amphipathic α helices can come together to form a pore in the membrane.However, the hydrophobic parts of the helix will interact with the hydrophobic hydrocarbon tails of the phospholipids within the lipid bilayer. That part of the protein is here shown in green.The parts of the protein shown in red, which line the water-filled pore, are the hydrophilic parts of the α helices.A polypeptide chain can also cross the lipid bilayer not as an α helix, but as a β sheet. In this case, the β sheets will be rolledinto a cylinder, forming a keglike structure called a β barrel. As with an α helix, the amino acid side chains that face the inside of a β barrel, and therefore line the aqueous channel, would be mostly hydrophilic, while those on the outside of the barrel, which contact the hydrophobic core of the lipid bilayer, would be exclusively hydrophobic.

Plasma membrane proteins that move ions in and out of cells using active transport are called 1. transporters. 2. receptors. 3. anchors. 4. channels.

transporters. Transporters move ions using active transport. Channels allow cells to move in and out of cells down their concentration gradient.

In a lipid bilayer, where do lipids rapidly diffuse? 1. within the plane of one monolayer and back and forth between the monolayers 2. within the plane of their own monolayer 3. in and out of the bilayer 4. back and forth from one monolayer to the other in the bilayer 5. not at all, because they remain in place within the bilayer

within the plane of their own monolayer The lipid bilayer acts like a two-dimensional fluid. Once phospholipids are inserted into the bilayer by the enzymes that synthesize them, they diffuse rapidly and continuously within the plane of the monolayer into which they were added. Phospholipids very rarely tumble from one monolayer to the other. The cell contains transporters that can move phospholipids from one membrane monolayer to the other as needed. Phospholipids also do not spontaneously pop in and out of the bilayer; their hydrophobic tails make interaction with the aqueous environment around the membrane unfavorable. Phospholipids can be added to or removed from the bilayer as part of lipid vesicles.