Cell Biology Exam 2

In this illustration of the structure of ATP synthase, which of the components rotate?

1 (baby blue)& 5(dark blue)

Shown is a schematic diagram of a membrane phospholipid. Which segment will always carry a negative charge?

B (phosphate group)

Assume five molecules of FADH2 are made in the citric acid cycle in a given amount of time. How many NADH are made during the same time interval?

15

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?

ATP export will decrease because its carrier exploits the difference in voltage across the inner membrane.

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?

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.

Not all fat cells are equivalent. Humans and other animals contain both white fat cells and brown fat cells, named after their color. Because increases in brown fat cells may aid weight loss, researchers are interested in factors that control the ratio of white fat cells and brown fat cells, as detailed in a 2017 article in Obesity Reviews. What is the mechanism for how brown fat cells aid weight loss?

Brown fat cell mitochondria contain an uncoupling protein.

To study the structure of a particular membrane protein, the target protein is usually removed from the membrane and separated from other membrane proteins. Shown below are three different proteins associated with the cell membrane. Treatment with high salt would release which protein or proteins from the bilayer?

C

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

Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate.

The diffusion of an integral membrane protein is studied by fluorescence recovery after photobleaching (FRAP). In this procedure, the protein of interest is labeled with a fluorescent marker, and the fluorescence in a small patch of membrane is then irreversibly "bleached" by a pulse of light from a focused laser. The time it takes for fluorescence to return to the bleached membrane patch provides a measure of how rapidly unbleached, fluorescently labeled proteins diffuse through the bilayer into the area. This "recovery" is plotted on a curve that shows fluorescence over time. For one protein, which acts as a receptor for an extracellular signal molecule, stimulation by its signal ligand causes the receptor to interact with other membrane proteins, forming a large protein signaling complex. Shown here is the FRAP result for the unstimulated receptor. Which of the following curves would most likely represent the behavior of the receptor once it has been activated by its signal molecule?

C (large gap between curves)

Which statement is true of glycogen phosphorylase?

It is inhibited by glucose 6-phosphate and by ATP.

When ATP and food molecules such as fatty acids are abundant, which will occur?

Enzymes involved in gluconeogenesis will use energy to produce glucose.

Which molecules are required for the citric acid cycle to fully oxidize the carbons donated by acetyl CoA?

GDP, O2, oxaloacetate, NAD+

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

H2O would diffuse in.

Weight loss can occur when glucose is oxidized to CO2 rather than being stored as glycogen. The first step in glucose oxidation is glycolysis. A 1930s diet drug, DNP, made the inner mitochondrial membrane permeable to protons, increasing the rate of glycolysis. What is the explanation for the DNP-induced increase in glycolysis?

High ADP activates phosphofructokinase.

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?

It slows the inactivation of voltage-gated Na+ channels.

Experiments performed by Hans Krebs in the 1930s revealed that the set of reactions that oxidize food molecules and produce CO2 occur in a cycle. In one experiment, Krebs exposed pigeon muscles to malonate, a compound that inhibits succinate dehydrogenase—the enzyme that converts succinate to fumarate, indicated in red in the linear representation of the reactions of the citric acid cycle (below). Which of the following observations, made in malonate-treated muscle, led Krebs to believe that this set of reactions is cyclical?

If fumarate were added, succinate would accumulate.

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?

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.

The drug scopolamine is used to treat dizziness, motion sickness, and smooth muscle spasms. When isolated muscle cells are incubated with scopolamine, addition of acetylcholine no longer depolarizes the muscle cell membrane or stimulates muscle cell contraction. Which would best explain how scopolamine exerts its muscle-relaxing effects?

It inhibits the opening of acetylcholine-gated Na+ channels in the muscle cell membrane.

When a neuron is activated by a stimulus, its plasma membrane will change until it reaches a membrane potential of about +40 mV. What is special about this value?

It is approximately the membrane potential at which the electrochemical gradient for Na+ is zero.

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?

NADH and the NADH dehydrogenase complex are bound to high-energy electrons while O2 and the cytochrome c oxidase complex are not.

Which statements below accurately describe an aspect of the citric acid cycle?

NADH is generated in steps 3, 4, and 8. Oxaloacetate is regenerated by the end of the citric acid cycle.

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

NaCl, because Na+ is needed for glucose entry.

Investigators introduce two proteins into the membrane of artificial lipid vesicles: (1) an ATP synthase isolated from the mitochondria of cow heart muscle, and (2) a light-activated proton pump purified from the prokaryote Halobacterium halobium. The proteins are oriented as shown in the diagram. When ADP and Pi are added to the external medium and the vesicle is exposed to light, would this system produce ATP?

No, because ATP synthase is not oriented correctly.

In the patch-clamp technique (shown here), a small glass electrode forms a tight seal with a portion of the cell membrane (A), allowing detection of ion flow through channels (B). This technique has clinical uses. For example, some individuals with high blood pressure (hypertension) have a genetic defect in an epithelial Na+ channel (ENaC) that makes the channel hyperactive (open more than normal). Such individuals can be treated with amiloride, which inhibits sodium ion movement through ENaC. Researchers predicted that amiloride would not effectively treat hypertension in individuals with normal ENaC activity and tested this prediction by analyzing ENaC activity in patch-clamped cells from individuals with high blood pressure, some of whom showed hyperactive ENaC and some of whom showed normal ENaC activity. Which of the following results align with researcher expectations?

Patch-clamp data showed normal ENaC activity prior to amiloride treatment; patient did not benefit from amiloride. Patch-clamp data showed hyperactive ENaC prior to amiloride treatment; patient benefited from amiloride.

Below is a table listing the reactions that constitute the 10 steps of glycolysis, along with the change in free energy (ΔG°) for each step. Based on the data, which steps in glycolysis are effectively irreversible?

Steps 1, 3, and 10

Fluorescence recovery after photobleaching (FRAP) is used to monitor the movement of fluorescently labeled molecules within the plane of a cell membrane. The molecules labeled are often proteins, but lipids can be labeled too. How would the curve that represents FRAP for labeled proteins compare to the curve representing labeled lipids?

The FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence.

Protons are pumped across the mitochondrial inner membrane as electrons are transferred through the mitochondrial electron transport chain. Which of the following statements about proton pumping are correct?

The NADH dehydrogenase, cytochrome b-c1, and cytochrome oxidase complexes all pump protons across the membrane. The pH inside the mitochondrial matrix is higher than in the intermembrane space. The mitochondria use the proton gradient to synthesize ATP.

In the technique called optogenetics, light-gated Na+ channels are introduced into the brains of living animals. Activation of these channels by light can depolarize the membranes of neurons that contain them, selectively activating these target cells.Since its inception, optogenetics has been expanded to include other types of light-gated channels, such as a channel that is selective for Cl- instead of Na+. If this light-gated Cl- channel were introduced into neurons in a region of the brain that stimulates feeding, what might you expect to see?

The animals would avoid eating, even when they are hungry—but only when the channels are activated by light.

Antibiotics should inhibit bacterial cell growth without generating side effects in the human patient, but that is not always the case. Some antibiotics that inhibit bacterial protein synthesis by binding to bacterial ribosomes induce negative side effects in patients. What is the most likely cause of these side effects?

The antibiotics interfere with mitochondrial ribosomes.

The main regulatory step of glycolysis occurs in step 3. Choose all of the following that correctly describe some aspect of step 3 in glycolysis.

The enzyme that catalyzes step 3 is phosphofructokinase. The enzyme uses an ATP. The reaction generates the product fructose 1,6-bisphosphate. The reaction is an irreversible reaction.

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?

The extracted lipids covered twice the surface area of the intact red blood cells.

The FRAP technique occurs in a series of steps. Select every statement that correctly describes a step in the FRAP procedure.

The molecule of interest is fluorescently labeled. The relative mobility of the fluorescently labeled molecule is measured.

When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol?

The orange monolayer will face the cytosol.

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

The pump ran out of Na+ to pump because it pumps 3 Na+ out for every 2 K+ pumped in.

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

The transporter oscillates randomly between states in which it is open to either the extracellular space or the cytosol.

In a patch of animal cell membrane about 10 μm in area, which will be true?

There will be more lipids than proteins.

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?

They will accumulate fats because cells can convert glycolytic metabolites into lipids.

Shown here is a data set representing genes involved in ribosome biogenesis and electron transport. Red indicates that supplementing the growth medium with glucose has increased the expression of the genes, whereas green indicates that the added glucose has decreased gene expression. Based on this data, what can be concluded about how yeast cells behave when grown in the presence of high concentrations of glucose?

Yeast cells exposed to high concentrations of glucose grow by fermentation.

Which membrane would show a more rapid recovery of fluorescence in a FRAP study?

a membrane containing a larger proportion of unsaturated fatty acids

Which of the following describes the resting membrane potential of a neuron?

a state in which the flow of positive and negative ions across the plasma membrane is precisely balanced

You join a laboratory to study muscle function. You decide to inhibit the pumping of Ca2+ into the sarcoplasmic reticulum to determine how excess cytosolic Ca2+ will affect muscle function. Which of the following strategies would be effective in blocking Ca2+ pumping?

block the phosphorylation of the conserved aspartate in the Ca2+ pump

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?

by activating a scramblase and inactivating a flippase in the plasma membrane

The buildup of lactic and formic acids generated by anaerobic fermentation likely favored the evolution of which of the following?

cells that could pump protons

Optogenetics inhibit depolarization

chloride channel and potassium channel

events in glycolysis

energy invested using atp, 6 carbon sugar split in2 energy captured as ATP & NADH 2 molecules of pyruvate generated

After an overnight fast, most of the acetyl CoA entering the citric acid cycle is derived from what type of molecule?

fatty acids

Intracellular condensates are non-membrane bound biochemical subcompartments that form due to phase separation among networks of weakly interacting molecules. Sabari et al., 2018, proposed that the transcriptional coactivator BRD4 helps form intracellular condensates containing other transcriptional proteins. A prediction of this proposal is that BRD4 should behave as a liquid within the condensate with rapid movement. Which procedure could be used to analyze movement of BRD2 in living cells?

fluorescence recovery after photobleaching (FRAP)

The chemistry of many metabolic reactions was deciphered using molecules labeled with radioactive isotopes. If acetyl CoA labeled with radioactive 14C in both carbon positions were fed into the citric acid cycle, where would the radioactivity be after one turn of the cycle?

in oxaloacetate

If acetyl CoA labeled with radioactive 14C in both carbon positions were fed into the citric acid cycle, where would the radioactivity be after two turns of the cycle?

in oxaloacetate and in CO2

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?

increase the proportion of phospholipids with unsaturated fatty acids

In cells, pyruvate can be converted to which of the following?

lactate glucose oxaloacetate acetyl CoA alanine

Order these components involved in electron transport from those with the lowest redox potential to the highest.

lowest NADH/NAD+ NADH dehydrogenase complex cytochrome c reductase cytochrome c oxidase H2o/O2

Sodium ions, oxygen (O2), and glucose pass directly through lipid bilayers at dramatically different rates. Which of the following choices presents the correct order, from fastest to slowest?

oxygen, glucose, sodium ions

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?

placing cells in an environment with a lower solute concentration than that in the cells

In cells that cannot carry out fermentation, which products derived from glycolysis will accumulate under anaerobic conditions?

pyruvate and NADH

Optogenetics promote depolarization

sodium channel and calcium channel

Based on these oxidation curves, place the listed components in the order in which electrons would be passed from one to the next along the electron-transport chain.

succinate, 1,2,3,4, o2

In the absence of oxygen, in cells that cannot carry out fermentation, glycolysis would halt at which step?

the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate

Why must all living cells carefully regulate the fluidity of their membranes?

to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides