Questions and Answers of Essential Cell Biology

Phospholipid bilayers form sealed, spherical vesicles in water. Assume you have constructed lipid vesicles that contain Na+ pumps as the sole membrane protein, and assume for the sake of simplicity
Name the three ways in which an ion channel can be gated.
The resting membrane potential of a typical animal cell is about –70 mV, and the thickness of a lipid bilayer is about 4.5 nm. What is the strength of the electric field across the membrane in
The neurotransmitter acetylcholine is made in the cytosol and then transported into synaptic vesicles, where its concentration is more than 100-fold higher than in the cytosol. When synaptic vesicles
Discuss the following statement: “The differences between a channel and a transporter are like the differences between a bridge and a ferry.”
Name at least one similarity and at least one difference between the following (it may help review the definitions of the terms using the Glossary):A. Symport and antiportB. Active transport
List the following compounds in order of decreasing lipid bilayer permeability: RNA, Ca2+, glucose, ethanol, N2, water.
Many catabolic and anabolic reactions are based on reactions that are similar but work in opposite directions, such as the hydrolysis and condensation reactions described in Figure 3–39. This is
Looking at the chemistry detailed in the overview of the citric acid cycle at the top of the first page of Panel 13–2, why do you suppose it is useful to link the two-carbon acetyl group to
The complete oxidation of sugar molecules by the cell takes place according to the general reactionC6H12O6 (glucose) + 6O2 → 6CO2 + 6H2O + energy.Which of the following statements are correct?
After looking at the structures of sugars and fatty acids, give an intuitive explanation as to why oxidation of a sugar yields only about half as much energy as the oxidation of an equivalent dry
A cyclic reaction pathway requires that the starting material be regenerated and available at the end of each cycle. If compounds of the citric acid cycle are siphoned off as building blocks to make
What, if anything, is wrong with the following statement? “The oxygen consumed during the complete oxidation of glucose in animal cells is returned as part of CO2 to the atmosphere.” How could
Which are membrane-enclosed intracellular organelles, need an acidic lumen in order to function. Acidification is achieved by an H+ pump in the endosomal membrane, which also contains Cl– channels.
Some bacterial cells can grow on either ethanol (CH3CH2OH) or acetate (CH3COO–) as their only carbon source. Dr. Schwips measured the rate at which the two compounds traverse the bacterial plasma
Arsenate (AsO4 3–) is chemically very similar to phosphate (PO43–) and can be used as an alternative substrate by many phosphate-requiring enzymes. In contrast to phosphate, however, an
At first glance, the final steps in fermentation appear to be unnecessary: the generation of lactate or ethanol does not produce any additional energy for the cell. Explain why cells growing in the
An exceedingly sensitive instrument (yet to be devised) shows that one of the carbon atoms in Charles Darwin’s last breath is resident in your bloodstream, where it forms part of a hemoglobin
During movement, muscle cells require large amounts of ATP to fuel their contractile apparatus. These cells contain high levels of creatine phosphate (Figure Q13–11), which has a standard
Yeast cells can proliferate both in the presence of O2 (aerobically) and in its absence (anaerobically). Under which of the two conditions could you expect the cells to proliferate better?
Which of the following changes takes place when a skeletal muscle contracts?A. Z discs move farther apart.B. Actin filaments contract.C. Myosin filaments contract.D. Sarcomeres become shorter.
In cells that can proliferate both aerobically and anaerobically, fermentation is inhibited in the presence of O2. Suggest a reason for this observation.
Dinitrophenol (DNP) is a small molecule that renders membranes permeable to protons. In the 1940s, small amounts of this highly toxic compound were given to patients to induce weight loss. DNP was
Identical pathways that make up the complicated sequence of reactions of glycolysis, shown in Panel 13–1, are found in most living cells, from bacteria to humans. One could envision, however,
The chemistry of most metabolic reactions was deciphered by synthesizing metabolites containing atoms that are different isotopes from those occurring naturally. The products of reactions starting
In cells that can proliferate both aerobically and anaerobically, fermentation is inhibited in the presence of O2. Suggest a reason for this observation.
The chemistry of most metabolic reactions was deciphered by synthesizing metabolites containing atoms that are different isotopes from those occurring naturally. The products of reactions starting
The remarkable properties that allow ATP synthase to run in either direction allow the interconversion of energy stored in the H+ gradient and energy stored in ATP to proceed in either direction.(A)
Calculate the number of usable ATP molecules produced per pair of electrons transferred from NADH to oxygen if(i) Five protons are pumped across the inner mitochondrial membrane for each electron
At many steps in the electron transport chain, Fe ions are used as part of he me or FeS clusters to bind the electrons in transit. Why do these functional groups that carry out the chemistry of
Explain why cyclic AMP must be broken down rapidly in a cell to allow rapid signaling.
Why do you suppose cells have evolved intracellular Ca2+ stores for signaling even though there is abundant extracellular Ca2+?
Would you expect to activate RTKs by exposing the exterior of cells to antibodies that bind to the respective proteins? Would your answer be different for GPCRs?
Two different diffusible electron carriers, ubiquinone and cytochrome c, shuttle electrons between the three protein complexes of the electron-transport chain. Could the same diffusible carrier, in
As your first experiment in the laboratory, your adviser asks you to reconstitute purified bacteriorhodopsin, a light-driven H+ pump from the plasma membrane of photosynthetic bacteria, and purified
You mix the following components in a reconstituted membrane-bound system. Assuming that the electrons must follow the path specified in Figure 14–14, in which experiments would you expect a net
As shown in the drawings in Figure 15–3, the lipid bilayer of the inner and outer nuclear membranes forms a continuous sheet, joined around the nuclear pores. As membranes are two-dimensional
A. Predict the membrane orientation of a protein that is synthesized with an un cleaved, internal signal sequence (shown as the red start-transfer sequence in Figure 15–17) but does not contain a
Some bacteria have become specialized to live in an environment of high pH (pH ~10). Do you suppose that these bacteria use a proton gradient across their plasma membrane to produce their ATP?
Summarizes the circuitry used by mitochondria and chloroplasts to interconvert different forms of energy. Is it accurate to sayA. that the products of chloroplasts are the substrates for
Iron (Fe) is an essential trace metal that is needed by all cells. It is required, for example, for synthesis of the heme groups and iron sulfur centers that are part of the active site of many
Which of the following statements are correct? Explain your answers.A. Ribosomes are cytoplasmic structures that, during protein synthesis, become linked by an mRNA molecule to form polyribosomes.B.
During mitosis, the nuclear envelope breaks down and intranuclear proteins completely intermix with cytosolic proteins. Is this consistent with the evolutionary scheme proposed in Figure 15–3?
Why do eukaryotic cells require a nucleus as a separate compartment when prokaryotic cells can manage perfectly well without?
Dr. Out on a limb’s claim to fame is her discovery of forgettin, a protein predominantly made by the pineal gland in human teenagers. The protein causes selective, short-term unresponsiveness and
Explain how an mRNA molecule can remain attached to the ER membrane while individual ribosomes translating it are released and rejoin the cytosolic pool of ribosomes after each round of translation.
The budding of clathrin-coated vesicles from eukaryotic plasma membrane fragments can be observed when adaptins, clathrin, and dynamin-GTP are added to the me membrane preparation. What would you
Why might it be advantageous to add a preassembled block of 14 sugar residues to a protein in the ER, rather than building the sugar chains step-by-step on the surface the protein by the
What would you expect to happen in cells that secrete large amounts of through the regulated secretory pathway if the ionic conditions in the ER lumen could changed to resemble those in the lumen of
Taking the evolutionary scheme in Figure 15–3 one step further, suggest how the Golgi apparatus could have evolved. Sketch a simple diagram to illustrate your ideas. For the Golgi apparatus to be
Some proteins shuttle back and forth between the nucleus and the cytosol. They need a nuclear export signal to get out of the nucleus. How do you suppose they get into the nucleus?
What would happen to proteins bound for the nucleus if there were insufficient energy to transport them? Figure Q15-22 (A) Ⓒ (D) G (E) (B F
Influenza viruses enter the cell by receptor-mediated endocytosis. The viruses are surrounded by a membrane that contains a fusion protein, which is activated by the acidic pH in the endosome. Upon
Consider the v-SNAREs that direct transport vesicles from the trans-Golgi network to the plasma membrane. They, like all other v-SNAREs, are membrane proteins that are integrated into the membrane of
A particular type of Drosophila mutant becomes paralyzed when the temperature is raised. The mutation affects the structure of dynamin, causing it to be inactivated at a higher temperature. Indeed,
Edit each of the following statements, if required, to make them true: “Because nuclear localization sequences are not cleaved off by proteases following protein import into the nucleus, they can
Consider a protein that contains an ER signal sequence at its N-terminus and a nuclear localization sequence in its middle. What do you think the fate of this protein would be? Explain your answer.
Compare and contrast protein import into the ER and into the nucleus. List at least two major differences in the mechanisms, and speculate why the ER mechanism might not work for nuclear import and
A protein that inhibits certain proteolytic enzymes (proteases) is normally secreted into the bloodstream by liver cells. This inhibitor protein, antitrypsin, is absent from the bloodstream of
If membrane proteins are integrated into the ER membrane by means of the ER protein translocator (which is itself composed of membrane proteins), how do the first protein translocation channels
To keep their action local, paracrine signal molecules must be prevented from straying too far from their points of origin. Suggest different ways by which this could be accomplished. Explain your
In principle, how might an intracellular signaling protein amplify a signal as it relays it onward?
GPCRs activate G proteins by reducing the strength of GDP binding to the G protein. This results in rapid dissociation of bound GDP, which is then replaced by GTP, because GTP is present in the
The contraction of the myosin actin system in cardiac muscle cells is triggered by a rise in intracellular Ca2+. Cardiac muscle cells have specialized Ca2+ channels called ryanodine receptors because
Which of the following statements are correct? Explain your answers.A. The extracellular signal molecule acetylcholine has different effects on different cell types in an animal and often binds to
Consider the structure of cholesterol, a small, hydrophobic molecule with a sterol backbone similar to that of three of the hormones shown in Figure 16−40, but possessing fewer polar groups such as
The Ras protein functions as a molecular switch that is set to its “on” state by other proteins that cause it to release its bound GDP and bind GTP. A GTPase-activating protein helps reset the
A. Compare and contrast signaling by neurons, which secrete neurotransmitters at synapses, with signaling carried out by endocrine cells, which secrete hormones into the blood.B. Discuss the relative
Two intracellular molecules, X and Y, are both normally synthesized at a constant rate of 1000 molecules per second per cell. Molecule X is broken down slowly: each molecule of X survives on average
In a series of experiments, genes that code for mutant forms of an RTK are introduced into cells. The cells also express their own normal form of the receptor from their normal gene, although the
Discuss the following statement: “Membrane proteins that span the membrane many times can undergo a conformational change upon ligand binding that can be sensed on the other side of the membrane.
What are the similarities and differences between the reactions that lead to the activation of G proteins and the reactions that lead to the activation of Ras?
Why do you suppose cells use Ca2+ (which is kept by Ca2+ pumps at a cytosolic concentration of 10–7 M) for intracellular signaling and not another ion such as Na+ (which is kept by the Na+ pump at
Dynein arms in a cilium are arranged so that, when activated, the heads push their neighboring outer doublet outward toward the tip of the cilium. Consider a cross section of a cilium (see Figure
It seems counterintuitive that a cell, having a perfectly abundant supply of nutrients available, would commit suicide if not constantly stimulated by signals from other cells. What do you suppose
Two protein kinases, K1 and K2, function in an intracellular signaling pathway. If either kinase contains a mutation that permanently inactivates its function, no response is seen in cells when an
A. Trace the steps of a long and indirect signaling pathway from a cell-surface receptor to a change in gene expression in the nucleus.B. Compare this pathway with an example of a short and direct
How does PI 3-kinase activate the Akt kinase after activation of an RTK?
A. Note that in Figure 17−46, troponin molecules are evenly spaced along an actin filament, with one troponin found every seventh actin molecule. How do you suppose troponin molecules can be
The signaling mechanisms used by a steroid-hormone-type nuclear receptor and by an ion-channel-coupled receptor are relatively simple as they have few components. Can they lead to an amplification of
If some cell-surface receptors, including Notch, can rapidly signal to the nucleus by activating latent transcription regulators at the plasma membrane, why do most cell surface receptors use long,
Animal cells and plant cells have some very different intracellular signaling mechanisms but also share some common mechanisms. Why do you think this is so?
Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm? Explain your answers.A. Amoeba proteus (a free-living amoeba)B. Skin
Why do you suppose it is much easier to add tubulin to existing microtubules than to start a new microtubule from scratch? Explain how γ-tubulin in the centrosome helps to overcome this hurdle.
Dynamic instability causes microtubules either to grow or to shrink rapidly. Consider an individual microtubule that is in its shrinking phase.A. What must happen at the end of the microtubule in
The formation of actin filaments in the cytosol is controlled by actin binding proteins. Some actin binding proteins significantly increase the rate at which the formation of an actin filament is
At the leading edge of a crawling cell, the plus ends of actin filaments are located close to the plasma membrane, and actin monomers are added at these ends, pushing the membrane outward to form
If both the actin and myosin filaments of muscle are made up of subunits held together by weak noncovalent bonds, how is it possible for a human being to lift heavy objects?
Compare the structure of intermediate filaments with that of the myosin-II filaments in skeletal muscle cells. What are the major similarities? What are the major differences? How do the differences
Which of the following statements are correct? Explain your answers.A. Kinesin moves endoplasmic reticulum (ER) membranes along microtubules so that the network of ER tubules becomes stretched
In a series of experiments, genes that code for mutant forms of an RTK are introduced into cells. The cells also express their own normal form of the receptor from their normal gene, although the
Why do eukaryotic cells, and especially animal cells, have such large and complex cytoskeletons? List the differences between animal cells and bacteria that depend on the eukaryotic cytoskeleton.
Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm? Explain your answers.A. Amoeba proteus (a free-living amoeba)B. Skin
There are no known motor proteins that move on intermediate filaments. Suggest an explanation for this.
When cells enter mitosis, their existing array of cytoplasmic microtubules has to be rapidly broken down and replaced with the mitotic spindle that forms to pull the chromosomes into the daughter
The drug Taxol, extracted from the bark of yew trees, has an opposite effect to the drug colchicine, an alkaloid from autumn crocus. Taxol binds tightly to microtubules and stabilizes them when added
Chloroplasts have a third internal compartment, the thylakoid space, bounded by the thylakoid membrane. This membrane contains the photosystems, reaction centers, electron-transport chain, and ATP
Both NADPH and the related carrier molecule NADH are strong electron donors. Why might plant cells have evolved to rely on NADPH, rather than NADH, to provide the reducing power for biosynthesis?
A. How do cells in plant roots survive, since they contain no chloroplasts and are not exposed to light?B. Unlike mitochondria, chloroplasts do not have a transporter that allows them to export ATP
Which of the following statements are correct? Explain your answers.A. After an electron has been removed by light, the positively charged chlorophyll the reaction center of the first

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