Molecular Biology of the Cell

Bruce Alberts, Alexander Johnson, Julian Lewis

Chapter 2 Cell Chemistry and Biosynthesis - all with Video Answers

Educators

Chapter Questions

01:29

Problem 1

Of the original radioactivity in a sample, only about $1 / 1000$ will remain after 10 half-lives.

Rabeya Zahid

Numerade Educator

02:12

Problem 2

A $10^{-8} \mathrm{M}$ solution of HCl has a pH of 8

Joanna Quigley

Numerade Educator

01:32

Problem 3

Most of the interactions between macromolecules could be mediated just as well by covalent bonds as by noncovalent bonds.

Joanna Quigley

Numerade Educator

00:41

Problem 4

Animals and plants use oxidation to extract energy from food molecules.

Joanna Quigley

Numerade Educator

00:38

Problem 5

If an oxidation occurs in a reaction, it must be accompanied by a reduction.

Joanna Quigley

Numerade Educator

02:55

Problem 6

Linking the energetically unfavorable reaction $A \rightarrow B$ to a second, favorable reaction $\mathrm{B} \rightarrow \mathrm{C}$ will shift the equilib rium constant for the first reaction.

Ummatul Choudary

Numerade Educator

01:38

Problem 7

The criterion for whether a reaction proceeds spontaneously is $\Delta G$ not $\Delta G^{\circ},$ because $\Delta G$ takes into account the concentrations of the substrates and products.

Joanna Quigley

Numerade Educator

02:19

Problem 8

Because glycolysis is only a prelude to the oxidation of glucose in mitochondria, which yiclds 15 -fold more ATP, glycolysis is not really important for human cells.

Ummatul Choudary

Numerade Educator

01:45

Problem 9

The oxygen consumed during the oxidation of glucose in animal cells is returned as $\mathrm{CO}_{2}$ to the atmosphere.

Joanna Quigley

Numerade Educator

01:34

The organic chemistry of living cells is said to be special for two reasons: it occurs in an aqueous environment and it accomplishes some very complex reactions. But do you suppose it is really all that much different from the organic chemistry carried out in the top laboratories in the world? Why or why not?

Joanna Quigley

Numerade Educator

05:07

Problem 11

The molecular weight of ethanol $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right)$ is 46 and its density is $0.789 \mathrm{g} / \mathrm{cm}^{3}$
A. What is the molarity of ethanol in beer that is $5 \%$ ethanol by volume? [Alcohol content of beer varies from about $4 \%$ (lite beer) to $8 \%$ (stout beer).
B. The legal limit for a driver's blood alcohol content varies, but $80 \mathrm{mg}$ of ethanol per $100 \mathrm{mL}$ of blood (usually referred to as a blood alcohol level of 0.08 ) is typical. What is the molarity of ethanol in a person at this legal limit?
C. How many $12-02(355-\mathrm{mL})$ bottles of $5 \%$ beer could a 70 -kg person drink and remain under the legal limit? $\mathrm{A} 70-\mathrm{kg}$ person contains about 40 liters of water. Ignore the metabolism of ethanol, and assume that the water content of the person remains constant.
D. Ethanol is metabolized at a constant rate of about 120 mg per hour per kg body weight, regardless of its concentration. If a $70-\mathrm{kg}$ person were at twice the legal limit (160 $\mathrm{mg} / 100 \mathrm{mL}$ ), how long would it take for their blood alcohol level to fall below the legal limit?

Ummatul Choudary

Numerade Educator

06:49

Problem 12

Specific activity refers to the amount of radioactivity per unit amount of substance, usually in biology expressed on a molar basis, for example, as Ci/mmol. IOne curie (Ci) corresponds to $2.22 \times 10^{12}$ disintegrations per minute (dpm).] As apparent in Table Q2-1, which lists properties of four isotopes commonly used in biology, there is an inverse relationship between maximum specific activity and half-life. Do you suppose this is just a coincidence or is there an underlying reason? Explain your answer.

Rabeya Zahid

Numerade Educator

05:49

Problem 13

By a convenient coincidence the ion product of water, $K_{w}=\left[\mathrm{H}^{+}\right]\left[\mathrm{OH}^{-}\right],$ is a nice round number: $1.0 \times 10^{-14} \mathrm{M}^{2}$
A. Why is a solution at $\mathrm{pH} 7.0$ said to be neutral?
B. What is the H' concentration and pH of a 1 mM solution of NaOH?
C. If the $\mathrm{pH}$ of a solution is $5.0,$ what is the concentration of $\mathrm{OH}^{-}$ ions?

Ummatul Choudary

Numerade Educator

02:11

Problem 14

Suggest a rank order for the p $K$ values (from lowest to highest) for the carboxyl group on the aspartate side chain in the following environments in a protein. Explain your ranking.
1. An aspartate side chain on the surface of a protein with no other ionizable groups nearby.
2. An aspartate side chain buried in a hydrophobic pocket on the surface of a protein.
3. An aspartate side chain in a hydrophobic pocket adjacent to a glutamate side chain.
4. An aspartate side chain in a hydrophobic pocket adjacent to a lysine side chain.

Mathew Botros

Montclair State University

01:07

Problem 15

A histidine side chain is known to play an important role in the catalytic mechanism of an enzyme; however, it is not clear whether histidine is required in its protonated (charged) or unprotonated (uncharged) state. To answer this question you measure enzyme activity over a range of $\mathrm{pH}$ with the results shown in Figure $Q 2-1 .$ Which form of histidine is required for enzyme activity?

Joanna Quigley

Numerade Educator

01:52

Problem 16

During an all-out sprint, muscles metabolize glucose anaerobically, producing a high concentration of lactic acid, which lowers the pH of the blood and of the cytosol and contributes to the fatigue sprinters experience well before their fuel reserves are exhausted. The main blood buffer against pH changes is the bicarbonate/CO $_{2}$ system.
To improve their performance, would you advise sprinters to hold their breath or to breathe rapidly for a minute immediately before the race? Explain your answer.

Ummatul Choudary

Numerade Educator

05:40

Problem 17

The three molecules in Figure $Q 2-2$ contain the seven most common reactive groups in biology. Most molecules in the cell are built from these functional groups. Indicate and name the functional groups in these molecules.

Joanna Quigley

Numerade Educator

02:09

Problem 18

"Diffusion" sounds slow-and over everyday distances it is- but on the scale of a cell it is very fast. The average instantaneous velocity of a particle in solution, that is, the velocity between collisions, is
\[v=(k T / m)^{1 / 2}\]
where $k=1.38 \times 10^{-16} \mathrm{g} \mathrm{cm}^{2} / \mathrm{K} \mathrm{sec}^{2}, T=$ temperature in $\mathrm{K}$
$\left(37^{\circ} \mathrm{C} \text { is } 310 \mathrm{K}\right), m=$ mass in $\mathrm{g} /$ molecule.

Alex Thoms

Numerade Educator

01:44

Problem 19

Pulymerization of tubulin subunits into microtubules occurs with an increase in the orderliness of the subunits (Figure $Q 2-3$ ). Yet tubulin polymerization occurs with an increase in entropy (decrease in order). How can that be?

Ummatul Choudary

Numerade Educator

01:19

Problem 20

A 70 -kg adult human $(154$ lb) could meet his or her entire energy needs for one day by eating 3 moles of glucose $(540 \mathrm{g}) .$ (We don't recommend this.) Each molecule of glucose generates 30 ATP when it is oxidized to $\mathrm{CO}_{2}$. The concentration of ATP is maintained in cells at about $2 \mathrm{mM}$, and a $70-\mathrm{kg}$ adult has about 25 liters of intracellular fluid. Given that the ATP concentration remains constant in cells, calculate how many times per day, on average, each ATP molecule in the body is hydrolyzed and resynthesized.

Ummatul Choudary

Numerade Educator

01:27

Problem 21

Assuming that there are $5 \times 10^{13}$ cells in the human body and that ATP is turning over at a rate of $10^{9}$ ATP per minute in each cell, how many watts is the human body consuming? (A watt is a joule per second, and there are 4.18 joules/calorie.) Assume that hydrolysis of ATP yields 12 kcal/mole.

Ummatul Choudary

Numerade Educator

01:36

Problem 22

Does a Snickers $^{\mathrm{TM}}$ candy bar $(65 \mathrm{g}, 325 \text { kcal })$ provide enough energy to climb from Zermatt (elevation $1660 \mathrm{m}$ ) to the top of the Matterhorn $(4478 \mathrm{m}, \text { Figure } \mathrm{Q} 2-4),$ or might you need to stop at Hörnli Hut $(3260 \mathrm{m})$ to eat another one? Imagine that you and your gear have a mass of $75 \mathrm{kg}$, and that all of your work is done against gravity (that is, you are just climbing straight up). Remember from your introductory physics course that
\[\text { work }(\mathrm{J})=\operatorname{mass}(\mathrm{kg}) \times g\left(\mathrm{m} / \mathrm{sec}^{2}\right) \times \text { height gained }(\mathrm{m})\]
where $g$ is acceleration due to gravity $\left(9.8 \mathrm{m} / \mathrm{sec}^{2}\right) .$ One joule is $1 \mathrm{kg} \mathrm{m}^{2} / \mathrm{sec}^{2}$ and there are $4.18 \mathrm{kJ}$ per kcal.
What assumptions made here will greatly underestimate how much candy you need?

Ummatul Choudary

Numerade Educator

03:39

Problem 23

glycolysis. After all, could cells growing in the absence of oxygen not simply discard pyruvate as a waste product? In the absence of fermentation, which products derived from glycolysis would accumulate in cells under anaerobic conditions? Could the metabolism of glucose via the glycolytic pathway continue in the absence of oxygen in cells that cannot carry out fermentation? Why or why not?

Ummatul Choudary

Numerade Educator

01:21

Problem 24

In the absence of oxygen, cells consume glucose at a high, steady rate. When oxygen is added, glucose consumption drops precipitously and is then maintained at the lower rate. Why is glucose consumed at a high rate in the absence of oxygen and at a low rate in its presence?

Joanna Quigley

Numerade Educator

01:36

Problem 25

The liver provides glucose to the rest of the body between meals. It does so by breaking down glycogen, forming glucose 6 -phosphate in the penultimate step. Glucose 6 phosphate is converted to glucose by splitting off the phosphate $\left(\Delta G^{\circ}=-3.3 \mathrm{kcal} / \mathrm{mole}\right) .$ Why do you suppose the liver removes the phosphate by hydrolysis, rather than reversing the reaction by which glucose 6 -phosphate (G6P) is formed from glucose (glucose $+\mathrm{ATP} \rightarrow \mathrm{G} 6 \mathrm{P}+\mathrm{ADP}, \Delta G^{\circ}=-4.0$
kcal/mole)? By reversing this reaction the liver could generate both glucose and ATP.

Ummatul Choudary

Numerade Educator

03:50

Problem 26

In 1904 Franz Knoop performed what was probably the first successful labeling experiment to study metabolic pathways. He fed many different fatty acids labeled with a terminal benzene ring to dogs and analyzed their urine for excreted benzene derivatives. Whenever the fatty acid had an even number of carbon atoms, phenylacetate was excreted (Figure $Q 2-5$ A). Whenever the fatty acid had an odd number of carbon atoms, benzoate was excreted (Figure $Q 2-5 B$ ).
From these experiments Knoop deduced that oxidation of fatty acids to $\mathrm{CO}_{2}$ and $\mathrm{H}_{2} \mathrm{O}$ involved the removal of two-carbon fragments from the carboxylic acid end of the chain.

Shazia Naz

Numerade Educator

03:31

Problem 27

Pathways for synthesis of amino acids in microorganisms were worked out in part by cross-feeding experiments among mutant organisms that were defective for individual steps in the pathway, Results of cross-feeding experiments for three mutants defective in the tryptophan pathway $-$ Trp $B$, Trp $D$, and Trp$E$ -are shown in Figure Q2-6. The mutants were streaked on a Petri dish and allowed to grow briefly in the presence of a very small amount of tryptophan, producing three pale streaks. As shown, heavier growth was observed at points where some streaks were close to other streaks. These spots of heavier growth indicate that one mutant can cross-feed (supply an intermediate) to the other one. From the pattern of cross-feeding shown in Figure $\mathrm{Q} 2-6$ deduce the order of the steps controlled by the products of the TrpB, TrpD, and TrpE genes. Explain your reasoning.

Khalida Dawar

Numerade Educator

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