Science Practice Challenge Questions

7.1 Energy in Living Systems


Combustion of carbohydrates, like in a fireplace, is a reduction-oxidation reaction in which the carbon atom is oxidized and the oxygen atom is reduced, producing water and carbon dioxide. Oxidative phosphorylation and glycolysis are also reduction-oxidation reactions that produce the same products. Explain the differences and similarities among these abiotic and biotic processes in terms of the changes in entropy and heat that contribute to the free energy extracted from chemical bonds, the spontaneity of each, and the role of catalysis.

7.3 Oxidation of Pyruvate and the Citric Acid Cycle


A. [Extension] Living systems require free energy to carry out cellular functions, and employ various strategies to capture, use, and store free energy. Explain the advantage that the higher energy efficiency per kg of the Krebs cycle provides to you compared to a metabolism based on glycolysis alone. Your explanation should make use of all the following facts:

  • ΔG for glycolysis is -135kJ per mole of glucose
  • ΔG for aerobic respiration is -2880kJ per mole glucose
  • the basal metabolic rate of mammals is often represented as -300kJ/day • m0.75
  • the molar mass of glucose is 180 g/mole

B. Explain the bioenergetic difference between aerobic and anaerobic respiration in terms of the difference between free-energy production and power. Your explanation should make use of all the following facts:

  • power is the rate of free-energy production
  • cancer cells derive most of their free energy from glycolysis
  • enzymes of the citric acid (Kreb’s) cycle form coordinate complexes on the cytoskeleton within the mitochondria

C. The life cycle of the human parasite Trypanosoma brucei is divided between the body of the tsetse fly and the human blood stream. The parasite causes “sleeping sickness” in Sub-Saharan Africa. Within the human bloodstream, the parasite depends on glycolysis, with enzymes compartmentalized in a membrane-bound organelle called the glycosome. In the insect host, the parasite utilizes glycolysis as well as substrate-level and oxidative phosphorylation. Explain the advantage of a life cycle in the human host that employs anaerobic respiration with a rate of free-energy production that is enhanced by compartmentalization in the glycosome and a life cycle in the insect host that is aerobic.

D. Predict the advantages of a biological system that uses both glycolysis and oxidative phosphorylation. Your prediction should make use of all the following facts:

  • signaling can be used to detect low-oxygen environments and to regulate response
  • some cells, such as muscle and blood cells, must function in both low- and high-oxygen environments
  • glycolysis is reversible
  • the citric acid cycle is not reversible
  • thermoregulation is needed for homeostasis

7.4 Oxidative Phosphorylation


Dinitrophenol (DNP) was used in the manufacture of munitions in World War I. In the 1930s, it was used as a weight loss drug. Use in the U.S. cannot be regulated by the FDA because DNP is considered a dietary supplement. Attempts to ban the drug in the U.K. following the death of four users in 2015 failed in Parliament. DNP is a small molecule that is soluble in the mitochondrial inner membrane. The hydroxyl group reversibly dissociates a proton.

The image shows the inner mitochondrial membrane, with the matrix on the right and the intermembrane space on the left. Two similar ring-shaped molecules, each with 6 carbon atoms and 3 double bonds, are embedded in the membrane. Both molecules have 2 N O 2 groups attached at nonadjacent carbon atoms. The first ring-shaped molecule, which is located on the left near the intermembrane space, also has an O H group attached. The second ring-shaped molecule, which is located on the right near the matrix, has
Figure 7.25

A. Predict the effect of DNP on the electrochemical gradient across the inner mitochondrial membrane.

B. Explain how DNP can be used to reduce weight.

C. The effects of DNP can be reversed by administering glucose. However, treatment with a combination of glucose and 2-deoxyglucose, which is an inhibitor of glycolysis, does not reverse the effects of DNP. Explain, in terms of the products of glycolysis, why this reversal of the effects of DNP was unexpected. (Hint: It might be useful to review the reactants and products of glycolysis.)

D. Obesity correlates with an epidemic of other health issues, such as elevated blood pressure, heart disease, and diabetes II. A slow-release form of DNP (CRMP) is patented. With slow-release technology, a drug can be delivered in small doses over time from a pill whose matrix limits solubility. A simple but nonscientific question that can be raised is: Will a slow-release drug retard progress toward behavioral changes that can reduce the magnitude of this epidemic? Scientific questions can be pursued by testing the outcomes predicted by possible answers. Refine this question for discussion in small groups. Be prepared to justify the merits of your question.


As shown in Figure 7.11, cyanide inhibits the electron transport chain by competing with O2 molecules for the cytochrome c oxidase heme group. Carbon monoxide (CO) has a similar effect. Both cyanide and carbon monoxide cause poisoning in victims of smoke inhalation.

A. Predict the effects of these poisons on the following properties of mitochondria just after exposure: the pH of the intermembrane space, the concentration of NADH, and the rate of production of ATP in the matrix. Justify your predictions.

B. Rotenone is a poison that blocks the transfer of electrons from Complex I of the electron transport chain to ubiquinone. Methylene blue is a molecule with many uses involving its reduction-oxidation properties. Recent studies show the effectiveness of methylene blue in increasing the body’s metabolic rate and as a treatment for Alzheimer’s patients. The oxidized form of methylene blue is reduced by NADH, and its reduced form is oxidized by O2. Explain the use of methylene blue as an antidote for rotenone poisoning.

7.5 Metabolism Without Oxygen


E. coli are enteric (gut-dwelling) facultative anaerobic bacteria. (Facultative anaerobes can grow either with or without free oxygen. Obligatory anaerobes grow only in the absence of free oxygen.) Researchers planned to grow cultures of E. coli under a range of conditions to model the transition from strictly anaerobic to aerobic respiration.

The oxygen content of atmospheres at constant total pressure will be controlled by volumes of nitrogen and oxygen gases. Ratios of volume, r = VO2/VN2 between 0 and 0.25 of shaken growth flasks can be measured in terms of optical density, which is the percent of transmission of light through a sample of the growing E. coli culture. A rule of thumb is that the range of strict anaerobes is when r 0.01, and the boundary for aerobic respiration is when r = 0.05. A large number of flasks that can be constantly shaken at fixed temperature, and from which samples can be taken without atmospheric contamination, are available for this study.

These results of the experiment will be used to infer growth rates of E. coli along the entire 7.5 m length of the average human intestine (small intestine and large intestine), where the oxygen content varies from atmospheric to anaerobic conditions. The retention time of food in the small intestine, whose average length is 2.5 m, is approximately four hours. The retention time of food over the entire length of the intestine is between 24 and 72 hours.

A. Describe and apply a mathematical model that can be used to represent the variation of oxygen environments of a bacterium that is being transported with the food along the length of the intestine.

B. Design the experimental sampling times in terms of growth intervals of interest in this study: (i) the time when the bacteria is passing the small-large intestine boundary; (ii) the time when the bacteria reaches the end of the large intestine; and (iii) the time when the bacteria reaches facultative anaerobic conditions, r 0.05.

C. Sketch a graph that predicts the distribution of aerobic, facultative anaerobic and obligatory anaerobic bacteria along the length of the entire intestine based on these parameters. Keep in mind that anaerobes have a lower respiration rate.

7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways


White snakeroot is a plant that contains chemicals that deactivate the enzyme lactate dehydrogenase. Humans who consume milk from cows or goats that eat white snakeroot can become ill. Symptoms of milk poisoning include vomiting, abdominal pain, and tremors, which become worse after exercise. Beyond childhood, most people do not express the enzyme lactase that catalyzes the breakdown of lactose into glucose and galactose. Consumption of milk can produce symptoms similar to those of milk poisoning. After a period of consumption of dairy foods, though, prebiotic adaptation (changes in the microbes in the intestine) imparts lactose tolerance. Since dairy foods are a valuable source of calcium, proteins, and vitamin D, considerable research has been conducted to characterize adaptation.

Explain the similarities and differences between the effect of milk poisoning by white snakeroot and lactose intolerance, and the possibility of prebiotic adaptation for each.