BioG 1440 Problem Set 2 (with answers) 1. What are the units for expressing the maximal rate of oxygen consumption (VO2max)? Answer: ml O2/kg•min 2. Why is VO2max measured on a weight-specific basis? Give an example of why this is useful. Answer: Doing so enables us to make meaningful comparisons of metabolic rates between animals that differ in mass. For example, we can compare the metabolic rates (per kg) of a running mouse and a running human, and see that even though a mouse consumes less oxygen to run a given distance, it consumes more oxygen per unit body mass. 3. Which of the following statements regarding maximal rate of oxygen consumption (VO2max) is false? Give an example that illustrates why it is false. a. It is the same for individuals within a species but is different between species. b. It determines the peak rate at which an animal can synthesize ATP in a pay-as-you-go mode. c. It determines how quickly an animal can cover large distances because it determines the maximum sustained speed of an animal. d. It provides a benchmark by which to judge the strenuousness of all aerobic physical activity. Answer: a is false. For example, individual humans vary considerably in the maximal rate of oxygen consumption (e.g. Olympic cross country skier vs. your average Joe or Jane). 4. In osmosis, why does water move across the membrane from the solution with a low solute concentration to the solution with a high solute concentration? Hint: think about what happens at the molecular level. Answer: The solution with the lower solute concentration has a higher water concentration. This means that there is a higher density of water molecules on the side of the membrane that has the lower solute concentration. Because this side of the membrane has a higher density of water molecules, each pore through which water can pass is more likely to be “hit” by a water molecule from this side of the membrane (theone with a higher water density) than from the other side of the membrane (the one with a lower water density). 5. Reverse osmosis is a process whereby pressure is applied to a solution with a higher concentration of solutes (e.g. seawater) and water accumulates on the other, unpressurized side of a semipermeable membrane, thereby producing a solution with a lower concentration of solutes (e.g. fresh water). Explain why reverse osmosis works. Why does increasing the pressure on the solution with a higher concentration of solute result in water moving from this solution into the one with a lower concentration of solute? Answer: The water moves as it does because the pressure applied to the solution with a higher solute concentration increases the density of the water molecules on the surface of the semipermeable membrane sufficiently so that water molecules on this side are more likely to “hit” and pass through the pores than are the water molecules on the other (unpressurized) side, where the density of the water molecules is lower. 6. Where exactly inside a cell is the oxygen consumed in aerobic metabolism? Answer: In the mitochondria, during the last steps of oxidate phosphorylation. 7. We inhale O2 and exhale CO2. Do the inhaled oxygen atoms leave our bodies in the CO2 that we exhale? If not, then how do the oxygen atoms in the O2 molecules that we breath in get back out of our bodies? And where do the carbon and oxygen atoms come from that are in the CO2 we exhale? Answer: No, the oxygen atoms we inhale leave our bodies in the water that is a product of aerobic metabolism, not in the CO2. The carbon and oxygen atoms that we exhale as CO2 come from the food molecules we metabolize. 8. Explain why in Fig. 7.7, part b, the label for the y-axis reads “Rate of O2 demand or supply” rather than “Rate of O2 consumption or supply.” Answer: It says “demand” rather than “consumption” because when an animal starts exercising not all of the ATP it uses is produced by consuming O2 from the environment. So how else is the ATP produced? Anaerobic gycolysis, stored O2, and phosphagen molecules.9. Ornithologists have described birds that are about to undertake long-distance migratory flights as “butter balls” because they have large fat stores. Why have these birds stored up energy as fats (lipids) rather than carbohydrates? And is there a difference in the peak rate of ATP production from carbohydrates and fats as fuels? If so, then why can they get by with fats? Answer: The energy density of fats is higher than that of carbohydrates, so fats are a less bulky way to carry the fuel for the flight. And while it is true that the peak rate of ATP production is lower for fats than carbohydrates, migrating birds are just cruising steadily along, not scrambling to avoid a danger, so they can get by with a lower rate of ATP production. 10. Lactate dehydrogenase causes a build up of lactic acid in the muscles of animals, causing some people to develop muscle cramps when exercising. Since this clearly hinders athletic performance, what is the advantage to having this enzyme in muscles? Answer: It enables cells to produce ATP without oxygen, which is useful since sometimes lots of ATP are needed in cells before oxygen can be delivered to the cells at a rate sufficient to produce the needed ATP aerobically. 11. You are a member of the Cornell women’s rowing crew, you weigh 50 kg, and you’ve just had your VO2max measured at the energetics facility in Martha Van Rensselaer Hall. You’ve been told that your VO2max is exactly 3 liters of O2 per minute. a) What is your maximum sustained power output, expressed in terms of watts (=joules per second)? Note: Assume that in aerobic catabolism the consumption of 1 liter of oxygen makes available to you 20 kilojoules of energy. Answer: 3 liters O2 consumed per minute x 20 kilojoules/liter O2 consumed = 60 kilojoules per minute 60 kilojoules per minute = 1 kilojoule per second 1 kilojoule per second = 1000 joules/sec = 1000 watts (= ten 100-watt light bulbs!) b) What is your weight-specific metabolic rate, measured in ml O2/kg•min? Answer:3 liters O2 per min = 3000 ml O2 /min 3000 ml O2/min /50 kg = 60 ml O2/kg•min (impressive!) c) Do you have an Olympic level VO2max? Almost! A world-class athlete in an event requiring tremendous aerobic capacity (e.g., cross-country skiing, long-distance running,