| Date | May 2010 | Marks available | 2 | Reference code | 10M.2.HL.TZ1.1 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Describe | Question number | 1 | Adapted from | N/A |
Question
During aerobic cell respiration, oxygen is consumed and carbon dioxide is produced inside cells. This generates concentration gradients between respiring cells and the environment, which cause diffusion of oxygen and carbon dioxide. Both oxygen and carbon dioxide are soluble in water. As the temperature rises, water becomes saturated at a lower concentration of the gas.
Laternula elliptica is a mollusc that lives on the sea bed in Antarctica. Its body temperature is always similar to that of the environment around it. To investigate the effect of temperature on Laternula elliptica, specimens were kept in temperature-controlled aquaria. The oxygen concentrations of water near the gills and in the body fluids were measured, at a range of temperatures from 0°C to 9°C. The graph below shows the mean results.
The world’s oceans can absorb large amounts of carbon dioxide. This process has been studied in the Pacific Ocean near Hawaii, by measuring carbon dioxide concentrations in the atmosphere and in surface water every month, from October 1988 onwards. The graph below shows the carbon dioxide concentration expressed as partial pressures (Pco2).
The concentration of carbon dioxide in the atmosphere is currently 385 ppm (parts per million). Variations in the concentration of carbon dioxide in the atmosphere can be studied using ice-cores. An ice-core record covering the last 400 000 years has been obtained from Vostok in the Antarctic. The graph below shows the carbon dioxide concentrations that were measured at different depths in the ice. Atmospheric temperatures are also shown on the graph. These were deduced from ratios of oxygen isotopes. The upper line on the graph shows CO2 concentrations and the lower line shows temperature.
Outline the relationship between temperature and oxygen concentration in the body fluids in Laternula elliptica.
Suggest two reasons for the relationship.
In its natural environment, Laternula elliptica buries itself in the mud on the sea bed. In this investigation, it was found that above 6°C it is unable to bury itself. Suggest a reason for this.
Describe the trends in atmospheric carbon dioxide concentration, shown in the graph.
Suggest two reasons for the trends that you have described.
Diffusion of carbon dioxide only occurs when there is a concentration gradient. Deduce the pattern of carbon dioxide diffusion, between water and atmosphere, from 1988 to 2002.
The graph provides evidence for the hypothesis that there will be no net diffusion of carbon dioxide between water and atmosphere by 2020. Explain this evidence.
State the highest carbon dioxide concentration shown on the graph.
State the highest temperature shown on the graph.
Using the data in the graph, deduce the relationship between atmospheric carbon dioxide concentration and temperature.
Using the data in this question, explain reasons for concern about the long-term survival of Antarctic species, such as Laternula elliptica.
Markscheme
oxygen concentration falls as temperature rises / negative correlation/inverse relationship;
steady decline below 4.2/4.3/4.4°C / vice versa:
rapid decrease between 4.2/4.3/4.4 °C and 5°C;
zero oxygen concentration at/above 9°C;
warmer water can hold less oxygen / lower oxygen solubility as temperature rises;
lower oxygen concentration of water reaching gills / less oxygen available from the water to diffuse into the gills;
higher metabolic rate / faster rates of respiration / more oxygen consumption as temperature rises;
not enough energy/ATP/aerobic respiration (for muscle contraction/movement)
rising trend overall;
annual rise and fall / fluctuations;
(CO2 emissions from) increased burning of fossil fuels/deforestation/other anthropogenic factor;
variation in photosynthesis rates during the year / variations in CO2 uptake in the oceans;
diffusion in both directions during each year;
diffusion from atmosphere to water during most of the year;
diffusion from water to atmosphere for part of year/autumn/fall/seasonal;
increasing diffusion from water to atmosphere in later years;
(no net diffusion because) concentrations will become equal / there will be no gradient;
water concentration higher than atmospheric concentration as often as atmospheric concentration higher than water concentration;
300 ppm (Allow answers in the range 295–305 ppm) unit must be included to earn mark.
3.3°C (Allow answers in the range 3.0–3.3°C) unit must be included to earn mark.
N.B. A maximum of [1] per exam can be deducted for a missing unit.
positive correlation / higher temperature with higher CO2 concentration
oceans may cease to act as sink / store for CO2;
atmospheric CO2 concentration may then rise more rapidly;
atmospheric CO2 concentration is higher than for at least 400 000 years/any time in recent (geological) time;
Antarctic temperatures will (probably) rise higher than at any time in 400 000 years/any time in recent (geological) time;
rising (sea water) temperature would reduce oxygen availability in water;
significant changes in habitat/abiotic factors;
populations may not be able to adapt;
Examiners report
Many candidates stopped at outlining one aspect of the data.
Candidate's gave one factual statement rather than the required two.
Many were able to successfully link the concept of aerobic respiration to the question.
There was poor understanding of the second graph. Even those who spotted the regular oscillation usually linked this to a 2 year interval rather than an annual cycle.
Some answers to (c) ii) were vague making reference to "pollution‟ as the cause. Some showed confusion by arguing that the general rise in CO2 was caused by the greenhouse effect rather than being the cause of the greenhouse effect. Candidates should be familiar with the atmosphere data as this is required in the syllabus.
For the greatest number of candidates, the pattern discussed in (d) i) was that of concentration rather than the pattern of diffusion. There was very little reference to time data.
Many were able to discern that the lines would intersect, so that there would be no net diffusion.
Very few failed to put in the correct units though many included the solidus in the units; i.e., the units were CO2 / ppm rather than just ppm.
Surprisingly some candidates managed to get the highest temperature wrong. Very few failed to put in the correct units though many included the solidus in the units; i.e., the units were CO2 / ppm rather than just ppm.
The greatest number of candidates earned this mark. The graph suggests that CO2 levels were the cause of the temperature. The markscheme did not penalize candidates for this, but candidates did confuse the dependent and independent relationship.
There were two aspects of the prompt that were commonly left unaddressed by candidates. “Using the data in this question....” invited the candidates to consider all of the data but many referenced just the first graph on page 2 of the booklet. “...Antarctic species, such as ...” was meant for candidates to discuss organisms more broadly than the particular species in the question. Some talked about habitats other than the Antarctic ocean.
