Question 21N.2.HL.TZ0.c
| Date | November 2021 | Marks available | [Maximum mark: 2] | Reference code | 21N.2.HL.TZ0.c |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Explain | Question number | c | Adapted from | N/A |
A community living in the water of an estuary was used to investigate how climate change may affect ecological systems. The food web in this community included phytoplankton (producers), zooplankton (consumers) and saprotrophic bacteria. Small plastic mesocosms were set up with water from the estuary containing only these three groups of organisms. The mesocosms were subjected to four different temperatures and two nutrient levels (control and nutrients added) to replicate local variations of the conditions in the estuary during springtime warming.
The graph shows the biomass of the community for each of the eight mesocosms at the end of the experimental period. Biomass was measured in terms of the amount of carbon present. The horizontal line indicates the initial biomass.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
The graph shows the mean biomass of autotrophs and heterotrophs in the eight mesocosms. The horizontal lines indicate the initial biomasses.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
Chlorophyll concentration was used as an estimate of the photosynthetic capacity of the community. The rate of photosynthesis and mass of chlorophyll per unit volume were measured in a mesocosm at three different temperatures.
[Source: adapted from O’Connor, M.I., Piehler, M.F., Leech, D.M., Anton, A. and Bruno, J.F., 2009.
PLOS Biology, [e-journal] 7(9). https://doi.org/10.1371/journal.pbio.1000178.]
Explain the effect of temperature on the rate of photosynthesis in this mesocosm.
[2]
rate of photosynthesis increases as temperature rises because:
a. temperature is the limiting factor for photosynthesis;
b. higher temperatures increase enzyme activity;
c. faster molecular motion/more molecular kinetic energy/more frequent enzyme-substrate collisions;
d. Calvin cycle/light independent reactions (of photosynthesis) speed up;
About half candidates merely stated the relationship shown in the graph, rather than actually explaining it. An explanation based on enzyme activity were expected. Performance in this question and in (d) correlated well with the overall performance of each candidate on the paper, probably because biological understanding was required. Other parts of question 1 were less well correlated, as is typical for data analysis questions.
