Fig. 1 shows a school investigation into decomposition in a freshwater pond. Students place equal masses of dead reed leaves in two mesh bags. One bag is he...
Question 1 Report
Fig. 1 shows a school investigation into decomposition in a freshwater pond. Students place equal masses of dead reed leaves in two mesh bags. One bag is held in flowing, oxygen-rich water near a fountain. The other rests in still mud at the pond edge. Both bags are recovered after three weeks and dried before their masses are compared. The mesh allows bacteria, fungi and small invertebrates to enter but prevents leaves washing away. The students also notice that the mud is darker and has a sour smell, suggesting little oxygen is present.
(a) Describe one reason for using mesh bags in this investigation. [2]
(b) Name two types of organism that may cause the leaves to decay. [3]
(c) Use Fig. 1 to predict which bag will have the smaller dry mass after three weeks. Explain your prediction. [4]
(d) Explain why drying the leaves before measuring mass improves the validity of the comparison. [4]
(e) What conclusion about oxygen and decay could the students make if bag A has a much smaller dry mass than bag B? [5]
Answer Details
(a) [2] Mesh bags allow decomposers and small invertebrates to enter, while preventing leaves being washed away and keeping them together for recovery.
(b) [3] Two organisms that may cause decay are bacteria and fungi. Detritivores or small invertebrates, such as freshwater shrimp, are also acceptable. These organisms act as decomposers. A third correct example gains the additional marking point.
(c) [4] Bag A should have the smaller dry mass. It is in flowing water near the fountain, where more dissolved oxygen is available. Aerobic microorganisms can respire and decay the leaves faster, leaving less leaf material after three weeks.
(d) [4] Leaves can hold different amounts of water. Wet mass would include this water, which is not leaf material. Drying means the measured mass is mainly the remaining leaf material. Therefore a mass difference is more validly attributed to decay rather than unequal water content.
(e) [5] If bag A has a much smaller dry mass, decay was faster in the oxygen-rich condition. Its smaller dry mass shows less leaf material remained. Oxygen allows decomposers to carry out aerobic respiration, releasing energy for growth and enzyme-controlled breakdown. Therefore the lack of oxygen in still mud slows decomposition.
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